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Impact of Spectral Nudging on the Downscaling of Tropical Cyclones in Regional Climate Simulations

doi: 10.1007/s00376-016-5061-y

  • This study investigated the simulations of three months of seasonal tropical cyclone (TC) activity over the western North Pacific using the Advanced Research WRF Model. In the control experiment (CTL), the TC frequency was considerably overestimated. Additionally, the tracks of some TCs tended to have larger radii of curvature and were shifted eastward. The large-scale environments of westerly monsoon flows and subtropical Pacific highs were unreasonably simulated. The overestimated frequency of TC formation was attributed to a strengthened westerly wind field in the southern quadrants of the TC center. In comparison with the experiment with the spectral nudging method, the strengthened wind speed was mainly modulated by large-scale flow that was greater than approximately 1000 km in the model domain. The spurious formation and undesirable tracks of TCs in the CTL were considerably improved by reproducing realistic large-scale atmospheric monsoon circulation with substantial adjustment between large-scale flow in the model domain and large-scale boundary forcing modified by the spectral nudging method. The realistic monsoon circulation took a vital role in simulating realistic TCs. It revealed that, in the downscaling from large-scale fields for regional climate simulations, scale interaction between model-generated regional features and forced large-scale fields should be considered, and spectral nudging is a desirable method in the downscaling method.
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  • Alexand ru, A., R. De Elia, R. Laprise, L. Separovic, S. Biner, 2009: Sensitivity study of regional climate model simulations to large-scale nudging parameters. Mon. Wea. Rev., 137, 1666- 1686.10.1175/ Previous studies with nested regional climate models (RCMs) have shown that large-scale spectral nudging (SN) seems to be a powerful method to correct RCMs’ weaknesses such as internal variability, intermittent divergence in phase space (IDPS), and simulated climate dependence on domain size and geometry. Despite its initial success, SN is not yet in widespread use because of disagreement regarding the main premises—the unconfirmed advantages of removing freedom from RCMs’ large scales—and lingering doubts regarding its potentially negative side effects. This research addresses the latter issue. Five experiments have been carried out with the Canadian RCM (CRCM) over North America. Each experiment, performed under a given SN configuration, consists of four ensembles of simulations integrated on four different domain sizes for a summer season. In each experiment, the effects of SN on internal variability, time means, extremes, and power spectra are discussed. As anticipated from previous investigations, the present study confirms that internal variability, as well as simulated-climate dependence on domain size, decreases with increased SN strength. Our results further indicate a noticeable reduction of precipitation extremes as well as low-level vorticity amplitude in almost all length scales, as a side effect of SN; these effects are mostly perceived when SN is the most intense. Overall results indicate that the use of a weak to mild SN may constitute a reasonable compromise between the risk of decoupling of the RCM internal solution from the lateral boundary conditions (when using large domains without SN) and an excessive control of the large scales (with strong SN).
    Camargo S. J., A. H. Sobel, 2004: Formation of tropical storms in an atmospheric general circulation model. Tellus A, 56, 56- 67.
    Camargo S. J., H. L. Li, and L. Q. Sun, 2007: Feasibility study for downscaling seasonal tropical cyclone activity using the NCEP regional spectral model. Int. J. Climatol., 27, 311- 325.10.1002/ The potential use of the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) for downscaling seasonal tropical cyclone (TC) activity was analyzed here. The NCEP RSM with horizontal resolution of 50 km, was used to downscale the ECHAM4.5 Atmospheric General Circulation Model (AGCM) simulations forced with observed sea surface temperature (SST) over the western North Pacific. An ensemble of ten runs for June-ovember 1994 and 1998 was studied. The representation of the TCs is much improved compared to the low-resolution forcing AGCM, but the TCs are not as intense as observed ones, as the RSM horizontal resolution is not sufficiently high. The large-scale fields of the RSM are examined and compared to both the AGCM and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The large-scale fields of RSM characteristics are in general similar to those of the reanalysis. Various properties of the TCs in the RSM are also examined such as first positions, tracks, accumulated cyclone energy (ACE) and duration. While the RSM does not reproduce the higher number of TCs in 1994 than in 1998, other measures of TC activity (ACE, number of cyclone days) in the RSM are higher in 1994 than in 1998. Copyright 2006 Royal Meteorological Society.
    Castro C. L., R. A. Pielke Sr., and G. Leoncini, 2005: Dynamical downscaling: Assessment of value retained and added using the regional atmospheric modeling system (RAMS). J. Geophys. Res., 110,D05108, doi: 10.1029/2004JD004721.10.1029/ value restored and added by dynamical downscaling is quantitatively evaluated by considering the spectral behavior of the Regional Atmospheric Modeling System (RAMS) in relation to its domain size and grid spacing. A regional climate model (RCM) simulation is compared with NCEP Reanalysis data regridded to the RAMS grid at each model analysis time for a set of six basic experiments. At large scales, RAMS underestimates atmospheric variability as determined by the column integrated kinetic energy and integrated moisture flux convergence. As the grid spacing increases or domain size increases, the underestimation of atmospheric variability at large scales worsens. The model simulated evolution of the kinetic energy relative to the reanalysis regridded kinetic energy exhibits a decrease with time, which is more pronounced with larger grid spacing. Additional follow-on experiments confirm that the surface boundary forcing is the dominant factor in generating atmospheric variability for small-scale features and that it exerts greater control on the RCM solution as the influence of lateral boundary conditions diminish. The sensitivity to surface forcing is also influenced by the model parameterizations, as demonstrated by using a different convection scheme. For the particular case considered, dynamical downscaling with RAMS in RCM mode does not retain value of the large scale which exists in the larger global reanalysis. The utility of the RCM, or value added, is to resolve the smaller-scale features which have a greater dependence on the surface boundary. This conclusion regarding RAMS is expected to be true for other RCMs as well.
    Cha D.-H., D.-K. Lee, 2009: Reduction of systematic errors in regional climate simulations of the summer monsoon over East Asia and the western North Pacific by applying the spectral nudging technique. J. Geophys. Res., 114,D14108, doi: 10.1029/2008JD011176.10.1029/ this study, the systematic errors in regional climate simulation of 28-year summer monsoon over East Asia and the western North Pacific (WNP) and the impact of the spectral nudging technique (SNT) on the reduction of the systematic errors are investigated. The experiment in which the SNT is not applied (the CLT run) has large systematic errors in seasonal mean climatology such as overestimated precipitation, weakened subtropical high, and enhanced low-level southwesterly over the subtropical WNP, while in the experiment using the SNT (the SP run) considerably smaller systematic errors are resulted. In the CTL run, the systematic error of simulated precipitation over the ocean increases significantly after mid-June, since the CTL run cannot reproduce the principal intraseasonal variation of summer monsoon precipitation. The SP run can appropriately capture the spatial distribution as well as temporal variation of the principal empirical orthogonal function mode, and therefore, the systematic error over the ocean does not increase after mid-June. The systematic error of simulated precipitation over the subtropical WNP in the CTL run results from the unreasonable positive feedback between precipitation and surface latent heat flux induced by the warm sea surface temperature anomaly. Since the SNT plays a role in decreasing the positive feedback by improving monsoon circulations, the SP run can considerably reduce the systematic errors of simulated precipitation as well as atmospheric fields over the subtropical WNP region.
    Cha D.-H., C.-S. Jin, D.-K. Lee, and Y.-H. Kuo, 2011: Impact of intermittent spectral nudging on regional climate simulation using Weather Research and Forecasting Model. J. Geophys. Res., 116,D10103, doi: 10.1029/2010JD015069.10.1029/ study examines simulated typhoon sensitivities to spectral nudging (SN) to investigate the effects on values added by regional climate models, which are not properly resolved by low-resolution global models. SN is suitably modified to mitigate its negative effects while maintaining the positive effects, and the effects of the modified SN are investigated through seasonal simulations. In the sensitivity experiments to nudging intervals of SN, the tracks of simulated typhoons are improved as the SN effect increases; however, the intensities of the simulated typhoons decrease due to the suppression of the typhoon developing process by SN. To avoid such suppression, SN is applied at intermittent intervals only when the deviation between the large-scale driving forcing and the model solution is large. In seasonal simulations, intermittent SN is applied for only 7% of the total time steps; however, this results in not only maintaining the large-scale features of monsoon circulation and precipitation corresponding to observations but also improving the intensification of mesoscale features by reducing the suppression.
