2011 Vol. 28, No. 3

Display Method:
2011, 28(3): 477-482. doi: 10.1007/s00376-010-9229-6
[Abstract](596) [PDF 542KB](1171)
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Roll vortices, which often appear when cold air outbreaks over warm ocean surfaces, are an important system for energy and substance exchange between the land surface and atmosphere. Numerical simulations were carried out in the study to simulate roll vortices in the convective boundary layer (CBL). The results indicate, that with proper atmospheric conditions, such as thermal instability in the CBL, stable stratification in the overlying layer and suitable wind shear, and a temperature jump between the two layers in a two-layer atmosphere, convective bands appear after adding initial pulses in the atmosphere. The simulated flow and temperature fields presented convective bands in the horizontal and roll vortices in the crosswind section. The structure of the roll vortices were similar to those observed in the cloud streets, as well as those from analytical solutions. Simulations also showed the influence of depth and instability strength of the CBL, as well as the stratification above the top of the CBL, on the orientation spacing and strength of the roll vortices. The fluxes caused by the convective rolls were also investigated, and should perhaps be taken into account when explaining the surface energy closure gap in the CBL.
2011, 28(3): 483-491. doi: 10.1007/s00376-010-9190-4
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This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008, during its formation stage. Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22--24 April 2008. The westerlies, together with strong tropical--subtropical easterlies, constituted a large-scale horizontal shear flow, creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis. This basic zonal flow in the lower troposphere was barotropically unstable, with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25--26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones. Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis.
2011, 28(3): 492-510. doi: 10.1007/s00376-010-0015-2
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The response of the upper-ocean temperatures and currents in the tropical Pacific to the spatial distribution of chlorophyll-a and its seasonal cycle is investigated using a coupled atmosphere-ocean model and a stand-alone oceanic general circulation model. The spatial distribution of chlorophyll-a significantly influences the mean state of models in the tropical Pacific. The annual mean SST in the eastern equatorial Pacific decreases accompanied by a shallow thermocline and stronger currents because of shallow penetration depth of solar radiation. Equatorial upwelling dominates the heat budget in that region. Atmosphere--ocean interaction processes can further amplify such changes. The seasonal cycle of chlorophyll-a can dramatically change ENSO period in the coupled model. After introducing the seasonal cycle of chlorophyll-a concentration, the peak of the power spectrum becomes broad, and longer periods (>3 years) are found. These changes led to ENSO irregularities in the model. The increasing period is mainly due to the slow speed of Rossby waves, which are caused by the shallow mean thermocline in the northeastern Pacific.
2011, 28(3): 511-520. doi: 10.1007/s00376-010-0091-3
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A dust deflation module was developed and coupled with the air quality modeling system RAMS-CMAQ to simultaneously treat all the major tropospheric aerosols (i.e., organic and black carbons, sulfate, nitrate, ammonia, soil dust, and sea salt). Then the coupled system was applied to East Asia to simulate Asian dust aerosol generation, transport and dry/wet removal processes during 14--25 March 2002 when two strong dust storms occurred consecutively. To evaluate model performance and to analyze the observed features of dust aerosols over the East Asian region, model results were compared to concentrations of suspended particulate matter of 10 μm or less (PM10; 1-h intervals) at four remote Japanese stations and daily air pollution index (API) values for PM10 at four large Chinese cities. The modeled values were generally in good agreement with observed data, and the model reasonably reproduced two dust storm outbreaks and generally predicted the dust onset and cessation times at each observation site. In addition, hourly averaged values of aerosol optical thickness (AOT) were calculated and compared with observations at four Aerosol Robotic Network (AERONET) stations to assess the model's capability of estimating dust aerosol column burden. Analysis shows that modeled and observed AOT values were generally comparable and that the contribution of dust aerosols to AOT was significant only with regard to their source regions and their transport paths.
2011, 28(3): 521-533. doi: 10.1007/s00376-010-9118-z
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Impacts of the MJO on winter rainfall and circulation in China are investigated using a real-time multivariate MJO index. Composite results using the daily rainfall anomalies and rainy day'' anomalies according to eight different MJO phases show that the MJO has considerable influence on winter rainfall in China. Rainfall anomalies show systematic and substantial changes (enhanced/suppressed) in the Yangtze River Basin and South China with the eastward propagation of the MJO convective center from the Indian Ocean to the western Pacific. When the MJO is in phase 2 and 3 (MJO convective center is located over the Indian Ocean), rainfall probability is significantly enhanced. While in phase 6 and 7 (MJO convective center is over the western Pacific), rainfall probability is significantly reduced. MJO in winter influences the rainfall in China mainly through modulating the circulation in the subtropics and mid-high latitudes. For the subtropics, MJO influences the northward moisture transport coming from the Bay of Bengal and the South China Sea by modulating the southern trough of the Bay of Bengal and the western Pacific subtropical high. For the mid-high latitudes, the propagation of the low frequency perturbations associated with the eastward-propagating MJO convection modulate the circulation in the mid-high latitudes, e.g. the East Asian winter monsoon and the low trough over central Asia.
