Simulation of the Electrification of a Tropical Cyclone Using the WRF-ARW Model：An Idealized Case

Evolution of the electrifi cation of an idealized tropical cyclone （TC） is simulated by using the Advanced Weather Research and Forecasting （WRF-ARW） model. The model was modifi ed by addition of explicit electrifi cation and a new bulk discharge scheme. The characteristics of TC lightning is further examined by analyses of the electrifi cation and the charge structure of the TC. The fi ndings thus obtained are able to unify most of the previous inconsisitent observational and simulation studies. The results indicate that the TC eyewall generally exhibits an inverted dipole charge structure with negative charge above the positive. In the intensifi cation stage, however, the extremely tall towers of the eyewall may exhibit a normal tripole structure with a main negative region between two regions of positive charge. The outer spiral rainband cells display a simple normal dipole structure during all the stages. It is further found that the diff erences in the charge structure are associated with diff erent updrafts and particle distributions. Weak updrafts, together with a coexistence region of diff erent particles at lower levels in the eyewall, result in charging processes that occur mainly in the positive graupel charging zone （PGCZ）. In the intensifi cation stage, the occurrence of charging processes in both positive and negative graupel charging zones is associated with strong updraft in the extremely tall towers. In addition, the coexistence region of graupel and ice crystals is mainly situated at upper levels in the outer rainband, so the charging processes mainly occur in the negative graupel charging zone （NGCZ）.     

An Application of the Adjoint Method to a Statistical-Dynamical Tropical-Cyclone Prediction Model （SD-90） Ⅱ： Real Tropical Cyclone Cases

In the first paper in this series, a variational data assimilation of ideal tropical cyclone （TC） tracks was performed for the statistical-dynamical prediction model SD-90 by the adjoint method, and a prediction of TC tracks was made with good accuracy for tracks containing no sharp turns. In the present paper, the cases of real TC tracks are studied. Due to the complexity of TC motion, attention is paid to the diagnostic research of TC motion. First, five TC tracks are studied. Using the data of each entire TC track, by the adjoint method, five TC tracks are fitted well, and the forces acting on the TCs are retrieved. For a given TC, the distribution of the resultant of the retrieved force and Coriolis force well matches the corresponding TC track, i.e., when a TC turns, the resultant of the retrieved force and Coriolis force acts as a centripetal force, which means that the TC indeed moves like a particle; in particular, for TC 9911, the clockwise looping motion is also fitted well. And the distribution of the resultant appears to be periodic in some cases. Then, the present method is carried out for a portion of the track data for TC 9804, which indicates that when the amount of data for a TC track is sufficient, the algorithm is stable. And finally, the same algorithm is implemented for TCs with a double-eyewall structure, namely Bilis （2000） and Winnie （1997）, and the results prove the applicability of the algorithm to TCs with complicated mesoscale structures if the TC track data are obtained every three hours.     

Maintenance of Tropical Cyclone Bill （1988） after Landfall

The mechanism for the maintenance of Tropical Cyclone Bill （1988） after landfall is investigated through a numerical simulation. The role of the large-scale environmental flow is examined using a scale separation technique, which isolates the tropical cyclone from the environmental flow. The results show that Bill was embedded in a deep easterly-southeasterly environmental flow to the north-northeast of a large-scale depression and to the southwest of the western Pacific subtropical high. The depression had a quasi-barotropic structure in the mid-lower troposphere and propagated northwestward with a speed similar to the northwestward movement of Bill. The moisture budgets associated with both the large-scale and the tropical cyclone scale motions indicate that persistent low-level easterly-southeasterly flow transported moisture into the inner core of the tropical cyclone. The low-level circulation of the tropical cyclone transported moisture into the eyewall to support eyewall convection, providing sufficient latent heating to counteract energy loss due to surface friction and causing the storm to weaken relatively slowly after landfall. Warming and a westward extension of the upper-level easterly flow led to westward propagation of the environmental flow in the mid-lower troposphere. As a result, Bill was persistently embedded in an environment of deep easterly flow with high humidity, weak vertical wind shear, convergence in the lower troposphere, and divergence in the upper troposphere. These conditions are favorable for both significant intensification prior to landfall and maintenance of the tropical cyclone after landfall.     