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    Craig G. C., S. L. Gray, 1996: CISK or WISHE as the mechanism for tropical cyclone intensification. J. Atmos. Sci., 53, 3528- 3540.10.1175/1520-0469(1996)053<3528:COWATM>2.0.CO; Examination of conditional instability of the second kind (CISK) and wind-induced surface heat exchange (WISHE), two proposed mechanisms for tropical cyclone and polar low intensification, suggests that the sensitivity of the intensification rate of these disturbances to surface properties, such as surface friction and moisture supply, will be different for the two mechanisms. These sensitivities were examined by perturbing the surface characteristics in a numerical model with explicit convection. The intensification rate was found to have a strong positive dependence on the heat and moisture transfer coefficients, while remaining largely insensitive to the frictional drag coefficient. CISK does not predict the observed dependence of vortex intensification rate on the heat and moisture transfer coefficients, nor the insensitivity to the frictional drag coefficient since it anticipates that intensification rate is controlled by frictional convergence in the boundary layer. Since neither conditional instability nor boundary moisture content showed any significant sensitivity to the transfer coefficients, this is true of CISK using both the convective closures of Ooyama and of Charney and Eliassen. In comparison, the WISHE intensification mechanism does predict the observed increase in intensification rate with heat and moisture transfer coefficients, while not anticipating a direct influence from surface friction.
    Dee D.P., Coruthors, 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
    Dudhia J., 1989: Numerical study of convection observed during the winter monsoon experiment using a mesoscale two-dimensional model. Atmos. Sci., 46, 3077-
    Emanuel K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady state maintenance. J. Atmos. Sci., 43, 585- 604.10.1175/1520-0469(1986)0432.0.CO;2af0a3265-005a-442d-9f3f-af90be5dccafc083923be7feefcc5f7f0953e0b912ae Observations and numerical simulators of tropical cyclones show that evaporation from the sea surface is essential to the development of reasonably intense storms. On the other hand, the CISK hypothesis, in the form originally advanced by Charney and Eliassen, holds that initial development results from the organized release of preexisting conditional instability. In this series of papers, we explore the relative importance of ambient conditional instability and air-sea latent and sensible heat transfer in both the development and maintenance of tropical cyclones using highly idealized models. In particular, we advance the hypothesis that the intensification and maintenance of tropical cyclones depend exclusively on self-induced heat transfer from the ocean. In this sense, these storms may be regarded as resulting from a finite amplitude air-sea interaction instability rather than from a linear instability involving ambient potential buoyancy. In the present paper, we attempt to show that reasonably intense cyclones may be maintained in a steady state without conditional instability of ambient air. In Part II we will demonstrate that weak but finite-amplitude axisymmetric disturbances may intensify in a conditionally neutral environment.
    Emanuel K. A., J. D. Neelin, and C. S. Bretherton, 1994: On large-scale circulations in convecting atmospheres. Quart. J. Roy. Meteor. Soc., 120, 1111- 1144.10.1002/ Available
    Feser F., H. von Storch, 2008: A dynamical downscaling case study for typhoons in Southeast Asia using a regional climate model. Mon. Wea. Rev., 136, 1806- 1815.10.1175/ Abstract This study explores the possibility to reconstruct the weather of SE Asia for the last decades using an atmospheric regional climate model, the Climate version of the Lokal Model (CLM). For this purpose global National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP-NCAR) reanalyses data were dy- namically downscaled,to 50 km,and in a double-nesting approach to 16.5 km,grid distance. To prevent the regional model from deviating to a great extent from the reanalyses for spacious weather phenomena, a spectral nudging technique was used which serves as a constraint exclusively for the large spatial scales of the regional simulation. The performance,of this technique in dealing with SE Asian typhoons is now ex- amined. First case studies indicate that tropical storms which are described by the reanalyses are correctly identied and tracked; considerably deeper core pressure and higher wind speeds are simulated compared,to the driving reanalyses. When the re- gional atmospheric model is run without spectral nudging, signicant intra-ensemble variability occurs; also additional, non-observed typhoons form. Several sensitivity experiments were performed concerning varied grid distances, different initial starting dates of the simulations and changed spectral nudging parameters. 2
    Haarsma R. J., J. F. B. Mitchell, and C. A. Senior, 1993: Tropical disturbances in a GCM. Climate Dyn., 8, 247- 257.10.1007/ We have analyzed the tropical disturbances in a 11-layer atmospheric general circulation model (GCM) on a 2.5 3.75 horizontal grid coupled to a 50 m-mixed layer ocean. Due to the coarse resolution, the GCM is unable to resolve adequately tropical cyclones. The tropical disturbances simulated by the GCM are much weaker and have a much larger horizontal extent. However, they still display much of the essential physics of tropical cyclones, including low-level convergence of mass and moisture, upper tropospheric outflow and a warm core. For most ocean basins the spatial and temporal distribution of the simulated tropical disturbances compares well with the observed tropical cyclones. On doubling the CO2 concentration, the number of simulated tropical disturbances increases by about 50%. There is a relative increase in the number of more intense tropical disturbances, whose maximum windspeed increases by about 20%. This agrees with the theoretical estimate of Emanuel. However, because the low-resolution of the GCM severely restricts their maximum possible intensity, simulated changes in tropical disturbance intensity should be interpreted cautiously.
    Harr P. A., R. L. Elsberry, 1995: Large-scale circulation variability over the tropical western North Pacific. Part I: Spatial patterns and tropical cyclone characteristics. Mon. Wea. Rev., 123, 1225- 1246.
    Hong S.-Y., J. Dudhia, and S.-H. Chen, 2004: A revised approach to ice microphysical processer for the bulk parameterization of clouds and precipitation. Mon. Wea. Rev., 132, 103- 120.
    Hong S.-Y., Y. Noh, and J. Dudhia, 2006: A new vertical diffusion package with an explicit treatment of entrainment processes. Mon. Wea. Rev., 134, 2318- 2341.10.1175/ This paper proposes a revised vertical diffusion package with a nonlocal turbulent mixing coefficient in the planetary boundary layer (PBL). Based on the study of Noh et al. and accumulated results of the behavior of the Hong and Pan algorithm, a revised vertical diffusion algorithm that is suitable for weather forecasting and climate prediction models is developed. The major ingredient of the revision is the inclusion of an explicit treatment of entrainment processes at the top of the PBL. The new diffusion package is called the Yonsei University PBL (YSU PBL). In a one-dimensional offline test framework, the revised scheme is found to improve several features compared with the Hong and Pan implementation. The YSU PBL increases boundary layer mixing in the thermally induced free convection regime and decreases it in the mechanically induced forced convection regime, which alleviates the well-known problems in the Medium-Range Forecast (MRF) PBL. Excessive mixing in the mixed layer in the presence of strong winds is resolved. Overly rapid growth of the PBL in the case of the Hong and Pan is also rectified. The scheme has been successfully implemented in the Weather Research and Forecast model producing a more realistic structure of the PBL and its development. In a case study of a frontal tornado outbreak, it is found that some systematic biases of the large-scale features such as an afternoon cold bias at 850 hPa in the MRF PBL are resolved. Consequently, the new scheme does a better job in reproducing the convective inhibition. Because the convective inhibition is accurately predicted, widespread light precipitation ahead of a front, in the case of the MRF PBL, is reduced. In the frontal region, the YSU PBL scheme improves some characteristics, such as a double line of intense convection. This is because the boundary layer from the YSU PBL scheme remains less diluted by entrainment leaving more fuel for severe convection when the front triggers it.
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    Kain J. S., J. M. Fritsch, 1993: Convective parameterization for mesoscale models: The Kain-Fritsch scheme. The Representation of Cumulus Convection in Numerical Models, K. A. Emanuel and D. J. Raymond, Eds., Amer. Meteor. Soc., 246 pp.
    Knutson T. R., J. J. Sirutis, S. T. Garner, I. M. Held, and R. E. Tuleya, 2007: Simulation of the recent multidecadal increase of Atlantic hurricane activity using an 18-km-grid regional model. Bull. Amer. Meteor. Soc., 88, 1549-;link_type=DOI
    Lee C.-S., K. K. W. Cheung, J. S. N. Hui, and R. L. Elsberry, 2008: Mesoscale features associated with tropical cyclone formations in the western North Pacific. Mon. Wea. Rev., 136, 2006- 2022.10.1175/ mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999092004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE09W), southwesterly flow (SW), monsoon confluence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE09W cases in which clustering of MCSs is found about 10009-200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies.
    Lee D. K., D. H. Cha, and H. S. Kang, 2004: Regional climate simulation of the 1998 summer flood over East Asia. J. Meteor. Soc.Japan, 82, 1735- 1753.10.2151/ this study, the severe flood case over East Asia during the 1998 summer was simulated using a regional climate model (SNURCM) with 60 km horizontal resolution (EX60), and the model performance in reproducing the extreme climate events was evaluated. An experiment with higher horizontal resolution of 20 km (EX20) was also performed in order to assess the impact of increased resolution on precipitation simulation of the severe flood.The model reproduced the severe precipitation events occurring in central China in June. In EX60, the temporal and spatial variations of the abnormal Meiyu monsoon fronts, which were well observed were also simulated reasonably except in southern China. The area-averaged daily precipitation and surface air temperatures were underestimated, but their temporal evolutions were in good agreement with observation. In the higher resolution experiment (EX20), simulated downward solar radiation, latent heat flux and convective rain were increased in the major severe rain area over the Yangtze River Basin. The increased precipitation in EX20, which was attributed mainly to the increase of convective rain, resulted in the enhanced precipitation intensity, but only slightly affected total precipitation amounts. The improvement in the higher horizontal resolution simulation appeared in precipitation resulting, in particular, from increased convective activity due to increased latent heat flux at the surface. Nevertheless, the model had significant precipitation bias in some areas with disagreement between the simulated precipitation patterns and distribution, and the observations. The model also had surface air temperature bias resulting from cold biases of the land surface model. With horizontal resolution increased to 20 km, the convective and non-convective precipitation was increased for the late afternoon and early evening time, increasing the total precipitation slightly.