2011, 28(3): 534-550. doi: 10.1007/s00376-010-0062-8
[Abstract](613) [PDF 974KB](1004)
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A Single Column Model (SCM) for Global and Regional Atmospheric Prediction Enhanced System (GRAPES) is constructed for the purpose of evaluating physical process parameterizations. Two observational datasets including Wangara and the third Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS-3) SCM field observations have been applied to evaluate this SCM. By these two numerical experiments, the GRAPES_SCM is verified to be correctly constructed. Furthermore, the interaction between the land surface process and atmospheric boundary layer (ABL) is discussed through the second experiment. It is found that CASE3 (CoLM land surface scheme coupled with ABL scheme) simulates less sensible heat fluxes and smaller surface temperature which corresponds with its lower potential temperature at the bottom of the ABL. Moreover, CASE3 simulates turbulence that is weaker during the daytime and stronger during nighttime, corresponding with its wind speed at 200 m which is bigger during daytime and smaller during nighttime. However, they are generally opposite in CASE2 (SLAB coupled with ABL). The initial profile of the water vapor mixing ratio is artificially increased by the experiment setup which results in the simulated water vapor mixing becoming wetter than actually observed. CASE1 (observed surface temperature taken as lower thermal forcing) and CASE2 have no soil moisture prediction and simulate a similar water vapor mixing ratio, while CASE3 has a soil moisture prediction and simulates wetter. It is also shown that the time step may affect the stabilization of the ABL when the vertical levels of the SCM are fixed.
2011, 28(3): 551-563. doi: 10.1007/s00376-010-0010-7
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The residual meridional circulation derived from the transformed Eulerian-mean thermodynamic equation and continuity equation can be separated into two parts, the slowly varying diabatic circulation and the transient circulation, as demonstrated by others. We calculated and composite-analyzed the transient and diabatic circulation for 14 stratospheric sudden warming (SSW) events from 1979--2002 by using the daily ECMWF reanalysis data. Specifically, the transient residual meridional circulation was calculated both with and without inclusion of the eddy heat transport term in the transformed Eulerian-mean thermodynamic equation to investigate the importance of the eddy heat transport term. The results showed that calculations of transient residual meridional circulation present rapid variations during SSWs, with or without inclusion of the eddy heat transport term. Although the patterns of transient residual meridional circulation with the eddy heat transport term were similar to that without the eddy heat transport term during SSW, the magnitudes in the upper stratosphere and high-latitude regions differed. As for the diabatic circulation, its daily variations were small during SSW events, and its patterns were in agreement with its monthly average.
2011, 28(3): 564-572. doi: 10.1007/s00376-010-9219-8
[Abstract](550) [PDF 506KB](1106)
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Phenological data on the First Flowering Date (FFD) of woody plants in Beijing from 1963--2007 are analyzed. The correlation between each species' yearly FFD and the mean monthly temperatures for every year over a 45-year period is used to identify the month in which temperature has the most effect on FFD. Through further analysis, the FFDs of 48 woody plant species are shown to have advanced an average of 5.4 days from 1990--2007 compared to 1963--1989. The results indicate that 70.8% of species flowered significantly earlier (7 days on average) during the period 1990--2007, while only one species (2.1%) flowered significantly later. Moreover, the responses of FFD to climate change are shown to be different in two climatic stages, defined by an abrupt climate change point. Thirty-three species which first flower in March and April are sensitive to temperature are examined. The correlation coefficients between FFD and temperature for 20 species during the latter period (1990--2007) are shown to be larger than during the former period (1963--1989), with a difference of around -0.87 days per 1oC on average. The paper concludes that with the warming of climate, the linear trend of FFD variation, as well as its responsiveness to temperature, became more prominent during 1990--2007 than 1963--1989. The data analyzed in this study present a strong biological indicator of climate change in Beijing, and provide further confirmation of previous results from regional and local studies across the Northern Hemisphere. Phenophase variations indicate that the climate is changing rapidly.