Responses of Vertical Structures in Convective and Stratiform Regions to Large-Scale Forcing during the Landfall of Severe Tropical Storm Bilis （2006）

The responses of vertical structures,in convective and stratiform regions,to the large-scale forcing during the landfall of tropical storm Bilis(2006) are investigated using the data from a two-dimensional cloud-resolving model simulation.An imposed large-scale forcing with upward motion in the mid and upper troposphere and downward motion in the lower troposphere on 15 July suppresses convective clouds,which leads to ～100% coverage of raining stratiform clouds over the entire model domain.The imposed forci...     

Sensitivity of the Grid-point Atmospheric Model of IAP LASG （GAMILI.I.0） Climate Simulations to Cloud Droplet Effective Radius and Liquid Water Path

This paper documents a study to examine the sensitivity to cloud droplet effective radius and liquid water path and the alleviation the energy imbalance at the top of the atmosphere and at the surface in the latest version of the Grid-point Atmospheric Model of the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics （LASG）, Institute of Atmospheric Physics （IAP） （GAMIL1.1.0）. Considerable negative biases in all flux components, and thus an energy imbalance, are found in GAMIL1.1.0. In order to alleviate the energy imbalance, two modifications, namely an increase in cloud droplet effective radius and a decrease in cloud liquid water path, have been made to the cloud properties used in GAMIL. With the increased cloud droplet effective radius, the single scattering albedo of clouds is reduced, and thus the reflection of solar radiation into space by clouds is reduced and the net solar radiation flux at the top of the atmosphere is increased. With the reduced cloud optical depth, the net surface shortwave radiation flux is increased, causing a net warming over the land surface. This results in an increase in both sensible and latent heat fluxes over the land regions, which is largely balanced by the increased terrestrial radiation fluxes. Consequently, the energy balance at the top of atmosphere and at the surface is achieved with energy flux components consistent with available satellite observations.     

Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis （2006）

The effects of environmental vertical wind shear(VWS)on the intensity and rainfall asymmetries in Tropical Storm(TS)Bilis(2006)have been analyzed based on TRMM/TMI-estimated surface rainfall data,QuikSCAT wind fields,850-and 200-hPa winds of the NCEP-NCAR reanalysis,precipitation data at 5-min intervals from automatic weather stations over mainland China,and the best track data of TS Bilis(2006). The results show that the simultaneous and 6-hour-lagged correlation coeffcients between VWS and storm intensity(the minimum central sea level pressure)are 0.59145 and 0.57438(P0.01),respectively.The averaged VWS was found to be about 11 m s-1 and thus suppressed the intensification of Bilis(2006). Distribution of precipitation in Bilis(2006)was highly asymmetric.The azimuthally-averaged rainfall rate in the partial eyewall,however,was smaller than that in a major outer rainband.As the storm intensified,the major rainband showed an unusual outward propagation.The VWS had a great impact on the asymmetric distribution of precipitation.Consistent with previous modeling studies,heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall,showing a strong wavenumber-one asymmetry,which was amplified as the VWS increased.     

Improving Simulation of a Tropical Cyclone Using Dynamical Initialization and Large-Scale Spectral Nudging： A Case Study of Typhoon Megi （2010）

In 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.     

The Vertical Structures of Atmospheric Temperature Anomalies Associated with El Nio Simulated by the LASG/IAP AGCM： Sensitivity to Convection Schemes

The vertical structures of atmospheric temperature anomalies associated with El Nio are simulated with a spectrum atmospheric general circulation model developed by LASG/IAP (SAMIL). Sensitivity of the model’s response to convection scheme is discussed. Two convection schemes, i.e., the revised Zhang and Macfarlane (RZM) and Tiedtke (TDK) convection schemes, are employed in two sets of AMIP-type (Atmospheric Model Intercomparison Project) SAMIL simulations, respectively. Despite some deficiencies in the up...     

