基于SWAN产品的短时强降水雷达特征及预警分析

利用SWAN产品和自动站雨量资料,详细对比分析了2010~2012年区域性暴雨中的短时强降水雷达特征,结果表明:(1)短时强降水具有反射率因子大,液态水含量高、回波顶高,强回波厚度大等特征,在每隔6min的SWAN拼图产品中,短时强降水通常满足:3km高度处CAPPI回波≥30dBZ,组合反射率CR中心强度≥40dBZ,VIL>5kg/m²,45dBZ以上的回波中心厚度(H)≥3km,这些参数的变化可以作为短时强降水的预警临近指标。(2)现有SWAN产品中的QPE/QPF产品对未来逐小时的雨量和落区具有一定的预报能力,但QPE产品估测1h累计降水量更接近于实况雨量,在监测到有上述强回波发展时,可通过分析QPE产品和回波特征及预警指标对短时强降水过程进行预报。     

MM5水平嵌套方案对降水预报影响的敏感性试验

利用MM5对2003-2005年华东地区的3次暴雨过程数值模拟,通过采用3种水平网格嵌套方案做对降水预报影响的敏感性试验,研究模式水平率对降水强度和落区预报的影响。结果表明:随着水平网格精度提高,模式对中小尺度天气系统的模拟能力增强,地形对天气过程的影响反映更加全面,模拟的雨带分布也越接近实况,雨量中心强度也随之增大;三层嵌套网格在暴雨落区和暴雨量级的预报上均优于两层和一层水平网格嵌套方案,尤其是暴雨落区预报有很大优势。     

基于不用分辨率和参数化方案的新疆区域数值模式性能初步评估

基于中尺度数值模式WRF,选取新疆两次强降水过程,设计三个试验方案,其中试验1为控制试验,试验2提高分辨率,试验3提高分辨率并调整物理参数化方案,初步评估不同分辨率和参数化方案对新疆区域2米温度、10米风速、降水预报的影响。结果表明：（1）提高分辨率对2米温度、10米风速模拟精度均有提高,2米温度预报精度提高约0.5℃,降低了日间温度模拟冷偏差;10米风速预报精度提高约0.5 m/s,降低了风速模拟正偏差;但提高分辨率后,模式出现虚假降水预报的情况。（2）提高分辨率并调整物理参数化方案后,2米温度模拟误差略有减小,模拟偏差减小约0.2℃;10米风速模拟误差增大约0.5 m/s,模拟偏差增大超过0.5 m/s;对降水落区、量级的模拟精度显著提高,减小了降水中心的模拟强度,对虚假降水预报有一定修正。     

华南区域高分辨率数值模式前汛期预报初步评估

基于华南地区自动站逐小时观测资料, 采用传统站点评分、邻域法等评估华南区域高分辨率数值模式(包括GRAPES_GZ_R 1 km模式和GRAPES_GZ 3 km模式)对降水、地面温度和风场等要素的预报能力。结果表明: GRAPES_GZ_R 1 km模式的降水预报技巧优于GRAPES_GZ 3 km模式, 模式预报以正偏差为主。对于不同起报时间的预报, 00时(世界时, 下同)起报的预报效果优于12时。GRAPES_GZ_R 1 km模式的TS评分是GRAPES_GZ 3 km模式的两倍以上, 对不同降水阈值的评分均较高。分数技巧评分(FSS)显示GRAPES_GZ_R 1 km模式6 h累计降水预报在0.1 mm、1 mm及5 mm以上的降水均可达到最低预报技巧尺度, 对所检验降水对象的空间位置把握能力更好。2 m气温和10 m风速检验结果表明两个模式均能较好把握广东省温度的分布特征, GRAPES_GZ_R 1 km模式对2 m气温预报结果优于GRAPES_GZ 3 km模式, 预报绝对误差更小; 两个模式对风速的预报整体偏强, 预报偏差在1~4 m/s之间, 但相比之下GRAPES_GZ 3 km模式在风场预报上表现更好。GRAPES_GZ_R 1 km模式的2 m气温和10 m风速预报偏差随降水过程存在明显波动, 强降水过后温度预报整体偏低, 风速预报偏强, 在模式产品订正、使用等需要考虑模式对主要天气系统的预报情况。总的来说, GRAPES_GZ_R 1 km模式的预报产品具有较好的参考价值。      