2011, 28(3): 573-590. doi: 10.1007/s00376-010-0035-y
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This study investigated the impact of multiple-Doppler radar data and surface data assimilation on forecasts of heavy rainfall over the central Korean Peninsula; the Weather Research and Forecasting (WRF) model and its three-dimensional variational data assimilation system (3DVAR) were used for this purpose. During data assimilation, the WRF 3DVAR cycling mode with incremental analysis updates (IAU) was used. A maximum rainfall of 335.0 mm occurred during a 12-h period from 2100 UTC 11 July 2006 to 0900 UTC 12 July 2006. Doppler radar data showed that the heavy rainfall was due to the back-building formation of mesoscale convective systems (MCSs). New convective cells were continuously formed in the upstream region, which was characterized by a strong southwesterly low-level jet (LLJ). The LLJ also facilitated strong convergence due to horizontal wind shear, which resulted in maintenance of the storms. The assimilation of both multiple-Doppler radar and surface data improved the accuracy of precipitation forecasts and had a more positive impact on quantitative forecasting (QPF) than the assimilation of either radar data or surface data only. The back-building characteristic was successfully forecasted when the multiple-Doppler radar data and surface data were assimilated. In data assimilation experiments, the radar data helped forecast the development of convective storms responsible for heavy rainfall, and the surface data contributed to the occurrence of intensified low-level winds. The surface data played a significant role in enhancing the thermal gradient and modulating the planetary boundary layer of the model, which resulted in favorable conditions for convection.
2011, 28(3): 591-600. doi: 10.1007/s00376-010-0037-9
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Decadal changes in the subseasonal evolution and the phase-locked climatological intraseasonal fluctuation of summertime rainfall over the Korean Peninsula before and after the mid-1990s are investigated. The activity and the migration speed of the monsoon rain band over the East Asian region are altered in the recent decade, resulting in the drier conditions in late spring and the earlier onset of Changma. In early August when a climatological monsoon break was clear in the earlier decade, the precipitation has increased dramatically with a meridional coherency. The response to the enhanced convection over the South China Sea and southeastern China provides a favorable condition for more precipitation in early August through the changes in moisture transport and tropical cyclone passage.
2011, 28(3): 601-611. doi: 10.1007/s00376-010-9056-9
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We describe how the Unit-Feature Spatial Classification Method (UFSCM) can be used operationally to classify cloud types in satellite imagery efficiently and conveniently. By using a combination of Interactive Data Language (IDL) and Visual C++ (VC) code in combination to extend the technique in three dimensions (3-D), this paper provides an efficient method to implement interactive computer visualization of the 3-D discrimination matrix modification, so as to deal with the bi-spectral limitations of traditional two dimensional (2-D) UFSCM. The case study of cloud-type classification based on FY-2C satellite data (0600 UTC 18 and 0000 UTC 10 September 2007) is conducted by comparison with ground station data, and indicates that 3-D UFSCM makes more use of the pattern recognition information in multi-spectral imagery, resulting in more reasonable results and an improvement over the 2-D method.
2011, 28(3): 612-622. doi: 10.1007/s00376-010-9089-0
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The main objective of this article is to establish a new mechanism of ENSO, as a self-organizing and self-excitation system, with two highly coupled processes. The first is the oscillation between the two metastable warm (El Nino phase) and cold events (La Nina phase), and the second is the spatiotemporal oscillation of the sea surface temperature (SST) field. The symbiotic interplay between these two processes gives rises the climate variability associated with the ENSO, leads to both the random and deterministic features of the ENSO, and defines a new natural feedback mechanism, which drives the sporadic oscillation of the ENSO. The new mechanism is rigorously derived using a dynamic transition theory developed recently by the authors, which has also been successfully applied to a wide range of problems in nonlinear sciences.
