THE WINDFIELD AND RAINFALL DISTRIBUTION INDUCED WITHIN A SMALL VALLEY: FIELD OBSERVATIONS AND 2-D NUMERICAL MODELLING

The distribution of rainfall over small-scale topography of uniformroughness was studied both numerically and from field observations. Field data haveshown that small-scale topographic inhomogeneities (hills and valleys of afew tens of metres in height) substantially influence the rainfalldistribution over the inhomogeneity itself and also some distance downwind from it.According to comprehensive rainfall measurements carried out in a smallwatershed, windward slopes received less conventionally-measured rain than lee-sides.Moreover, on the windward slope rainfall decreased uphill, to apronounced minimum near the crest. In addition, directional raingages,situated at the bottom of both slopes revealed a pattern suggesting a reversedsurface flow on the lower slopes. This pattern persisted through a variety ofbackground conditions.To explain the observed data, a numerical simulation of both the windfieldand of drop trajectories was carried out. The windfield was generated insimulations using the Colorado State University RAMS model with a very highresolution (5 m in the horizontal). Drop trajectories were calculated usingan equation for the motion of drops in a flow field that incorporates currentexperimental results.A strong effect of small-scale topography on drop trajectories wasfound. The simulation results are encouraging and help to explain manycharacteristic features of precipitation distributions observed in fieldexperiments.     

0216号台风降水及其影响降水机制的数值模拟试验

文中用MM5模式对 0 2 16号台风 (森拉克 )降水进行了数值模拟 ,取得了较好的效果。模拟得到的台风降水中心强度和位置以及降水的分布与实况基本相符。文中还对影响热带气旋降水的机制进行数值模拟试验。结果表明 :冷空气入侵热带气旋外围可以大幅度增加热带气旋外围及倒槽的降水 ,但入侵到热带气旋中心附近的冷空气使热带气旋强度减弱 ,造成中心附近降水明显减少 ,但其外围与倒槽的降水明显加大 ;地形作用使迎风坡及降水中心增加雨量 ,背风坡雨量减少 ,从而使降水分布更不对称、更不均匀 ;大陆及其近海的湿度场对热带气旋降水有较大的影响。     

Boundary-layer effects on mountain waves: a new look at some historical studies

Early studies of mountain waves reported various results that have rarely been investigated since. These include: large-amplitude mountain waves above an unstable boundary layer much higher than the mountains; a repeated downwind drift and upwind jump of mountain waves; and larger vertical wind magnitude near sunrise and/or sunset. These are investigated using over 3,000 radiosondes and meso-strato-troposphere (MST) radar. Superadiabatic temperature gradients are found beneath mountain waves, explainable by convection which appears to raise the mountain-wave launching height. Movement of mountain-wave patterns is studied by a new method using height–time vertical wind data. A swaying motion of mountain waves, with period of a few minutes, appears to be equally upwind and downwind, rather than asymmetric at the heights measurable. Also, vertical wind shows no change in mean, variance or extreme values near sunrise and sunset, despite the expected diurnal changes of boundary-layer structure. An explanation for differences between MST radar and other measurements and models of mountain waves is suggested in terms of more than one variety of mountain wave. Type 1 has stable air near the ground; type 2 is above a convective/turbulent boundary layer of significant height as compared to the mountains.     

Spatial variability of rainfall: Variations within a single radar pixel

As a part of a Local Area Weather Radar (LAWR) calibration exercise 15 km south of Århus, Denmark, the variability in accumulated rainfall within a single radar pixel (500 by 500 m) was measured using nine high-resolution rain gauges. The measured values indicate up to a 100% variation between neighbouring rain gauges within the pixel over a 4-day period.     

