Experiments with the Improved Dynamical-Statistical-Analog Ensemble Forecast Model for Landfalling Typhoon Precipitation over South China

In recent work, three physical factors of the Dynamical-Statistical-Analog Ensemble Forecast Model for Landfalling Typhoon Precipitation (DSAEF_LTP model) have been introduced, namely, tropical cyclone (TC) track, TC landfall season, and TC intensity. In the present study, we set out to test the forecasting performance of the improved model with new similarity regions and ensemble forecast schemes added. Four experiments associated with the prediction of accumulated precipitation were conducted based on 47 landfalling TCs that occurred over South China during 2004-2018. The first experiment was designed as the DSAEF_LTP model with TC track, TC landfall season, and intensity (DSAEF_LTP-1). The other three experiments were based on the first experiment, but with new ensemble forecast schemes added (DSAEF_LTP-2), new similarity regions added (DSAEF_LTP-3), and both added (DSAEF_LTP- 4), respectively. Results showed that, after new similarity regions added into the model (DSAEF_LTP-3), the forecasting performance of the DSAEF_LTP model for heavy rainfall (accumulated precipitation ≥250 mm and ≥100 mm) improved, and the sum of the threat score (TS250 + TS100) increased by 4.44%. Although the forecasting performance of DSAEF_LTP-2 was the same as that of DSAEF_LTP-1, the forecasting performance was significantly improved and better than that of DSAEF_LTP-3 when the new ensemble schemes and similarity regions were added simultaneously (DSAEF_LTP-4), with the TS increasing by 25.36%. Moreover, the forecasting performance of the four experiments was compared with four operational numerical weather prediction models, and the comparison indicated that the DSAEF_LTP model showed advantages in predicting heavy rainfall. Finally, some issues associated with the experimental results and future improvements of the DSAEF_LTP model were discussed.     

西北太平洋热带气旋路径的历史模拟

基于IBTrACS提供的热带气旋最佳路径数据集,在统计分析历史热带气旋的发生年频次、发生位置、路径移动及强度变化等的基础上,建立了西北太平洋热带气旋轨迹合成模型。模型包括生成模型、移动模型、消亡模型及强度模型4个部分,并从地理轨迹密度、年登陆率、登陆风速分布三个方面,对模拟的气旋路径与历史气旋路径进行比较,以验证模型的准确性和可靠性。结果表明,构建的西北太平洋热带气旋全路径统计模拟模型稳健可靠,可进一步应用于研究区热带气旋的定量精细化的风险评估,能提高气旋风险灾害评估的可信度。     

登陆热带气旋路径和强度预报的效益评估初步研究

近年来有关热带气旋(TC)灾情的评估指标和方法的研究取得明显进展,但较少涉及TC预报对减少灾害损失的贡献(即效益)分析。基于中央气象台的TC实时路径和强度预报,针对登陆中国大陆的TC,初步分析了TC的路径和强度预报误差与其造成的直接经济损失之间的可能关系,并在此基础上建立了包含TC路径和强度预报误差的TC直接经济损失的预估模型。TC登陆前后24 h的路径和强度预报误差与TC所致直接经济损失均呈正相关关系;对于单个登陆TC而言,若24 h TC路径预报误差每减小1 km可减少因灾直接经济损失约0.97亿元,若强度预报每减小1 m/s可减少因灾直接经济损失约3.8亿元(以2014年为基准年)。可见,提高TC路径和强度预报精度对于减灾的效益巨大,且当前尤以提高强度预报能力的效益为佳。      

NUMERICAL ANALYSIS ON THE ROLE OF TROPICAL STORM NAMTHEUN IN THE UNUSUAL TRACKS OF THREE TROPICAL CYCLONES IN 2010

Tropical cyclones (TCs) Lionrock, Kompasu, and Namtheun were formed successively within 40 hours in 2010. Over the next several days afterwards, these TCs exhibited unusual movements which made operational prediction difficult. Verifications are performed on the forecasts of the tracks of these TCs with six operational models, including three global and three regional models. Results showed that the trends of TC tracks could be reproduced by these models, whereas trajectory turning points and landfall locations were not simulated effectively. The special track of Lionrock should be associated with its direct interaction with Namtheun, according to a conceptual model of binary TC interaction. By contrast, the relation between Kompasu and Namtheun satisfied the criteria for a semi-direct interaction. Numerical experiments based on the Global and Regional Assimilation and Prediction System-Tropical Cyclone forecast Model (GRAPES-TCM) further confirmed the effect of Namtheun on the unusual tracks of Lionrock and Kompasu. Finally, the physical mechanism of binary TC interaction was preliminarily proposed.     