2011, 28(3): 623-635. doi: 10.1007/s00376-010-9194-0
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The cloud variations under subtropical high (STH) conditions during summers over a ten-year period are studied using combined data from the International Satellite Cloud Climatology Project and the National Centers for Environmental Prediction. The results reveal that clouds mainly experience an isolated evolution in the STHs, which is designated in this study by the 1540 gpm geopotential lines at 850 hPa. In the STH domain throughout the Northern Hemisphere, the average amount of total clouds exceeds 30%. Low clouds dominate in the STH domain, contributing over 60% of total cloud amount within the Pacific subtropical high and over 40% within the Atlantic subtropical high. The prevalence of low clouds in above regions is determined by the circulation pattern around 150o--180oE and 850 hPa, which suppresses both the upward development of the cloud tops and the water vapor divergences near the surface. Furthermore, clouds present great geographical incoherence within the STH domain. In the eastern STHs, the amount of middle and low clouds increases to peak in the early morning and decreases to a trough in the afternoon, while the amount of high clouds remains stable throughout the day. Conversely, in the western STHs, the diurnal amplitude of low and middle clouds is less than three, while high clouds dramatically reach the maximum in the afternoon and drop to the minimum in the evening. Among the nine cloud categories, stratocumulus clouds with greater optical thickness account for the most under STH conditions, no matter their occurrence or amount, causing more shortwave cloud radiative forcing to cool the local atmosphere and surface as a consequence.
2011, 28(3): 636-652. doi: 10.1007/s00376-010-0029-9
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An empirical atmospheric model (EAM) based on the singular value decomposition (SVD) method is evaluated using the composite El Nino/Southern Oscillation (ENSO) patterns of sea surface temperature (SST) and wind anomalies as the target scenario. Two versions of the SVD-based EAM were presented for comparisons. The first version estimates the wind anomalies in response to SST variations based on modes that were calculated from a pair of global wind and SST fields (i.e., conventional EAM or CEAM). The second version utilizes the same model design but is based on modes that were calculated in a region-wise manner by separating the tropical domain from the remaining extratropical regions (i.e., region-wise EAM or REAM). Our study shows that, while CEAM has shown successful model performance over some tropical areas, such as the equatorial eastern Pacific (EEP), the western North Pacific (WNP), and the tropical Indian Ocean (TIO), its performance over the North Pacific (NP) seems poor. When REAM is used to estimate the wind anomalies instead of CEAM, a marked improvement over NP readily emerges. Analyses of coupled modes indicate that such an improvement can be attributed to a much stronger coupled variability captured by the first region-wise SVD mode at higher latitudes compared with that captured by the conventional one. The newly proposed way of constructing the EAM (i.e., REAM) can be very useful in the coupled studies because it gives the model a wider application beyond the commonly accepted tropical domain.
2011, 28(3): 653-664. doi: 10.1007/s00376-010-0049-5
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Two years of mid-latitude cirrus cloud macrophysical and optical properties over North China are described from Earth-orbiting Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite measurements. Global cloud climatological studies based on active remote sensing data sets benefit from more accurate resolution of vertical structure and more reliable detection of optically thin layers. The mean values for cirrus cases over North China are 0.19±0.18 for infrared emittance, 0.41±0.68 for visible optical depth, 0.26±0.12 for integrated depolarization ratio, and 0.72±0.22 for integrated color ratio. When studied using reasonable assumptions for the relationship between extinction and ice crystal backscatter coefficients, our results show that most of the cirrus clouds profiled using the 0.53 μm channel data stream correspond with an optical depth of less than 1.0. The dependence of cirrus cloud properties on cirrus cloud mid-cloud temperature and geometry thickness are generally similar to the results derived from the ground-based lidar, which are mainly impacted by the adiabatic process on the ice cloud content. However, the differences in macrophysical parameter variability indicate the limits of spaceborne-lidar and dissimilarities in regional climate variability and the nature and source of cloud nuclei in different geographical regions.
2011, 28(3): 665-681. doi: 10.1007/s00376-010-9142-z
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The purpose of this paper is to explore how a tropical cyclone forms from a pre-existing large-scale depression which has been observed and associated with cross-equatorial surges in the western North Pacific. Tropical cyclone Bilis (2000) was selected as the case to study. The research data used are from the results of the non-hydrostatic mesoscale model (MM5), which has successfully simulated the transformation of a pre-existing weak large-scale tropical depression into a strong tropical storm. The scale separation technique is used to separate the synoptic-scale and sub-synoptic-scale fields from the model output fields. The scale-separated fields show that the pre-existing synoptic-scale tropical depression and the sub-synoptic scale tropical cyclone formed later were different scale systems from beginning to end. It is also shown that the pre-existing synoptic-scale tropical depression did not contract to become the tropical cyclone. A series of weak, sub-synoptic-scale low and high pressure systems appeared and disappeared in the synoptic-scale depression, with one of the low systems near the center of the synoptic-scale depression having deepened to become the tropical cyclone. The roles of the synoptic-scale flow and the sub-synoptic scale disturbances in the formation of the tropical cyclone are investigated by diagnoses of the scale-separated vertical vorticity equation. The results show that the early development of the sub-synoptic scale vortex was fundamentally dependent on the strengthening synoptic-scale environmental depression. The depression was strengthened by cross-equatorial surges, which increased the convergence of the synoptic-scale depression at low levels and triggered the formation of the tropical cyclone.