Scaling of tropical rainfall as observed by TRMM precipitation radar

We used a three-year (1998–2000) dataset of TRMM Precipitation Radar observations to investigate the scaling properties of spatial rainfall fields. This dataset allows consideration of spatial scales ranging from about 4.3 km to 138 km and short temporal scales corresponding to the sensor overpasses. The focus is on the marginal spatial moment scaling, which allows estimation of the scaling parameters from a single scene of data. Here we present a global perspective of the scaling properties of tropical rainfall in terms of its spatial variability, atmospheric forcing, predictability, and applicability. Our results reveal the following: 1) the scaling parameters exhibit strong variability associated with land/ocean contrast and mean precipitation at the synoptic scale; 2) there exists a one-to-one relationship between the scaling parameters and the large-scale spatial average rain rate of a universal functional form; 3) the majority of the scenes are consistent with the hypothesis of scale invariance at the moment orders of 0 and 2; 4) relatively there are more scale-invariant rain scenes over land than over ocean; and 5) for the scenes that are non-scale-invariant, deviation from scale-invariance mainly arises from the increasingly intermittent behavior of rainfall as spatial scale decreases. These results have important implications for the development and calibration of downscaling procedures designed to reproduce rainfall properties at different spatial scales and lead to a better understanding of the nature of tropical rainfall at various spatial resolutions.     

近56 a武汉市降水气候变化特征分析

以武汉市1951—2006年逐日降水资料为基础,采用累积距平、线性趋势、移动T检验、5％分位数、小波分析等方法,分析了近56a来武汉市降水气候变化特征。结果表明：（1）近56年来,武汉市年降水量、降水强度呈增加趋势,而降水日数呈减少趋势;（2）除春季外,其它季节以及汛期、梅雨期、伏旱期等时段的降水量均有所增加;降水日数在春、秋季、汛期呈下降趋势,其余时段则为增加趋势;降水强度在夏季、伏早期呈减小趋势,其余时段均为增大趋势;（3）梅雨期、年的降水量变化较为一致,其周期性变化明显,主要表现为10a年代际周期,突变点约在1979年;（4）1960、1970年代暴雨日数较少,在1979年前后突变增多后,进人多暴雨阶段;（5）历年最大日降水量、5％分位数极端降水强度、暴雨平均强度变化略有减少趋势但不显著,而大暴雨平均强度减弱趋势明显。     

The spatial distribution of annual rainfall in Israel — a review

Summary This paper summarizes relevant studies dealing with the analysis of the annual rainfall map of Israel. With the aid of multiple regression analysis it was possible to quantify the contribution of each of the five geographical factors determining the annual rainfall map. Although the variance of the north-south component on the rainfall map looks most impressive, the orographic factor seems to be the more important contributor. The contribution of the spatially discrete urban factor has to be isolated from the deliberated cloud seeding augmentation. The statistical tools, spatial regression model and distance correlation matrix technique, were applied for the Central Coastal Plain and the Haifa area downwind, respectively. By applying sophisticated remote sensing methods, it was possible to classify and determine the rainfall yields of the variety of cloud formations appearing in the cold air mass following the cold front.With 8 Figures     

中国大陆降水日变化研究进展

文章概述了中国大陆降水日变化的最新研究成果,给出了中国大陆降水日变化的整体图像,指出目前数值模式模拟降水日变化的局限性,为及时了解和掌握降水日变化研究进展、开展相关科学研究和进行降水预报服务提供了有价值的科学依据和参考。现有研究表明：（1）中国大陆夏季降水日变化的区域特征明显。在夏季,东南和东北地区的降水日峰值主要集中在下午;西南地区多在午夜达到降水峰值;长江中上游地区的降水多出现在清晨;中东部地区清晨、午后双峰并存;青藏高原大部分地区是下午和午夜峰值并存。（2）降水日变化存在季节差异和季节内演变。冷季降水日峰值时刻的区域差异较暖季明显减小,在冷季南方大部分地区都表现为清晨峰值;中东部地区暖季降水日变化随季风雨带的南北进退表现出清晰的季节内演变,季风活跃（间断）期的日降水峰值多发生在清晨（下午）。（3）持续性降水和局地短时降水的云结构特性以及降水日峰值出现时间存在显著差异。持续性降水以层状云特性为主,地表降水和降水廓线的峰值大多位于午夜后至清晨;短时降水以对流降水为主,峰值时间则多出现在下午至午夜前。（4）降水日变化涉及不同尺度的山-谷风、海-陆风和大气环流的综合影响,涉及复杂的云雨形成和演变过程,对流层低层环流日变化对降水日变化的区域差异亦有重要影响。（5）目前数值模式对中国降水日变化的模拟能力有限,且模拟结果具有很强的模式依赖性,仅仅提高模式水平分辨率并不能总是达到改善模拟结果的目的,关键是要减少存在于降水相关的物理过程参数化方案中的不确定性问题。     