    Miguez-Macho G., G. L. Stenchicov, and A. Robock, 2004: Spectral nudging to eliminate the effects of domain position and geometry in regional climate model simulations. J. Geophys. Res., 109,D13104, doi: 10.1029/2003JD004495.10.1029/ is well known that regional climate simulations are sensitive to the size and position of the domain chosen for calculations. Here we study the physical mechanisms of this sensitivity. We conducted simulations with the Regional Atmospheric Modeling System (RAMS) for June 2000 over North America at 50 km horizontal resolution using a 7500 km 脳 5400 km grid and NCEP/NCAR reanalysis as boundary conditions. The position of the domain was displaced in several directions, always maintaining the U.S. in the interior, out of the buffer zone along the lateral boundaries. Circulation biases developed a large scale structure, organized by the Rocky Mountains, resulting from a systematic shifting of the synoptic wave trains that crossed the domain. The distortion of the large-scale circulation was produced by interaction of the modeled flow with the lateral boundaries of the nested domain and varied when the position of the grid was altered. This changed the large-scale environment among the different simulations and translated into diverse conditions for the development of the mesoscale processes that produce most of precipitation for the Great Plains in the summer season. As a consequence, precipitation results varied, sometimes greatly, among the experiments with the different grid positions. To eliminate the dependence of results on the position of the domain, we used spectral nudging of waves longer than 2500 km above the boundary layer. Moisture was not nudged at any level. This constrained the synoptic scales to follow reanalysis while allowing the model to develop the small-scale dynamics responsible for the rainfall. Nudging of the large scales successfully eliminated the variation of precipitation results when the grid was moved. We suggest that this technique is necessary for all downscaling studies with regional models with domain sizes of a few thousand kilometers and larger embedded in global models.
    Miguez-Macho G., G. L. Stenchicov, and A. Robock, 2005: Regional climate simulations over North America: Interaction of local processes with improved large-scale flow. J.Climate, 18, 1227- 1246.10.1175/ reasons for biases in regional climate simulations were investigated in an attempt to discern whether they arise from deficiencies in the model parameterizations or are due to dynamical problems. Using the Regional Atmospheric Modeling System (RAMS) forced by the National Centers for Environmental Predictionational Center for Atmospheric Research reanalysis, the detailed climate over North America at 50-km resolution for June 2000 was simulated. First, the RAMS equations were modified to make them applicable to a large region, and its turbulence parameterization was corrected. The initial simulations showed large biases in the location of precipitation patterns and surface air temperatures. By implementing higher-resolution soil data, soil moisture and soil temperature initialization, and corrections to the Kainritch convective scheme, the temperature biases and precipitation amount errors could be removed, but the precipitation location errors remained. The precipitation location biases could only be improved by implementing spectral nudging of the large-scale (wavelength of 2500 km) dynamics in RAMS. This corrected for circulation errors produced by interactions and reflection of the internal domain dynamics with the lateral boundaries where the model was forced by the reanalysis.
    Mlawer E. J., S. J. Taubman, P. D. Brown, M. J. Iacono, and S. A. Clough, 1997: Radiative transfer for inhomogeneous atmosphere: RRTM, a validated correlated-k model for the longwave. J. Geophys. Res., 102( D14), 16 663- 16 682.10.1029/97JD0023733ca9d21-1574-4293-8d4b-e7ef17cddd90f4120d12c2c63e7bc01d0dead8c1827e A rapid and accurate radiative transfer model (RRTM) for climate applications been developed and the results extensively evaluated. The current version of RRTM calculates fluxes and cooling rates for the longwave spectral region (10-3000 cm-1) for an arbitrary clear atmosphere. The molecular species treated in the model are water vapor, carbon dioxide, ozone, methane, nitrous oxide, and the common halocarbons. The radiative transfer in RRTM is performed using the correlated-k method: the k distributions are attained directly from the LBLRTM line-by-line model, which connects the absorption coefficients used by RRTM to high-resolution radiance validations done with observations. Refined methods have been developed for treating bands containing gases with overlapping absorption, for the determination of values of the Planck function appropriate for use in the correlated-k approach, and for the inclusion of minor absorbing species in a band. The flux and cooling rate results of RRTM are linked to measurement through the use of LBLRTM, which has been substantially validated with observations. Validations of RRTM using LBLRTM have been performed for the midlatitude summer, tropical, midlatitude winter, subarctic winter, and four atmospheres from the Spectral Radiance Experiment campaign. On the basis of these validations the longwave accuracy of RRTM for any atmosphere is as follows: 0.6 W m-2 (relative to LBLRTM) for net flux in each band at all altitudes, with a total (10-3000 cm-1) error of less than 1.0 W m-2 at any altitudes; 0.07 K d-1 for total cooling rate error in the troposphere and lower stratosphere, and 0.75 K d-1 in the upper stratosphere and above. Other comparisons have been performed on RRTM using LBLRTM to gauge its sensitivity to changes in the abundance of specific species, including the halocarbons and carbon dioxide. The radiative forcing due to doubling the concentration of carbon dioxide is attained with an accuracy of 0.24 W m-2, an error of less than 5%. The speed of execution of RRTM compares favorably with that of other rapid radiation models, indicating that the model is suitable for use in general circulation models.
    Monin A. S., A. M. Obukhov, 1954: Basic laws of turbulent mixing in the surface layer of the atmosphere. Contributions of the Geophysical Institute of the Slovak Academy of Sciences, 24( 151), 163-
    Nguyen K. C., K. J. E. Walsh, 2001: Interannual, decadal and transient greenhouse simulation of tropical cyclone-like vortices in a regional climate model of the South Pacific. J.Climate, 14, 3043- 3054.10.1175/1520-0442(2001)014<3043:IDATGS>2.0.CO; This study aims to determine the effect of human mesenchymal stem cell (hMSC) labeling with the fluorescent dye DiD and the iron oxide nanoparticle ferucarbotran on chondrogenesis. Procedures hMSCs were labeled with DiD alone or with DiD and ferucarbotran (DiD/ferucarbotran). hMSCs underwent confocal microscopy, optical imaging (OI), and magnetic resonance (MR) imaging. Chondrogenesis was induced by transforming growth factor-b and confirmed by histopathology and glycosaminoglycan (GAG) production. Data of labeled and unlabeled hMSCs were compared with a t test. Results Cellular uptake of DiD and ferucarbotran was confirmed with confocal microscopy. DiD labeling caused a significant fluorescence on OI, and ferucarbotran labeling caused a significant T2* effect on MR images. Compared to nonlabeled controls, progenies of labeled MSCs exhibited similar chondrocyte morphology after chondrogenic differentiation, but the labeled cells demonstrated significantly reduced GAG production ( p <0.05). Conclusion DiD and DiD/ferucarbotran labeling of hMSC does not interfere with cell viability or morphologic differentiation into chondrocytes, but labeled cells exhibit significantly less GAG production compared to unlabeled cells.
    Oncley S. P., J. Dudhia, 1995: Evaluation of surface fluxes from MM5 using observations. Mon. Wea. Rev., 123, 3344- 3357.10.1175/1520-0493(1995)123<3344:EOSFFM>2.0.CO; Available
    Oouchi K., J. Yoshimura, H. Yoshimura, R. Mizuta, S. Kusunoki, and A. Noda, 2006: Tropical cyclone climatology in a global-warming climate as simulated in a 20 km-mesh global atmospheric model: Frequency and wind intensity analyses. J. Meteor. Soc.Japan, 84, 259- 276.10.2151/ changes in the tropical cyclones in a future, greenhouse-warmed climate are investigated using a 20km-mesh, high-resolution, global atmospheric model of MRI/JMA, with the analyses focused on the evaluation of the frequency and wind intensity. Two types of 10-year climate experiments are conducted. One is a present-day climate experiment, and the other is a greenhouse-warmed climate experiment, with a forcing of higher sea surface temperature and increased greenhouse-gas concentration. A comparison of the experiments suggests that the tropical cyclone frequency in the warm-climate experiment is globally reduced by about 30% (but increased in the North Atlantic) compared to the present-day-climate experiment. Furthermore, the number of intense tropical cyclones increases. The maximum surface wind speed for the most intense tropical cyclone generally increases under the greenhouse-warmed condition (by 7.3 ms^<-1> in the Northern Hemisphere and by 3.3 ms^<-1> in the Southern Hemisphere). On average, these findings suggest the possibility of higher risks of more devastating tropical cyclones across the globe in a future
