Rain event properties in nature and in rainfall simulation experiments: a comparative review with recommendations for increasingly systematic study and reporting

In hydrology and geomorphology, less attention has been paid to rain event properties such as duration, mean and peak rain rate than to rain properties such as drop size or kinetic energy. A literature review shows a lack of correspondence between natural and simulated rain events. For example, 26 studies that report event statistics from substantial records of natural rain reveal a mean rain rate of just 3·47 mm h^?1 (s.d. 2·38 mm h^?1). In 17 comparable studies dealing with extreme rain rates including events in cyclonic, tropical convective, and typhoon conditions, a mean maximum rain rate (either hourly or mean event rain rate) of 86·3 mm h^?1 (s.d. 57·7 mm h^?1) is demonstrated. However, 49 studies using rainfall simulation involve a mean maximum rain rate of 103·1 mm h^?1 (s.d. 81·3 mm h^?1), often sustained for > 1 h, exceeding even than of extreme rain events, and nearly 30 times the mean rain rate in ordinary, non‐exceptional, rain events. Thus rainfall simulation is often biased toward high rain rates, and many of the rates employed (in several instances exceeding 150 mm h^?1) appear to have limited relevance to ordinary field conditions. Generally, simulations should resemble natural rain events in each study region. Attention is also drawn to the raindrop arrival rate at the surface. In natural rain, this is known to vary from < 100 m^?2 s^?1 to > 5000 m^?2 s^?1. Arrival rate may need to be added to the list of parameters that must be reproduced realistically in rainfall simulation studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Probability distribution function of a forced passive tracer in the lower stratosphere

The probability distribution function （PDF） of a passive tracer, forced by a ＂mean gradient＂, is studied. First, we take two theoretical approaches, the Lagrangian and the conditional closure formalisms, to study the PDFs of such an externally forced passive tracer. Then, we carry out numerical simulations for an idealized random flow on a sphere and for European Center for Medium-Range Weather Forecasts （ECMWF） stratospheric winds to test whether the mean-gradient model can be applied to studying stratospheric tracer mixing in midlatitude surf zones, in which a weak and poleward zonal-mean gradient is maintained by tracer leakage through polar and tropical mixing barriers, and whether the PDFs of tracer fluctuations in midlatitudes are consistent with the theoretical predictions. The numerical simulations show that when diffusive dissipation is balanced by the mean-gradient forcing, the PDF in the random flow and the Southern-Hemisphere PDFs in ECMWF winds show time-invariant exponential tails, consistent with theoretical predictions. In the Northern Hemisphere, the PDFs exhibit non-Gaussian tails. However, the PDF tails are not consistent with theoretical expectations. The long-term behavior of the PDF tails of the forced tracer is compared to that of a decaying tracer. It is found that the PDF tails of the decaying tracer are time-dependent, and evolve toward flatter than exponential.     

基于MODIS产品的中国陆地冰云季节变化特征

利用2011年11月-2016年10月Terra卫星MODIS（moderate-resolution imaging spectroradiometer）3级大气产品数据（MOD08_M3）对中国陆地区域冰云发生概率、有效粒子半径、光学厚度和冰水路径的水平分布与季节变化进行分析。结果表明:冰云特性的水平分布和季节变化特征与东亚季风和强对流天气的发生存在一定联系。近5年冰云发生概率呈上升趋势,季节性变化规律明显,高值区出现在青藏高原东北部;冰云有效粒子水平分布呈现由西南向东北逐渐增加的趋势,总体季节性变化特点不明显,但在纬度较高地区出现随季节变化特征;冰云光学厚度与冰水路径水平分布和季节变化趋势大致相同,呈东南向西北递减趋势,总体季节性变化明显。     

Improving multi-model ensemble probabilistic prediction of Yangtze River valley summer rainfall

Seasonal prediction of summer rainfall over the Yangtze River valley(YRV) is valuable for agricultural and industrial production and freshwater resource management in China, but remains a major challenge. Earlier multi-model ensemble(MME) prediction schemes for summer rainfall over China focus on single-value prediction, which cannot provide the necessary uncertainty information, while commonly-used ensemble schemes for probability density function(PDF) prediction are not adapted to YRV summer rainfall prediction. In the present study, an MME PDF prediction scheme is proposed based on the ENSEMBLES hindcasts. It is similar to the earlier Bayesian ensemble prediction scheme, but with optimization of ensemble members and a revision of the variance modeling of the likelihood function. The optimized ensemble members are regressed YRV summer rainfall with factors selected from model outputs of synchronous 500-h Pa geopotential height as predictors. The revised variance modeling of the likelihood function is a simple linear regression with ensemble spread as the predictor. The cross-validation skill of 1960–2002 YRV summer rainfall prediction shows that the new scheme produces a skillful PDF prediction, and is much better-calibrated, sharper, and more accurate than the earlier Bayesian ensemble and raw ensemble.     

