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Upper-Tropospheric Environment-Tropical Cyclone Interactions over the Western North Pacific: A Statistical Study

doi: 10.1007/s00376-015-5148-x

  • Based on 25-year (1987-2011) tropical cyclone (TC) best track data, a statistical study was carried out to investigate the basic features of upper-tropospheric TC-environment interactions over the western North Pacific. Interaction was defined as the absolute value of eddy momentum flux convergence (EFC) exceeding 10 m s-1 d-1. Based on this definition, it was found that 18% of all six-hourly TC samples experienced interaction. Extreme interaction cases showed that EFC can reach ∼120 m s-1 d-1 during the extratropical-cyclone (EC) stage, an order of magnitude larger than reported in previous studies. Composite analysis showed that positive interactions are characterized by a double-jet flow pattern, rather than the traditional trough pattern, because it is the jets that bring in large EFC from the upper-level environment to the TC center. The role of the outflow jet is also enhanced by relatively low inertial stability, as compared to the inflow jet. Among several environmental factors, it was found that extremely large EFC is usually accompanied by high inertial stability, low SST and strong vertical wind shear (VWS). Thus, the positive effect of EFC is cancelled by their negative effects. Only those samples during the EC stage, whose intensities were less dependent on VWS and the underlying SST, could survive in extremely large EFC environments, or even re-intensify. For classical TCs (not in the EC stage), it was found that environments with a moderate EFC value generally below ∼25 m s-1 d-1 are more favorable for a TC's intensification than those with extremely large EFC.
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  • Barcikowska M., F. Feser, and H. von Storch, 2012: Usability of best track data in climate statistics in the western North Pacific. Mon. Wea. Rev., 140, 2818- 2830.10.1175/ Tropical cyclone (TC) activity for the last three decades shows strong discrepancies, deduced from different best track datasets (BTD) for the western North Pacific (WNP). This study analyzes the reliability of BTDs in deriving climate statistics for the WNP. Therefore, TC lifetime, operational parameters [current intensity (CI) number], and tracks are compared (for TCs identified concurrently) in BTD provided by the Joint Typhoon Warning Center (JTWC), the Japan Meteorological Agency (JMA), and the China Meteorological Administration (CMA). The differences between the BTD are caused by varying algorithms used in weather services to estimate TC intensity. Available methods for minimizing these discrepancies are not sufficient. Only if intensity categories 2-5 are considered as a whole, do trends for annually accumulated TC days show a similar behavior. The reasons for remaining discrepancies point to extensive and not regular usage of supplementary sources in JTWC. These are added to improve the accuracy of TC intensity and center position estimates. Track and CI differences among BTDs coincide with a strong increase in the number of intense TC days in JTWC. These differences are very strong in the period of intensive improvement of spatiotemporal satellite coverage (1987-99). Scatterometer-based data used as a reference show that for the tropical storm phase JMA provides more reliable TC intensities than JTWC. Comparisons with aircraft observations indicate that not only homogeneity, but also a harmonization and refinement of operational rules controlling intensity estimations, should be implemented in all agencies providing BTD.
    Bosart L. F., J. A. Bartlo, 1991: Tropical storm formation in a baroclinic environment. Mon. Wea. Rev., 119, 1979- 2013.10.1175/1520-0493(1991)119<1979:TSFIAB>2.0.CO; An analysis is presented of the large-scale conditions associated with the initial development of Tropical Storm Diana (September 1984) in a baroclinic environment. Ordinary extratropical wave cyclogenesis began along an old frontal boundary east of Florida after 0000 UTC 7 September and culminated in tropical cyclogenesis 48 h later. Water-vapor satellite imagery showed that the initial cyclogenesis and incipient tropical storm formation was nearly indistinguishable from a classical midlatitude development. Cyclogenesis occurred in three stages. A large-scale cold trough and associated frontal system crossed the Atlantic coast, while a small potential vorticity maximum aloft fractured off the main trough and stalled over central Florida in the first stage. As the main trough sheared off eastward, cyclogenesis began along the southwestern end of the stalled frontal zone east of Florida. Anticyclogenesis to the north in the wake of the shearing trough allowed a surge of cooler and drier air to flow southeastward behind the front toward the developing cyclone. Combined surface sensible and latent heat fluxes in excess of 1000 W m 2 acted on this inflowing air, producing a warming and moistening of the boundary layer. Cyclogenesis intensified during the second stage in response to positive potential vorticity advection aloft ahead of the slow moving cutoff cyclone over Florida. The maximum ascent was centered near 300 mb, indicative of deep tropospheric ascent and cyclonic vorticity production by convergence in midlevels. The ascent occurred along uplifted isentropic surfaces that defined the cold dome associated with the potential vorticity anomaly aloft. Low-level potential vorticity was generated in the vicinity of the developing storm below the presumed level of maximum diabatic heating. The third stage of cyclogenesis was marked by the collapse of the mid- and upper-tropospheric cold dome and associated potential vorticity maximum and the simultaneous initiation of a warm thickness ridge. This occurred in response to the widespread outbreak of convection at the southwestern end of the baroclinic zone, where the greatest destabilization occurred for air parcels subject to prolonged surface sensible and latent heat fluxes in the persistent northeasterly flow. Upright ascent associated with the convection short-circuited the slantwise ascent ahead of the advancing potential vorticity anomaly, triggering warming aloft and the eventual disappearance of the potential vorticity anomaly and associated cold dome. Tropical storm development and intensification occurred as the low-level vorticity center (potential vorticity maximum) moved northwestward to become situated beneath the midlevel vortex embedded within a local 500-200 mb warm thickness anomaly. The interaction of the upper- and lower-level potential vorticity anomalies appeared to be important in the initial strengthening of the tropical cyclone. The interpretation is equivalent to earlier energetic arguments by Riehl and others that tropical cyclogenesis is often preceded by the collapse of a nearby cold dome.
    Bosart L. F., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev., 128, 322-
    Bracken W. E., L. F. Bosart, 2000: The role of synoptic-scale flow during tropical cyclogenesis over the North Atlantic Ocean. Mon. Wea. Rev., 128, 353-
    Challa M., R. L. Pfeffer, 1980: Effects of eddy fluxes of angular momentum on model hurricane development. J. Atmos. Sci., 37, 1603-
    Chan J. C. L., F. M. F. Ko, and Y. M. Lei, 2002: Relationship between potential vorticity tendency and tropical cyclone motion. J. Atmos. Sci., 59, 1317- 1336.10.1175/1520-0469(2002)0592.0.CO; This paper proposes a consistent conceptual framework to explain tropical cyclone (TC) motion based on the concept of potential vorticity tendency (PVT) and to verify this framework based on analyses of different observational datasets. The framework suggests that a TC is likely to move toward an area of maximum wavenumber-1 (WN1) PVT, which is mainly contributed by the corresponding WN1 components of potential vorticity (PV) advection and diabatic heating (DH). The PV advection process consists of advection of symmetric PV by the asymmetric flow [AASPV, which includes, but is not limited to, the environmental -teering flow- and the beta-induced circulation (the so-called ventilation flow)] and the advection of asymmetric PV by the symmetric flow (SAAPV). The asymmetric PV includes any asymmetry in the TC circulation, the beta gyres and contributions from asymmetric convective heating. The modification of PVT by the DH process depends on the vertical gradient of convective heating and the coupling between horizontal gradient of convective heating and vertical wind shear. In steady (i.e., without much change in direction or speed) TC motion, the PV advection processes are generally dominant while the contribution by DH is usually less significant. However, the latter process becomes important for irregular TC motion. Changes in TC motion are then not only caused by those in steering, but can also be induced by variations in the other processes. Composites of the Met Office operational analyses associated with TCs that had similar and relatively steady motion are first made to verify the contribution by the advection terms. In all motion categories examined, while the magnitude of the AASPV term is found to be generally dominant, its maximum is not downstream of the TC motion. The SAAPV term also contributes to the overall PV advection. The sum of these two terms gives a maximum at a location that generally aligns with the direction of TC motion. The contribution of the DH process to PVT, and hence TC motion, is then examined using satellite-derived temperatures from high-resolution geosynchronous satellite images for individual TCs. It is found that DH appears to be important especially for slow-moving TCs. Track oscillations as well as irregular track changes may be explained by changes in the convection pattern that lead to variations in the location of maximum WN1 DH. The entire PVT concept is further investigated using analyses from the Tropical Cyclone Motion Experiment TCM-90 for individual TCs with different track types. The results are consistent with those from the composites (for straight-moving cases) as well as from the satellite image analyses (for the irregular-moving case). Further, in the recurving case, the locations of the maximum in the advection terms rotate ahead of the turning motion of the TC, which is consistent with previous studies of TC motion based on the concept of absolute vorticity conservation. An integration of all these observational analyses generally verifies the validity of the proposed conceptual framework, which appears to explain most types of TC motion.
    Chen X. M., Y. Q. Wang, and K. Zhao, 2015: Synoptic flow patterns and large-scale characteristics associated with rapidly intensifying tropical cyclones in the South China Sea. Mon. Wea. Rev., 143, 64-
    Choi Y., K. -S. Yun, K. -J. Ha, K. -Y. Kim, S. -J. Yoon, and J. C. L. Chan, 2013: Effects of asymmetric SST distribution on straight-moving Typhoon Ewiniar (2006) and recurving Typhoon Maemi (2003). Mon. Wea. Rev., 141, 3950- 3967.10.1175/ effects of asymmetric sea surface temperature (SST) distribution on the tropical cyclone (TC) motion around East Asia have been examined using the Weather Research and Forecasting Model for the straight-moving Typhoon Ewiniar (2006) and recurving Typhoon Maemi (2003). The SST-TC motion relationships associated with the two different TCs and the physical mechanism of recurvature are investigated in the context of the potential vorticity tendency framework. A zonally asymmetric SST distribution alters the TC translating direction and speed, which is ascribable to the interaction between a TC and the environmental current associated with asymmetric SST forcing. A north-south SST gradient has an insignificant role in the TC motion. It is noted that the straight-moving (i.e., northward moving) TC deflects toward the region of warmer SST when SST is zonally asymmetric. A contribution of the horizontal advection including asymmetric flow induced by asymmetric forcing is dominant for the deflection. The recurving TC reveals northeastward acceleration and deceleration after the recurvature point in the western warming (WW) and eastern warming (EW) experiments, respectively. When it comes to a strong southerly vertical wind shear under the recurvature condition, diabatic heating can be a significant physical process associated with the downward motion over the region of upshear right. The enhanced (reduced) southwesterly flow effectively produces the acceleration (deceleration) of northeastward movement in WW (EW) after recurvature.
    Dee, D. P., Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553- 597.10.1002/ ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright 2011 Royal Meteorological Society
    DeMaria M., J. -J. Baik, and J. Kaplan, 1993: Upper-level eddy angular momentum fluxes and tropical cyclone intensity change. J. Atmos. Sci., 50, 1133- 1147.10.1175/1520-0469(1993)050<1133:ULEAMF>2.0.CO; The eddy flux convergence of relative angular momentum (EFC) at 200 mb was calculated for the named tropical cyclones during the 1989-1991 Atlantic hurricane seasons (371 synoptic times). A period of enhanced EFC within 1500 km of the storm center occurred about every 5 days due to the interaction with upper-level troughs in the midlatitude westerlies or upper-level, cold lows in low latitudes. Twenty-six of the 32 storms had at least one period of enhanced EFC. In about one-third of the cases, the storm intensified just after the period of enhanced EFC. In most of the cases in which the storm did not intensify the vertical shear increased, the storm moved over cold water, or the storm became extratropical just after the period of enhanced EFC. A statistically significant relationship (at the 95% level) was found between the EFC within 600 km of the storm center and the intensity change during the next 48 h. However, this relationship could only be determined using a multiple regression technique that also accounted for the effects of vertical shear and sea surface temperature variations. The EFC was also examined for the ten storms from the 1989-1991 sample that had the largest intensification rates. Six of the ten periods of rapid intensification were associated with enhanced EFC. In the remaining four cases the storms were intensifying rapidly in a low shear environment without any obvious interaction with upper-level troughs.
    DeMaria M., J. A. Knaff, and C. Sampson, 2007: Evaluation of long-term trends in tropical cyclone intensity forecasts. Meteor. Atmos. Phys., 97, 19- 28.10.1007/ National Hurricane Center and Joint Typhoon Warning Center operational tropical cyclone intensity forecasts for the three major northern hemisphere tropical cyclone basins (Atlantic, eastern North Pacific, and western North Pacific) for the past two decades are examined for long-term trends. Results show that there has been some marginal improvement in the mean absolute error at 24 and 4865h for the Atlantic and at 7265h for the east and west Pacific. A new metric that measures the percent variance of the observed intensity changes that is reduced by the forecast (variance reduction, VR) is defined to help account for inter-annual variability in forecast difficulty. Results show that there have been significant improvements in the VR of the official forecasts in the Atlantic, and some marginal improvement in the other two basins. The VR of the intensity guidance models was also examined. The improvement in the VR is due to the implementation of advanced statistical intensity prediction models and the operational version of the GFDL hurricane model in the mid-1990s. The skill of the operational intensity forecasts for the 5-year period ending in 2005 was determined by comparing the errors to those from simple statistical models with input from climatology and persistence. The intensity forecasts had significant skill out to 9665h in the Atlantic and out to 7265h in the east and west Pacific. The intensity forecasts are also compared to the operational track forecasts. The skill was comparable at 1265h, but the track forecasts were 2 to 5 times more skillful by 7265h. The track and intensity forecast error trends for the two-decade period were also compared. Results showed that the percentage track forecast improvement was almost an order of magnitude larger than that for intensity, indicating that intensity forecasting still has much room for improvement.