基于风廓线雷达的湖北梅雨期暴雨中小尺度特征

针对2016年湖北梅雨期3次(“6·19”、“7·5”和“7·19”)暴雨过程,首先对比了汉口站探空数据与汉口、咸宁两个风廓线雷达站水平风速、风向,发现“6·19”和“7·5”过程汉口风廓线雷达站3 km以下水平风速和探空数据较为接近,而3次过程中咸宁风廓线雷达站8 km以下水平风向、风速和汉口站探空数据基本吻合。在此基础上利用风廓线雷达资料并结合常规、加密自动气象站资料,对3次过程中水平风场、平均垂直速度及其变率、水平风速垂直切变、大气折射率结构常数(C■)等进行分析。结果表明：(1)降水开始前西南风速明显增大,中层干冷空气入侵和地面冷池形成的中尺度偏东气流是“6·19”过程50站出现大于等于17.2 m·s^-1大风的主要原因,“7·5”和“7·19”过程西南急流长时间维持及1 km以下的偏东气流则是短时强降水持续时间较长的诱因;(2)梅雨期暴雨期间风廓线雷达观测的水平风速垂直切变、平均垂直速度及其变率随高度变化较小,较强上升运动区域主要集中在4 km高度以下;(3)C■显示强降水发生前大气水汽含量有一增加过程,且整层水汽含量深厚,C■大值区的消失对应降水结束。     

国家级格点化定量降水预报系统

2020年7月18—19日,江淮地区出现一次特大暴雨过程,欧洲中期天气预报中心全球确定性预报模式（以下简称EC模式）、华东区域数值模式（以下简称华东模式）和国家气象中心Grapes高分辨区域模式（3 km分辨率,以下简称G3模式）预报的暴雨落区均明显偏北,且降水强度偏弱。通过对模式的风场及降水预报进行检验发现：模式天气形势预报的优劣,很多时候与模式的降水预报优劣是直接相关的,尤其是天气形势中的中低层风场,很容易受模式中降水潜热反馈过程的影响,导致错误的预报订正指引;比较而言,模式预报的高层流场,受潜热反馈过程影响较小,是值得在天气分析环节中加以重视的预报着眼点;此外,对于由中小尺度天气系统传播所致的大暴雨或特大暴雨,分辨率更高的区域模式预报结果可能具有更好的预报参考性,也是今后类似暴雨预报过程中应该重视的着眼点之一。

     

Fine-Scale Observations of Summertime Precipitation in an Intra-Alpine Region

Summary ?Intra-mountain summertime precipitation was studied in the Alps in a 40×20 km² area centered around Innsbruck, Austria, from June through September 1997. An observational network with a mean separation distance of 9 km and forecasts from the ECMWF model were used to examine the role the strong forcing from the lower boundary plays in creating “hot spots” for the formation of thunderstorms and the location of heavy precipitation as well as systematic precipitation patterns for different weather situations, which can be used to downscale forecasts from global scale routine numerical weather prediction models. Received March 16, 1999/Revised August 20, 1999     

Experiments on numerical modeling of intense convection

Results of numerical simulations using the WRF-ARW nonhydrostatic model are presented for eight episodes of intense convection over European Russia in the summer of 2007. The calculations were performed on four nested grids with horizontal grid meshes of 27, 9, 3, and 1 km. Convection was parametrized on the first two grids and explicitly resolved on the other two. It has been found that simulations on finer grids with explicit calculation of convective flows make it possible to reproduce heavy rainfalls and strong-wind zones in the areas of intense convection. A preliminary verification of the short-range predictions of convective systems shows that the maximum 12-h precipitation totals and the maximum winds at 10 m are close, in the order of magnitude, to the observed values. Prediction of convection centers is the weakest point. Difficulties in the model verification associated with the absence of data with high space-time resolution are discussed.     

