基于多普勒雷达垂直风廓线产品的风暴相对螺旋度对山西暴雨的诊断

介绍了风暴相对螺旋度（SRH）的原理及基于多普勒天气雷达垂直风廓线（VWP）产品计算螺旋度的方法,探讨了不同时间分辨率对螺旋度应用效果的影响,最后重点对山西2个典型夏季暴雨个例分别做螺旋度诊断分析。结果表明：利用VWP产品计算的SRH具有较好的时间分辨率,可用于山西夏季暴雨的短时临近预报研究,相较于15min和30min,1h分辨率的螺旋度数据稳定、曲线平滑,更有利于直观分析和业务化;螺旋度强弱变化趋势与降水大小的变化趋势比较吻合,螺旋度变化一般提前于降水变化,有1—2h的提前预报量,可以作为短时（临近）预报降水开始、维持、结束的一个有效预报因子。     

应用雷达产品计算风暴相对螺旋度

风暴相对螺旋度（SRH）反映了一定气层厚度内环境风场的旋转程度和输入到对流体内环境涡度的多少，对雷暴、龙卷和大范围暴雨的分析与预报有一定的实用价值。首先探讨了由多普勒天气雷达提供的垂直风廓线（VWP）产品计算SRH的方法和步骤。根据此方法，分别计算、分析了暴雨、冰雹、大风三个天气个例的SRH。结果表明：SRH与大面积降水过程的暴雨雨强有很好的对应关系，降水强度的变化滞后SRH强度的变化约半小时左右，可以由SRH大致估计降水加强及消亡的时间；SRH对尺度非常小的冰雹、大风等强对流天气有提前10～20分钟的预报作用。应用VWP产品计算出的SRH，可以作为实际业务工作中暴雨、冰雹、大风等强对流天气的预报因子，给预测人员预报强对流天气提供宝贵时间。     

GRAPES_GFS模式全球降水预报的主要偏差特征

利用2017年1、4、7、10月“全球降水观测（global precipitation measurement,GPM）计划”每日08时（北京时）的24 h累计降水量和逐30 min降水量观测产品,从降水量和频率等角度,对同期GRAPES全球模式（GRAPES_GFS）第1（D1）、3（D3）、5天（D5）的全球降水预报性能和偏差特征进行细致评估与分析,且对低纬度暖池和北半球中纬度风暴路径区进行了重点观察,初步探讨了降水预报偏差特征在低纬度和中纬度明显不同的可能原因。结果显示:（1）GRAPES_GFS的D1—D5预报对全球日降水（量和频率）分布描述合理,能准确再现纬向平均降水（量和频率）的典型特征—降水“双峰”极大位于南北纬20°之间,次极大位于南北纬40°—50°地区的特征,以及关键区日降水时、空演变和降水日循环逐日演变的主要趋势特征。（2）低纬度的纬向平均湿日（≥0.1 mm/d）频率预报正偏差很小,但日降水量和强降水日（＞25 mm/d）频率预报的正偏差明显、偏差极大值“双峰”位置恰是相应观测极大值所在处（南北纬5°—10°）;中纬度的纬向平均日降水量预报基本无偏,但明显的湿日降水频率预报正偏差（20%—30%）和强降水日频率负偏差出现在南北纬40°—60°。降水偏差正、负分布特征随季节和预报时效基本保持不变,预报均方根误差数倍于平均误差,暗示模式降水预报偏差有系统性且性能表现波动较大。（3）日循环中,模式在暖池的降水量预报正偏差缘于降水强度预报偏强,降水频率预报的弱负偏差主要与降水落区预报偏小有关;而模式在北半球风暴路径区降水频率预报的正偏差则是降水落区预报偏大和空报弱降水事件两方面因素造成。（4）模式降水（量和频率）预报偏差特征在低纬度和中纬度的明显差异与模式次网格尺度和网格尺度降水比例失调有关,改进线索指向模式对流参数化方案中深对流的启动和深对流降水量的处理以及对流参数化方案与云微物理方案的协同问题。      

CMA-SH9在川渝地区的降水日变化预报效果评估

利用CMA-SH9模式逐小时降水预报数据和地面自动站-CMORPH卫星融合降水数据,开展该模式对2020年暖季（5~9月）川渝地区降水日变化的预报效果评估。结果表明:CMA-SH9模式可以再现小时平均降水量在四川盆地偏小、盆地周边陡峭地形处偏大的空间分布特征;显著的预报正偏差分布于青藏高原东坡至四川盆地西南部一带和四川盆地以东地区,偏差来自降水频率和降水强度的共同贡献;预报负偏差分布于四川盆地,主要来自模式对降水强度的低估;降水日变化峰值时间自西向东呈午夜到上午的滞后,模式预报的降水日变化峰值时间超前于观测;模式能够较好地把握青藏高原东坡至四川盆地西南部一带和四川盆地的单峰型日变化位相,以及盆地以东地区的双峰型日变化位相,但预报的降水量值和观测存在一定偏差。      