    Eliassen A., 1952: Slow thermally or frictionally controlled meridional circulation in a circular vortex. Astrophysica Norvegica, 5, 19- NORVEGICA VOL.V NO.2 SLOW THERMALLY OR FRICTIONALLY CONTROLLED MERIDIONAL CIRCULATION IN A CIRCULAR VORTEX BY ARNT ELIASSEN (MANUSCRIPT RECEIVED DECEMBER 8, 1950.) A bstract: A quasi-static theory of meridional motion in
    Elsberry R. L., G. J. Holland , H. Gerrish, M. DeMaria, C. P. Guard, and K. A. Emanuel, 1992: Is there any hope for tropical cyclone intensity prediction? panel discussion. Bull. Amer. Meteor. Soc., 73, 264- 275.
    Emanuel K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance. J. Atmos. Sci., 43, 585-
    Emanuel K. A., 2000: A statistical analysis of tropical cyclone intensity. Mon. Wea. Rev., 128, 1139-;cpsidt=1349927
    Erickson C. O., 1967: Some aspects of the development of Hurricane Dorothy. Mon. Wea. Rev., 95, 121- 130.10.1175/1520-0493(1967)095<0121:SAOTDO>2.3.CO; Dorothy, July 1966, possessed both extratropical and tropical features. A number of factors contributed to storm development, including a well-defined pre-existing disturbance, high-level advection of vorticity and kinetic energy, baroclinicity of both the extratropical and tropical-storm types, and a moderate degree of latent instability. 1.
    Fitzpatrick P. J., 1997: Understanding and forecasting tropical cyclone intensity change with the typhoon intensity prediction scheme (TIPS). Wea.Forecasting, 12, 826- 846.10.1175/1520-0434(1997)0122.0.CO; A multiple regression scheme with tropical cyclone intensity change as the dependent variable has been developed. The new scheme is titled the Typhoon Intensity Prediction Scheme (TIPS) and is similar to one used operationally at the National Hurricane Center. However, TIPS contains two major differences: it is developed for the western North Pacific Ocean, and utilizes digitized satellite data; the first time such satellite information has been combined with other predictors in a tropical cyclone multiple regression scheme. It is shown that the satellite data contains vital information that distinguishes between fast and slow developing tropical cyclones. The importance of other predictors (such as wind shear, persistence, climatology, and an empirical formula dependent on sea surface temperature) to intensity change are also clarified in the statistical analysis. A normalization technique reveals threshold values useful to forecasters. It is shown that TIPS may be competitive with the Joint Typhoon Warning Center in forecasting tropical cyclone intensity change.
    Fitzpatrick P. J., J. A. Knaff, C. W. Land sea, and S. V. Finley, 1995: Documentation of a systematic bias in the aviation model's forecast of the Atlantic tropical upper-tropospheric trough: Implications for tropical cyclone forecasting. Wea.Forecasting, 10, 433- 446.10.1175/1520-0434(1995)0102.0.CO; This study uncovers what appears to be a systematic bias in the National Meteorological Center's aviation (AVN) model at 200 mb over the Caribbean Sea. In general, the 48-h forecast in the vicinity of the Tropical Upper Tropospheric Trough (TUTT) underpredicts the magnitude of the westerly 200-mb winds on the order of 5-10 m s 611 . This unrealistic weakening of the TUTT and associated cold lows by the AVN results in erroneous values of the vertical (850-200 mb) wind shear. These systematic errors are in the same order of magnitude as the minimum shear threshold for tropical cyclone genesis and development. Thus, 48-h tropical cyclone formation and intensity forecasts based upon the AVN model are often incorrect in the vicinity of the TUTT. Knowing the correct future upper-wind regime is also crucial for track forecasting of more intense tropical cyclones, especially in cases of recurvature. It is shown that simple persistence or climatology of the 200-mb winds south of a TUTT axis is superior to the AVN model's 48-h forecast. Until this bias in the AVN is successfully removed, the tropical cyclone forecaster for the Atlantic basin should be aware of this systematic error and make subjective changes in his/her forecasts. For 200-mb west winds greater than or equal to 10 m s 611 , forecasts based on persistence are best, while for west winds less than 10 m s 611 , half climatology and half persistence is the preferable predictor. If the TUTT is weak such that 200-mb easterly winds occur, climatology tends to be the best predictor as it nudges the forecast back to a normal westerly wind regime.
    Hanley D. E., 2002: The evolution of a hurricane-trough interaction from a satellite perspective. Wea.Forecasting, 17, 916- 926.
    Hanley D., J. Molinari, and D. Deyser, 2001: A composite study of the interactions between tropical cyclones and upper-tropospheric troughs. Mon. Wea. Rev., 129, 2570- 2584.10.1175/1520-0493(2001)129<2570:ACSOTI>2.0.CO; The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (≤26°C) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300–600-km radial range is greater than 10 (m s 611 ) day 611 . The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison,
    Holland G.J., R. T. Merrill, 1984: On the dynamics of tropical cyclone structural changes. Quart. J. Roy. Meteor. Soc., 110, 723- 745.10.1002/ Tropical cyclone structural change is separated into three modes: intensity, strength and size. The possible physical mechanisms behind these three modes are examined using observations in the Australian/south-west Pacific region and an axisymmetric diagnostic model. We propose that, though dependent on moist convection and other internal processes, the ultimate intensity, strength or size of a cyclone is regulated by interactions with its environment. Possible mechanisms whereby these interactions occur are described.
    Hoskins B. J., M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111, 877- 946.10.1002/ The two main principles underlying the use of isentropic maps of potential vorticity to represent dynamical processes in the atmosphere are reviewed, including the extension of those principles to take the lower boundary condition into account. the first is the familiar Lagrangian conservation principle, for potential vorticity (PV) and potential temperature, which holds approximately when advective processes dominate frictional and diabatic ones. the second is the principle of &lsquo;invertibility&rsquo; of the PV distribution, which holds whether or not diabatic and frictional processes are important. the invertibility principle states that if the total mass under each isentropic surface is specified, then a knowledge of the global distribution of PV on each isentropic surface and of potential temperature at the lower boundary (which within certain limitations can be considered to be part of the PV distribution) is sufficient to deduce, diagnostically, all the other dynamical fields, such as winds, temperatures, geopotential heights, static stabilities, and vertical velocities, under a suitable balance condition. the statement that vertical velocities can be deduced is related to the well-known omega equation principle, and depends on having sufficient information about diabatic and frictional processes. Quasi-geostrophic, semigeostrophic, and &lsquo;nonlinear normal mode initialization&rsquo; realizations of the balance condition are discussed. an important constraint on the mass-weighted integral of PV over a material volume and on its possible diabatic and frictional change is noted. Some basic examples are given, both from operational weather analyses and from idealized theoretical models, to illustrate the insights that can be gained from this approach and to indicate its relation to classical synoptic and air-mass concepts. Included are discussions of (a) the structure, origin and persistence of cutoff cyclones and blocking anticyclones, (b) the physical mechanisms of Rossby wave propagation, baroclinic instability, and barotropic instability, and (c) the spatially and temporally nonuniform way in which such waves and instabilities may become strongly nonlinear, as in an occluding cyclone or in the formation of an upper air shear line. Connections with principles derived from synoptic experience are indicated, such as the &lsquo;PVA rule&rsquo; concerning positive vorticity advection on upper air charts, and the role of disturbances of upper air origin, in combination with low-level warm advection, in triggering latent heat release to produce explosive cyclonic development. In all cases it is found that time sequences of isentropic potential vorticity and surface potential temperature charts&mdash;which succinctly summarize the combined effects of vorticity advection, thermal advection, and vertical motion without requiring explicit knowledge of the vertical motion field&mdash;lead to a very clear and complete picture of the dynamics. This picture is remarkably simple in many cases of real meteorological interest. It involves, in principle, no sacrifices in quantitative accuracy beyond what is inherent in the concept of balance, as used for instance in the initialization of numerical weather forecasts.
    Kimball S. K., J. L. Evans, 2002: Idealized numerical simulations of hurricane-trough interaction. Mon. Wea. Rev., 130, 2210- 2227.10.1175/1520-0493(2002)130<2210:INSOHT>2.0.CO; three-dimensional, nonhydrostatic, fine-resolution model, with explicitly resolved convective processes, is used to investigate the evolution of (a) a hurricane in two sheared flows, and (b) a hurricane interacting with four different upper-level lows. The negative impact of vertical shear on hurricane intensification is confirmed. The hurricanes display asymmetries that are most pronounced in higher shear flow. In both shear cases, the hurricane asymmetries seem to be related to a single upper-tropospheric outflow jet forcing convective activity below its right entrance region. Weak subsidence is confined to only part of the eye. Less eye subsidence leads to less inner-core warming, and hence a smaller fall in central surface pressure. A hurricane in zero flow (control) displays subsidence in the entire eye leading to a symmetric storm with a deep, strong warm core temperature anomaly and lower central surface pressure. In the weak shear and control cases, the radius of maximum wind (RMW) contracts as the storms intensifies via the mechanism of symmetric intensification. In the high-shear case the RMW and intensity remain almost steady. When hurricanes interact with troughs, asymmetries are evident in the hurricanes and their RMWs expand as the storms slowly intensify. During the interaction, the troughs are deformed by the hurricane flow. Remnants of the deformed troughs prevent an outflow channel from developing on the eastern side of the hurricanes, hampering storm intensification in three of the four cases. In the fourth case, a strong and shallow trough merges with the hurricane causing a three-dimensional split of the trough, reduction of vertical shear over the vortex, followed by rapid intensification and RMW contraction. This vortex reaches the highest intensity of all four trough-interaction cases and comes close in intensity to the comparable no-trough case.
    Leroux M. -D., M. Plu, D. Barbary, F. Roux, and P. Arbogast, 2013: Dynamical and physical processes leading to tropical cyclone intensification under upper-level trough forcing. J. Atmos. Sci., 70, 2547- 2565.10.1175/ The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC-搕rough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum. It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.
    Lewis B. M., D. P. Jorgensen, 1978: Study of the dissipation of Hurricane Gertrude (1974). Mon. Wea. Rev., 106, 1288- 1306.10.1175/1520-0493(1978)1062.0.CO;<1288%3ASOTDOH>2.0.CO%3B2Abstract Meteorological events in the upper and lower troposphere in Hurricane Gertrude and vicinity are examined for causal effects related to the sudden dissipation of Hurricane Gertrude. Mesoscale and synoptic-scale meteorological observations reveal that the hurricane rapidly decreased in intensity as it overtook a westward propagating upper tropospheric trough. Quantized radar observations are presented, which show the marked decrease in storm-generated precipitation which occurred as Gertrude approached the vicinity of this trough. This study indicates that the dissipation of Gertrude resulted from large vertical wind shear and upper level synoptic-scale convergence with accompanying subsidence in the upper troposphere in the vicinity of the storm. The marked decrease in convective activity and storm organization occurred in spite of favorable sea surface temperatures, favorable lower troposphere stability, and convergence of air toward the storm center in the boundary layer. This study reveals the amount of control that upper atmospheric motion has on storm development.
    Li Y., L. S. Chen, and X. T. Lei, 2006: Numerical study on impacts of upper-level westerly trough on the extratropical transition process of Typhoon Winnie (1997). Acta Meteorologica Sinica, 64, 552- 563. (in Chinese)10.11676/
    Martin J. D., W. M. Gray, 1993: Tropical cyclone observation and forecasting with and without aircraft reconnaissance. Wea.Forecasting, 8, 519- 532.10.1175/1520-0434(1993)0082.0.CO; The contributions of aircraft reconnaissance to the accuracy of tropical cyclone center positioning, motion, and intensity determinations are examined, along with their impact on the accuracy of track and intensity forecasting. The analyses concentrate on differences in cyclone position and intensity diagnosis, as well as track forecasting during periods when aircraft measurements were made versus times when aircraft data were not available. Northwest Pacific data for the period 1979-86, which contain over 200 tropical cyclone cases with approximately 5000 center fix positions, were used for the analyses. Aircraft versus no-aircraft situations are examined with respect to the class of satellite data that were available and for day versus night measurements. Differences in positioning and intensity estimates made from simultaneous independent satellite observations are also examined. Results show that satellite analysts operating independently frequently obtain large differences in their estimates of tropical cyclone positions, as well as their intensity estimates. Aircraft reconnaissance of cyclone position and intensity, as were flown in the western Pacific, does not appear to improve track forecasts beyond 24 h, nor does it affect the current 12-h motion vector estimate. Other areas of tropical cyclone warning services, including estimates of current position and intensity as well as short-term estimates of motion, especially for recurvature forecasts, appear to be improved by aircraft data.
    McTaggart-Cowan R., L. F. Bosart, C. A. Davis, E. H. Atallah, J. R. Gyakum, and K. A. Emanuel, 2006: Analysis of Hurricane Catarina (2004). Mon. Wea. Rev., 134, 3029- 3053.10.1175/ The development of Hurricane Catarina over the western South Atlantic Ocean in March 2004 marks the first time that the existence of a hurricane has been confirmed by analysis and satellite imagery in the South Atlantic basin. The storm undergoes a complex life cycle, beginning as an extratropical precursor that moves east-southeastward off the Brazilian coast and toward the midlatitudes. Its eastward progress is halted and the system is steered back westward toward the Brazilian coast as it encounters a strengthening dipole-blocking structure east of the South American continent. Entering the large region of weak vertical shear that characterizes this blocking pattern, Catarina begins a tropical transition process over anomalously cool 25C ocean waters above which an elevated potential intensity is supported by the cold upper-level air associated with the trough component of the block. As the convective outflow from the developing tropical system reinforces the ridge component of the dipole block, the storm is accelerated westward toward the Santa Catarina province of Brazil and makes landfall there as a nominal category-1 hurricane, causing extensive damage with its heavy rains and strong winds. The complex evolution of the system is analyzed using a suite of diagnostic tools, and a conceptual model of the tropical transition and steering processes in the presence of a dipole block is developed. Once the essential properties of the upper-level flow are established, an analog study is undertaken to investigate lower-atmospheric responses to similar blocking regimes. Persistent dipole-blocking structures are found to be rare east of South America; however, the evolution of systems occurring during these periods is shown to be complex and to exhibit various subtropical development modes.