AN OPERATIONAL STATISTICAL SCHEME FOR TROPICAL CYCLONE INDUCED RAINFALL FORECAST

A non-parametric method is used in this study to analyze and predict short-term rainfall due to tropical cyclones(TCs) in a coastal meteorological station. All 427 TCs during 1953-2011 which made landfall along the Southeast China coast with a distance less than 700 km to a certain meteorological station- Shenzhen are analyzed and grouped according to their landfalling direction, distance and intensity. The corresponding daily rainfall records at Shenzhen Meteorological Station(SMS) during TCs landfalling period(a couple of days before and after TC landfall) are collected. The maximum daily rainfall(R-24) and maximum 3-day accumulative rainfall(R-72) records at SMS for each TC category are analyzed by a non-parametric statistical method, percentile estimation. The results are plotted by statistical boxplots, expressing in probability of precipitation. The performance of the statistical boxplots is evaluated to forecast the short-term rainfall at SMS during the TC seasons in 2012 and 2013. Results show that the boxplot scheme can be used as a valuable reference to predict the short-term rainfall at SMS due to TCs landfalling along the Southeast China coast.     

Dynamical downscaling forecasts of Western North Pacific tropical cyclone genesis and landfall

This study evaluates the potential use of the regional climate model version 3 (RegCM3) driven by (1) the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data during 1982–2001 and (2) the NCEP Climate Forecast System Version 2 (CFS2) hindcast data during 2000–2010 in forecasting Western North Pacific (WNP) tropical cyclone (TC) activity. The first experiment is conducted to investigate the ability of the model in generating a good climatology of TC activity in spatial and temporal scales, so the model could be used in the second experiment to test its ability in forecasting TC genesis and landfall. Both experiments extend through the May to October WNP-TC season. Results show that the use of RegCM3 driven by the CFSR achieves a better simulation on the temporal and spatial variation of WNP-TC genesis during 1982–2001, as compared to previous studies using the same model but driven by the ERA40 reanalysis. In addition, diagnoses on the use of RegCM3 driven by the CFS2 point out that the 2000–2010 WNP-TC genesis locations and numbers from the model are very similar to those from the observations. The skill of RegCM3 in the forecasts of landfalling TCs is higher over the Southeast Asian region than over the other sub-regions of East Asia. Potential causes for such regional differences are discussed. Most importantly, statistical analyses show that the use of RegCM3 driven by the CFS2 gives a better forecast skill than the use of CFS2 alone for the prediction of WNP-TCs making landfall in East Asia. This indicates that the use of a dynamical downscaling method for the global forecast data would likely lead to a higher forecast skill of regional TC landfalls in most of the East Asian region.     

基于降水极端预报指数的福建台风极端降水预报研究

应用1961—2017年中国气象局热带气旋最佳路径数据集、国家地面气象观测站日降水观测资料和2015年8月—2017年12月欧洲中期天气预报中心（ECMWF）集合预报系统降水极端预报指数（EFI）数据,根据百分位法定义台风影响期间福建省各站点的台风极端降水阈值,采用最小阈值法剔除台风极端降水时EFI箱线图中的异常值,保留最小值作为台风极端降水EFI阈值,建立基于EFI阈值的台风极端降水预报方法。用该方法分别对2015年8月—2017年12月和2018年登陆或影响福建台风进行台风极端降水回报和预报试验,采用TS、空报率、漏报率对回报和预报结果进行检验。结果表明:福建台风极端降水阈值由沿海向西北内陆逐渐减小,其中中北部沿海地区的阈值最大;台风日降水量与日降水EFI预报、台风极端降水时的日降水量与日降水EFI预报均存在明显的正相关,日降水EFI预报的箱线图差异指数（I_bd）也表明EFI可以较好地区分台风极端降水和非极端降水;预报试验20时（北京时）起报时效12—36、36—60、60—84和84—108 h的台风极端降水预报TS分别达到0.26、0.22、0.20和0.19,总体上略优于回报试验;台风极端降水越显著,台风极端降水预报效果越好,回报和预报都存在的不足是空报率高,主要出现在台风极端降水不明显的个例中。降水EFI预报对台风极端降水预报具有较好的指示意义,基于降水EFI阈值的台风极端降水预报产品可在业务中作为台风极端降水预报的参考。      