    Qian J.-H., A. Seth, and S. Zebiak, 2003: Reinitialized versus continuous simulations for regional climate downscaling. Mon. Wea. Rev., 131, 2857- 2874.10.1175/1520-0493(2003)131<2857:RVCSFR>2.0.CO; methodology for dynamical climate downscaling is studied using the second-generation regional climate model (RegCM2). The question addressed is, in order to simulate high-resolution details as accurately as possible, what strategy should be taken: continuous long-term integration in climate prediction mode or consecutive short-term integrations in weather forcasting mode? To investigate this problem, the model was run for 5 months in three different ways: 1) a 5-month continuous simulation, 2) monthly reinitialized simulations, and 3) 10-day reinitialized simulations. Compared to the observed precipitation, the 10-day reinitialized simulation results in the smallest error, while the continuous run shows larger error. Analysis shows that the long-term continuous simulation is contaminated by the systematic errors associated with the steep Andes Mountains and the uncertainties in the moisture processes in the planetary boundary layer near the coast. The method of 10-day reinitialization effectively mitigates the problem of systematic errors and makes a difference in the subtle precipitation processes in the regional climate model, therefore improving the accuracy in dynamic downscaling.
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    Skamarock W. C., J. B. Klemp, J. Dudhia, D. O. Gill, D. M. Barker, W. Wang, and J. G. Powers, 2005: A description of the Advanced Research WRF Version 2. NCAR Tech. Note TN-468+STR.10.5065/ development of the Weather Research and Forecasting (WRF) modeling system is a multiagency effort intended to provide a next-generation mesoscale forecast model and data assimilation system that will advance both the understanding and prediction of mesoscale weather and accelerate the transfer of research advances into operations. The model is being developed as a collaborative effort ort among the NCAR Mesoscale and Microscale Meteorology (MMM) Division, the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Prediction (NCEP) and Forecast System Laboratory (FSL), the Department of Defense's Air Force Weather Agency (AFWA) and Naval Research Laboratory (NRL), the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma, and the Federal Aviation Administration (FAA), along with the participation of a number of university scientists. The WRF model is designed to be a flexible, state-of-the-art, portable code that is an efficient in a massively parallel computing environment. A modular single-source code is maintained that can be configured for both research and operations. It offers numerous physics options, thus tapping into the experience of the broad modeling community. Advanced data assimilation systems are being developed and tested in tandem with the model. WRF is maintained and supported as a community model to facilitate wide use, particularly for research and teaching, in the university community. It is suitable for use in a broad spectrum of applications across scales ranging from meters to thousands of kilometers. Such applications include research and operational numerical weather prediction (NWP), data assimilation and parameterized-physics research, downscaling climate simulations, driving air quality models, atmosphere-ocean coupling, and idealized simulations (e.g boundary-layer eddies, convection, baroclinic waves).*WEATHER FORECASTING
    Tsutsui J. I., A. Kasahara, 1996: Simulated tropical cyclones using the national center for atmospheric research community climate model. J. Geophys. Res., 101, 15 013- 15 032.10.1029/ possibility of simulating tropical cyclones (TCs) using the National Center for Atmospheric Research community climate model (CCM2) is explored. Daily outputs from two long-term simulation runs using the standard T42 resolution CCM2 are examined to identify simulated tropical cyclones (STCs) using a search scheme that selects qualified STCs resembling observed TCs. The two simulation cases are a 20-year run driven by climatological sea surface temperatures (SSTs) and a 10-year run, corresponding to the decade from 1979 to 1988, with the same model configuration except for the use of observed SSTs. A composite technique is adopted to reveal the horizontal and vertical structures of well-developed STCs, and a comparison with those of observed TCs is presented. Then, the climatologies of STCs from the two simulation cases are discussed in terms of their genesis, movements, and seasonal and interannual variations through the comparisons with observed TC statistics. Despite obvious shortcomings of the standard CCM2, such as a coarse horizontal resolution, the structure and climatology of STCs identified in both climate runs are in reasonably good agreement with those of observed TCs. The annual STC frequency shows a better agreement with the observed SST run than the climatological SST run, while many other aspects of STCs in the two climate runs are comparable.
    Tulich S. N., G. N. Kiladis, and A. Suzuki-Parker, 2011: Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics. Climate Dyn.,36, 185-203, doi: 10.1007/s00382-009-0697-2.10.1007/ study evaluates the performance of a regional climate model in simulating two types of synoptic tropical weather disturbances: convectively-coupled Kelvin and easterly waves. Interest in these two wave modes stems from their potential predictability out to several weeks in advance, as well as a strong observed linkage between easterly waves and tropical cyclogenesis. The model is a recent version of the weather research and forecast (WRF) system with 36-km horizontal grid spacing and convection parameterized using a scheme that accounts for key convective triggering and inhibition processes. The domain spans the entire tropical belt between 45°S and 45°N with periodic boundary conditions in the east–west direction, and conditions at the meridional/lower boundaries specified based on observations. The simulation covers 6years from 2000 to 2005, which is long enough to establish a statistical depiction of the waves through space-time spectral filtering of rainfall data, together with simple lagged-linear regression. Results show that both the horizontal phase speeds and three-dimensional structures of the waves are qualitatively well captured by the model in comparison to observations. However, significant biases in wave activity are seen, with generally overactive easterly waves and underactive Kelvin waves. Evidence is presented to suggest that these biases in wave activity (which are also correlated with biases in time–mean rainfall, as well as biases in the model’s tropical cyclone climatology) stem in part from convection in the model coupling too strongly to rotational circulation anomalies. Nevertheless, the model is seen to do a reasonable job at capturing the genesis of tropical cyclones from easterly waves, with evidence for both wave accumulation and critical layer processes being importantly involved.
    von Storch, H.H. Langerberg, F. Feser, 2000: A spectral nudging technique for dynamical downscaling purposes. Mon. Wea. Rev., 128, 3664-
    Walsh K. J. E., K.-C. Nguyen, and J. L. McGregor, 2004: Fine-resolution regional climate model simulations of the impact of climate change on tropical cyclones near Australia. Climate Dyn., 22, 47- 56.10.1111/ Available
    Wang H., Y. Q. Wang, and H. M. Xu, 2013: Improving simulation of a tropical cyclone using dynamical initialization and large-scale spectral nudging: A case study of Typhoon Megi (2010). Acta Meteorologica Sinica,27(4), 455-475, doi: 10.1007/s13351-013-0418-y.10.1007/ this study, an approach combining dynamical initialization and large-scale spectral nudging is proposed to achieve improved numerical simulations of tropical cyclones (TCs), including track, structure, intensity, and their changes, based on the Advanced Weather Research and Forecasting (ARW-WRF) model. The effectiveness of the approach has been demonstrated with a case study of Typhoon Megi (2010). The ARW-WRF model with the proposed approach realistically reproduced many aspects of Typhoon Megi in a 7-day-long simulation. In particular, the model simulated quite well not only the storm track and intensity changes but also the structure changes before, during, and after its landfall over the Luzon Island in the northern Philippines, as well as after it reentered the ocean over the South China Sea (SCS). The results from several sensitivity experiments demonstrate that the proposed approach is quite effective and ideal for achieving realistic simulations of real TCs, and thus is useful for understanding the TC inner-core dynamics, and structure and intensity changes.
    Weng H., A. Sumi, Y. N. Takayabu, M. Kimoto, and C. Li, 2004: Interannual-interdecadal variation in large-scale atmospheric circulation and extremely wet and dry summers in China/Japan during 1951-2000 part I: spatial patterns. J. Meteor. Soc.Japan, 82, 775- 788.
    Wu G. X., N. C. Lau, 1992: A GCM simulation of the relationship between tropical storm formation and ENSO. Mon. Wea. Rev., 120, 958-
    Yesubabu V., C. V. Srinivas, S. S. V. S. Ramakrishna, and K. B. R. R. Hari Prasad, 2014: Impact of period and timescale of FDDA analysis nudging on the numerical simulation of tropical cyclones in the Bay of Bengal. Natural Hazards, 74, 2109- 2128.10.1007/ In this study, the impact of four-dimensional data assimilation (FDDA) analysis nudging is examined on the prediction of tropical cyclones (TC) in the Bay of Bengal to determine the optimum period and timescale of nudging. Six TCs (SIDR: November 13&ndash;16, 2007; NARGIS: April 29&ndash;May 02, 2008; NISHA: November 25&ndash;28, 2008; AILA: May 23&ndash;26, 2009; LAILA: May 18&ndash;21, 2010; JAL: November 04&ndash;07, 2010) were simulated with a doubly nested Weather Research and Forecasting (WRF) model with a horizontal resolution of 9 km in the inner domain. In the control run for each cyclone, the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis and forecasts at 0.5 resolution are used for initial and boundary conditions. In the FDDA experiments available surface, upper air observations obtained from NCEP Atmospheric Data Project (ADP) data sets were used for assimilation after merging with the first guess through objective analysis procedure. Analysis nudging experiments with different nudging periods (6, 12, 18, and 24 h) indicated a period of 18 or 24 h of nudging during the pre-forecast stage provides maximum impact on simulations in terms of minimum track and intensity forecasts. To determine the optimum timescale of nudging, two cyclone cases (NARGIS: April 28&ndash;May 02, 2008; NISHA: November 25&ndash;28, 2008) were simulated varying the inverse timescales as 1.0e-4 to 5.0e-4 s-1 in steps of 1.0e-4 s-1. A positive impact of assimilation is found on the simulated characteristics with a nudging coefficient of either 3.0e-4 or 4.0e-4 s-1 which corresponds to a timescale of about 1 h for nudging dynamic (u,v) and thermodynamical (t,q) fields.
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Manuscript received: 05 July 2015
Manuscript revised: 16 December 2015
Manuscript accepted: 08 January 2016
通讯作者: 陈斌,
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Impact of Spectral Nudging on the Downscaling of Tropical Cyclones in Regional Climate Simulations