    Merrill R. T., 1988a: Characteristics of the upper-tropospheric environmental flow around hurricanes. J. Atmos. Sci., 45, 1665- 1677.10.1175/1520-0469(1988)045<1665:COTUTE>2.0.CO; The upper-tropospheric flow out to a radius of 2000 km around Atlantic hurricanes is described using rotated coordinate composite analysis of the NOAA National Hurricane Center operational wind set. The rotated coordinate methodology, designed to preserve some of the asymmetry of hurricane outflow during compositing, is described in detail. A rotated coordinate composite of all hurricanes from a five-year period is used to study the general properties of the hurricane outflow layer. Coordinate rotation improves the representation of the outflow jet and the associated extrema of radial and tangential wind, but tends to obscure the geographically persistent features of the upper-tropospheric environment such as the midlatitude westerlies. The amplitude of the asymmetric radial wind is twice that of the symmetric, while the amplitudes of tangential winds are equivalent. A comparison of geographic and rotated coordinate composites indicates that both the outflow jet and the midlatitude westerlies are important structures for the import of angular momentum into the hurricane by horizontal eddy fluxes. Separate composites of eight characteristic outflow patterns are also presented. Pattern variability arises from the juxtaposition of the hurricane circulation with surrounding synoptic features.
    Merrill R. T., 1988b: Environmental influences on hurricane intensification. J. Atmos. Sci., 45, 1678- 1687.10.1175/1520-0469(1988)045<1678:EIOHI>2.0.CO; Although driven by internal processes, hurricanes are also regulated by conditions in their oceanic and atmospheric surroundings. Sea surface temperature determines an upper bound on the intensity of hurricanes, but most never reach this potential, apparently because of adverse atmospheric conditions. Winds measured by satellite cloud tracking, commercial aircraft, and rawinsondes are composited using a rotated coordinate system designed to preserve the asymmetries in the upper-tropospheric environment. Composites of upper-tropospheric environmental flows for intensifying and nonintensifying hurricanes for a five-year period are compared. Nonintensifying composites indicate stronger mean environmental flow relative to the hurricane motion, unidirectional flow over and near the hurricane center, and slightly weaker radial outflow and/or more pronounced anticyclonic flow surrounding the center in the upper troposphere.
    Molinari J., D. Vollaro, 1989: External influences on hurricane intensity. Part I: Outflow layer eddy angular momentum fluxes. J. Atmos. Sci., 46, 1093- 1104.10.1175/1520-0469(1989)046<1093:EIOHIP>2.0.CO; Outflow layer winds were objectively analyzed every 12 h for 6 days during the life cycle of Hurricane Elena (1985). A high correlation was found between angular momentum fluxes by azimuthal eddies at large radii and central pressure changes in the storm 27-33 h later. Momentum flux by eddies exceeded that by the azimuthal mean outside the 800 km radius, while vortex spinup by the eddies reached instantaneous magnitudes as large as 25 m s1/day. Outflow maxima and minima repeatedly appeared more than 1000 km from the hurricane center and tracked inward with time. The results provide evidence of significant environmental control on the behavior of the storm.After reaching hurricane strength, Elena experienced a major secondary intensification associated with a large inward cyclonic eddy momentum flux produced by the passage of a middle latitude trough north of the hurricane. An outflow maximum appeared radially inside of the eddy momentum source, consistent with balanced vortex theory, and tracked inward with the eddy momentum source during the following 24 h. When the outflow maximum reached the storm core, an extended period of rapid pressure fails followed. It is speculated that these pressure falls represented a response to midlevel spinup forced by the outflow layer momentum sourcers.Although environmental forcing dominated the later stages of Elena, the rapid initial intensification of the storm as it moved from land to water appeared to be a precursor to subsequent environmental interactions. The enhanced anticyclonic outflow from this initial deepening reduced the outflow-layer inertial stability, allowing a more radially extended region for external forcing. The secondary intensification of Elena is thus viewed as a cooperative interaction between mesoscale events at the hurricane core and synoptic-scale features in the upper tropospheric environment.
    Molinari J., D. Vollaro, 1990: External influences on hurricane intensity. Part II: Vertical structure and response of the hurricane vortex. J. Atmos. Sci., 47, 1902-
    Molinari J., D. Vollaro, 1993: Environmental controls on eye wall cycles and intensity change in Hurricane Allen (1980). Tropical Cyclone Disasters, J. Lighthill et al., Eds., Peking University Press, 328- 337.
    Molinari J., D. Vollaro, and F. Robasky, 1992: Use of ECMWF operational analyses for studies of the tropical cyclone environment.Meteor. Atmos. Phys., 47, 127- 144.10.1007/ study examined ECMWF operational analyses of the outflow layer of two tropical cyclones (Allen, 1980; Elena, 1985) during their passage across the Atlantic and Caribbean. Wind fields and related derived quantities were compared to those from objective analyses of specialized data sets. Errors in center position and storm motion from the ECMWF analyses were also evaluated. Analyses of wind and angular momentum flux in 1985, subsequent to upgrading of the operational model, were superior to those from 1980. High-resolution, uninitialized analyses from 1985, however, provided no advantages over lower resolution, initialized analyses for the same time period. For all ECMWF analyses, azimuthally averaged (mean) tangential velocity, and thus mean vorticity, were well represented. Mean radial velocity and mean divergence were poorly represented. Problems with the latter arose primarily due to underestimation of outflow, especially in the 1980 analyses. Azimuthaleddy fluxes of angular momentum in the ECMWF analyses quantitatively differed from but qualitatively resembled, the control analyses. Vorticity maxima at 850 mb in the operational analyses most accurately defined the center position of the storms, with a mean error less than or equal to one grid point. In contrast, surface pressure minima failed to provide reliable estimates. Over open ocean and at early stages of storms, analysis quality was uneven, with occasional large position errors and widely varying locations of vorticity maxima in the vertical. Nevertheless, in regions surrounded by even a few rawinsondes, such as the Caribbean or Gulf of Mexico, ECMWF analyses contained sufficient information to allow individual case studies of the tropical cyclone environment. In the same regions, estimates of the eddy flux convergence of angular momentum were found to be accurate enough to aid in operational hurricane intensity prediction. Enhancements in resolution and model initialization at ECMWF since 1985 should further improve operational analyses of the tropical cyclone environment.
    Molinari J., S. Skubis, and D. Vollaro, 1995: External influences on hurricane intensity. Part III: Potential vorticity structure. J. Atmos. Sci., 52, 3593-
    Molinari J., S. Skubis, D. Vollaro, F. Alsheimer, and H. E. Willoughby, 1998: Potential vorticity analysis of tropical cyclone intensification. J. Atmos. Sci., 55, 2632- 2644.10.1175/1520-0469(1998)0552.0.CO; a study which examined the tropical storm Danny in 1985, during its marginal interaction with an upper-tropospheric positive potential vorticity (PV) anomaly. What the intensification of the storm was attributed to; Examination of the behavior of a weaker storm interacting with a smaller-scale upper PV anomaly; Assessment of the roles of vertical wind shear and diabatic heating in the observed evolution.
    Molinari J., P. Dodge, D. Vollaro, and K. L. Corbosiero, 2006: Mesoscale aspects of the downshear reformation of a tropical cyclone. J. Atmos. Sci., 63, 341- 354.10.1175/ The downshear reformation of Tropical Storm Gabrielle (2001) was investigated using radar reflectivity and lightning data that were nearly continuous in time, as well as frequent aircraft reconnaissance flights. Initially the storm was a marginal tropical storm in an environment with strong 850-200-hPa vertical wind shear of 12-13 m s 1 and an approaching upper tropospheric trough. Both the observed outflow and an adiabatic balance model calculation showed that the radial-vertical circulation increased with time as the trough approached. Convection was highly asymmetric, with almost all radar return located in one quadrant left of downshear in the storm. Reconnaissance data show that an intense mesovortex formed downshear of the original center. This vortex was located just south of, rather than within, a strong downshear-left lightning outbreak, consistent with tilting of the horizontal vorticity associated with the vertical wind shear. The downshear mesovortex contained a 972-hPa minimum central pressure, 20 hPa lower than minimum pressure in the original vortex just 3 h earlier. The mesovortex became the new center of the storm, but weakened somewhat prior to landfall. It is argued that dry air carried around the storm from the region of upshear subsidence, as well as the direct effects of the shear, prevented the reformed vortex from continuing to intensify. Despite the subsequent weakening of the reformed center, it reached land with greater intensity than the original center. It is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough. In addition, the new center formed much closer to the coast and made landfall much earlier than predicted. Such vertical-shear-induced intensity and track fluctuations are important to understand, especially in storms approaching the coast.
    Montgomery M. T., R. K. Smith, 2014: Paradigms for tropical cyclone intensification. Australian Meteorological and Oceanographic Journal, 64, 37- 66.10.1175/ We review four paradigms of tropical-cyclone intensification that have emerged over the past five decades, discussing the relationship between them and highlighting their positive aspects and limitations. A major focus is on a new paradigm articulated in a series of recent papers by ourselves and colleagues. Unlike the three previous paradigms, all of which assumed axial symmetry, the new one recognizes the importance of rotating deep convection, which possesses local buoyancy relative to the azimuthally-averaged virtual temperature of the warm-cored vortex. This convection comes under increasing rotational control as the vortex intensifies. It exhibits also a degree of randomness that has implications for the predictability of local asymmetric features of the developing vortex. While surface moisture fluxes are required for intensification, the postulated 'evaporation-wind' feedback process that forms the basis of an earlier paradigm is not. The details of the intensification process as well as the structure of the mature vortex are sensitive to the boundary-layer parameterization used in the model. The spin up of the inner-core winds in the new paradigm occurs within the boundary layer and is associated with the convergence of absolute angular momentum in this layer, where absolute angular momentum is not materially conserved. This spin up is coupled with that of the winds above the boundary layer through boundary-layer dynamics. Balanced and unbalanced contributions to the intensification process are discussed. An application of the new paradigm is given to help describe and understand a simulated intensification process in a realistic numerical weather prediction model.
    Pfeffer R. L., M. Challa, 1981: A numerical study of the role of eddy fluxes of momentum in the development of Atlantic hurricanes. J. Atmos. Sci., 38, 2393- 2398.10.1175/1520-0469(1981)038<2393:ANSOTR>2.0.CO; The results of numerical integrations of Sundqvist's (1970) symmetric model for hurricane development modified to include parameterized large-scale eddy fluxes of momentum are presented. The initial wind and moisture distributions, and the prescribed eddy fluxes of momentum, were taken from atmospheric observations of Atlantic developing (prehurricane) and non-developing tropical disturbances as composited by McBride (1981a,b) and McBride and Zehr (1981). For the purposes of the present study, the data for individual stages in the evolution of developing and non-developing disturbances were combined to form a single composite developing hurricane and a single composite non-developing disturbance. The data reveal the presence of intense, well organized inward eddy fluxes of momentum in developing Atlantic hurricanes and weak, poorly organized fluxes in non-developing disturbances. In the developing disturbances, the eddy fluxes of momentum are organized such that they act as a forcing function for driving the radial circulation, drawing moist air in toward the center of the vortex in the lower troposphere and pumping drier air outward aloft, thereby providing fuel for the explosive growth of the hurricane. In order to test the efficacy of this mechanism, and of Ekman suction and cooperative instability, numerical integrations were performed using the data for the composite developing hurricane, with and without the observed eddy fluxes of momentum, and for the composite non-developing disturbance with the observed eddy fluxes corresponding to this disturbance. Without eddy flux forcing, the prehurricane developing vortex fails to intensify into a hurricane, even after 20 days of integration. With the observed eddy fluxes of momentum, the same initial vortex intensifies rapidly, reaching hurricane strength within 4 days. Moreover, because of the weak and diffuse pattern of the eddy fluxes of momentum in non-developing tropical disturbances, the initial vortex characterizing these disturbances also fails to develop into a hurricane. The kinetic energy budgets corresponding to the integrations with the composite developing and non-developing disturbances are presented as a function of time. The calculations reveal that, during the early stages of development of the model hurricane, the conversion ( E k ) from eddy kinetic energy to the kinetic energy of the mean hurricane circulation is larger than the conversion ( C A ) from potential to kinetic energy. The eddy process is, therefore, directly responsible for the early growth of the model hurricane. This is followed by an explosive increase in the rate of conversion from potential to kinetic energy and in the rate of kinetic energy dissipation ( F ). During the latter period, C A and F become almost an order of magnitude greater than the peak attained earlier by E k , and the kinetic energy tendency reaches its peak. Without the eddy momentum flux forcing, no such explosive growth takes place. The results of these integrations provide evidence that properly organized large-scale eddy fluxes of momentum may be an essential ingredient id the development of Atlantic hurricanes.