西北太平洋TC移动速度异常及预报误差特征的分析

利用国家气象局和上海台风研究所(CMA-STI)整编的西北太平洋1970—2009年热带气旋(TC,Tropical Cyclone)及TC最佳路径数据集和2005—2010年的TC路径预报误差资料,应用百分位法,确定TC移动速度异常指标,分析了40 a来西北太平洋TC移动速度及其变化异常发生的时空分布特征,研究了TC速度预报误差对路径预报误差的影响及其与大尺度引导气流之间的关系。结果显示:1)西北太平洋TC移速及移速变化累积概率达95%(5%)分位数的阈值分别为10.8 m·s~(-1)(1.43 m·s~(-1))和2.42m·s~(-1)(-1.72 m·s~(-1))。2)快速移动及加速的TC大都出现在日本海地区,而缓慢的和减速移动TC主要出现在南海区域。3)TC移动速度异常的季节变化表现为,快速移动的TC在5月出现的频率达到最高,缓慢移动的TC在10月频率达到最高,加速移动的TC在6月频率达到最高。4)近6 a的TC移速预报误差对TC路径预报误差的贡献平均约为41.6%。5)对TC路径预报误差偏大,且移速预报误差贡献大的个例分析显示,该个例大尺度环境引导气流偏弱使TC移动速度偏慢。而如果预报的大尺度环境引导气流偏强,使预报的TC移速偏快,那么就容易导致大的路径预报误差。     

The Simulation of Five Tropical Cyclones by Sample Optimization of Ensemble Forecasting Based on the Observed Track and Intensity

The quality of ensemble forecasting is seriously affected by sample quality. In this study, the distributions of ensemble members based on the observed track and intensity of tropical cyclones (TCs) were optimized and their influence on the simulation results was analyzed. Simulated and observed tracks and intensities of TCs were compared and these two indicators were combined and weighted to score the sample. Samples with higher scores were retained and samples with lower scores were eliminated to improve the overall quality of the ensemble forecast. For each sample, the track score and intensity score were added as the final score of the sample with weight proportions of 10 to 0, 9 to 1, 8 to 2, 7 to 3, 6 to 4, 5 to 5. These were named as "tr", "91", "82", "73", "64", and "55", respectively. The WRF model was used to simulate five tropical cyclones in the northwestern Pacific to test the ability of this scheme to improve the forecast track and intensity of these cyclones. The results show that the sample optimization effectively reduced the track and intensity error, "55" usually had better performance on the short-term intensity prediction, and "tr" had better performance in short-term track prediction. From the overall performance of the track and intensity simulation, "91" was the best and most stable among all sample optimization schemes. These results may provide some guidance for optimizing operational ensemble forecasting of TCs.     

华东登陆热带气旋降水不同分布的对比分析

利用CMORPH降水资料，将热带气旋（TC）登陆后的降水分为路径左侧降水（L型）和右侧降水（R型）两类，并针对登陆华东地区TC的 L型和R型降水的大气环流场、环境水平风垂直切变以及台风环流内的动热力条件进行对比分析，结果表明:2005～2014年间登陆华东地区的20例TC中包括12例L型和8例R型。总体来看，大气环流因子对于登陆华东TC降水分布起主要作用。L型降水TC高层南亚高压主要呈纬向带状分布，在登陆过程中路径左侧维持偏东风高空辐散气流，中层西风槽偏东，西太平洋副热带高压（简称副高）偏南，环境水平风垂直切变指向西南。R型降水TC高层南亚高压断裂，呈经向分布。TC路径左侧风场较均匀，右侧东南风高空辐散气流明显。副高的位置偏北呈块状，同时环境水平风垂直切变指向东北，有利于路径右侧降水。台风环流内，低层冷暖平流输送以及水汽辐合与降水落区也有较好对应关系。L型TC低层暖平流的输送使TC西南象限低层增暖，大气稳定度降低。同时水汽辐合区也主要位于西南象限，有利于TC路径左侧降水。而R型TC副高位置偏北可将南侧的东南暖湿气流向台风环流更西部输送，东北象限维持暖平流，有利于路径右侧降水发生。     

热带气旋的路径及登陆预报

用几个非线性数学模型制作热带气旋短期路径预报及热带气旋个数、登陆时段、地段的短期气候预报。5年多的研究和预报试验结果表明：用指数曲线模型制作热带气旋路径预报，准确率较高。24h预报，199次平均误差123km，达到国内先进水平。用多项式等非线性模型，制作登陆我国及登陆广东热带气旋的年、月个数预测，经过3年实际应用检验，准确率达到70％～90％。用非线性预测模型的逐日气压场、逐日雨量场长期预测结果进行分析，制作广东热带气旋登陆时段、地段和南海海面热带气旋出现时间的预报，准确率达到70％～80％，2002年热带气旋的预报，采用长中短期预报相结合，数值预报与统计预报相结合，预报效果较佳。     

Effects of a Dry-Mass Conserving Dynamical Core on the Simulation of Tropical Cyclones

The accurate forecasting of tropical cyclones(TCs) is a challenging task. The purpose of this study was to investigate the effects of a dry-mass conserving(DMC) hydrostatic global spectral dynamical core on TC simulation. Experiments were conducted with DMC and total(moist) mass conserving(TMC) dynamical cores. The TC forecast performance was first evaluated considering 20 TCs in the West Pacific region observed during the 2020 typhoon season. The impacts of the DMC dynamical core on forecasts o...     