一次淮河流域梅雨锋暴雨的大别山地形敏感性试验

利用NCEP／NCAR提供的TRMM资料、FNL资料和再分析资料,应用WRFV3．2中尺度模式对2011年6月23—24日淮河流域梅雨锋暴雨进行了数值模拟,并通过一系列地形敏感性试验,详细分析了大别山地形对暴雨的影响。结果表明：1）大别山地形会强迫底层西南暖湿气流绕流和抬升,形成扰动并使其所含水汽和不稳定能量沿途释放,形成带状降水;2）大别山地形会使暖湿气流与偏北气流交汇形成带状分布的小槽,降水与气流辐合带方向一致;3）当地形增高时,降水中心位置变化不大,但对流更加剧烈;若使地形降低或消失,江苏淮河流域降水中心明显东移,说明大别山减弱了降水系统的东移,并使大值降水分布相对集中。     

NUMERICAL STUDY OF COLD AIR IMPACT ON RAINFALL REINFORCEMENT ASSOCIATED WITH TROPICAL CYCLONE TALIM (2005): I. IMPACT OF DIFFERENT COLD AIR INTENSITY

In 2005,significant rainfall reinforcement and severe disaster was induced by tropical cyclone(TC) Talim after it made landfall on the east of China.Observational analyses show that it has relationship with cold air intrusion.For investigating the impact of cold air intensity,we make use of Weather Research and Forecasting(WRF) model,the synthesizer of NCEP/NCAR reanalysis data and Japan regional spectral model data,to carry out numerical experiments.Results show that rainfall reinforcement occurs in all experiments.Different intensity of cold air can modify the rainfall distribution and intensity significantly.In the rainfall center,the increment maximum of rainfall is twice as large as that of the minimum.Moderate cold air intrusion may result in the strongest rainfall reinforcement.Different cold air intensity can lead to different motion of low-level convergence lines and fronts.There is a good relationship between the rainfall region and the eastern part of the front.On one hand,strong cold air weakens the TC intensity by its intrusion into the TC center and results in weak convergence and a convergent zone and a rain band shifted southward.On the other hand,weak cold air reduces the convergence and moves the convergent zone and rain band northward.Moderate cold air intrusion maintains strong low-level convergence and high-level divergence,keeping strong upward motion over certain regions.Consequently,the rain band begins to stagnate and rainfall reinforces abruptly therein.     

斯堪的纳维亚遥相关对我国西南西部深秋降水的影响

使用中国台站降水数据和NCEP/NCAR再分析资料,分析了1961~2012年深秋斯堪的纳维亚遥相类型(SCA)与同期我国西南西部降水的关系,以及SCA型环流异常影响西南西部降水的具体物理过程。结果表明:SCA型遥相关与西南西部降水存在显著的负相关关系,且具有非对称性:SCA正位相时西南西部降水偏少,而SCA负位相与西南西部降水的关系不显著。SCA正位相年西南西部的降水偏少与该地区对流层中下层的异常下沉运动有关。垂直运动方程诊断结果表明该异常下沉运动主要是由冷平流异常维持。气候态风场对温度异常场的平流在西南西部的冷平流异常中起主要作用。SCA正位相时,西南西部地区的东侧有暖异常,而其西侧的印度半岛北部及伊朗高原地区为冷异常,在平均西风作用下,西南西部有冷平流异常,造成该地区的异常下沉运动,从而降水偏少。     

不同校准方法检验雷达定量估测降水的效果对比

应用雷达低仰角基本反射率资料和地面加密自动站降水量资料,采用最优化方法,根据天津地区降水特点和不同降水类型,建立适用本地的雷达Z-I关系。经实际应用检验,积混降水类型Z-I关系具实用性。在天津本地化Z-I关系基础上,通过了对比分析6种不同校准方法在天津夏季降水估测中的检验效果。结果表明：Z-I关系校准法和最大集成法对降水的估测偏高,误差较大;最优插值法的估测精度最高,平均绝对误差和均方根误差最小;但计算不同校准方法与实况相关性表明,变分校准法的估测效果与雨量计降水量的相关性最好。同时,所有估测校准法对小雨量级的降水均出现了不同程度的偏高估测。     