    Qian Y. K., C. X. Liang, Q. Q. Liang, L. X. Lin, and Z. J. Yuan, 2011: On the forced tangentially-averaged radial-vertical circulation within vortices. Part II: The transformation of Tropical Storm Haima (2004). Adv. Atmos. Sci.,28, 1143-1158, doi: 10.1007/s00376-010-0060-x.10.1007/ real case study for the transformation of Tropical Storm (TS) Haima (2004) into an extratropical cyclone (EC) is carried out numerically since,after landfall,Haima (2004) (as an EC) brought severe weather to a large area (from the south to the north) in China during 13-16 September 2004.With the linear diagnostic model (derived in a previous study) for the tangentially-averaged radial-vertical circulation within vortices moving on the spherical Earth,Haima's (2004) life cycle is reconstructed noticeably well.Therefore,the major contributor could be identified confidently for Haima's (2004) extratropical transition based on the diagnostic model outputs.The quantitative comparison shows that up to a 90% contribution to the innerregion updraft and a 55% contribution to the upper-layer outflow come from latent heating during Haima's (2004) TS stage.Up to a 90% contribution to the inner-region updraft and nearly a 100% contribution to the upper-layer outflow come from the upper-layer eddy angular momentum advection (EAMA) during Haima's (2004) EC stage.Representing the asymmetric structure of the storm,the predominantly positive contribution of the upper-layer EAMA to Haima's (2004) transformation is closely associated with the Sshaped westerlies in the upper layer with two jets.One jet in the cyclonic-curvature area carries cyclonic angular momentum into the storm,and the other jet in the anticyclonic-curvature area carries anticyclonic angular momentum out of the storm.Consequently,the newly-increased cyclonic tangential wind is deflected by the Coriolis force to the right to form the upper-layer outflow accompanied by the central-area rising motion,leading to Haima's (2004) extratropical transition after its landfall.
    Rappin E. D., M. C. Morgan, and G. J. Tripoli, 2011: The impact of outflow environment on tropical cyclone intensification and structure. J. Atmos. Sci., 68, 177- 194.10.1175/ In this study, the impacts of regions of weak inertial stability on tropical cyclone intensification and peak strength are examined. It is demonstrated that weak inertial stability in the outflow layer minimizes an energy sink of the tropical cyclone secondary circulation and leads to more rapid intensification to the maximum potential intensity. Using a full-physics, three-dimensional numerical weather prediction model, a symmetric distribution of environmental inertial stability is generated using a variable Coriolis parameter. It is found that the lower the value of the Coriolis parameter, the more rapid the strengthening. The lower-latitude simulation is shown to have a significantly stronger secondary circulation with intense divergent outflow against a comparatively weak environmental resistance. However, the impacts of differences in the gradient wind balance between the different latitudes on the core structure cannot be neglected. A second study is then conducted using an asymmetric inertial stability distribution generated by the presence of a jet stream to the north of the tropical cyclone. The initial intensification is similar, or even perhaps slower, in the presence of the jet as a result of increased vertical wind shear. As the system evolves, convective outflow from the tropical cyclone modifies the jet resulting in weaker shear and more rapid intensification of the tropical cyclone-et couplet. It is argued that the generation of an outflow channel as the tropical cyclone outflow expands into the region of weak inertial stability on the anticyclonic shear side of the jet stream minimizes the energy expenditure of forced subsidence by ventilating all outflow in one long narrow path, allowing radiational cooling to lessen the work of subsidence. Furthermore, it is hypothesized that evolving conditions in the outflow layer modulate the tropical cyclone core structure in such a way that tropical cyclone outflow can access weak inertial stability in the environment.
    Rodgers E. B., S. W. Chang, J. Stout, J. Steranka, and J.-J. Shi, 1991: Satellite observations of variations in tropical cyclone convection caused by upper-tropospheric troughs. J. Appl. Meteor., 30, 1163- 1184.10.1175/1520-0450(1991)030<1163:SOOVIT>2.0.CO; Satellite observations and numerical model results have been used to study the relationship between upper-tropospheric forcing and the oscillation of convection of tropical cyclones Florence (1988) and Irene (1981) during their mature stage over open warm oceans (SST greater than or equal to 26 C). It is suggested that the initiation and maintenance of intense convective outbreaks in tropical cyclones are related to the channeling and strengthening of their outflow by upper-tropospheric troughs. It is possible to enhance the convection in response to the outflow jet-induced import of eddy relative angular momentum and ascending motion associated with the thermally direct circulation. Both Florence and Irene are found to intensify after the onset of these convective episodes. It is also suggested that the cessation in the convection of the two tropical cyclones occurs when the upper-tropospheric troughs move near or over the tropical cyclones, resulting in the weakening of their outflow and the entrainment of dry upper-tropospheric air into their inner core.
    Rodgers E. B., W. S. Olson, V. M. Karyampudi, and H. F. Pierce, 1998: Satellite-derived latent heating distribution and environmental influences in Hurricane Opal (1995). Mon. Wea. Rev., 126, 1229- 1247.10.1175/1520-0493(1998)126<1229:SDLHDA>2.0.CO; The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane’s rapid intensification and decay stages (3–5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal’s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal’s middle- and upper-tropospheric region during these convective burst episodes allowed Opal’s eyewall to become more buoyant, enhanced the generation of kinetic energy and, thereby, rapidly intensified the system. The observations also suggest that Opal’s intensity became more responsive to the convective burst episodes (i.e., shorter time lag between LHR and intensity and greater maximum wind increase) as Opal became more intense. Analyses of SSM/I-retrieved parameters, sea surface temperature observations, and the European Centre for Medium-Range Weather Forecasts (ECMWF) data reveal that the convective rainband (CRB) cycles and sea surface and tropopause temperatures, in addition to large-scale environmental forcing, had a profound influence on Opal’s episodes of convective burst and its subsequent intensity. High sea surface (29.7°C) and low tropopause (192 K) temperatures apparently created a greater potential for Opal’s maximum intensity. Strong horizontal moisture flux convergence within Opal’s outer-core regions (i.e., outside 333-km radius from the center) appeared to help initiate and maintain Opal’s CRBs. These CRBs, in turn, propagated inward to help generate and dissipate the eyewall convective bursts. The first CRB that propagated into Opal’s eyewall region appeared to initiate the first eyewall convective burst. The second CRB propagated to within 111 km of Opal’s center and appeared to dissipate the first CRB, subjecting it to subsidence and the loss of water vapor flux. The ECMWF upper-tropospheric height and wind analyses suggest that Opal interacted with a diffluent trough that initated an outflow channel, and generated high values of upper-tropospheric eddy relative angular momentum flux convergence. The gradient wind adjustment processes associated with Opal’s outflow channel, in turn, may have helped to initiate and maintain the eyewall convective bursts. The ECMWF analyses also suggest that a dry air intrusion within the southwestern quadrant of Opal’s outer-core region, together with strong vertical wind shear, subsequently terminated Opal’s CRB cycle and caused Opal to weaken prior to landfall.
    Sawyer J. S., 1956: The vertical circulation at meteorological fronts and its relation to frontogenesis. Proc. Roy. Soc.London, 234A, 346- 362.10.1098/ first small nematodes to be described, free-living in the seventeenth century and plant-parasitic in the eighteenth, are identifiable only from their peculiar habitats; taxonomy came relatively late in nematology because adequate optical equipment was a prerequisite. In the study of the free-living and the plant-parasitic species, the development of two readily-presented aspects of taxonomy, figure drawing and mensuration ( each showing oscillation between paucity and excess of detail), is traced in the work of five founders, all of whom were living in 1904: Bastian, Butschli, de Man, Cobb and Goodey. Possible future developments in the identification of species and strains, the investigation of host-parasite relationships, and the training of nematologists are briefly discussed.
    Shi J. J., S. Chang, and S. Raman, 1997: Interaction between Hurricane Florence (1988) and an upper-tropospheric westerly trough. J. Atmos. Sci., 54, 1231- 1247.10.1175/1520-0469(1997)054<1231:IBHFAA>2.0.CO; the interaction between Hurricane Florence (1988) and its upper-tropospheric environment. Use of the Naval Research Laboratory limited-area numerical prediction system; Synoptic review of Hurricane Florence; Simulated structure of the outflow layer of Hurricane Florence; Angular momentum transport in the outflow layer; Structure of the outflow layer of tropical cyclones.
    Smith R. K., M. T. Montgomery, 2015: Toward clarity on understanding tropical cyclone intensification. J. Atmos. Sci., 72, 3020- 3031.
    Sundqvist H., 1970: Numerical simulation of the development of tropical cyclones with a ten-level model. Part I. Tellus, 22, 359- 390.10.1111/
    Titley D. W., R. L. Elsberry, 2000: Large intensity changes in tropical cyclones: A case study of Supertyphoon Flo during TCM-90. Mon. Wea. Rev., 128, 3556- 3573.10.1175/1520-0493(2000)1282.0.CO; unique dataset, recorded during the rapid intensification and rapid decay of Typhoon Flo, is analyzed to isolate associated environmental conditions and key physical processes. This case occurred during the Tropical Cyclone Motion (TCM-90) field experiment with enhanced observations, especially in the upper troposphere beyond about 300 km. These data have been analyzed with a four-dimensional data assimilation technique and a multiquadric interpolation technique. While both the ocean thermal structure and vertical wind shear are favorable, they do not explain the rapid intensification or the rapid decay. A preconditioning phase is defined in which several interrelated factors combine to create favorable conditions: (i) a cyclonic wind burst occurs at 200 mb, (ii) vertical wind shear between 300 and 150 mb decreases 35%, (iii) the warm core is displaced upward, and (iv) 200-mb outflow becomes larger in the 400-1200-km radial band, while a layer of inflow develops below this outflow. These conditions appear to be forced by eddy flux convergence (EFC) of angular momentum, which appears to act in a catalyst function as proposed by Pfeffer and colleagues, because the EFC then decreases to small values during the rapid intensification stage. Similarly, the outer secondary circulation decreases during this stage, so that the rapid intensification appears to be an internal (within 300 km) adjustment process that is perhaps triggered during the preconditioning phase. Rapid decay occurred over open ocean when the environmental factors of ocean thermal structure, and vertical wind shear, positive 200-mb EFC, and vigorous outflow into the midlatitudes appear favorable. However, the EFC extending down to 500 mb and inducing a second shallower secondary circulation is hypothesized to account for the rapid decay by leading to a less efficient energy conversion.
    Willoughby H. E., J. A. Clos, and M. G. Shoreibah, 1982: Concentric eye walls, secondary wind maxima, and the evolution of the hurricane vortex. J. Atmos. Sci., 39, 395- 411.10.1175/1520-0469(1982)039<0395:CEWSWM>2.0.CO; Research aircraft observations in recent hurricanes support the model of Shapiro and Willoughby (1982) for the tropical cyclone's response to circularly symmetric, convective heat sources (convective rings). In both nature and the numerical model the tangential wind commonly increases rapidly just inside the radius of maximum wind and decreases inside the eye near the central axis of the vortex. Thus both secondary outer wind maxima and eyewall wind maxima often contract as they intensify. This response is independent of the horizontal spatial scale of the maximum. An outer maximum is frequently observed to constrict about a pre-existing eye and replace it. This chain of events often coincides with a weakening, or at least a pause in intensification, of the vortex as a whole. The concentric eye phenomenon is a common, but by no means universal, feature of tropical cyclones. It is most frequently observed in intense, highly symmetric systems.
    Wu C.-C., H.-J. Cheng, 1999: An observational study of environmental influences on the intensity changes of Typhoons Flo (1990) and Gene (1990). Mon. Wea. Rev., 127, 3003-
    Wu L. G., B. Wang, 2000: A potential vorticity tendency diagnostic approach for tropical cyclone motion. Mon. Wea. Rev., 128, 1899- 1911.10.1175/1520-0493(2000)128<1899:APVTDA>2.0.CO; order to understand the roles of various physical processes in baroclinic tropical cyclone (TC) motion and the vertical coupling between the upper- and lower-level circulations, a new dynamical framework is advanced. A TC is treated as a positive potential vorticity (PV) anomaly from environmental flows, and its motion is linked to the positive PV tendency. It is shown that a baroclinic TC moves to the region where the azimuthal wavenumber one component of the PV tendency reaches a maximum, but does not necessarily follow the ventilation flow (the asymmetric flow over the TC center). The contributions of individual physical processes to TC motion are equivalent to their contributions to the wavenumber one PV component of the PV tendency. A PV tendency diagnostic approach is described based on this framework. This approach is evaluated with idealized numerical experiments using a realistic hurricane model. The approach is capable of estimating TC propagation with a suitable accuracy and determining fractional contributions of individual physical processes (horizontal and vertical advection, diabatic heating, and friction) to motion. Since the impact of the ventilation flow is also included as a part of the influence of horizontal PV advection, this dynamical framework is more general and particularly useful in understanding the physical mechanisms of baroclinic and diabatic TC motion.
    Yu H., H. J. Kwon, 2005: Effect of TC-trough interaction on the intensity change of two typhoons. Wea.Forecasting, 20, 199- 211.10.1175/ Using large-scale analyses, the effect of tropical cyclone–trough interaction on tropical cyclone (TC) intensity change is readdressed by studying the evolution of upper-level eddy flux convergence (EFC) of angular momentum and vertical wind shear for two TCs in the western North Pacific [Typhoons Prapiroon (2000) and Olga (1999)]. Major findings include the following: 1) In spite of decreasing SST, the cyclonic inflow associated with a midlatitude trough should have played an important role in Prapiroon’s intensification to its maximum intensity and the maintenance after recurvature through an increase in EFC. The accompanied large vertical wind shear is concentrated in a shallow layer in the upper troposphere. 2) Although Olga also recurved downstream of a midlatitude trough, its development and maintenance were not strongly influenced by the trough. A TC could maintain itself in an environment with or without upper-level eddy momentum forcing. 3) Both TCs started to decay over cold SST in a large EFC and vertical wind shear environment imposed by the trough. 4) Uncertainty of input adds difficulties in quantitative TC intensity forecasting.
    Zeng Z. H., Y. Q. Wang, and C. -C. Wu, 2007: Environmental dynamical control of tropical cyclone intensityn observational study. Mon. Wea. Rev., 135, 38- 59.
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Manuscript received: 13 June 2015
Manuscript revised: 16 September 2015
Manuscript accepted: 22 October 2015
通讯作者: 陈斌,
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