风暴相对螺旋度与强对流天气类型的关系分析

对风暴相对螺旋度（SRH）的定义进行了介绍。通过对2007—2008年石家庄地区27次强对流天气过程的统计分析,找出短时暴雨、冰雹和大风任意组合时高、低层SRH的不同特征,得到不同天气现象的预报指标,并利用2009年的几次强对流天气过程进行了验证。结果表明：出现短时暴雨时,SRH低经常连续较长时间为正值,但数值较小,SRH高也是以正值为主;伴有冰雹或者大风时,SRH低、SRH高的差值迅速增大,SRH高可达到100 m^2·s^-2或以上。     

广州逐日降水振荡及其延伸期预报的简谐波模型

利用1951—2010年广州逐日降水资料,采用功率谱、小波变换、Lanczos 滤波器和简谐波拟合等方法研究了广州逐日降水的振荡特征及其延伸期预报方法。结果表明,广州逐日降水振荡主要表现为2～4.1 d、8 d、13.3 d、40～60 d和120 d变化等5种时间尺度的准周期振荡,其中准单周变化、11～21 d、21～80 d的季节内振荡具有振幅和周期接近的1～2个振荡周期性循环出现的特征。利用小波变换揭示出近期逐日降水距平5 d滤波序列变化的主要周期,采用简谐波拟合的方法建立了广州逐日降水5 d滤波序列的延伸期预报模型。对1990—2009年近20年的后报结果分析表明,所建立的延伸期预报简谐波模型除第24 d和29 d外,前33 d的预报值与低通滤波值之间的相关达到0.05的显著性水平。对未来34 d降水低频分量极大值的预报与实况低通滤波极大值相差在0～2 d的概率最高,可为中期与延伸期降水过程出现时段的预报提供重要参考。      

基于时空不确定性的对流尺度集合预报效果评估检验

针对对流尺度天气系统的高度非线性特征和高分辨率模式预报结果存在时、空不确定性现象,以及当前邻域概率法主要考虑高分辨率预报结果的空间位移误差,而不能有效解决预报结果存在时间超前与滞后问题,将时间因素引入到邻域概率法中,结合一次强飑线过程进行对流尺度集合预报试验,并基于改进后的新型邻域概率法与分数技巧评分,对降水预报进行了不同时、空尺度的效果评估检验。结果表明:（1）邻域集合概率法和概率匹配平均法在极端降水的分数技巧评分远高于传统集合平均,弥补了集合平均对极端降水预报能力偏低的缺陷。（2）对于此类飑线过程的对流尺度天气系统而言,邻域半径为15—45 km的空间尺度能够改善降水位移误差的空间不确定性,并使其预报效果达到最优,其中15—30 km的邻域半径对于尺度更小的大量级降水事件预报能力更强。（3）对流尺度降水预报考虑时间尺度与降水强度存在着对应关系,不同时间尺度可以捕获到不同量级降水的时间不确定性。同时,时间尺度与空间尺度对于降水预报效果的影响是相互关联的。（4）改进的邻域概率法能够同时体现高分辨率模式预报结果在对流尺度降水事件上存在的时、空不确定性,实现了对流尺度降水在时、空尺度上的综合评估,并能为不同量级降水提供与其时、空尺度相匹配的概率预报结果。      

基于CFS预报产品的广东省季节降水统计降尺度预测

美国国家环境预报中心（NCEP）开发的气候预报系统（CFS）预报数据资料有1981—2008年共28年历史预报数据,有实时的预报产品（含有未来9个月的预报值）。与NCEP资料相比,CFS能较好模拟季风环流的季节变化,能超前几个月模拟出ENSO发展和衰减时期的海温异常发展,可以用于广东季节降水预测。采用CFS预报产品开发基于最优子集回归和多元均生函数的广东季节降水的两种统计降尺度预报方法。经过分析检验,分别选取海平面气压场、风场和位势高度场显著影响区域作为同期预报因子,从多年（2001—2008年）的历史回报检验来看,虽然两种预测模型对于个别季节存在年内预报效果不稳定性,但综合而言,大部分季节降水的气候预测评分总体平均在64分以上。2009/2010年的实时预报检验表明,两种预测模型均达到较好的预测水平,降水预测结果与实况较接近。与基于NCEP观测资料的传统统计方法比较,CFS预报产品具有实时性、更新快等优点。基于CFS预报产品的降尺度统计方法可以超前三个季预报广东降水,丰富了传统统计方法,但预报的稳定性还需进一步改进。      