登陆广东北上路径热带气旋强降水的分布特征

利用1981-2018年中国气象局上海台风研究所热带气旋最佳路径资料和国家基本气象站逐日降水资料,对登陆广东北上路径热带气旋活动及强降水特征进行统计分析.结果表明北上热带气旋从登陆时到登陆后48 h是陆地强降水产生的集中时段,登陆时24 h是陆地大暴雨最集中的时段.强降水从沿海向内陆逐渐减少,一般登陆后热带气旋持续时间...     

1949—2009年西北太平洋热带气旋气候特征

利用1949—2009年中国气象局热带气旋（TC:Tropical Cyclone）最佳路径数据集,对西北太平洋TC生成源地的时空分布、生成和登陆我国TC的年、月频数分布、强度分布和地理分布等气候特征进行统计分析。研究结果表明：在这61年中生成的TC呈减少趋势,登陆我国的TC数量比较稳定,不过近10年来登陆的较强TC数量却呈现出上升趋势;每年6—10月是TC高发期,强度等级越高的TC生成季节越偏晚,而8月是生成和登陆数量最多的月份;TC平均生命期随强度等级的增加而增大,且逐渐表现出单峰值分布特征;TC频数的地理分布以我国南海和菲律宾以东洋面为中心,向四周呈辐射状减少,近10年来其活动范围有所西伸加强。     

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。     

Changes in Typhoon Regional Heavy Precipitation Events over China from 1960 to 2018

In earlier studies,objective techniques have been used to determine the contribution of tropical cyclones to precipitation(TCP)in a region,where the Tropical cyclone Precipitation Event(TPE)and the Regional Heavy Precipitation Events(RHPEs)are defined and investigated.In this study,TPE and RHPEs are combined to determine the Typhoon Regional Heavy Precipitation Events(TRHPEs),which is employed to evaluate the contribution of tropical cyclones to regional extreme precipitation events.Based on the Objective Identification Technique for Regional Extreme Events(OITREE)and the Objective Synoptic Analysis Technique(OSAT)to define TPE,temporal and spatial overlap indices are developed to identify the combined events as TRHPE.With daily precipitation data and TC best-track data over the western North Pacific from 1960 to 2018,86 TRHPEs have been identified.TRHPEs contribute as much as 20%of the RHPEs,but100%of events with extreme individual precipitation intensities.The major TRHPEs continued for approximately a week after tropical cyclone landfall,indicating a role of post landfall precipitation.The frequency and extreme intensity of TRHPEs display increasing trends,consistent with an observed positive trend in the mean intensity of TPEs as measured by the number of daily station precipitation observations exceeding 100 mm and 250 mm.More frequent landfalling Southeast and South China TCs induced more serious impacts in coastal areas in the Southeast and the South during 1990-2018 than1960-89.The roles of cyclone translation speed and"shifts"in cyclone tracks are examined as possible explanations for the temporal trends.     

西北太平洋热带气旋残留低压对中国大陆地区降水的影响

西北太平洋热带气旋(TC)是影响中国降水的重要天气系统,其减弱后的残留低压(TCRL)仍然携带大量的水汽和能量,给其经过之处带来强降水。为分析热带气旋残留低压对中国降水的影响,采用客观方法从ERA-40和ERA-Interim再分析资料中识别出热带气旋对应的气旋,从而得到热带气旋残留低压活动资料。将残留低压的活动路径分为东北路径、东南沿海路径和西行路径3类,采用客观天气图分析法(OSAT)得到残留低压影响下的降水分布,对残留低压的活动路径和降水的分析结果表明,文中采用的方法能追踪得到热带气旋的完整生命史,在1958-2014年进入警戒区的718个热带气旋中,追踪得到706个对应的气旋,追踪的气旋中心与热带气旋中心平均距离131 km;共443个热带气旋有对应的残留低压,平均持续时间48.5 h,1 d以上的残留低压共293个,占66.1%,残留低压的持续时间与对应的热带气旋强度没有显著的关系,夏季残留低压持续时间较冬季长;对内陆地区的影响残留低压较热带气旋更为显著,降水影响自东南向西北减少,影响范围较热带气旋西扩,强度更大;东北路径主要进入地区为东北和华东地区,影响中国东部地区的降水;东南沿海路径影响范围最广,影响降水的强度最大;西行路径进入范围仅华南和西南地区,主要影响中国南部地区的降水,降水强度最小。      