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Upper-Tropospheric Environment-Tropical Cyclone Interactions over the Western North Pacific: A Statistical Study

  • 1. Center for Monsoon and Environmental Research/Department of Atmospheric Science, Sun Yat-sen University, Guangzhou 510275
  • 2. State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301
  • 3. South China Sea Marine Prediction Center, South Oceanic Administration, Guangzhou 510300
  • 4. Civil Aviation Flight University of China, Guanghan 618307
  • 5. Luogang District Meteorological Bureau, Guangzhou 510530

Abstract: Based on 25-year (1987-2011) tropical cyclone (TC) best track data, a statistical study was carried out to investigate the basic features of upper-tropospheric TC-environment interactions over the western North Pacific. Interaction was defined as the absolute value of eddy momentum flux convergence (EFC) exceeding 10 m s-1 d-1. Based on this definition, it was found that 18% of all six-hourly TC samples experienced interaction. Extreme interaction cases showed that EFC can reach ∼120 m s-1 d-1 during the extratropical-cyclone (EC) stage, an order of magnitude larger than reported in previous studies. Composite analysis showed that positive interactions are characterized by a double-jet flow pattern, rather than the traditional trough pattern, because it is the jets that bring in large EFC from the upper-level environment to the TC center. The role of the outflow jet is also enhanced by relatively low inertial stability, as compared to the inflow jet. Among several environmental factors, it was found that extremely large EFC is usually accompanied by high inertial stability, low SST and strong vertical wind shear (VWS). Thus, the positive effect of EFC is cancelled by their negative effects. Only those samples during the EC stage, whose intensities were less dependent on VWS and the underlying SST, could survive in extremely large EFC environments, or even re-intensify. For classical TCs (not in the EC stage), it was found that environments with a moderate EFC value generally below ∼25 m s-1 d-1 are more favorable for a TC's intensification than those with extremely large EFC.

1. Introduction
  • Track and intensity forecasts are two major concerns in tropical cyclone (TC) research. Particular emphasis has been placed on the former of these two aspects, leading to a substantial increase in the skill of TC track prediction. Meanwhile, TC intensity forecasts have received virtually no improvement (e.g., Emanuel, 2000). This lagging behind of intensity forecast skill, which is a widely accepted fact (e.g., Elsberry et al., 1992; Fitzpatrick, 1997; DeMaria et al., 2007; Montgomery and Smith, 2014; Smith and Montgomery, 2015), means that greater effort should be put into researching TC intensity.

    It has long been known that upper-tropospheric environmental flow is an important factor that could modulate the intensity change of a TC. Two reasons are proposed, based on Sawyer-Eliassen balance (SEB) vortex theory (Eliassen, 1952; Sawyer, 1956). The first is that, in the upper troposphere, a TC does not retain its axisymmetric structure and thus environmental asymmetric forcing is generally stronger than that of the middle or lower troposphere (e.g., Pfeffer and Challa, 1981). The second is that the inertial stability in the upper layer, representing the resistance of the vortex axisymmetric response to environmental forcings, is much weaker than that in the middle or lower layers (e.g., Holland and Merrill, 1984). Therefore, upper-level asymmetric forcings will excite larger vortex responses, which could easily penetrate into the core region and thus result in TC's intensity change. This phenomenon is also termed as upper-level environment-TC interaction.

    A large number of interaction cases have been reported in the literature, such as Hurricanes (or Tropical Storms) Dorothy (1966) (Erickson, 1967), Elena (1985) (Molinari and Vollaro, 1989), Danny (1985) (Molinari et al., 1998), Florence (1988) (Rodgers et al., 1991; Shi et al., 1997), Opal (1995) (Rodgers et al., 1998; Bosart et al., 2000), Bertha (1996) (Hanley, 2002) and Gabrielle (2001) (Molinari et al., 2006), over the Atlantic basin; Typhoons (or Tropical Storms) Flo (1990) (Wu and Cheng, 1999), Gene (1990) (Wu and Cheng, 1999), Winnie (1997) (Li et al., 2006), Olga (1999) (Yu and Kwon, 2005), Prapiroon (2000) (Yu and Kwon, 2005) and Haima (2004) (Qian et al., 2011), over the western North Pacific basin; as well as Tropical Cyclone Dora (2007) (Leroux et al., 2013) over the southwest Indian Ocean. These cases also show that intensification induced by upper-level environments can be found in all stages of TCs, such as tropical depression formation (Bracken and Bosart, 2000), tropical depression (Bosart and Bartlo, 1991) or tropical storm (Shi et al., 1997; Molinari et al., 1998) to hurricane transition, rapid intensification to a category 5 hurricane (Bosart et al., 2000) or supertyphoon (Titley and Elsberry, 2000), and tropical storm (Qian et al., 2011) or typhoon (Li et al., 2006) to extratropical cyclone transition.

    Early case studies (e.g., Erickson, 1967) identified interaction visually from weather maps, by observing an upper-level westerly trough approaching a TC. Thus, such interaction is also termed TC-trough interaction (e.g., Kimball and Evans, 2002; Yu and Kwon, 2005). Later, a typical case, Hurricane Elena (1985), was studied at length by Molinari and colleagues (Molinari and Vollaro, 1989, 1990; Molinari et al., 1995). In this case, they adopted the SEB vortex model to diagnose the relationship between eddy fluxes and Elena's intensity change and found that 200-hPa eddy angular momentum flux convergence (EFC) significantly increased as an upper-level trough swept over Elena's northern part. From then on, the EFC value, which can be computed quantitatively and objectively, has been commonly used as a diagnostic for identifying TC-trough interaction, although the role of EFC in TC intensification was also explored in pioneering works (e.g., Sundqvist, 1970; Challa and Pfeffer, 1980; Pfeffer and Challa, 1981) that used idealized models.

    Environmental EFC larger than a certain value is usually adopted to identify whether interaction has occurred (e.g., Molinari et al., 2006; Chen et al., 2015). Although many of the above cases underwent intensification when they interacted with upper-level flow using the EFC criterion, there were also weakened or decayed cases (e.g., Lewis and Jorgensen, 1978). Therefore, interaction with upper-level flows does not guarantee TC intensification. In order to verify the effect of upper-level flow on TC intensity, statistical studies are also carried out. Merrill (1988a, b) systematically investigated a large sample of observed outflow-layer wind data near Atlantic hurricanes. The common features of the upper-tropospheric flow pattern for intensifying and non-intensifying hurricanes were summarized, using a rotated-coordinate composite technique rather than the EFC diagnostic. (DeMaria et al., 1993) investigated the relation between intensity change and 200-hPa EFC with 3-yr records of Atlantic hurricanes. They found that about 1/3 of the TCs intensified just after the enhanced EFC. The reason for TCs not intensifying was mostly increased vertical wind shear (VWS). (Hanley et al., 2001) also examined TC-trough interaction, but using 12-yr records of Atlantic hurricanes and the composite technique. After excluding records that were over cold water or close to land, they separated the interaction events into four composites. Their results showed that 78% of superposition and 61% of distant interaction cases deepened. Although these results confirm that interaction favors TC intensification in a statistical sense, to identify whether or not a particular trough interaction is conducive to TC intensification is still not straightforward (Hanley et al., 2001; Leroux et al., 2013). Besides, these two studies ignored the role of upper-level inertial stability, emphasized in a number of other studies (e.g., Holland and Merrill, 1984; Rappin et al., 2011).

    Since the comprehensive work of (Hoskins et al., 1985), the perspective of isentropic potential vorticity (PV) has been introduced for viewing dynamical processes in the atmosphere. The PV perspective is not only applicable to TC motion (e.g., Wu and Wang, 2000; Chan et al., 2002; Choi et al., 2013), but also to TC intensity change (e.g., Molinari et al., 1998). Molinari et al. (1998, 1995) also applied the PV perspective to interaction cases, in which interaction could be viewed as an upper-level synoptic PV anomaly (the environment) interacting with a lower-level mesoscale PV anomaly (the TC). From such a perspective, although interaction is clearly and effectively shown as a superposition of two PV anomalies, there is no quantitative way of identifying whether interaction has occurred. Therefore, the EFC diagnostic is more frequently used than the PV perspective.

    To date, statistical studies in this field have been relatively less common——especially for the western North Pacific, where TCs are most active compared with other basins. Therefore, the climatology of TC-upper-level-flow interactions and its possible impact on TC intensity over this basin remain unclear. Before addressing the "good trough/bad trough" issue, we should first carry out a preliminary but thorough study to reveal more basic features of upper-level environment-TC interactions, especially for the western North Pacific basin. For example: What are the characteristics of upper-level flow patterns during interactions, besides the well-known trough signature? What is the relationship between the upper-level environmental factor and other large-scale factors (e.g., SST and VWS) in controlling TC intensity over this basin? What is the nature of inertial stability during interactions? The present paper aims to address these aspects by examining 25-yr TC records and revealing statistical characteristics of upper-tropospheric environment-TC interaction over the western North Pacific.

    Section 2 describes the data and method. Section 3 presents the climatology of interactions. Section 4 describes the characteristics of upper-level flow patterns. The relationship between environmental factors and TC intensity change follows in section 5, and conclusions are given in section 6.

2. Data and method
  • The TC "best track" data over the western North Pacific were obtained from the Regional Specialized Meteorological Centre (RSMC), Tokyo. Given that aircraft reconnaissance of TCs in this basin terminated in 1987, making observations of TCs thereafter completely dependent on satellite retrieval (Martin and Gray, 1993), for consistency, only data after 1987 were used. Specifically, 25 years of data, from 1987 to 2011, including six-hourly TC positions, near-center maximum surface wind speed, and minimum sea level pressure, were chosen for the present study.