风电场风速降尺度预报方法对比分析

使用中尺度数值天气预报业务模式9 km和3 km分辨率的模式输出产品,分别应用小尺度模式CALMET模式和双线性插值(BLI)方法将预报风速进行降尺度处理,并对比预报风速和风塔观测资料。结果表明：WRF模式9 km分辨率的模式输出经过CALMET模式降尺度以后得到的风速预报效果比3 km分辨率的模式输出略好。同时,由于中尺度数值预报模式分辨率本身较高,使用BLI也可以得到较好的风速预报。将风速分为0 m·s^-1≤风速<5 m·s^-1,5 m·s^-1≤风速<10 m·s^-1和风速≥10 m·s^-1共3个等级,检验3个风速等级的预报偏差百分比得出,CALMET模式和BLI方法对10 m·s^-1以上的大风的预报效果相对较差;如何对大风预报进行订正对风速预报准确率的提高具有重要的意义。     

鲁中地区分类强对流天气环境参量特征分析

将山东中部地区16 a暖季(4-9月)106次伴随瞬时风力不低于8级的强对流个例划分为雷暴大风、冰雹雷暴大风和强降水混合型等3种类型,利用常规探空资料和地面观测资料,通过箱须图的形式分别讨论3种类型对应的一系列关键环境参数的分布特征和预报阈值。进一步,又将上述106次个例中的特强对流个例,包括产生25 m/s以上瞬时大风的特强雷暴大风个例、产生不小于20 mm直径冰雹的特强冰雹个例以及50 mm/h或以上强度的特强短时强降水个例提取出来构成一个子集,讨论其关键环境参数分布特征和预报阈值,并与全部对流个例的相应关键环境参数进行比较。最后,对鲁中地区强对流系统的触发机制进行了简要阐述和讨论。结果表明:(1)雷暴大风型、冰雹雷暴大风型和强降水混合型对应的850和500 hPa温差的最低阈值为25℃; 3种类型对应的地面露点最低阈值分别为13、16和24℃; 相应的大气可降水量最低阈值分别为20、24和32 mm; 相应对流有效位能的最低阈值分别为300、900和1300 J/kg; 相应的0-6 km风垂直切变最低阈值分别为12.0、12.5和8.0 m/s。(2)通过地面露点、大气可降水量以及暖云层厚度等关键参数的分布特征可以将上述3种类型的前两种与第3种类型即强降水混合型进行一定程度的区分,但要通过各个关键参数的分布特征区分前两种强对流天气是困难的。(3)对于伴随冰雹的强对流天气,适宜的融化层高度为3.0-3.9 km; (4)特强雷暴大风、特强冰雹和特强短时强降水等3种特强对流类型与全部强对流个例的3种类型相比,其条件不稳定度明显增大,体现为850和500 hPa温差的增大、水汽条件有所加强、对流有效位能明显增大,3种类型特强对流天气对应的对流有效位能最低阈值分别为1000、1100和2000 J/kg; 相应的0-6 km风垂直切变最低阈值分别为16、12和11 m/s,即特强雷暴大风型和特强短时强降水型的风垂直切变阈值明显增大。上述工作构成了山东中部伴随雷暴大风的强对流天气短时预报的一个基础,结合各类强对流天气发生的气候概率,可以通过决策树或模糊逻辑方法制作成适合于地、市气象台的分类强对流天气短时预报系统。      