  • 1. Atmospheric Sciences Program, School of Earth and Environmental Sciences, Korea-USA Weather and Climate Research Center Seoul National University, Seoul 151-747, Korea

Abstract: This study investigated the simulations of three months of seasonal tropical cyclone (TC) activity over the western North Pacific using the Advanced Research WRF Model. In the control experiment (CTL), the TC frequency was considerably overestimated. Additionally, the tracks of some TCs tended to have larger radii of curvature and were shifted eastward. The large-scale environments of westerly monsoon flows and subtropical Pacific highs were unreasonably simulated. The overestimated frequency of TC formation was attributed to a strengthened westerly wind field in the southern quadrants of the TC center. In comparison with the experiment with the spectral nudging method, the strengthened wind speed was mainly modulated by large-scale flow that was greater than approximately 1000 km in the model domain. The spurious formation and undesirable tracks of TCs in the CTL were considerably improved by reproducing realistic large-scale atmospheric monsoon circulation with substantial adjustment between large-scale flow in the model domain and large-scale boundary forcing modified by the spectral nudging method. The realistic monsoon circulation took a vital role in simulating realistic TCs. It revealed that, in the downscaling from large-scale fields for regional climate simulations, scale interaction between model-generated regional features and forced large-scale fields should be considered, and spectral nudging is a desirable method in the downscaling method.

1. Introduction
  • The western North Pacific (WNP) is home to the most vigorous tropical cyclone (TC) activity. Severe TCs are of considerable interest for many Asian countries because of the serious impact of their high winds, associated storm surges, excessive rain, and flooding. The coarse resolution of GCMs limits them to simulating TC-like vortices that are broader and weaker than actual TCs. This is often inadequate for simulating meaningful TC activity. The simulated vortices in GCMs usually tend to be lacking in terms of storm track length, a distinct eye, and rain bands, compared with observation, although the frequency of simulated TCs is generally similar to that observed (Wu and Lau, 1992; Haarsma et al., 1993; Tsutsui and Kasahara, 1996; Camargo and Sobel, 2004). In order to address the question of TC activity more directly, RCMs, with their relatively higher spatial resolution, have attracted attention (Camargo et al., 2007; Knutson et al.,2007; Feser and von Storch, 2008). As a dynamical downscaling strategy, RCMs can reproduce regional weather details that are influenced by topography, land-sea contrast, and small-scale atmospheric features. They are forced by large-scale information from global coupled model simulations, and provide regional details embedded within low-resolution lateral boundary data, as well as allow the reproduction of their large-scale variability.

    In regional climate simulations, strong internal variability can be generated. In East Asia and the WNP, regional features such as complicated topography, various land surface conditions, warm local ocean conditions, strong seasonal monsoon circulation, and thermal convection, affect internal variability in regional climate simulations. These regional features can lead to systematic errors in long-term integrations of regional climate simulations. For example, (Qian et al., 2003) found that the systematic errors of precipitation in their long-term simulation were associated with the steep topography and uncertainties in the moisture processes. (Cha and Lee, 2009) showed that errors in precipitation were induced by the enhanced surface latent heat flux caused by the warm SST anomaly. The spectral nudging technique can reduce these problems, maintaining the added values generated by the limited-area model, as discussed by (von Storch et al., 2000), (Miguez-Macho et al., 2004), (Castro et al., 2005), and (Cha et al., 2011). The results of regional models using the spectral nudging technique do not deviate from large-scale forcing but sustain the numerical balance, while significant modifications in small-scale features are allowed in regional models.

    Although this framework clearly depends on the various sources of large-scale information that are forcing the RCM, it is necessary to determine how successfully RCMs can be used for extended simulations of TCs when using the observed atmospheric state and SSTs as perfect boundary conditions. Modeling studies have been conducted previously for individual TCs as case studies for using the nudging technique, which focused on its positive effects. For example, (Wang et al., 2013) examined the effect of the spectral nudging technique in improving a TC simulation over seven days. (Yesubabu et al., 2014) conducted three-day simulations for several TC cases using grid nudging. Although the simulations of such studies are successful in terms of mesoscale features, such as track, intensity, or structure of the TC, the effect of the spectral nudging technique on extended-range simulations has not been examined. In cases of long-term simulation studies, a number of studies have reported that the RCM produces bias in terms of TC frequency. For example, (Walsh et al., 2004) simulated the regional climate of TCs over eastern Australia for a 30-year period without inserting artificial vortices or relaxing the large-scale component of the observed atmospheric state. Despite the lack of generally accepted threshold detection criteria, they concluded that the model generated too many TCs, as compared to observation. (Knutson et al., 2007) also simulated the Atlantic hurricane activity during the TC seasons of a 26-year period. The model resulted in a systematic error of generating a higher TC frequency than observed. (Tulich et al., 2011) also showed significant bias in TC frequency in their six-year simulation using the tropical channel of the Advanced Research WRF Model (WRF-ARW), which has only meridional boundaries.