    ERA-Interim (Dee et al., 2011) data were employed to describe the TC environment. The benefits of using these reanalysis data for studying the upper-level environment of TCs have been demonstrated in a number of studies (Molinari and Vollaro, 1990; Molinari et al., 1992). In the present study, six-hourly pressure-level wind fields, as well as SST data, on a 1.5°× 1.5° grid, were used.

  • In order to identify upper-level environment-TC interactions, EFC was computed in storm-relative cylindrical coordinates. The cylindrical coordinates contained 36 grids in the azimuthal direction, with 10° intervals and 28 grids in the radial direction with 0.3° intervals (about 33.3 km). Latitude/longitude gridded data were interpolated to the cylindrical gridded data using a 16-neighbouring-point bicubic polynomial. EFC was calculated following (Molinari and Vollaro, 1990): \begin{equation} {EFC}=-\dfrac{1}{r^2}\dfrac{\partial}{\partial r}r^2\overline{u'v'} , (1)\end{equation} where v and u are the tangential and radial components of the storm-relative wind vector (i.e., after subtracting the translation velocity of a TC from the full wind vector), r is the distance to the TC center, the overbar is the azimuthal mean, and the prime is the deviation from the mean. Consistent with previous studies (e.g., Hanley et al., 2001), the units of measurement for EFC is m s-1 d-1. (Molinari and Vollaro, 1989) showed that the errors of EFC calculated in the inner radii (within 300 km) may exceed 40%. EFC evaluated at large radii is more reliable; however, it may not have an immediate impact on TC intensity. Therefore, the present study used a radial average from 300 to 600 km to identify TC-upper-flow interactions, following (Hanley et al., 2001).

    According to the SEB theory, within a slowly evolving TC, the TC's axisymmetric secondary circulation is largely balanced by several forcings, including EFC. This implies that if other forcings are not important, larger EFC will result in a stronger response of the TC's secondary circulation. If the strength of the TC's secondary circulation is positively correlated with its intensity, which is generally true, then one may expect a simple rule that larger EFC is more likely to enhance the outflow and thus intensify the TC (e.g., Challa and Pfeffer, 1980; Pfeffer and Challa, 1981; Molinari and Vollaro, 1990; Qian et al., 2011). However, when EFC becomes large, other factors may come into play.

    One of the important factors concerning a TC's interaction with upper-level environmental flow, besides EFC, is the inertial stability, usually defined in the SEB theory (e.g., Rappin et al., 2011) as \begin{equation} \label{eq1} I=\overline{\zeta_{a}}\left(f+\dfrac{2\overline{v}}{r}\right) , (2)\end{equation} in which the azimuthal-averaged absolute vorticity \(\overline\zeta_a\) is defined as \begin{equation} \label{eq2} \overline{\zeta_{a}}=\overline{f}+\overline{\zeta}=\overline{f}+\dfrac{\partial\overline{v}}{\partial r}+\dfrac{\overline{v}}{r} ,(3) \end{equation} where ζ is relative vorticity and f is planetary vorticity. Notice that the factor of \((f+2\overline v/r)\) also appears in the momentum forcing functions [e.g., Molinari and Vollaro, 1990; see the first term on the rhs of their Eq. (2)]. When simultaneously taking into account the forcing effect of momentum source and the resistance effect of inertial stability, the factor of \((f+2\overline v/r)\) can be eliminated. Therefore, the axisymmetric outflow is proportional to a forcing function in terms of EFC [rather than EFC multiplied by \((f+2\overline v/r)\)] and inversely proportional to the inertial stability, proxied by \(\overline\zeta_a\) (rather than I). In the SEB theory, it is generally required that the azimuthal-averaged absolute vorticity \(\overline\zeta_a>0\) (inertial stable). Smaller \(\overline\zeta_a\) indicates less resistance to the environmental forcing (e.g., EFC) and a larger radial extent of the vortex response (e.g., Holland and Merrill, 1984). In the present study, the absolute vorticity ζ a before azimuthal averaging was used to represent localized inertial stability: \begin{equation} \label{eq3} \zeta_{a}=f+\zeta=f+\left(\dfrac{1}{r}\dfrac{\partial vr}{\partial r}-\dfrac{1}{r}\dfrac{\partial u}{\partial\lambda}\right) , (4)\end{equation} where Λ is the azimuth.

    Besides EFC and inertial stability, two other environmental factors, SST and VWS, were also computed in cylindrical coordinates for each six-hourly TC record. According to (Molinari and Vollaro, 1993), VWS is defined as the difference between the 200- and 850-hPa wind vector averaged within 500 km from the TC's center: \begin{equation} \label{eq4} {VWS}=\sqrt{[\langle u\rangle_{200}-\langle u\rangle_{850}]^2+[\langle v\rangle_{200}-\langle v\rangle_{850}]^2} . (5)\end{equation} In this definition, the angled brackets indicate area-weighted averaging: $$ \langle A\rangle=\dfrac{1}{\sum_{j=J_0}^{j=J}\sin\beta_j}\sum_{j=J_0}^{j=J}\overline{A_j}\sin\beta_j , $$ where βj=0.3j is the radial angle, in degrees (interval of 33.3 km), and j is the radial grid index, starting from J0 to J. J0=0 and J=15 are used for averaging within 500 km. Similarly, SST is also averaged within 500 km.

    Figure 1.  Distribution of 200-hPa EFC averaged over a 300-600 km radius. There \small \parbox[t]10cmwere 21 685 samples in total from 1987 to 2011.

3. Climatology of upper-tropospheric TC-environment interaction
  • The "best track" dataset from the RSMC used in the resent study contains 628 TCs. Records at 0000, 0600, 1200 and 1800 UTC were chosen, yielding 21 685 six-hourly samples, including tropical depressions (TD), tropical storms (TS), typhoons (TY), and extratropical cyclones (EC). Since the interaction could occur at any stage of a TC (Hanley et al., 2001), keeping all these records yields more samples for reliable statistics. TC intensity change is defined as the forward difference of sea-level minimum central pressure (SLP) i.e., ∆ p(t)=p(t+∆ t)-p(t) where ∆ t=6 h. Negative (positive) pressure change means intensification (weakening), and zero means no change.

    Firstly, the 200-hPa EFC averaged within the 300-600 km radial band for all 21 685 samples was calculated. Figure 1 shows the distribution of the results. Following previous studies (e.g., DeMaria et al., 1993; Hanley et al., 2001), an EFC value of 10 m s-1 d-1 is usually defined for identifying interactions. In the present study, interaction is defined similarly, but an EFC value above 10 m s-1 d-1 means positive interaction, while EFC below -10 m s-1 d-1 means negative interaction. According to this definition, about 17.7% of the samples experience interaction: 3.7% negative and 14.0% positive. The proportion of positive interaction (14.0%) is smaller than that (23%) over the Atlantic basin, as shown by (Hanley et al., 2001). More positive than negative samples over the western North Pacific basin, which is quite similar to the situation over the Atlantic basin, indicates that a positive EFC environment is more favorable for the maintenance of TCs than a negative one. However, a larger EFC value does not guarantee a greater number (or larger proportion) of intensifying samples, as can be seen from Fig. 1.

    Figure 2 shows the percentage of different TC types, including TDs, TSs, TYs and ECs, as identified by the RSMC. The TS and TY samples account for 37% and 25% of the total, respectively, resulting in 62% of samples being canonical tropical cyclones (i.e., TSs and TYs). Besides, 26% of samples are TDs and 12% are ECs. Each sample type has a proportion that interacts with upper-level flows. This result verifies that interaction can occur at any stage of development. We can also see that, although the proportion of EC samples is least, almost 60% of them experience interaction with upper-level flows. This ratio is significantly larger than for the other three types, showing that EC samples are more likely to interact with upper-level flow, while it is relatively rare to see interaction in the other three types.

  • The spatial distribution of interaction samples over the western North Pacific is shown in Fig. 3a. The southwest to northeast orientation of the distribution from the South China Sea to the Bering Sea suggests that interactions generally occur after a TC's recurvature to the northeast. The maximum frequency of occurrence of 0.5 per year per grid can be found around Japan. The interaction active regions are within (25°-45°N, 125°-165°E), with two peaks at about 30°N and 40°N (Fig. 3b), and 138°E and 151°E (Fig. 3c), respectively. These results indicate that most interactions occur when TCs move into the midlatitudes, right under the influence of upper-level westerlies, but before dissipating. Notice that there are still many interaction samples (434) south of 20°N, over both the north South China Sea and east of the Philippines. More than 60% (276) are during the genesis or development stages (before reaching their peak intensities) when interaction occurs.

    Figure 2.  Distributions of different TC types. There were 21 685 samples in total and 3890 interaction samples from 1987 to 2011.

    Figure 3.  (a) Spatial (interval: 0.05 per square grid per year; 9-point smoothing operator applied), (b) latitudinal and (c) longitudinal distributions of the interaction samples from 1987 to 2011. The plots are on a $1.5^\circ\times 0.5^\circ$ grid.

4. Characteristics of upper-level flow patterns during interaction
  • Previous studies (e.g., Hanley et al., 2001) have already identified the most common upper-level flow, i.e., a westerly trough pattern during interaction. Therefore, the interaction is also named TC-trough interaction. However, EFC is an abstract index and EFC larger than a certain threshold does not necessarily mean the flow would be characterized by a westerly trough. There could be other types of flow patterns with different synoptic signatures, although they may be related more or less to a "V"-shaped trough in the vicinity of a TC. Summarizing these features of the flow pattern will help to identify interactions using upper-level wind fields only.

  • Firstly, extreme interactions (extremely large magnitudes of EFC) from 1987 to 2011 were identified. Table 1 details the basic information of three negative and three positive extreme interaction samples. The three negative samples have EFC values of about -70 m s-1 d-1. The three positive interactions show EFC values larger than 120 m s-1 d-1, almost twice that of the negative interactions in magnitude. The identified extreme positive EFC values are much larger than those reported previously in the literature [e.g., 40 m s-1 d-1, as shown by (Wu and Cheng, 1999)]. Except for TC Andy (1989), which was in the TS stage, the other five samples were all in the EC stage. Low SST and large VWS are their common environmental features. It is noticeable that the negative extreme interaction samples could undergo filling (∆ p>0), while the positive ones could intensify further. Since these interaction samples were mainly in the EC stage, their intensity (i.e., minimum SLP) may not have depended as strongly on SST and VWS as it would in the TS or TY stages, for example. Extremely large positive EFC from upper-level flows would possibly have been responsible for their intensification.

    Figure 4.  200-hPa wind fields (m s$^{-1}$; shaded for wind speed $>$50 m s$^{-1}$) for (a-c) three TC samples experiencing extreme negative EFC values and (d-f) three TC samples experiencing extreme positive EFC values. Black dots show TC locations; thick solid lines indicate upper-layer troughs.

    Figure 4 shows the 200-hPa flows for three extreme negative interaction samples (Figs. 4a-c) and three positive samples (Figs. 4d-f). All six samples show that extreme EFC is associated with upper-level westerly troughs. For extreme negative interaction, TCs are right under the influence of northwesterly wind upstream of the trough axes and tend to dissipate. In contrast, the three positive interactions cases (Figs. 4d-f) were under the influence of southwesterly wind ahead of the troughs axis and retained a relatively high level of intensity compared to the negative cases (Table 1). Experiencing extremely large EFC, these TC samples were mostly close to westerly jet streams (shaded areas in Fig. 4), or even inside the jet core regions. Although strong wind speed associated with the jets exerted large asymmetric forcing that resulted in large EFC, it also brought strong VWS to the TCs (Table 1). In such a strongly sheared environment, TCs cannot maintain their typical structure and usually undergo transition into ECs.

  • According to the SEB theory, positive (negative) interaction would transport cyclonic (anticyclonic) angular momentum from the upper-level environment to TCs and enhance (reduce) their outflows. Due to its possible effects on TC intensification, previous studies have tended to concentrate on positive interaction. Following this line, we focused on upper-level flow patterns of all positive interaction samples. To extract the characteristics of the flow pattern from a large number of samples, the composite technique was used. Although the composite may smooth out some important characteristics of individual TCs, it emphasizes the signatures that repeatedly appear.

    There were several considerations regarding the composite procedure. Firstly, as EFC is computed using storm-relative wind (i.e., the translation speed of the TC is subtracted), it would be more meaningful to examine the storm-relative wind pattern. Secondly, to reduce the smoothing effect of the composite technique and to obtain more flow patterns, the samples were divided into eight groups according to the storm-relative wind directions averaged within 500 km of TCs. It is worth noting that different synoptic patterns or different positions of troughs will be somehow smoothed in the composite. However, there is no best way to group individual samples into different flow patterns, and different composite criteria would also suffer from the same problem. The present choice was shown to be effective in the sense of keeping flow features in the regions near the TC center and minimizing the blurring effect not too far from TCs. Besides, the most dominant flow type, the trough pattern, should be most frequently identified, in accordance with previous studies. Thirdly, different TCs may locate at different latitudes. Thus, traditional composite analysis using the latitude-longitude gridded wind field at different latitudes may be compromised by the problem of the meridian converging at high latitudes, and thus blurring some flow features. Taking this into account, all the composites were performed in cylindrical coordinates on a sphere with respect to their origins (i.e., the TCs' centers).