长江上游暴雨短期集合预报系统试验与检验

基于PSU/NCAR的高分辨率MM5模式,采用多物理方案构建长江上游中尺度集合预报系统,于2004年8月16日—9月30日进行了预报试验。降水集合预报检验表明,在25mm以上级别的降水预报中,集合预报能改进单一模式的预报能力。对“9·3”暴雨过程的检验表明,降水集合预报平均对暴雨过程的开始、持续、结束时间均有预报指示意义,特别是大于50mm的降水概率分布区域和值的大小对预报大降水的范围有指导作用。     

微波辐射计观测数据在降水预报中的应用

利用兰州大学半干旱气候与环境观测站2007~2010年夏季地基12通道微波辐射计观测数据,对半干旱区降水前和非降水过程的水汽含量和云液态水含量的变化特征进行分析。结果表明:(1)水汽含量达到2.20 cm可以作为半干旱区降水预报的阈值。水汽含量随时间变化比较平缓,变率约为0.06 cm/h,且降水前24 h内水汽含量均2.20 cm;或者水汽含量开始较小,但降水前水汽含量随时间出现拐点,之后变率开始增加,可达0.19 cm/h以上,进而使水汽含量2.20 cm。这2种情况均可预报可能有降水产生;(2)云液态水含量达到0.20 mm可以作为降水预报的阈值,且降水前云液态水含量随时间变化出现拐点,之后变率开始增加,可以预报1 h后可能有降水。     

The climatic characteristics of extreme precipitations for short-term intervals in the watershed of Lake Maggiore

Alpine and Mediterranean areas are undergoing a profound change in the typology and distribution of rainfall. In particular, there has been an increase in consecutive non-rainy days, and an escalation of extreme rainy events. The climatic characteristic of extreme precipitations over short-term intervals is an object of study in the watershed of Lake Maggiore, the second largest freshwater basin in Italy (located in the north-west of the country) and an important resource for tourism, fishing and commercial flower growing. The historical extreme rainfall series with high-resolution from 5 to 45 min and above: 1, 2, 3, 6, 12 and 24 h collected at different gauges located at representative sites in the watershed of Lake Maggiore, have been computed to perform regional frequency analysis of annual maxima precipitation based on the L-moments approach, and to produce growth curves for different return-period rainfall events. Because of different rainfall-generating mechanisms in the watershed of Lake Maggiore such as elevation, no single parent distribution could be found for the entire study area. This paper concerns an investigation designed to give a first view of the temporal change and evolution of annual maxima precipitation, focusing particularly on both heavy and extreme events recorded at time intervals ranging from few minutes to 24 h and also to create and develop an extreme storm precipitation database, starting from historical sub-daily precipitation series distributed over the territory. There have been two-part changes in extreme rainfall events occurrence in the last 23 years from 1987 to 2009. Little change is observed in 720 min and 24-h precipitations, but the change seen in 5, 10, 15, 20, 30, 45, 60, 120, 180 and 360 min events is significant. In fact, during the 2000s, growth curves have flattened and annual maxima have decreased.     

DIURNAL CYCLES OF CONVECTIVE AND STRATIFORM PRECIPITATION OVER NORTH CHINA DURING SUMMER

The diurnal cycles of precipitation over north China during summer in four strong rainfall years are examined using two-dimensional cloud-resolving modeling data. The diurnal signals are analyzed in terms of precipitation budget, fractional rainfall coverage and rain intensity over convective and stratiform rainfall area. The analysis of precipitation budget shows that the diurnal cycles of convective and stratiform precipitation mainly correspond respectively to those of water vapor convergence and transport of hydrometeor from convective rainfall area to stratiform rainfall area in 1964, 1994 and 1995, whereas they mainly correspond to those of water vapor convergence in 2013. The diurnal cycles of convective and stratiform precipitation are mainly associated with those of rain intensity in 1964, 1994 and 1995. In 2013, the diurnal cycle of stratiform precipitation is mainly related to that of fractional rainfall coverage over stratiform rainfall area. The multiple peaks of convective precipitation mainly correspond to the rain intensity maxima associated with strong water vapor convergence.     

1967-2006年中国东南沿海盛夏降水强度变化特征分析

利用中国地面台站逐日和逐时降水资料,对中国东南沿海地区近40年(1967-2006年)盛夏(7-8月)降水强度变化特征进行了分析.逐日降水资料的分析结果表明我国东南沿海盛夏的降水量呈显著增加趋势,且主要是由日降水强度增强所致,日降水频次的贡献不显著.结合逐时降水资料的分析结果发现,东南沿海地区虽然降水日的平均降水时数显著增加,平均逐时降水强度也显著增强.通过按降水持续时数确定的降水事件分类分析发现,短持续降水(≤4h)平均小时强度显著增强,具体表现为弱小时强度降水减少和强降水增多.长持续性降水(≥15h)平均小时强度减弱,但降水频次增加.由于长持续性降水的平均小时降水强度远大于短时降水平均小时强度,对整体小时强度增强是正贡献.总之,我国东南沿海盛夏平均降水强度增强主要来自长持续性降水频次的增多、短时强降水频次的增多和短时弱降水频次的减少.     