    In this context of meaningful predictions of TC activity in an RCM, the objectives of the present study were to examine the ability of the limited-area WRF-ARW model to simulate the three-month TC frequency and track over the WNP for the three-month period of 18 June to 18 September 2002. We analyzed the effect of spectral nudging on the regional climate simulation for TC activity, as well as atmospheric circulation and precipitation over the WNP, and investigated the reasons why an overestimated TC frequency and relative systematic errors were generated in the simulation without spectral nudging.

    The organization of the paper is as follows. The next section describes the RCM, experiment setup and observational data used in this study. Section 3 presents the simulation results over the WNP, and section 4 summarizes and concludes the findings.

2. Model description, experiments and data
  • Version of WRF-ARW (Skamarock et al., 2005) was used in this study, employing the Kain-Fritsch cumulus parameterization scheme for subgrid-scale convection (Kain and Fritsch, 1990, 1993), WSM3 (Hong et al., 2004) for moist processes of grid-scale cloud and precipitation, the Yonsei University scheme (Hong et al., 2006) for the PBL, similarity theory (Monin and Obukhov, 1954) for the surface layer, the NOAH land-surface model (Chen and Dudhia, 2001) for land surfaces, and the RRTM longwave (Mlawer et al., 1997) together with the Dudhia shortwave (Dudhia, 1989) schemes for atmospheric radiation processes. The WRF model was run for a domain of 30-km horizontal grid meshes with 200× 240 grid points, covering most of the WNP, and 36 vertical layers with the model top at 50 hPa (Fig. 1).

    Figure 1.  Track density of TCs in (a) OBS, (b) FNL analysis, (c) CTL and (d) SPN, in the simulation period of 2002. First positions of TCs are marked.

    For this study, the spectral nudging method developed by (Miguez-Macho et al., 2004) was used. The equation for spectral nudging is $$ \dfrac{d\alpha}{dt}=X(\alpha)+G_\alpha w(\eta)\sum_{|n|\le N}\sum_{|m|\le M}K_{mn}(\alpha_{{o},mn}-\alpha_{mn})e^{ik_mx}e^{ik_ny} , $$ where α is any of the prognostic variables being nudged, X is the model operator, Gα is the nudging coefficient as inverse time, and α o is the reanalysis variable. Here, αmn and α o,mn are the spectral coefficients of α and α o, respectively. Kmn is the scale-selective nudging coefficient of zero or one. For this study, this was set to 1 when the m≤ 7 (zonal) and n≤ 6 (meridional) wave numbers corresponded to wavelengths larger than 1000 km, which is the synoptic scale. km and kn are the wave vector components in the x and y directions, respectively. Note that in this study the characteristic time of the relaxation corresponded to around 3 h, which corresponded to a half cycle of the driving analysis data, available every 6 h. This meant that Gα=0.0001 s-1. Because the characteristic time is a kind of free parameter, Miguez-Macho et al. (2004, 2005) set it at 5000 s, approximately 1.39 h. Since using a short relaxation time (large weight coefficient) could suppress the model-generating small-scale details, we chose to use a relatively small value. (Alexandru et al., 2009) showed that the model internal variability decreases with an increased coefficient. Also, we used w(η)=(1-η)2 for a vertical weight function of relaxations, where η is the vertical coordinate from 1 (surface) to 0 (top). In this study, the spectral nudging was applied to the horizontal wind component only.

    Model experiments were performed for the three-month period of 18 June to 18 September 2002, during the WNP active typhoon season, to investigate the seasonal march of simulated typhoons with and without the application of spectral nudging. The TC season in 2002 was chosen as the target period because the annual TC frequency over the WNP that year was 26, which is very close to the 30-year climatological frequency of 26.09. The experiment without spectral nudging is referred to here as the CTL run, and the experiment with spectral nudging is termed the SPN run. During the simulation period, the observed TC frequency was 14 in the area of interest; however, two TCs were not included within the model domain because they did not influence the East Asian continent. The remaining 12 typhoons are discussed in this paper.

    The initial and boundary conditions for the model simulations were obtained from the NCEP Final (FNL) Operational Global analysis data; the data were divided into 1°× 1° grids on 26 standard pressure levels from 1000 to 10 hPa ( In order to evaluate the simulated wind and geopotential height, ERA-Interim data (Dee et al., 2011) were used. In addition, the 3-h 0.25°× 0.25° merged TRMM and other satellite estimation data of the version 6 3B42 product were used to evaluate the simulated precipitation. The observational data were the best-track data created by the Regional Specialized Meteorological Center (RSMC) Tokyo-Typhoon Center; the data were used to compare the center positions, maximum wind speeds and minimum pressure of the simulated typhoons. Tropical depressions were included in the best-track dataset; however, only TCs with tropical storm intensity or higher are presented in this study.

    Figure 2.  Number of TCs (NTC) versus minimum central pressure (hPa) in OBS, CTL and SPN.

    Figure 3.  The three-month mean (a-c) precipitation (mm d$^-1$) and (d-f) 850 hPa wind vector, 200 hPa wind speed (m s$^-1$; contours and shading) and 5880 geopotential height contour (thick solid line), in the (a, d) observation, (b, e) CTL run and (c, f) SPN run. The observed precipitation and atmospheric variables are from TRMM and ERA-Interim data, respectively.

    To define TCs in the simulated results, we used the objective algorithms suggested by (Nguyen and Walsh, 2001) and (Oouchi et al., 2006). In order to evaluate the model performance for the system corresponding to the observed intensity of TCs, some modifications were made. The algorithm defines a simulated TC in an RCM as follows: (1) there must be a local minimum of SLP; (2) the vorticity must be at least 3.5× 10-5 s-1 at 850 hPa; (3) a point must be warm-core, i.e., the total tropospheric temperature anomaly calculated by summing temperature anomalies at 700, 500 and 300 hPa around the center of the storm must be greater than zero; (4) the mean wind speed in a 500 km2 region around the center of a point at 850 hPa must be higher than that at 300 hPa; (5) the total tropospheric temperature anomaly has to be greater than 1.5 K; (6) the duration of a model TC satisfying the above conditions must last at least one day; and (7) the surface wind speed has to exceed 17.5 m s-1, which corresponds to tropical storm intensity or higher in the RSMC scale. A simulated TC was identified if all the above criteria were satisfied. After a storm was identified, its track was traced from the identified point.

3. Results
  • Figure 1 shows the first position of all TCs appearing in the RSMC best-track data (OBS), FNL analysis, CTL and SPN at the time when each TC reached the strength of a tropical storm. During the simulation period, 12 TCs were observed within the model domain (Fig. 1a). In the FNL analysis data, which were used for the model boundary conditions, 9 TCs were found using the objective search algorithms of this study (Fig. 1b). Although a lower number of TCs was represented in the FNL data, the first position was in good agreement with OBS. However, the two experiments showed a large difference in the number of simulated TCs. CTL simulated 25 TCs, and SPN simulated 16 TCs (Figs. 1b and c). The spuriously simulated TCs in CTL were concentrated in the area of (10°-20°N, 125°-155°E), east of the Philippines. The formation of a few spurious TCs also occurred in SPN.

    To examine the tracks of the TCs in the models and observations, the probability density of TC tracks is also shown in Fig. 1. CTL simulated a relatively high probability density of tracks over the basin of (10°-20°N, 110°-160°E), with an east-west extended pattern. TC tracks in CTL tended not only to have longer curvatures but were also shifted eastward, showing approximate south-north paths. However, SPN showed a high probability density of tracks with a southeast-northwest direction of the two main streams as shown in OBS. The maximum location of the track density over the East China Sea in SPN was comparable to that observed. Furthermore, in SPN the area related to landfall over the Korean Peninsula agreed well with observations.