    Figure 5.  Eight composites of 200-hPa storm-relative flow (arrows; m s$^{-1}$) and local EFC (shaded, m s$^{-1}$ d$^{-1}$) for all positive interaction samples. Black bold arrows indicate wind speeds $>$15 m s$^{-1}$. Samples are grouped according to the averaged wind direction (shown in the lower-left corner of each panel) within 500 km of TCs. The composites were produced in cylindrical coordinates on a sphere, but the results were interpolated onto $1^\circ\times 1^\circ$ grids for a better visual. Black dots indicate TC centers and two black concentric circles in each panel show 300 km and 600 km radii from TC centers. The background map is shown for reference only.

    Table 2 lists the statistics of the eight composites and Fig. 5 shows the corresponding composite flow patterns as well as localized EFC. Local EFC is defined similar to Eq. (1), except that no azimuthal average is taken so that the local EFC is also a function of azimuth. Not surprisingly, the southwest composite contains 1359 samples, which is almost half of all positive interaction samples (Table 2), indicating that classical positive interactions usually occur right under the southwesterly wind between a westerly trough and a ridge (Fig. 5h). Two westerly jets (black bold arrows) locate southwest and northeast of the TC's center, respectively, and the TC's axisymmetric outflow tends to enhance the northeast outflow jet but weaken the southwest inflow one. The southwest jet imports cyclonic eddy angular momentum to the TC, while the northeast jet exports anticyclonic eddy angular momentum from the TC. Both jets result in positive EFC channels (red areas), so that the 300-600 km radial band has the largest mean EFC value (26.9 m s-1 d-1) among the eight composites. The west composite (Fig. 5a), containing 983 samples, shows a strong westerly jet crossing the TC's center. A smaller amplitude of the synoptic disturbance also indicates a smaller curvature of the flow and larger wind speeds, as compared to the southwest composite. The south composite has 323 samples, which is the third highest among the eight composites. The corresponding flow pattern (Fig. 5g) is similar to that of the southwest composite, except that the trough to the west of the TC center intensifies with its bottom extending to the south of the TC. The wind near the TC center is a uniform southerly, but weaker compared to that of the southwest composite, leading to a relatively weak mean VWS (15.1 m s-1), but still retaining a large value of mean EFC (24.8 m s-1 d-1). The whole pattern resembles an "S". This "S"-shaped flow has been reported to favor the intensification process of TCs during the extratropical transition stage (Qian et al., 2011). However, from the statistical point of view, only 18.8% of the samples intensified and more than 43% of the samples decayed. The northwest composite (Fig. 5b), which is the least among those composites (southwest, west, south and northwest) related to westerlies, contains only 204 samples. The flow features are quite similar, except that the inflow jet shifts to the northwest and the outflow jet shifts to the southeast.

    These four types, generally related to the upper-level westerly wind, contain more samples than the remaining four. A more interesting signature other than the trough in these composites is that there are two jets in the vicinity of the TC, like a dipole, resulting in an obvious azimuthal wavenumber-2 structure of eddy flux convergence. Their roles can be clearly identified by the local EFC in which no azimuthal average was calculated in Fig. 5: the two jets bring in two local EFC maxima (red areas) with respect to the TC center, regardless of jet directions (i.e., inflow or outflow). However, inside the trough regions (northwest quadrants between two jets), only negative local EFC (blue area) is found, and thus the troughs act to reduce the EFC when averaging within the 300-600 km radial band. Therefore, this double-jet signature, with the outflow anticyclonic curved and the inflow cyclonic curved, results in a large EFC value rather than the traditional trough signature. These two jets cannot be ascribed to the interaction between the TC and trough because there would be such a flow pattern even without a TC. That is why their influence is also viewed as external (e.g., Molinari and Vollaro, 1989).

    The remaining four composites of north, northeast, east and southeast (Figs. 5c-f) have relatively fewer samples. Such few samples may not produce a statistically significant percentage of intensifying or weakening samples, especially for the east composite. It seems that these interactions occur at lower latitudes where the prevailing upper-level wind has an easterly component (Figs. 5c-f) rather than synoptic waves embedded in westerlies. These TC samples are probably in their early stage as TDs because the underlying SST is relatively high (above 27°C, except that of the southeast composite). The previously mentioned double-jet signature also applies to these composites, except that the jets are much weaker, but the traditional trough pattern can barely be identified in these four composites.

    Figure 6 shows the local inertial stability, i.e., ζ a superposed by the composite wind fields. In terms of local inertial stability, the role of each jet, located in different quadrants, could be different because relatively weak inertial stability over a certain quadrant of the TC could minimize the energy expenditure and facilitate the formation of an outflow channel in that quadrant (Rappin et al., 2011). From Fig. 6 we can see that all composite ζ a values are positive, although this may not be true for each individual sample. Thus, the composite wind flows are generally inertially stable. However, the azimuthal distribution of ζ a is quite uneven, showing obvious azimuthal wavenumber-1 asymmetry. In general, the jet cores separate the ζ a into two parts——a larger part that is more inertially stable and locates inside the troughs, and a smaller part that is less stable. As can be seen from Fig. 6, the roles of the two jets are different in terms of inertial stability because the outflow jet is roughly within the less inertially stable region while the inflow jet is inside the more stable region. Given roughly the same environmental EFC forcing, the contribution of the outflow jet to the TC's axisymmetric secondary circulation is more significant than that of the inflow jet due to weaker inertial stability. Besides, as mentioned previously, smaller axisymmetric \(\overline\zeta_a\), after azimuthal averaging, indicates less resistance to the environmental EFC forcing and a stronger vortex response in the TC's radial-vertical circulation. These conditions apply to the four groups of north, northeast, east, and southeast composites (Figs. 6c-f), as these interactions occur at relatively low latitudes (Table 2), characterized by smaller planetary vorticity.

    It is worth mentioning that there are some similarities between the eight flow patterns identified here and those proposed in previous studies. For example, (Holland and Merrill, 1984) showed the manner in which a cyclone interacts with the subtropical westerlies and produces a poleward outflow channel (see their Fig. 15, and notice that their illustration is in the Southern Hemisphere). They also emphasized the effect of the subtropical jet on enhancing the TC's outflow, which is quite similar to Figs. 5f and g shown here. Figures 5f and g are similar to Holland and Merrill's (1984) illustration in the sense that the subtropical jet is located just north of the TC center and the TC keeps a certain distance away from the jet (heavy black arrows). However, the maximum radius is 1000 km in the composites. Therefore, the complete nature of the jet in Fig. 5f cannot be shown. Notice that there are also jet maxima just to the north of the TC center in Fig. 5h, but TCs are already inside the westerly jet core with large VWS, which is different from the picture of (Holland and Merrill, 1984). Another example was given by (McTaggart-Cowan et al., 2006), who presented a conceptual model of tropical transition in a dipole-blocking environment (see the right-hand column of their Fig. 2). Their flow pattern is quite similar to Figs. 5f and g shown here, in which the anticyclone to the northeast of the TC center develops intensively so that it is gradually cut off from the westerlies and forms a cutoff blocking. Such a blocking flow pattern, as noted by (McTaggart-Cowan et al., 2006), would facilitate a midlatitude vortex precursor transiting into a TC (known as tropical transition), whereas the similar flow pattern in Fig. 5g is likely to facilitate the transition of a TC into an EC (known as extratropical transition), depending on the accompanying VWS and underlying SST. A third example is the typical tropical upper-tropospheric trough (TUTT) flow pattern given by (Fitzpatrick et al., 1995). The TUTT pattern (see their Fig. 2), in which the axes of the shallow troughs lie in the northeast quadrants and also tilt northeast, resembles those of the north and northeast composites shown in Figs. 5c and d here.

    Figure 6.  As in Fig. 5 but for local absolute vorticity $\zeta_a$ (shaded; 10$^-5$ s$^-2$), defined in Eq. (2), as a proxy for local inertial stability.

    The EFC criterion, as shown here, actually does not guarantee the occurrence of westerly troughs but reflects azimuthal shears of the flow crossing the TC's center, especially those induced by wavenumber-1 asymmetry. Curvatures of the flow (i.e., cyclonic curved inflow and anticyclonic curved outflow) may eventually decide the sign of EFC. It was also found that extremely large EFC only occurs in those environments with large wind magnitude (e.g., Fig. 4), usually inside westerly jets. Therefore, we emphasize the feature of a double-jet signature of upper-level flows when interaction occurs because it is the curved jets, rather than troughs, that import large EFC from the environment to the TC (Fig. 5), resulting in significant upper-level asymmetric forcing on the TC's axisymmetric secondary circulation.

5. Environmental factors and TC intensity change
  • Studies (e.g., Wu and Cheng, 1999; Chen et al., 2015) have shown that SST, VWS and EFC are three large-scale environmental factors affecting TC intensity. Their effects may overlap to result in a more favorable environment for TC strengthening, or be cancelled to lead to a less favorable one. Therefore, the relationship between EFC and other environmental factors was investigated, based on the 25-year dataset.

    According to the SEB theory (section 2.2), large EFC would favor TC intensification. However, from Table 2 we can see that there is no simple "large EFC-large percentage of intensifying samples (or small percentage of decaying samples)" relationship, primarily due to two reasons. One reason is that larger EFC is generally accompanied by greater inertial stability (see the last two columns in Table 2). Inertial stability is a measure of the resistance of vortex responses to eddy forcing. High inertial stability means that strong forcing will result in limited responses. As a TC intensifies, especially when doing so rapidly, it requires low inertial stability (e.g., Rappin et al., 2011); the higher inertial stability shown here is partially responsible for reducing the positive effect of large EFC. Another reason is that TC intensity change is also controlled by two other environmental factors, i.e., SST and VWS. Larger EFC corresponds to lower SST and larger VWS so that EFC's positive effect on TC intensification is generally cancelled out by the negative effect of low SST and large VWS. All these facts can be explained by the variation in the latitude at which the TC is located, as a higher latitude usually means larger \(\overline\zeta_a\) (larger planetary vorticity f), lower SST, larger wind speed, and thus larger VWS and EFC.

    Figure 7 shows scatter plots of EFC versus SST for TC samples grouped by different TC stages. For the TD, TS and TY types (Figs. 7a-c), a large proportion of samples occur in a favorable environment of a warm ocean (SST >25°C). There are only a few samples whose underlying SSTs are lower than 20°C. The mean EFCs are slightly larger than 0, showing a skewness towards a positive EFC environment. For the EC type samples, as they have actually moved over cold water at higher latitudes, SST is much lower than those of TDs, with a mean of 16°C and large variance. Their mean EFC is close to 20 m s-1 d-1, already exceeding the threshold of positive interaction, which means that ECs are essentially different from other TC types and large upper-level EFC may be one of their characteristics. For all four types, it was found that the magnitude of EFC does not exceed 40 m s-1 d-1 if SST is higher than 25°C. However, extremely large EFCs occur frequently in lower SST environments. For the TS and TY samples, the highest EFC, at 80-120 m s-1 d-1, was in the SST range of 15°C-20°C. For the EC samples, this was even more prominent, in that EFC could reach above 120 m s-1 d-1 when SST was below 15°C.

    The relationship between VWS and EFC (Fig. 8) is the reverse of that between SST and EFC. Extremely large EFC usually occurs in a large VWS environment, but large VWS does not necessarily induce large EFC, as EFC also requires some curvature of the wind flow. According to the SEB theory, large EFC is usually expected to significantly enhance a TC's axisymmetric outflow, strengthen the updraft near the TC center, and lead to TC intensification. Here, it is shown that extremely large EFC (>40 m s-1 d-1) is usually accompanied by relatively low SST and strong VWS, so that the positive contribution of EFC to a TC's intensification is cancelled out by the negative contribution of the low SST and strong VWS. Therefore, EFC larger than 40 m s-1 d-1 does not guarantee TC intensification. On the contrary, such an environment would destroy a TC's warm-core structure and possibly lead to its transformation into an EC.

  • In order to investigate the relationship between environmental factors and TC intensity change, intensifying and weakening TC samples were selected. Generally, whether a TC is intensifying or weakening can be identified from the sign of pressure changes. Taking into account that the pressure records in the RSMC best track dataset are discrete, with a minimum interval of 2 hPa, and that the records may contain some uncertainties, records with |∆ p|>3 hPa were considered as the threshold for intensity change.

    Figure 7.  Scatter diagrams of SST ($^\circ$C) averaged within 500 km versus 200-hPa EFC (m s$^{-1}$ d$^{-1}$) averaged within 300-600 km for (a) TD, (b) TS, (c) TY and (d) EC samples. Samples of landing or near land (within 200 km) are excluded since no reliable SST can be obtained, yielding 15 999 samples. Black crosses indicate the mean EFC and SST for each panel.

    Figure 8.  As in Fig. 7 but for VWS (m s$^{-1}$).