利用热带降雨测量卫星的微波成像仪观测资料反演陆地降水

利用热带降雨测量卫星的微波成像仪资料，结合淮河流域试验加密观测期的阜阳地面天 气雷达雨量资料，建立了以散射指数和极化订正温度为主要参数的降水反演算法。对文 中所做反演试验与日本ＮＡＳＤＡ用微波成像仪和星载测雨雷达反演的雨强进行了比较。结果表明 ，文中所用的方法在反演陆地下垫面的降雨强度的分布和降雨区域的确定是比较成功的。     

用神经网络进行多波段卫星信息的降水估测

本文用BP神经网络对日本GMS-5的VIS(可见光)、IR1(红外1)、WV(水汽波段)、及红外1与红外2波段亮温差资料进行降水强度与降水落区的估测。经1998年夏季6～8月降水期间的1208个检验样本的检验，无雨、小到中雨和大到暴雨的三等级降水的估测准确率分别达到89．04％、70．34％、60．1％。     

Selection of regional historical rainfall time series as input to urban drainage simulations at ungauged locations

It has become established practice during the past 20 years to use high-resolution historical rainfall time series as input to hydrological model packages for detailed simulation of urban drainage systems. However, sufficiently long rain series are rarely available from the exact catchment in question and simulations are hence often based on available rain series from other locations. Extreme rainfall properties of importance to the performance of urban storm drainage systems vary significantly even in regions with only minor physiographic differences. Part of this variation can be explained by regional variations of the mean annual rainfall and the remaining statistical residue can be interpreted as statistical uncertainty.In Denmark, more than 75 high-resolution rain gauges are installed across a total area of 43,000 m. About 40 gauges had sufficiently long records to be included in a comprehensive national investigation where newly developed statistical regionalisation procedures were used to model the regional variation of extreme rainfalls. On this basis, a spreadsheet model was made available for estimation of extreme design rainfalls and the associated uncertainty at any location in the country. Statistics were furthermore computed to classify historical rainfall time series according to the developed regional model, and this makes it possible to assess the uncertainty related with using different historical rain series for simulations at ungauged locations.This research indicates that use of historical point rainfall data at ungauged locations introduces a significant uncertainty that is largely overlooked in today's practice. The engineering recommendation is to select historical rain series based on an evaluation of the local physiographic characteristics (e.g., the mean annual rainfall) and a (pre-defined) desired safety level of the simulations.     

一次沿川西高原东麓地带的强降雨过程分析

利用NCEP1°×1°再分析资料和地面加密自动站资料及卫星资料,对2012年8月16~18日盆地西北部沿龙门山脉的连续特大暴雨的形成机制进行探讨,此次暴雨过程出现在青藏高原东侧陡峭地形向盆地的过渡带,具有突出的地域特点。重点分析了青藏高原切变线东移期间,副高西北侧暴雨区内的对流触发机制和地形作用。分析表明:副热带高压前期的维持稳定与高原低值系统东移是产生强降雨的环流背景,在强降雨区域低层具有明显的风速风向辐合,东北—西南向的龙门山带即青藏高原东侧陡峭地形引起了盆地低层东南气流强烈的垂直上升运动。青藏高原东侧暴雨区最显著的热力特征是低层具有明显的高温高湿和大气不稳定层结。此次强降雨具有典型的“上干下湿,上冷下暖”的结构,正是强对流天气形成的有利条件。      

Pattern characteristics of Indian monsoon rainfall using principal component analysis (PCA)

In the present study the Principal Component Analysis (PCA) is used to determine the dominant rainfall patterns from rainfall records over India. Pattern characteristics of seasonal monsoon rainfall (June–September) over India for the period 1940 to 1990 are studied for 68 stations. The stations have been chosen on the basis of their correlation with all India seasonal rainfall after taking the ‘t’ Student distribution test (5% level). The PCA is carried out on the rainfall data to find out the nature of rainfall distribution and percentage of variance is estimated. The first principal component explains 55.50% of the variance and exhibits factor of one positive value throughout the Indian subcontinent. It is characterized by an area of large positive variation between 10°N and 20°N extending through west coast of India. These types of patterns mostly occur due to the monsoon depression in the head Bay of Bengal and mid-tropospheric low over west coast of India. The analysis identifies the spatial and temporal characteristics of possible physical significance. The first eight principal component patterns explain for 96.70% of the total variance.     