Intrinsic properties of Sahel precipitation anomalies and rainfall

The estimation of the future projected precipitation and rainfall on short- and long-term basis is crucial because their substantial changes are closely associated with severe socioeconomic and ecological consequences. For this reason, the detrended fluctuation analysis is applied on the Sahel precipitation and standardized rainfall anomalies in order to explore the intrinsic properties of their temporal variability. The results obtained show that the Sahel precipitation anomalies the period 1900–2010 exhibit persistent long-range correlations for all the time lags between 4 months and 28 years. This result states that the fluctuations of the Sahel precipitation anomalies in small time intervals are positively correlated to those in longer time intervals in a power law fashion. In opposite, the Sahel standardized rainfall fluctuations during the periods 1948–2001 show an almost random walk behavior. It should be emphasized that these findings could substantially contribute to the precipitation forecast and the advanced simulation of the variability of the global climate system. For instance, the data of the precipitation forecast modeling must exhibit the long-range correlations dictated by the precipitation data in the past. A detailed analysis on this topic will be published elsewhere.     

Surface rainfall and cloud budgets associated with mei-yu torrential rainfall over eastern China during June 2011

Surface rainfall and cloud budgets associated with three heavy rainfall events that occurred over eastern China during the mei-yu season in June 2011 were analyzed using 2D cumulus ensemble model simulation data.Model domain mean rainfall showed three peaks in response to three prescribed ascending motion maxima,primarily through the mean moisture convergence during the torrential rainfall period.Prescribed ascending motion throughout the troposphere produced strong convective rainfall during the first （9 June） and third （17-18 June） rainfall events,whereas strong prescribed ascending motion in the mid and upper troposphere and weak subsidence near the surface generated equally important stratiform and convective rainfall during the second rainfall event （14 June）.The analysis of surface rainfall budgets reveals that convective rainfall was associated with atmospheric drying during the first event and moisture convergence during the third event.Both stratiform and convective rainfall responded primarily to moisture convergence during the second event.An analysis of grid data shows that the first and third mean rainfall maxima had smaller horizontal scales of the precipitation system than the second.     

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.     

一次强降水过程中尺度系统发生发展动力机制的诊断分析

利用1°×1°经纬度NCEP/NCAR再分析资料、地面1h观测降水资料和TRMM卫星产品资料,分析了2009年6月8~9日引发贵州南部的强降水天气过程的中尺度对流系统活动,并进一步研究了大气正压非平衡强迫、湿斜压热动力耦合强迫在强降水天气发生发展与维持过程中的作用。结果表明:8~9日贵州南部的强降水主要是由中尺度系统的发生发展引起的。南海季风不仅给贵州南部强降水区带来充足的水汽,同时也带来足够的能量。正压非平衡强迫在降水的开始阶段与降水落区有较好的对应关系,是强降水的启动机制。斜压热动力耦合强迫在降水的维持阶段,特别是当大气层结转为弱湿中性时,其分布和强度与中尺度对流系统和降水的强度与落区有较好的对应关系,可能是强降水的维持机制。      

城市地表特征对京津冀地区夏季降水的影响研究

本文利用京津冀地区24个气象站的日降水资料和耦合有单层城市冠层模式(SLUCM)的中尺度数值模式WRF的模拟结果,研究了城市地表特征对京津冀地区夏季降水的影响。结果表明,在京津冀城市面积迅速增长的近三十年(1981~2010),该地区大部分站点的降水量都呈现减少的趋势,减少最明显的站点主要集中在京津唐城市区域,其中≥50 mm的降水量减少趋势占总降水量减少趋势的50%以上。城市扩张可能是造成京津冀降水时空格局改变的因素之一。通过对比分析控制试验与敏感性试验的模拟结果,发现城市化引起的地表特征的改变使北京、天津、唐山主要城市地区的降水量和降水频次都有明显减少,而城市群下风向的降水量和降水强度则明显增加和增强,其中50 mm以上等级的降水量变化最为显著,贡献率在60%以上。城市地表特征使北京、天津和唐山地区50 mm以上等级降水量的百分比下降了6%~20%,下风向地区增加了8%。城市地表特征也影响了主要城市和城市群下风向地区降水量的日变化结构,使北京和唐山几乎所有时段的降水量都有所减少,而城市群下风向降水量的增加主要发生在白天。研究发现城市地表特征对深对流的抑制(加强)可能是造成京津冀地区降水减少(增多)的重要原因,而由于城市地表蒸发量的改变引起的潜热通量和对流有效位能的改变则可能是引起深对流变化的重要因素。