热带气旋路径集合预报试验

以MM5模式为试验模式，分别用增长模繁殖法（BGM）和模式物理过程扰（MPP）形成12个集合成员，对2005年登陆我国的8个热带气旋进行了52次路径集合预报试验，以寻找适合西北太平洋热带气旋路径的集合预报方法。结果表明:BGM方法的集合预报结果总体上好于控制试验结果，其中对强度较弱的热带气旋的集合预报效果更好。MPP方法的集合预报结果总体上不如控制试验结果，但对初始时刻强度达到台风的热带气旋的集合预报效果较好，集合预报相对于控制试验的改进效果明显。基于BGM方法和MPP方法集合预报的不同特点，将两种方法相结合形成扰动成员，这一综合方法的集合预报效果好于单一方法的集合预报效果。BGM方法和MPP方法的系统发散度总体上都偏小。     

AN OBSERVATIONAL STUDY ON DISTRIBUTION OF PRECIPITATION ASSOCIATED WITH LANDFALLING TROPICAL CYCLONES AFFECTING CHINA

In order to provide an operational reference for tropical cyclone precipitation forecast,this study investigates the spatial distributions of precipitation associated with landfalling tropical cyclones(TCs) affecting China using Geostationary Meteorological Satellite 5(GMS5)-TBB dataset.All named TCs formed over the western North Pacific that made direct landfall over China during the period 2001-2009 are included in this study.Based on the GMS5-TBB data,this paper reveals that in general there are four types of distribution of precipitation related to landfalling TCs affecting China.(a) the South-West Type in which there is a precipitation maximum to the southwestern quadrant of TC;(b) the Symmetrical South Type in which the rainfall is more pronounced to the south side of TC in the inner core while there is a symmetrical rainfall distribution in the outer band region;(c) the South Type,in which the rainfall maxima is more pronounced to the south of TC;and(d) the North Type,in which the rainfall maxima is more pronounced to the north of TC.Analyses of the relationship between precipitation distributions and intensity of landfalling TCs show that for intensifying TCs,both the maximum and the coverage area of the precipitation in TCs increase with the increase of TC intensity over northern Jiangsu province and southern Taiwan Strait,while decreasing over Beibu Gulf and the sea area of Changjiang River estuary.For all TCs,the center of the torrential rain in TC shifts toward the TC center as the intensity of TC increases.This finding is consistent with many previous studies.The possible influences of storm motion and vertical wind shear on the observed precipitation asymmetries are also examined.Results show that the environmental vertical wind shear is an important factor contributing to the large downshear rainfall asymmetry,especially when a TC makes landfall on the south and east China coasts.These results are also consistent with previous observational and numerical studies.     

Potential use of a regional climate model in seasonal tropical cyclone activity predictions in the western North Pacific

This study investigates the potential use of a regional climate model in forecasting seasonal tropical cyclone (TC) activity. A modified version of Regional Climate Model Version 3 (RegCM3) is used to examine the ability of the model to simulate TC genesis and landfalling TC tracks for the active TC season in the western North Pacific. In the model, a TC is identified as a vortex satisfying several conditions, including local maximum relative vorticity at 850?hPa with a value?≥450?×?10^?6?s^?1, and the temperature at 300?hPa being 1°C higher than the average temperature within 15° latitude radius from the TC center. Tracks are traced by following these found vortices. Six-month ensemble (8 members each) simulations are performed for each year from 1982 to 2001 so that the climatology of the model can be compared to the Joint Typhoon Warning Center (JTWC) observed best-track dataset. The 20-year ensemble experiments show that the RegCM3 can be used to simulate vortices with a wind structure and temperature profile similar to those of real TCs. The model also reproduces tracks very similar to those observed with features like genesis in the tropics, recurvature at higher latitudes and landfall/decay. The similarity of the 500-hPa geopotential height patterns between RegCM3 and the European Centre for Medium-Range Weather Forecasts 40 Year Re-analysis (ERA-40) shows that the model can simulate the subtropical high to a large extent. The simulated climatological monthly spatial distributions as well as the interannual variability of TC occurrence are also similar to the JTWC data. These results imply the possibility of producing seasonal forecasts of tropical cyclones using real-time global climate model predictions as boundary conditions for the RegCM3.