Modeling heavy precipitation in complex terrain

Summary A numerical prediction model is described which uses the full set of prognostic equations on a domain roughly the size of the United States with a 96 km horizontal grid resolution and six sigma-coordinate levels. Within this grid resides a nested domain of approximately 1000×1000 km with 24 km horizontal resolution. In this nested grid only modifications to the wind field by the better resolved terrain are considered on the lowest two sigma levels. The terrain effects necessitate adjustments in the location of these two sigma levels. Adjusted wind fields cause modifications in the mass and moisture divergence fields, hence in precipitation. These modifications are averaged into the appropriate meteorological fields on the larger grid.The algorithms used by our model allow continuous interaction between both grids with high computational efficiency.The relative importance of synoptic forcing and terrain is demonstrated for the cases of the Big Thompson, Colorado, flood of 1976 and the Cheyenne, Wyoming, flood of 1985.With 15 Figures     

Numerical Study of the Mesoscale Systems in the Spiral Rainband of 0509 Typhoon Matsa

The Advanced Weather Research and Forecasting Model (ARW) is used to simulate the local heavy rainstorm process caused by Typhoon Matsa over the northeastern coast of Zhejiang Province in 2005. The results show that the rainstorm was caused mainly by the secondary spiral rainband of the Stationary Band Complex (SBC) structure. Within the secondary spiral rainband there was a strong meso-β-scale convergence line generated in the boundary layer, corresponding very well to the Doppler radar echo band. The convergence line comprised several smaller convergence centers, and all of these convergence columns inclined outward. Along the convergence line there was precipitation greater than 20 mm occurring during the following one hour. During the heavy rainstorm process, the Doppler radar echo band, convergence line, and the precipitation amount during the following one hour, moved and evolved synchronously. Further study reveals that the vertical shear of radial wind and the low-level jet of tangential wind contributed to the genesis and development of the convergence columns. The combined effect of the ascending leg of the clockwise secondary circulation of radial wind and the favorable environment of the entrance region of the low-level jet of tangential wind further strengthened the convergence. The warm, moist inflow in the lower levels was brought in by the inflows of the clockwise secondary circulation and uplifted intensely at the effect of convergence. In the convectively instable environment, strong convection was triggered to produce the heavy rainstorm.     

Extended-Range Forecasts of the Principal 20–30-Day Oscillation of the Circulation over East Asia During the Summer of 2002

Daily 850-hPa meridional wind fields in East Asia from March to September 2002 were used to establish a model of the principal oscillation pattern (POP). This model was then used to conduct independent extended-range forecasts of the principal temporal and spatial variations in the low-frequency meridional wind field on a time scale of 20-30 days. These variations affect the occurrence of heavy precipitation events in the lower reaches of the Yangtze River valley (LYRV). The results of 135 forecast experiments during the summer half year show that the predicted and observed anomalies are strongly correlated at a lead time of 20 days (mean correlation greater than 0.50). This strong correlation indicates that the model is capable of accurately forecasting the low-frequency variations in meridional wind that corresponded to the 3 heavy precipitation events in the LYRV during the summer of 2002. Further forecast experiments based on data from multiple years with significant 20-30-day oscillations show that these prediction modes are effective tools for forecasting the space-time evolution of the low-frequency circulation. These findings offer potential for improving the accuracy of forecasts of heavy precipitation over the LYRV at lead times of 3-4 weeks.     

Analysis of a Heavy Rainfall Event over Beijing During 21-22 July2012 Based on High Resolution Model Analyses and Forecasts

The heaviest rainfall over 61 yr hit Beijing during 21-22 July 2012.Characterized by great rainfall amount and intensity,wide range,and high impact,this record-breaking heavy rainfall caused dozens of deaths and extensive damage.Despite favorable synoptic conditions,operational forecasts underestimated the precipitation amount and were late at predicting the rainfall start time.To gain a better understanding of the performance of mesoscale models,verification of high-resolution forecasts and analyses from the WRFbased BJ-RUCv2.0 model with a horizontal grid spacing of 3 km is carried out.The results show that water vapor is very rich and a quasi-linear precipitation system produces a rather concentrated rain area.Moreover,model forecasts are first verified statistically using equitable threat score and BIAS score.The BJ-RUCv2.0forecasts under-predict the rainfall with southwestward displacement error and time delay of the extreme precipitation.Further quantitative analysis based on the contiguous rain area method indicates that major errors for total precipitation( 5 mm h~(-1)) are due to inaccurate precipitation location and pattern,while forecast errors for heavy rainfall( 20 mm h~(-1)) mainly come from precipitation intensity.Finally,the possible causes for the poor model performance are discussed through diagnosing large-scale circulation and physical parameters(water vapor flux and instability conditions) of the BJ-RUCv2.0 model output.     