    The characteristics of the simulated TC intensity were examined in terms of the number of TCs versus the minimum central pressure (Fig. 2). The observational distribution of the minimum central pressure was classified by two groups in which a peak of weaker (stronger) TCs appears in the range of 990-980 hPa (940-930 hPa). Both experiments overestimated the mid-intensity TCs. On average, the minimum central pressure in OBS was 956.7 hPa, while those in CTL and SPN were 960.9 and 970.1 hPa, respectively. The CTL central pressure was close to that observed, but that of SPN was about 14 hPa less than observed. Although CTL seemed to show good ability in simulating TCs with intense minimum central pressure, it was found that CTL produced especially higher numbers of weaker TCs, which could be mitigated in SPN. It was notable that the stronger TCs were not sufficiently simulated in SPN. Since the spectral nudging technique provides additional forcing for the model solution to have realistic large-scale flows, the nudged large-scale forcing may dilute small-scale forcing, which is necessary for a TC's strength in its mature stage. On the other hand, the spurious TCs due to erroneous mutual interaction between large-scale flows and small-scale flows could be controlled in SPN.

  • Figure 3 shows the observed and simulated three-month mean precipitation and wind fields over the entire model domain, excluding the buffer zone. Observationally, the distribution of the precipitation was characterized by two rain bands and a dry region over the ocean. One of the rain bands extended from the South China Sea to Korea and Japan, which was related to the East Asian summer monsoon (EASM). The other was over the WNP along 10°N, which resulted from the WNP summer monsoon (WNPSM). A relatively dry region appeared in the subtropical high over the WNP, south of Japan, between 20°N and 40°N.

    In CTL, the precipitation area related to the EASM was extended eastward over the subtropical WNP, with overestimated precipitation over the dry region southeast of Japan. In addition, the WNPSM precipitation area was overestimated considerably and shifted northward. The overestimated precipitation in CTL was attributed to inaccurate simulations of atmospheric monsoon circulation, as well as TCs. CTL simulated stronger monsoon westerly flows in the area along 10°N, from the west of the Philippines, compared to those observed. The strong westerly flow prevented the WNP subtropical high (WNPSH) from extending westward to East Asia. The simulated 5880 geopotential height contour was displaced to the east by approximately 15°. Consequently, the confluence of westerly and easterly flows over the southern vicinity of the WNPSH turned northward and shifted more eastward than observed. This resulted in intensified cyclonic circulation near 20°N, south of Japan. The overestimated precipitation over the dry area to the southeast of Japan was likely simulated by the migration of convective cells due to the intensified cyclonic circulation, and also by the paths of TCs along the incorrect cyclonic flows (refer to Fig. 1). These results indicate that the mean low-level flow, upper-level flow, and subtropical WNP were not correctly simulated in CTL, which was an unfavorable environment for the development of TCs over the WNP.

    In SPN, the overestimated precipitation of the EASM and WNPSM in CTL was considerably reduced. SPN improved the low-level monsoon circulation, and therefore westerly flows along the western boundary of the subtropical high were much weakened, and the anticyclonic circulation of the WNPSH was much intensified. Thus, the eastward extension of the EASM precipitation was significantly corrected. The overestimated WNPSM precipitation was also reduced. It is notable that the precipitation over southern China in SPN was increased and that the precipitation over central and northern China was decreased as compared to CTL. This indicates the importance of interactions between model scale and large-scale variables when an RCM is forced by large-scale fields. The improvement of the simulation of precipitation is evidenced by the spatial correlation coefficients between the simulations and observation. The spatial correlation coefficient over ocean (land) increased from 0.61 (0.61) to 0.78 (0.90) when the spectral nudging technique was applied.

    Figure 4 shows the eigenvector of the EOF and the time series of the corresponding first-mode eigenvalue of a six-hourly 925-hPa wind vector during the simulated period. In the observation (ERA-Interim), the first eigenvector accounted for 14.0% of total variance, which was characterized by a dipole pattern of anticyclonic circulation between 30°N and 45°N, and cyclonic circulation along 20°N between 120°E and 160°E (Fig. 4a). The active cyclonic circulation indicated an active monsoon trough, which was also coupled with the anticyclonic circulation. This reflects the intensification of the monsoon trough. The southern branch of the cyclonic circulation extended eastward from the Philippine Sea, thereby covering the southwestern North Pacific. This pattern was also noticeable in the study of (Harr and Elsberry, 1995). However, in CTL, these patterns were not reproduced, as they were somewhat distorted in the simulation (Fig. 4b). That is, in CTL, the simulated cyclonic circulation dominated over the ocean in the model domain, and the observed anticyclonic circulation east of Japan was not simulated. Thus, an exaggerated strong cyclonic flow was simulated in the southwestern North Pacific in CTL. The observed circulation pattern was reasonably reproduced in SPN but the cyclonic circulation was intensified, with its center having moved eastward (Fig. 4c). It should be noted that in SPN the eigenvector pattern and the corresponding principal component (PC) time series of the 925-hPa wind were closely comparable with observations (Fig. 4d). The PC time series of the observation and SPN indicated intra-seasonal variability with an approximate 20-day cycle, implying that SPN was able to simulate the temporal variability of the atmospheric circulation.

    Figure 4.  First-mode eigenvectors and its percentage of total variance, which is shown in the parenthesis, from the EOF analysis of the six-hourly 925-hPa wind vector (units: m s$^-1$) during the total simulated period in (a) OBS (ERA-Interim), (b) CTL and (c) SPN. (d) Corresponding PC time series in OBS (thick solid line), CTL (thin solid line) and SPN (dotted line).

    Figure 5.  Temporal variation in the cross-correlation coefficient of geopotential height at 500 hPa between the simulations and ERA-Interim data. The TC marks indicate when TCs appeared for the first time. The duplicated red lines of CTL are marked to indicate the time when TCs under 945 hPa existed. P1 indicates the period of 2-6 September, and P2 is the period of 11-14 September, which are also referred to in Fig. 7.

    The good agreement of SPN was indicated well by the temporal variation of the cross-correlation coefficient of 500 hPa geopotential height between the simulations and ERA-Interim data (Fig. 5). The simulated atmospheric flows in CTL without the adjustment of the large-scale pattern were readily deviated from the lateral boundary conditions because of a lack of interaction between the large-scale field and TCs within the model. It was noticeable that the mature stage of TCs being under 945 hPa was likely to decrease the cross-correlation coefficient in CTL. Although CTL allowed the model-generating small-scale details more than SPN, the stronger TCs in CTL tended to reduce the reliability of the simulations, with inaccurate large-scale wind fields.

    Figure 6 shows the 3-month mean spectral variances of the kinetic energy over the entire domain at the height of 10 m, and the corresponding ratio of CTL to SPN. There was little difference between the two simulations at the small scale, under the wavelength of 1000 km (wave number of 7), which corresponds to the cutoff wave number of the spectral nudging technique used in this study. However, the difference at the large scale was greater because of the distorted atmospheric flows. Although the variance of kinetic energy was small at the small scale, the ratio of the spectra of CTL to those of SPN showed a large value at the small scale, compared with that at the large scale (Fig. 6b). It can be seen that the ratio had a peak at the wave number of 17 (wavelength of 450 km), which can correspond to an average TC radius. CTL resolved more kinetic energy at this wave number than SPN, as more TCs were simulated. Thus, it was concluded that small-scale circulations in CTL were readily affected by the large amount of kinetic energy in the large-scale circulations.

  • To investigate the large-scale patterns at the formation of TCs, the classification method of (Lee et al., 2008) was used for the experiments in this study. Through the use of the low-level wind flow and surge direction of the satellite-based wind data at the first time of the best-track data (Table 1), (Lee et al., 2008) classified the formation of TCs into six synoptic patterns; easterly wave (EW); northeasterly flow (NE); coexistence of northeasterly and southwesterly flow (NE-SW); southwesterly flow (SW); monsoon confluence (MC); and monsoon shear (MS). The set of criteria was established on the magnitude of the zonal and meridional wind components in the four 5°× 5° latitude-longitude quadrants. In this study, the same criteria were used for classifying model-simulated synoptic flow patterns associated with TC formation (Table 1), but the formation time was taken at the first time of an isolated circular of local minimum SLP associated with an identified TC occurrence, which was archived by backward tracing from the first time of the TC track. On average, the time interval between the formation time and the first time of simulated TC tracks was about 48 hours. A more detailed description for the classification method can be found in (Lee et al., 2008).

    Figure 6.  (a) Spectral variances of the kinetic energy at the height of 10 m in CTL (solid) and SPN (dashed). (b) Ratio of the spectra between CTL and SPN (CTL/SPN). The wave number 7 is the top wave number to nudge. The wave numbers of 15 to 19 correspond to a radius of TC, which is indicated by arrows of the $x$-axis.