基于LLS的多回击地闪及其雷电流幅值分布特征

为进一步研究多回击地闪参数分布特征,以便为雷电防护工程设计和雷电物理研究提供参考,根据湖北省雷电定位系统(LLS)2007年1月-2018年12月监测资料,采用计算机编程处理和数理统计方法,对多回击地闪次数、多重回击次数和不同类型多回击地闪雷电流幅值等参数进行了统计分析.结果 表明:多回击正地闪、负地闪和总地闪次数占其...     

强降水和冰雹对流许可尺度集合预报个例试验

利用对流许可尺度集合预报系统,针对2015年4月28日夜间移动到江苏南部和上海地区,伴随短时强降水和冰雹的一次强对流过程,使用初始多源融合分析场对集合预报结果影响进行了分析。结合上海南汇双偏振雷达基数据观测,对12～14 h预报时效的反射率因子、差分反射率及冰雹集合预报结果进行了定性和定量的评估,分析了改进初始水物质分布,同时增加小尺度信息对于模式预报结果的影响。主要结果为:(1)对反射率因子预报的评估显示,初始场调整了水物质分布且增加了小尺度信息的试验(以下简称ADAS试验),对降水的范围、分布特征及评分都有明显改进(2)由于差分反射率在较小的距离内变化剧烈,对其准确预报难度较大,ADAS试验虽然预报强度偏强,但整体的位置和强度与实况更为接近,特别在大粒子预报方面具有更高的技巧,能够对微物理过程相关特征更好地进行描述;(3)使用地面人工观测和双偏振雷达观测对冰雹概率预报评估的结果显示,ADAS试验预报的高概率降雹区与观测落区接近,对冰雹落区预报具有一定的指示意义。通过多源融合分析调整初始水物质分布并增加小尺度信息的集合预报试验改善了较长预报时效的强降水和冰雹概率预报,具有更高的可信度,双偏振变量预报具有区分强降水与冰雹的优势,通过与观测的对比可以更好地评估模式对微物理过程描述的准确性。     

人工神经网络台风预报系统

系统在经大量数据训练、运算基础上，选用了ＢＰ网络模型。为适应台风路径预报，对ＢＰ网络进行了改进，应用改进后的ＢＰ网络，自动训练到２１５９次后，由神经网络输出的判定台风移向趋势－西进，北上，西北移与实际历史移行路径概括率达９７％。在业务实用时，只需在指定硬盘路径中给出系统运行的３个文件，包括天气系统主要特征参数，系统即可以人机对话形式给出当前台风移动路径概率值。     

基于贝叶斯方法的四川暴雨集合概率预报产品释用

在贝叶斯概率决策理论的基础上,探索了一种提高四川暴雨预报准确率的方法,该方法利用四川境内1951—2004年147站暴雨的气候概率对西南区域中尺度集合预报模式提供的≥50mm集合降水概率预报产品进行了修正。从2008年6—9月的连续性预报试验结果来看:基于贝叶斯方法修正后的集合概率预报产品在一定程度上消除了空报,尽管相比于区域中尺度集合预报系统直接提供的暴雨概率预报产品实际提高暴雨准确率的效果不明显,但在如何有效地利用数值集合预报产品提高四川暴雨预报的准确率以及如何为预报员提供更有价值的暴雨预警决策方法上值得进一步探索。     

1960-2005年长江流域降水极值概率分布特征

Based on the daily observational precipitation data of 147 stations in the Yangtze River basin for 1960-2005,and the projected daily data of 79 grids from ECHAM5/MPI-OM in the 20th century,time series of precipitation extremes which contain annual maximum(AM)and Munger index(MI)were constructed.The distribution feature of precipitation extremes was analyzed based on the two index series.Research results show that(1)the intensity and probability of extreme heavy precipitation are higher in the middle Mintuo River sub-catchment,the Dongting Lake area,the mid-lower main stream section of the Yangtze River,and the southeastern Poyang Lake sub-catchment;whereas,the intensity and probability of drought events are higher in the mid-lower Jinsha River sub-catchment and the Jialing River sub-catchment;(2)compared with observational data,the averaged value of AM is higher but the deviation coefficient is lower in projected data,and the center of precipitation extremes moves northwards;(3)in spite of certain differences in the spatial distributions of observed and projected precipitation extremes,by applying General Extreme Value(GEV)and Wakeby(WAK)models with the method of L-Moment Estimator(LME)to the precipitation extremes,it is proved that WAK can simulate the probability distribution of precipitation extremes calculated from both observed and projected data quite well.The WAK could be an important function for estimating the precipitation extreme events in the Yangtze River basin under future climatic scenarios.     

天气预报评分方法评述

本文按定性预报、百分率预报和形态预报3类预报形式，分别概述了常用的验证评分方法。提出了预报评分方法应遵循的7条准则，并以此为依据，针对强对流天气等小概率事件的预报，讨论了各种评分方法的适用性。最后，对各类预报形式的验证评分，提出了最佳的方案。