Monthly and annual statistical distributions of daily rainfall at the Fabra Observatory (Barcelona, NE Spain) for the years 1917–1999

Summary Three empirical distributions of the daily rainfall collected at the Fabra Observatory from 1917 to 1999 are fitted to different statistical models. The first two are designated as the distributions of cumulative amounts and cumulative times. The third distribution accounts for the time interval between two consecutive rainy days with rain amounts equalling or exceeding a threshold amount. Whereas the distribution of cumulative amounts follows an exponential model at monthly and annual scale, except for a few cases, the distribution of the cumulative times is well modelled by a Weibull function, whether monthly or annual scales are considered. The distribution of time intervals also follows a Weibull distribution for the different thresholds considered. In addition, the combination of the two first distributions leads to the normalised rainfall curve, NRC, which is also reproduced satisfactorily by a beta (type 1) distribution. It is worth mentioning that the NRCs follow the expected behaviour with respect to the coefficient of variation of daily rain amounts at monthly and annual scales. In addition, a better understanding of fluctuations and time trends affecting the daily pluviometric regime is achieved by analysing the annual NRCs. The impact of some features of this rain regime, developed for Barcelona, a crowded metropolitan area, on many human activities, may provides the focus of future interdisciplinary analyses.     

MJO对我国东部春季降水影响的分析

利用澳大利亚气象局的MJO(Madden-Julian Oscillation)指数,通过位相合成及对比分析研究了MJO对我国东部春季降水的影响.研究表明,当MJO传播至中东印度洋时,我国长江中下游地区的春季降水为正异常,当其进一步东传至中南半岛-印尼群岛一带时,我国华南地区的春季降水为正异常,而在其他活动阶段不利于我国东部的春季降水.对比分析表明,MJO的活动主要通过引起大尺度环流异常、对流层中低层涡度及水汽输送的异常,进而对我国东部春季降水产生明显的影响.     

Large-scale Circulation Anomalies Associated with Interannual Variation in Monthly Rainfall over South China from May to August简

Interannual variation in summer rainfall over South China （SC） was investigated on the monthly timescale.It was found that monthly rainfall from May to August exhibits different features of variation,and the amounts are basically independent of each other.There is a significant negative correlation,however,between May and July SC rainfall,which is partially related to the developing phases of ENSO events.It was also found that stronger （weaker） lower-tropospheric winds over SC and the upstream parts are responsible for more （less） SC rainfall in every month from May to August.Despite this monthly consistent enhancement of horizontal winds,the wind anomalies exhibit distinct differences between May-June and July-August,due to the remarkable change in climatological winds between these two periods.More SC rainfall is associated with a lower-tropospheric anticyclonic anomaly over the SCS and the Philippine Sea in May and June,but with a cyclonic anomaly centered over SC in July and August.     

THE IMPACTS OF MADDEN-JULIAN OSCILLATION ON SPRING RAINFALL IN EAST CHINA

Phase composite analyses are conducted to investigate the possible effect of the Madden–Julian oscillation(MJO)on the spring rainfall anomalies in East China by using the Real-time Multivariate MJO(RMM)index from Australian Meteorological Bureau.The results show that the rainfall anomalies over the mid-and lower-valley of Yangtze River are positive when the MJO shifts eastward to the mid-and eastern-Indian Ocean,and anomalous precipitation over South China are positive when the MJO moves further eastward to the maritime continent,whereas spring rainfall anomalies over East China are negative in the other MJO episodes.The MJO impacts on the precipitation over East China result from the changes in large-scale atmospheric circulation as well as vorticity and water vapor transportation in the mid-and lower-troposphere.     

Some Statistical Characteristics of Monthly and Annual Pluviometric Irregularity for the Spanish Mediterranean Coast

Summary  Seven series of monthly pluviometric amounts, sometimes exceeding recording periods of 100 years and compiled by the Instituto Nacional de Meteorología (Spain), are used to study the irregularity of the pluviometric regime along the Spanish Mediterranean coast and nearby Atlantic coast. First of all, three statistical functions (gamma, log-normal and a combination of Poisson and gamma distributions) and moment-ratio diagrams are used to model the monthly and annual empirical distributions of precipitation amounts, each distribution being tested by means of the Kolmogorov-Smirnov test. It is noteworthy that, whereas most of the monthly cases require the gamma distribution, the pluviometric behaviour of the summer months is well described by the Poisson-gamma distribution. Moreover, both the log-normal and the gamma distributions satisfactorily model empirical annual amounts. Consequently, rainfall amounts are not identically distributed along a year for each gauge tested. Second, temporal trends deduced for annual and seasonal amounts are computed and their statistical significance evaluated. The most notable fact is that, although some linear trends are close to 1 mm/year, their significance levels exceed the assumed threshold value and, excepting the winter season for Barcelona, they are considered non-significant from a statistical point of view. Finally, by again using monthly and annual amounts, three temporal irregularity indexes are computed for each pluviometric series, the temporal disparity of the rainfall patterns of the Mediterranean region being enhanced as a result. It addition to the temporal irregularity, a change with latitude is observed both in the parameters of the statistical distributions and the temporal irregularity indexes for the rain gauges analysed. The two most southerly rain gauges constitute a special case in comparison with the remaining stations, because they also receive the Atlantic influences due to their proximity to this ocean. Received February 25, 1999/Revised August 2, 1999     