2011, 28(3): 682-690. doi: 10.1007/s00376-010-9184-2
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A regional ocean reanalysis system for the coastal waters of China and adjacent seas has been developed by the National Marine Data and Information Service (NMDIS). It produces a dataset package called CORA (China ocean reanalysis). The regional ocean model used is based on the Princeton Ocean Model with a generalized coordinate system (POMgcs). The model is parallelized by NMDIS with the addition of the wave breaking and tidal mixing processes into model parameterizations. Data assimilation is a sequential three-dimensional variational (3D-Var) scheme implemented within a multigrid framework. Observations include satellite remote sensing sea surface temperature (SST), altimetry sea level anomaly (SLA), and temperature/salinity profiles. The reanalysis fields of sea surface height, temperature, salinity, and currents begin with January 1986 and are currently updated every year. Error statistics and error distributions of temperature, salinity and currents are presented as a primary evaluation of the reanalysis fields using sea level data from tidal gauges, temperature profiles, as well as the trajectories of Argo floats. Some case studies offer the opportunity to verify the evolution of certain local circulations. These evaluations show that the reanalysis data produced provide a good representation of the ocean processes and phenomena in the coastal waters of China and adjacent seas.
2011, 28(3): 691-698. doi: 10.1007/s00376-010-9152-x
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The heat storage terms in the soil--vegetation--atmosphere system may play an important role in the surface energy budget. In this paper, we evaluate the heat storage terms of a subalpine meadow based on a field experiment conducted in the complex terrain of the eastern Qilian Mountains of Northwest China and their impact on the closure of the surface energy balance under such non-ideal conditions. During the night, the average sum of the storage terms was -5.5 W m-2, which corresponded to 10.4% of net radiation. The sum of the terms became positive at 0730 LST and negative again at about 1500 LST, with a maximum value of 19 W m-2 observed at approximately 0830 LST. During the day, the average of the sum of the storage terms was 6.5 W m-2, which corresponded to 4.0% of net radiation. According to the slopes obtained when linear regression of the net radiation and partitioned fluxes was forced through the origin, there is an imbalance of 14.0% in the subalpine meadow when the storage terms are not considered in the surface energy balance. This imbalance was improved by 3.4% by calculating the sum of the storage terms. The soil heat storage flux gave the highest contribution (1.59%), while the vegetation enthalpy change and the rest of the storage terms were responsible for improvements of 1.04% and 0.77%, respectively.
2011, 28(3): 699-708. doi: 10.1007/s00376-010-0024-1
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In order to re-examine some trends related to tropical cyclones (TCs) over the western North Pacific since 1949, the unreliable maximum sustained wind (Vmax) recorded in the 1949--1978 TC best-track data from the Shanghai Typhoon Institute was modified based on the wind--pressure relationships (WPRs) in this study. Compared to the WPR scheme based on the cyclostrophic balance, the WPR scheme based on the gradient balance could give a better fit to TCs under higher wind speeds and could introduce smaller estimated errors for TCs locating at higher latitudes as well as TCs landing on the continent. After the Vmax modification based on minimum sea-level pressure and TC center latitude, the revised annual number of category 4--5 typhoons shows no long-term trend, while the potential destructiveness measured by power-dissipation index decreases slightly, and this trend is not significant in the period 1949--2008.
2011, 28(3): 709-724. doi: 10.1007/s00376-010-0026-z
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Energy budgets were analyzed to study the development of an eastward propagating southwest vortex (SWV) associated with heavy rainfall over southern China (11--13 June 2008). The results show that kinetic energy (KE) generation and advection were the most important KE sources, while friction and sub-grid processes were the main KE sinks. There was downward conversion from divergent to rotational wind KE consistent with the downward stretching of SWVs. The Coriolis force was important for the formation and maintenance of the SWV. Convergence was also an important factor for maintenance, as was vertical motion during the mature stage of the SWV and the formation stage of a newly formed vortex (vortex B). The conversion from available potential energy (APE) to KE of divergent wind can lead to strong convection. Vertical motion influenced APE by dynamical and thermal processes which had opposite effects. The variation of APE was related to the heavy rainfall and convection; in this case, vertical motion with direct thermal circulation was the most important way in which APE was released, while latent heat release and vertical temperature advection were important for APE generation.