    Figure 7.  Composite 925-hPa streamlines and wind speed (m s$^-1$; shaded) for the (a, g, l) NE, (b, h, m) NE-SW, (c, i, n) SW, (d, j) MC and (e, k, o) MS synoptic patterns, in (a-e) CTL, (g-k) SPN and (l-o) OBS. (f) "Others" indicates the non-classified patterns. $x$ indicates a grid length in unit of 30-km.

    Figure 8.  925-hPa streamline and wind speed (m s$^-1$; shaded) in the CTL at (a) 0000 UTC 3 September 2002 and (b) 0000 UTC 6 September 2002 for the period P1, and (c) 0000 UTC 11 September 2002 and (d) 0000 UTC 14 September 2002 for the period P2.

    As a result of this classification, five classes of low-level flow patterns were identified for both experiments (all except the EW pattern) (Table 2). However, four cases were only identified in the observation (all except the EW and MC pattern). It is important to note that, regarding the classification in the observation, FNL analysis data were used for the low-level wind flow and surge direction, and the first time and location of the RSMC best-track data were also employed. In CTL (SPN), there were 5(3) NE cases, 4(4) NE-SW cases, 1(2) SW cases, 10(5) MC cases, and 1(2) MS cases. CTL simulated the MC pattern twice more than SPN, and even 4 TCs of non-classified patterns, which were not shown in SPN. Figure 7 shows the composite 925 hPa flows for each pattern. The low-level flow patterns of this study correspond well with those analyzed in (Lee et al., 2008). CTL enhanced the southern quadrants with respect to the center of a TC system, as compared to SPN. In CTL, the non-classification patterns indicated TC cases that were generated near the eastern boundary of the model domain, due to the lack of interaction with large-scale flows forced from the lateral boundary. A typical example of a non-classified pattern is shown in Fig. 7f.

    Higher numbers of CTL TCs in the MC case were mostly attributed to the enhanced wind speed in the southern quadrant, which resulted in an inaccurately active monsoon trough (refer to Fig. 3e). (Harr and Elsberry, 1995) pointed out that the enhancement of the monsoon trough was associated with active TCs in lower latitudes of the WNP. It should also be noted that monsoon-related flows are responsible for multiple TC geneses. For example, during the period P1 (2-6 September) and P2 (11-14 September), the 925 hPa streamlines and wind speed successfully diagnosed the synoptic conditions associated with the simultaneous formation (Fig. 8). For the first date of P1, strong westerly flows along 10°N sustained for four days prior to the formation of the TCs in CTL (Fig. 8a). A time series of confluent regions was collocated with the maximum gradient of the zonal flow and the cyclonic shear extending from east of the Philippines to the western boundary of the model domain. The zonal and meridional winds were also gradually intensified over the four days, leading to the simultaneous formation of two NE-SW pattern TCs and two MC TCs (Fig. 8b). About five days later, a similar streamline and wind speed appeared again in the same region in CTL (Figs. 8c and d). For the first date of P2, the monsoonal westerly flow in CTL was strong in the region of the monsoon shear line. In P2, the simultaneous formation of three TCs of the MC pattern occurred, while the wind speed was intensified over three days. In the observation and SPN, however, there was no significant change in flow patterns during the two periods (not shown).

    The erroneous wind induced increased surface fluxes of moisture and latent heat though friction-induced convergence, which resulted in increased equivalent potential temperature at the surface. In the surface layer scheme used in this study (Oncley and Dudhia, 1995), the parameterization for the surface moisture flux is based on eddy perturbation quantities, such as the friction velocity (u*) and characteristic moisture (q*). The surface moisture flux is then given by u*q*. The enhanced friction velocity by the erroneous wind and wind shear in CTL played a role in increasing the surface fluxes of moisture and latent heat. This relationship is known as a wind-induced surface heat exchange (WISHE) in several studies (Emanuel, 1986; Emanuel et al., 1994; Craig and Gray, 1996). Figure 9 shows the temporal variations of the three-hourly frictional velocity, surface moisture flux, surface latent heat flux, and equivalent potential temperature at the surface, averaged over the ocean at (10°-20°N, 125°-165°E), in which most of the simulated TCs were formed. The temporal variations of all variables were significantly correlated with each other. CTL tended to simulate a stronger friction velocity over the entire period, compared with SPN. The stronger friction velocity tended to simulate more surface moisture and more latent heat in CTL than SPN. Due to the moisture and heat enhancement from the sea surface, the equivalent potential temperature at the lowest model level also increased, which could have fueled the initiation and intensification of TCs.

    Figure 9.  Time series of (a) frictional velocity, (b) moisture flux, (c) latent heat flux and (d) equivalent potential temperature, at the lowest model level in CTL (thick line) and SPN (dashed line). The TC marks indicate when TCs appeared for the first time.

4. Summary and conclusions
  • In this study, we investigated the simulated formation and development of TCs over the WNP for a 3-month period using the WRF model and boundary data forced by FNL analysis data. The effect of the spectral nudging method on the seasonal frequency and intensity of simulated TCs was analyzed. The simulation without spectral nudging (CTL) showed not only an overestimated frequency and unrealistic tracks of TCs, but unreasonable seasonal mean fields of precipitation and low-level circulation too. In CTL, the summer monsoon precipitation along 10°N over the WNP was considerably overestimated and northward-shifted due to inaccurately simulated atmospheric monsoon circulation, as well as TC frequency and tracks. CTL simulated strong westerly flow and a weakened WNPSH. The maximum intensity of TCs also tended to be more excessive. The number of spurious TCs was increased due to a kind of climate drift effect in the later part of the simulation period.

    The overestimated number of TCs in CTL was attributed to the multiple TC formation associated with the inaccurate monsoon trough, which was mainly modulated by enhanced large-scale flows greater than 1000 km. The enhanced kinetic energy of the large-scale flow in CTL implied that the model could not correctly reproduce the observed atmospheric circulation due to the lack of interaction between large-scale flows provided by the lateral boundary and model-generated flows. Consequently, the model simulated incorrect or dynamically imbalanced large-scale flows in the long integration of the model. These results are consistent with several studies (e.g., Rinke and Dethloff, 2000; Lee et al., 2004; Miguez-Macho et al., 2004). (Rinke and Dethloff, 2000) indicated that most errors in climate simulations over the Arctic using a regional model resulted from differences in wavelengths longer than 1000 km, which corresponds to the cutoff of large-scale flows in this study.

    In the classification of the low-level wind patterns at the formation of TCs, CTL simulated twice as many TCs of the MC pattern defined by (Lee et al., 2008) than those in SPN. More simulated TCs of the MC pattern in CTL was mostly attributed to enhanced westerly monsoon flows in the southern quadrant with respect to the TC center. Furthermore, enhanced wind speed caused the model to produce more moisture content and increased temperature at the lowest level over the ocean, which in turn initiated more typhoons by increasing the conditional instability.

    It should be emphasized that a realistic monsoon circulation takes a vital role in simulating realistic TCs. Owing to the realistic low-level monsoon circulation in SPN, the westerly flow along the western boundary of the model domain and the anticyclonic circulation of the WNPSH were much improved. SPN reduced the exaggerated number of TCs and erroneous rainfall of the major rain bands of the EASM and WNPSM, due to the simulated spurious wind field compared to CTL. The interaction between the large-scale environment and internal variability in the regional model was reasonable as a result of using the spectral nudging technique.

    The summer monsoon over East Asia and the WNP is affected by many factors, such as the subtropical high in the WNP, the midlatitude low-pressure systems and blocking highs, land surface processes related to soil moisture and snow depth, and the SST (Weng et al., 2004). (Cha and Lee, 2009) investigated the systematic errors of simulated precipitation associated with the summer monsoon by applying the spectral nudging technique. In this study, the spurious TC formation, which was attributed to inaccurate monsoonal atmospheric flow fields, was corrected by the proper simulation of the synoptic background using the spectral nudging technique.

    Regional climate simulations are also dependent on physical parameterization schemes, especially cumulus parameterization and grid-scale resolvable schemes. Used in this study, the Kain-Fritsch scheme, whose convection is determined by CAPE, can overestimate convection and precipitation over the ocean. Strong convection can trigger unreasonable positive feedback during a long period of simulation. In the future, the effect of convection in longer simulations of TCs should be investigated, and it is also necessary to further investigate the intensity of simulated TCs through the coupling of RCMs with ocean models.




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