用PR资料分析热带气旋卡特里娜降水特征

利用TRMM (Tropical Rainfall Measuring Mission) 卫星降水雷达 (Precipitation Radar,PR) 资料对2005年8月发生在墨西哥湾的热带气旋卡特里娜 (Katrina) 初生、发展和变性3个阶段的层状云和对流云的近地面降水和降水垂直廓线进行分析。结果表明：在热带气旋的整个生命期,对流性降水个数约是层状云降水个数的四分之一;随着气旋的发展,对流性和层状云降水的平均强度逐渐增强, 在登陆前有所减弱,登陆后对流性降水和总平均降水均增强,层状云降水稍有减弱;大部分降水廓线都随高度升高均呈现先略增大 (到4 km高度)、再减小的趋势,在6~7 km处由于冻结层的存在使得降水达到最小值。     

城市效应对登陆热带气旋妮妲降水影响的模拟

运用中尺度数值模式WRF耦合城市冠层模式（urban canopy model,UCM）,对2016年登陆深圳的热带气旋妮妲（1604）（以下简称妮妲）进行数值模拟。高分辨率数值模拟较好地再现了妮妲登陆前后的强度、路径和累积降水。利用城市化过程当中城市冠层对热带气旋降水的敏感性试验结果表明:城市冠层会减弱对流运动和水汽的输送,导致热带气旋登陆后珠江口城市群区域累积降水量略减少。应用最新的土地利用资料进行的城市下垫面敏感性试验结果表明:由于城市下垫面粗糙度增加,造成登陆地面风的减速,强度减弱,潜热通量与2 m高度比湿相应减小;城市下垫面粗糙度增加会加强该区域垂直对流运动以及不稳定能量增加,有利于降水增强,尤其在城市化下垫面处,热带气旋登陆后6 h累积降水增加量最大可超过20 mm。总体而言,对登陆热带气旋降水而言,耦合城市冠层使城市区域热带气旋降水减少,但在数值模拟中城市冠层影响作用不显著。城市化下垫面对登陆热带气旋暴雨的增幅作用明显,在登陆热带气旋降水预报中应重视。     

中国台风降水分离客观方法的改进研究

针对中国台风降水的分离问题,对原客观分离方法进行改进研究。首先提出判别指标,然后对主、客观方法分离台风降水结果进行对比分析,在此基础上确定了将原客观方法中两个重要的参数由固定参数调整为可变参数的方案思路,并最终确定了最佳改进方案。结果显示,客观分离方法的改进效果十分明显。此外,对客观方法进行个例检验,并引入了美国卫星综合降水（CMORPH）资料,结果表明改进后客观方法在双台风降水和台风与中纬度系统相互作用产生的降水识别上均表现出较强的能力。     

The Genesis of Tropical Cyclone Bilis (2000) Associated with Cross-equatorial Surges

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 subsynoptic 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 synopticscale 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.     

水汽输送影响登陆热带气旋维持和降水的数值试验

Bilis(0010)是一个登陆后长久维持的热带气旋,其维持期间与一条西南风低空急流水汽通道长时间保持联结.用中尺度模式MM5V3对其维持过程进行模拟,并通过多个敏感性试验,研究不同方向水汽输送对热带气旋在陆上维持和降水的影响.结果表明:外界持续的水汽输送有利于热带气旋气旋性环流在陆地上维持,减缓其登陆后的强度衰减.水汽输送通过影响热带气旋的热力结构来影响热带气旋强度,充沛的水汽供应有利于热带气旋暖心维持.外界水汽输送有助于热带气旋雨带中的强对流活动,使雨量加强.而对流活动过程中水汽凝结潜热释放是热带气旋获得能量继续维持的重要条件.结果还表明,南边界水汽输送对热带气旋维持和降水的影响最为明显,其次是东边界.来自北界和西界水汽的影响微弱.     