    Scatter plots of the intensifying (∆ p<-3 hPa) and weakening (∆ p>3 hPa) TC samples and their environmental factors are shown in Fig. 9. For the intensifying samples (right-hand column in Fig. 9), there are obvious scatter clouds within regions of SST >25°C (the circle in Fig. 9b) and VWS <15 m s-1 (the circle in Fig. 9d). These clouds consist of samples from classic TC types, i.e., TD, TS and TY, while the EC samples are generally located outside the clouds (see Figs. 7d and 8d). For the classic TC types, environments with SST >25°C and VWS <15 m s-1 are generally known as necessary conditions for their maintenance or development. Their EFC magnitudes seldom exceed 25 m s-1 d-1. For the EC-type samples outside the dense clouds, it is surprising that all intensifying samples are in a positive EFC environment. Other factors, such as VWS or SST, place no obvious constraint on the distribution of these samples. This can be explained by the fact that ECs are baroclinic systems and gain energy primarily from baroclinic instability, while classic TC types (TD, TS and TY) gain energy mainly from latent (diabatic) heating. For weakening samples (left-hand column in Fig. 9), the scatter clouds are not as concentrated as their intensifying counterparts, and there are no obvious features in selecting an environment.

    Since ECs are different from other TC types in response to their environments, we should exclude them for further study. It is also known that large EFC may not correlate with simultaneous TC intensity, but has a lead-lag impact on later intensity change (e.g., Molinari and Vollaro, 1990). Therefore, it is more appropriate to define an interaction event as three consecutive six-hourly samples experiencing at least 10 m s-1 d-1 EFC. This definition groups those interaction samples into events and excludes samples that only once or twice exceed the 10 m s-1 d-1 threshold, which has little persistent effect on TC intensity. Samples with maximum sustained wind speed of <17.2 m s-1 were not considered, so as to exclude most EC samples, since ECs are different from other types. This procedure also excluded TDs, as there are large uncertainties in intensity records of weak intensity samples (e.g., Barcikowska et al., 2012).

    There were only 166 interaction events identified using the above constraints. The corresponding statistics of these events are listed in Table 3. The number of weakening events was twice as many as intensifying events. Therefore, drawing the simple conclusion that EFC >10 m s-1 d-1 may facilitate TC intensification is still not possible. Other factors, such as SST and VWS, seem more favorable for intensification. After excluding EC samples, the mean EFCs for both intensifying and weakening events remains at 25 m s-1 d-1. This value can be viewed as a threshold, and if environmental EFC exceeds this value the positive effect of EFC would be offset by accompanying large VWS, as well as low SST, possibly leading to extratropical transition of TCs.

    It is known that TCs usually weaken when close to land or are already landing. Thus, we added another condition that interaction events near land, or that had already landed, should be excluded. It is also known that, given an underlying SST, TCs have their maximum potential intensity (MPI). If a TC is close to its MPI (usually measured as surface maximum wind speed), then any atmospheric environment is hostile to a TC's intensity (Merrill, 1988b). That is, there is no room for the TC to develop. We imposed the final constraint that TCs should be at least 30 m s-1 away from their MPI, which was estimated using the empirical relation given by (Zeng et al., 2007) for the western North Pacific as: $$ {MPI}=A+B\exp[C(T_{s}-T_0)] , $$ where A=15.69 m s-1, B=98.03 m s-1, C=0.1806°C-1, T s is SST, and T0=30.0°C.

    Figure 9.  Scatter diagrams of (a, b) SST ($^\circ$C) and (c, d) VWS (m s$^{-1}$) averaged within 500 km versus 200-hPa EFC (m s$^{-1}$ d$^{-1}$) averaged within 300-600 km for (a, c) weakening samples ($\Delta p>3$ hPa) and (b, d) intensifying samples ($\Delta p<-3$ hPa). TC samples landing or near land (within 200 km) are excluded since no reliable SST can be estimated. Black crosses indicate the mean VWS, SST and EFC for each panel. Black circles highlight the scatter clouds in panels (b, d).

    Table 3 shows that when the "no-landing" condition is added, the number of weakening events is greatly reduced from 111 to 67, and the rate of SLP change is also reduced from 5 hPa to 4.5 hPa per six hours. After the "have-potential" condition is applied, the number of weakening events drops to 26, roughly half of that of intensifying events. Besides, the intensifying rate increases from 1.4 hPa to 1.7 hPa per six hours. However, even with these constraints, it is barely possible to draw the conclusion that higher EFC values will result in more intensifying events, because the mean EFC for an intensifying event is 16.2 m s-1 d-1, which is relatively smaller than that of weakening events. Thus, regardless of these constraints, the intensifying events seem to be affected more by the SST and VWS rather than by EFC environments. As noted earlier, EFC exceeding the 25 m s-1 d-1 threshold is likely to induce very large VWS and low SST, and thus negative effects of the environment are more pronounced than positive ones, leading to weakening TCs or even deforming them into ECs. Here, EFC larger than this value tends to slow down the intensification rate or increase the possibility of filling a TC. So, we regard a moderate EFC environment, larger than 10 m s-1 d-1 but generally below 25 m s-1 d-1, as a relatively good environment for TC intensification.

6. Conclusions
  • A preliminary statistical survey of the interactions between TCs and upper-level environmental flows over the western North Pacific is presented in this paper using a 25-year (1987-2011) best track dataset and ERA-Interim data. Interactions are defined as the magnitude of EFC exceeding 10 m s-1 d-1. According to this definition, it was found that 17.7% of 21 685 six-hourly TC samples experienced interaction: 14% positive and 3.7% negative, according to their EFC signs. These interactions were frequently identified around Japan, where TCs recurve into midlatitude westerlies.

    Six extreme interaction cases showed that negative (positive) interactions are characterized by upper-level northwest (southwest) cross-center wind associated with the upstream (downstream) flow of a trough. Extreme negative EFC values reach -70 m s-1 d-1, whose magnitude is only half that of positive ones (120 m s-1 d-1). These negative interactions, arising from slightly modifying the interaction definition, as compared to previous studies, also provides new features when EFC is negative. This kind of interaction also relates to upper-level troughs (Figs. 4a-c). The main difference is that the TC locates upstream of the trough, rather than downstream as in traditionally defined (i.e., positive) interaction.

    Composite analysis showed that positive interactions are characterized by an apparently curved upper-level crossing-center flow that consists of two jets. These two jets, identified in the present study through analyzing localized EFC, are a more important signature than the traditionally proposed trough pattern because large positive local EFC comes from these two jets rather than from the trough itself. Therefore, the interaction strength (e.g., EFC value) largely depends on the jet strength as well as their curvature. It is also shown that the outflow jet, exporting anticyclonic eddy angular momentum, is more important than the inflow one, importing cyclonic momentum, as the axisymmetric outflow enhances the outflow jet strength and lowers the adjacent inertial stability. That is why it is common to observe an enhanced outflow channel in the northern part of a TC without seeing any clear inflow channel (e.g., Rappin et al., 2011, Fig. 17).

    Generally, according to the SEB theory, large positive EFC in the upper troposphere means that large cyclonic angular momentum is transported towards the TC from its vicinity and is thus favorable for TC axisymmetric outflow enhancement or even intensification. Previous studies (e.g., DeMaria et al., 1993) have already pointed out that such a positive effect will be offset by accompanying strong VWS and low SST. Here, we suggested separating the interactions into extreme cases (EFC larger than 25 m s-1 d-1) and moderate cases (10-25 m s-1 d-1), according to Figs. 7 and 8. It was found that extreme interactions generally occur at high latitudes, meaning the environmental SST is very low and the VWS is quite strong. Besides, the inertial stability is also large at high latitudes (Table 2), which means that the positive effect of extremely large EFC will be scaled down. During such cases, the TC tends to transform into an EC. Only those ECs after transformation, which are less dependent on underlying high SST and low VWS, can survive or intensify in extremely large EFC environments (Fig. 9b). For moderate interactions that occur in the TS and TY stages, it is shown that their responses to EFC environments are quite different from that of the EC type. These traditional TCs can only survive in environments with small-to-moderate VWS, to maintain their warm core structures. The intensities of these TCs are also more sensitive to the underlying SST than EFC. Therefore, the "good trough/bad trough" issue still remains unclear for these traditional TCs.

    The EFC threshold of 25 m s-1 d-1, empirically identified here, also distinguishes the interaction case of Hurricane Elena (1985) reported by Molinari and Vollaro (1989, 1990) as a canonical one, because this case only just reached the proposed upper limit of favorable interaction and then experienced a rapid intensification. It is also interesting to find cases over the western North Pacific that are similar to this canonical one, which may help us identify these "good" troughs. We selected TC cases with EFC larger than 20 m s-1 d-1 and VWS smaller than 10 m s-1 for at least one six-hour snapshot. There were only 153 samples (5% of positive interactions) that met these requirements, including several consecutive records in a single TC. Six representative interacting TCs——two during the formation stage (cases 1 and 2), two during the mature stage (cases 3 and 4), and two during the dissipation stage (cases 5 and 6)——were selected (shown in Fig. 10). As we can see, favorable interaction (cases 1-4) does not indicate a westerly trough in the vicinity of the TC (e.g., cases 2 and 3). Even in the subtropics, flows with certain curvature can cause interaction. The enhanced EFC (green shaded areas) generally lead to subsequent intensifying processes. This is similar to the case of Hurricane Elena (1985), in which an EFC spike acted as a triggering effect for deepening the TC. Obviously, unfavorable interaction (cases 5 and 6) can be attributed to the increasing VWS and decreasing SST; although, at the interaction times (vertical gray lines), VWS does not exceed 10 m s-1. It also seems coincident that the 200-hPa flows in cases 5 and 6 resemble the non-intensifying pattern summarized by Merrill (1988b, Fig. 11), in which the anticyclones east of the TCs have closed streamlines.

    Based on the conclusions drawn from the present study, as well as these representative cases, we can gain some insights into the "good" trough issue. Favorable interactions for ECs should be characterized by very large environmental EFC (at least above 25 m s-1 d-1), and the "larger is better" conclusion may apply for ECs regardless of the associated VWS and underlying SST. However, for traditional TC types, such as TDs, TSs or TYs, favorable interaction occurs with only moderate EFC above 10 m s-1 d-1, but generally below 25 m s-1 d-1, in favor of low VWS. This requires that TCs should keep a certain distance from the disturbed westerly jet (usually with a trough embedded upstream) and not penetrate into the jet core regions (such as in extreme interaction cases), as was schematically illustrated by Holland and Merrill (1984, Fig. 15) and also exemplified by Rappin et al. (2011, Fig. 17), so that their warm core structure will not be destroyed by persistently strong VWS. Besides, interaction should not last more than around two days, playing its role as a triggering effect for initiating wind-induced surface heat exchange (Emanuel, 1986) or the eyewall replacement cycle (Willoughby et al., 1982; Leroux et al., 2013), as long-lasting interaction will eventually bring persistent VWS, which is destructive to a TC's warm core. Finally, interaction should appear at low latitudes so that environmental inertial stability is relatively weak and thus the TC's axisymmetric secondary circulation is more sensitive to the upper-level asymmetric forcing.

    Figure 10.  Six selected interaction TCs (one row for each). The left-hand column shows the tracks for each case and 200-hPa flow fields (vectors; m s$^{-1}$) valid at interaction times labelled in the titles. Black dots show the TC positions at those times. The right-hand column shows the temporal evolution of TC intensity i.e., minimum SLP (black line; hPa), as well as the three environmental factors of SST (red line; $^\circ$C), VWS (blue line; m s$^{-1}$) and EFC (green line; m s$^{-1}$ d$^{-1}$). Vertical gray lines in the right-hand column indicate the time shown in the titles of the left-hand column. The green shaded areas emphasize the interaction periods of interest with EFC larger than the 10 m s$^{-1}$ d$^{-1}$ threshold.

    The present study focused primarily on the dynamical effect of interaction in terms of EFC and inertial stability. In fact, a westerly trough approaching a TC will also import cold and dry air——also known as cold air intrusion. Such intrusion of cold and dry air will certainly alter the thermodynamic structure of a TC and exert some influence on its axisymmetric secondary circulation. (Molinari and Vollaro, 1990) found that the eddy heat flux induced by cold advection could contribute in the same direction as eddy momentum flux within 500 km, but opposite outside. Therefore, Molinari et al. (1995, 1998) later analyzed interactions by adopting the Eliassen-Palm (EP) flux diagnostic, which has two components: eddy angular momentum flux (dynamical component) and eddy heat flux (thermodynamical component). As there is uncertainty as to whether the contributions of these two components could be the same or opposite (e.g., Molinari and Vollaro, 1990; Qian et al., 2011), one way to address this issue is to use the EP flux diagnostic and its divergence. This diagnostic tool also requires the whole vertical structure of the TC, rather than a single upper-layer wind field, so that it will provide greater insights into TC-environment interaction. Using such a diagnostic in a statistical study of interaction, rather than EFC alone, may help identify "good" troughs and favorable interactions.




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