Analysis of a Heavy Rainfall Event over Beijing During 21-22 July 2012 Based on High Resolution Model Analyses and Forecasts

The heaviest rainfall over 61 yr hit Beijing during 21-22 July 2012.Characterized by great rainfall amount and intensity,wide range,and high impact,this record-breaking heavy rainfall caused dozens of deaths and extensive damage.Despite favorable synoptic conditions,operational forecasts underestimated the precipitation amount and were late at predicting the rainfall start time.To gain a better understanding of the performance of mesoscale models,verification of high-resolution forecasts and analyses from the WRFbased BJ-RUCv2.0 model with a horizontal grid spacing of 3 km is carried out.The results show that water vapor is very rich and a quasi-linear precipitation system produces a rather concentrated rain area.Moreover,model forecasts are first verified statistically using equitable threat score and BIAS score.The BJ-RUCv2.0forecasts under-predict the rainfall with southwestward displacement error and time delay of the extreme precipitation.Further quantitative analysis based on the contiguous rain area method indicates that major errors for total precipitation（〉 5 mm h~（-1）） are due to inaccurate precipitation location and pattern,while forecast errors for heavy rainfall（〉 20 mm h~（-1）） mainly come from precipitation intensity.Finally,the possible causes for the poor model performance are discussed through diagnosing large-scale circulation and physical parameters（water vapor flux and instability conditions） of the BJ-RUCv2.0 model output.     

MM5、WRF-RUC及T639模式对山东沿海风力预报分级检验

为客观评价不同的数值模式对山东沿海风的预报性能,结合中国气象局降水分级预报评分办法,定义了一种风力预报分级检验办法.对MM5、WRF-RUC和T639模式在山东沿海9个精细化海区代表站的日最大风速预报进行了检验,结果发现:各模式普遍存在对于小风天气预报偏大、大风天气预报偏小的特点.T639模式风力预报偏弱,因此,对于4级以下的风预报评分较高,而对于8级以上大风几乎没有预报能力.MM5和WRF-RUC模式对于4级以上的较强风力的预报结果明显好于T639模式,其中WRF-RUC模式预报准确率稍高于MM5模式,但风力越大,各模式均漏报越多.各模式分析场以及24 h风力预报与实况的一致性检验表明:5级以下的风力,MM5和WRF模式预报风力与实况基本为一致,但对于6级以上的大风,MM5模式预报较分散,WRF模式预报更接近实况风力.综合各模式对于风力预报的平均绝对误差,WRF-RUC模式预报误差最小,具有较高的参考价值.MM5模式预报准确率稍低于WRF-RUC模式,且存在一定的不稳定性.     

Lessons Learned from the Tragedy during the 100 km Ultramarathon Race in Baiyin,Gansu Province on 22 May 2021

Twenty-one runners died of hypothermia during the 100 km Ultramarathon Mountain race in Baiyin, Gansu Province on 22 May 2021. The hypothermia was caused by a combination of low temperatures, precipitation, and high winds associated with a typical large-scale cold front passing by the race site that morning. Based on historical hourly records of13 meteorological surface stations over the past six years, temperature(3.0°C) and apparent temperature(-5.1°C) at 1200 LST as well as gust wind speed(11.2 m s-1) at 1100 LST on the day of the tragedy were found to be within the top or bottom 5 th percentile for the month of May. The precipitation was only moderate at this time, but when temperature lower than 3.0°C,gust wind speed greater than 11.2 m s-1, and precipitation greater than 0.1 mm for any adjacent three hours were combined together, 1200 LST 22 May fell within the top 0.1% of cases. The European Centre for Medium-range Weather Forecasting model produced reasonably good forecasts of the low temperature and high wind one day and seven days before the event,respectfully. Based on this study, lessons that can be learned from this tragedy are summarized from an academic perspective: Hazard and impact forecasts of high-impact weather events should be developed to increase the value of weather forecasts. Probability forecasts should be issued by government weather agencies and communicated well to the public. And more importantly, knowledge of how to evaluate the impact of weather should be delivered to the public in the future.We would like to extend our deepest condolences to the families and loved ones of the people who lost their lives in this tragedy, including 21 runners and one officer. May our efforts honor those who lost their lives by highlighting the value of weather forecasting and calling for greater action in the future.     