Impact of Mid- and Upper-Level Dry Air on Tropical Cyclone Genesis and Intensification: A Modeling Study of Durian (2001)

The impact of mid- and upper-level dry air, represented by low relative humidity (RH) values, on the genesis of tropical cyclone (TC) Durian (2001) in the South China Sea was investigated by a series of numerical experiments using the Weather Research and Forecasting model. The mid-level RH was lowered in different regions relative to TC Durian (2001)'s genesis location. Results suggest that the location of dry air was important to Durian (2001)'s genesis and intensification. The rapid development of the TC was accompanied by sustained near-saturated mid- and upper-level air, whereas low humidity decelerated its development. Water vapor budget analysis showed that moisture at mid and upper levels was mainly supplied by the vertical convergence of moisture flux and the divergence terms, and consumed by the condensation process. The horizontal convergence of moisture flux term supplied moisture in the air moistening process but consumed moisture in the air drying process. With a dryer mid- and upper-level environment, convective and stratiform precipitation were both inhibited. The upward mass fluxes and the diabatic heating rates associated with these two precipitation types were also suppressed. Generally, convection played the dominant role, since the impact of the stratiform process on vertical mass transportation and diabatic heating was much weaker. The vorticity budget showed that the negative vorticity convergence term, which was closely related to the inhibited convection, caused the vorticity to decrease above the lower troposphere in a dryer environment. The negative vorticity tendency is suggested to slow down the vertical coherence and the development rate of TCs.     

一次南方冷季降水过程揭示的热带水汽羽与降水的关系

利用逐日降水量资料、FY-2E卫星资料以及NCEP/NCAR再分析资料,对2012年1月13~16日的一次南方冷季降水过程进行分析,剖析了热带水汽羽形成的水汽输送源和动力机制,及其与降水的关系。分析表明,热带水汽羽不但表征了中高层孟加拉湾水汽的输送,它实际上反映了对流层整层水汽的输送、辐合,低层南海水汽输送带为水汽羽的维持和降水的产生做出了贡献;高空槽前急流与低空切变线的耦合促使了上升运动的形成,导致降水的产生。后期高空槽的移动填塞致使急流轴发生移动及转向,从而引导了热带水汽羽的移动,最终导致热带水汽羽的断裂消亡;热带水汽羽上不存在强对流云团,未形成强对流过程,但由于水汽输送条件较好,配合弱的上升运动产生了这次强降水过程。     

热带气旋干湿特征的对比分析

热带气旋属于热带天气系统，由于其生成地及生命史的大部分时间基本都在洋面或海面上，所以就其自身来说，有着充分的水汽资源，但在它的移动路径中，随着环境场的不断变化，其影响地区也并不是都有很强的降水产生，针对这一问题，引入了干湿热带气旋概念，并选取了３个较为典型的个例，对它们的天气形势及物理量场作了对比分析，得到了些有有益的结果。     

Urban Impact on Landfalling Tropical Cyclone Precipitation: A Numerical Study of Typhoon Rumbia (2018)

Coastal urban areas are prone to serious disasters caused by landfalling tropical cyclones(TCs). Despite the crucial role of urban forcing in precipitation, how fine-scale urban features impact landfalling TC precipitation remains poorly understood. In this study, high-resolution ensemble simulations of Typhoon Rumbia(2018), which crossed the Yangtze River Delta urban agglomeration, were conducted to analyze the potential urban impact on TC precipitation. Results show that the inner-core rainfal...     

热带气旋碧丽斯(2000)发生的数值模拟:非对称流的发展及转换

采用NCAR/PSU研制的非静力双向嵌套的仔尺度模式MM5,模拟了热带气旋碧丽斯(2000)从热带弱低压发展为热带风暴及台风的过程.分析表明非对称流在低层发展并通过涡动通量项向对称流转换,使得热带弱低压快速发展达到热带风暴强度.进一步的分析表明:涡动通量项不仅依赖于非对称流的强度,还取决于非对称流型.当非对称流场为气旋式流入或反气旋式流出时,非对称动量向对称动量转换,非对称流减弱,轴对称流加强;反之,当非对称流场为反气旋式流入或气旋式流出时,动量由对称流向非对称流转换.