Rainfall yield characteristics of electrical storm observed in the Spanish Basque Country area during the period 1992–1996

This paper is focused on the study of rainfall yield characteristics of electrical storms observed over the Northern Iberian Peninsula during 1992–1996. To this aim Principal Components Analysis (PCA) and Self-Organizing Maps (SOM) method have been used. The SOM method is a group of artificial neural networks based on the topological properties of the human brain. Results clearly suggest that there exist three different meteorological patterns that are linked to the characteristics of electrical events found in the study area. In winter, most of the electrical events are formed under oceanic advection (NW air fluxes). On these cases, mean rainfall yield estimates reach values of 700 10⁴ m³ per cloud to ground lightning flash (CG flash). During summer most frequent electrical storms are associated to local instability shooting by surface heating with advection of humidity coming from the Iberian Peninsula. Under these meteorological situations, rain is scarcer if compared with oceanic events but lightning CG counts reach the maximum values found in the area (about 10 CG counts per 20 × 20 km² and day) giving this way the smallest rainfall yield with a mean value of 15 10⁴ m³ per CG flash. Iberian air fluxes associated with cold air in upper parts of the atmosphere represent the third meteorological pattern found. This pattern is most common in spring and autumn but is not unusual in the rest of the seasons. In those cases mean rainfall yield in the area is about 150 10⁴ m³ per CG flash. In all electrical episodes K instability index is greater than 15 °C but in the most lightning producing events, this index reaches in the area values greater than 24 °C. PCA results pointed out that there exists a relationship between rain and CG counts expressed by the first principal component computed from standardized data. However, we must notice that no event is solely linked to this axis, since a seasonal influence which decreases lightning production when rain increases is always present. Results found are of great interest for short term forecasting of flashfloods in mountainous areas like the Spanish Basque Country region.     

1215号“布拉万”台风暴雨及降水非对称性分布的成因分析

利用中尺度非静力模式WRF对2012年第15号台风“布拉万”在中国东北地区造成的暴雨过程进行了数值模拟,结合观测资料对模拟结果进行了验证,利用模式输出的高分辨率资料,对“布拉万”台风造成的强降水及其非对称性分布的成因进行了诊断分析。结果表明,模式很好地再现了台风登陆过程中的路径、强度变化和降水分布,受中纬度西风槽带来的干冷空气影响,“布拉万”台风登陆后的降水和环流结构具有明显的不对称性,降水主要集中在台风中心西北侧的能量锋区附近。水汽散度通量和水汽螺旋度能够较好地描述强降水过程的发生、发展及其非对称性分布的时空特征,在强降水区,水汽散度通量表现为正值强信号,而水汽螺旋度表现为负值强信号,在非降水区和弱降水区,两者均表现为弱信号。等熵位涡分析显示,对流不稳定只是此次台风暴雨前期和初始阶段的不稳定条件,而湿位涡(MPV)的湿斜压项(MPV₂)则是暴雨增强和出现非对称性分布的主要机制。在暴雨形成过程中,由于冷空气侵入造成了在台风环流西北侧湿等熵面的陡立倾斜和水平风垂直切变的增强,导致了气旋性涡度的显著增强,气旋性涡度增强造成的强烈上升运动将降水区东南侧输送过来的暖湿空气向上输送,从而导致了暴雨的发生,这其中条件性对称不稳定是降水得以加强的一种重要不稳定机制。     

Intensified reduction in summertime light rainfall over mountains compared with plains in Eastern China

Based on daily rainfall data from 1960 to 2007, this study investigated the difference in rainfall trends between seven mountain stations and 21 nearby plain stations in eastern China for the months June–August. The amount and frequency of light rain (≤2.5 mm/day) over the mountain areas showed a greater decreasing trend than over the surrounding plain regions. The trend of light-rainfall frequency at mountain stations is ??4.8%/decade, approximately double that at plain stations (??2.3%/decade). The trend in light-rainfall amount at mountain stations is ??5.0%/decade, approximately three times that at plains station (??1.4%/decade). Reduced wind speed may explain the enhanced decrease in light rainfall over mountain areas through the weakened orographic lifting. Further study is needed to determine whether the precipitation difference between mountain and plain (urban) regions is exacerbated by air pollution in East China through its indirect effects and influence on regional air stability and wind speed.