A regional model for studies of atmosphere-ice-ocean interaction in the western Arctic

Summary Asa step in the development of a fully coupled regional model of the atmosphere-ice-ocean system, atmospheric and sea ice models have been adapted to a western Arctic domain centered on the Bering Strait. Lateral boundary conditions derived from operational analyses drive the models through simulations on grids having horizontal resolutions of 21 km and 7 km. Sensitivities to the presence of sea ice are large after only 48 hours, by which time the surface temperatures in the Bering and Chukchi Seas are 10–15°C higher without sea ice than with sea ice. The temperatures, in turn, modify the fields of sea level pressure, surface wind and precipitation. By influencing the surface wind stress through the static static stability, the surface state feeds back to the surface momentum exchange, ice/ocean transport, and the rate of formation of new ice. The results also show a resolution-dependence of the surface winds, precipitation rates and new ice formation rates, particularly in areas in which the coastal configuration and topography are spatially complex. The experiments will be augmented by the implementation of an ocean model on the same grids.With 12 Figures     

Object-based evaluation of precipitation forecast quality

The method of object-based diagnostic evaluation (MODE) is used to estimate the quality of forecasts of intensive precipitation zones using the WRF-ARW mesoscale nonhydrostatic model. The model is integrated on the horizontal 3-km grid without the cumulus parameterization. The prognostic fields of the reflectivity at the level of 700 hPa are compared with the observed radar reflectivity at the level of 3 km. Objects in the simulated and observed fields obtained by means of the convolution (smoothing) and thresholding (>20 dBZ) were matched according to the distance separating their centers of gravity (centroids). This distance was 50 km or lower in 12% of the total number of forecasts and 100 km or lower, in 30% of forecasts. It is revealed that the model overestimates the heavy precipitation areas.     

Analysis of quantitative precipitation forecasts using the Dynamic State Index

The German Weather Service (DWD) has two non-hydrostatic operational weather prediction models with different spatial resolution and precipitation parametrisations. The coarser COSMO-EU model has a spatial resolution of 7 km, whereas the higher-resolution COSMO-DE model has a gridspace of 2.8 km and explicitly resolves deep convection. To improve the numerical weather prediction (NWP) models it is necessary to understand precipitation processes. A central goal is the statistical evaluation of precipitation forecasts with dynamic parameters. Here, the Dynamic State Index (DSI) is used as a dynamic threshold parameter. The DSI theoretically describes the change of atmospheric flow fields as deviations from a stationary adiabatic solution of the primitive equations (Névir, 2004). For seasonal area means the DSI shows a remarkably high correlation with the precipitation forecasts provided by the COSMO-DE model. This is especially the case for the summer of 2007. The same analysis has been performed with the COSMO-EU forecast data and the results were compared with those from the COSMO-DE model. Moreover, an independent precipitation analysis, with a resolution corresponding to 7 km and 2.8 km, has been compared with respect to modelled precipitation and the DSI. In addition, correlations between the DSI and modelled as well as observed precipitation as a function of the forecast time for the different grid resolutions are also presented. The results show, that after 12 h, the correlation of the persistence forecast with the DSI reaches two thirds of the initial value. Thus, the DSI offers itself as a new dynamic forecast tool for precipitation events.