初夏东亚-太平洋大气热源与长江流域及邻近地区7、8月降水异常的关系

用1958～2000年NCEP/NCAR再分析资料、中国160站降水量及1958～1998年月平均海温资料分析了中国夏季相邻月份降水异常型的相关特征，及其与大气热源的关系和相关物理过程。结果表明，7月长江流域的降水异常与8月长江和黄河之间地区的降水异常有很好的同号性。7、8月长江流域及附近地区持续性偏旱（涝）与太平洋洋盆尺度的大气热源异常有关，并与前期5、6月热带中、东太平洋大范围的热源异常、青藏高原热源异常也有密切的联系，即当5、 6月赤道东太平洋的大气热源正异常，而赤道中太平洋北侧的热源负异常，则中国7月长江中下游偏涝，8月长江中上游与江淮流域和内蒙古东部偏涝，华南偏旱；反之亦然。前期热带中、东太平洋上空的热源异常中心和与之联系的异常垂直运动中心的西扩和西移，以及青藏高原东部的热源异常中心是影响我国7、8月持续偏旱（涝）的重要环流异常特征。另外，南海－西太平洋海温在前期也已经具有我国夏季长江流域发生旱涝对应的同期海温异常分布型的特征。     

Principal Component Analysis of precipitation and rainfall regionalization in Spain

Summary A T-mode Principal Component Analysis (PCA) based on a network including 68 pluviometric gauges and their 12 mean monthly amounts of rainfall is attempted in order to describe the main patterns governing precipitation in Spain. The procedure is applied to a 12 × 12 intermonth covariance matrix; the unrotated components and two additional solutions deduced after varimax and oblimin rotations are presented and discussed. In all cases component scores are computed and their spatial distribution is discussed. Two regionalizations of Spanish rainfall are then obtained and compared in terms of group homogeneity. There follows a discussion concerning the main pluviometric characteristics of each region deduced from the best division.With 7 Figures     

盛夏四川盆地西部地区降水年际变化及其对应的环流异常

利用7、8月全国756站站点降水资料、NCEP/NCAR再分析资料的月平均资料,研究了盛夏四川盆地西部地区降水年际变化的基本特征及其对应的环流异常,并分析了该地区与江南地区降水年际变化的关系及其对应的700 hPa环流异常。结果表明：四川盆地西部地区盛夏降水偏多年,四川盆地上空为显著的南风异常,说明西南涡较常年活跃,同时西太平洋副热带高压偏北;而降水偏少时,上述环流异常符号相反,但更加明显,说明西南涡明显弱于常年,西太平洋副热带高压偏南。此外,四川盆地西部盛夏降水年际变化和我国西北、华北地区降水呈明显的正相关关系,而与江南地区降水有明显的负相关关系。这些关系与对流层低层环流异常有关。     

Some distinct effects of tropical monsoon clouds as derived from atmospherics

Summary The electrical effects due to monsoon clouds in conjunction with the VLF atmospherics data have been extensively analyzed. The cloud distribution and rainfall pattern during the SW monsoon period are examined. The diurnal curves of rainfall show that the maximum rain generally occurs in the afternoon hours between 13 to 18 IST. The coefficient of variation (CV) of monsoon rainfall plotted against rainfall amount reveals that CV decreases with increasing rainfall amounts upto about 40 inches. The differences in the mean dry bulb temperature as well as mean relative humidity values at the standard levels between strong and weak monsoon are studied. The monthly median of the hourly average together with the respective upper and lower decile values of atmospherics have been considered. Also, the frequency dependence of afternoon maximum (or late afternoon minimum) to morning minimum in the sferics level is taken into account to determine the seasonal variation. During monsoon months the sferics level with higher cloud amount (4 okta) increases considerably but the width of the sferics is reduced. The results are interpreted by considering the activity of the sources involved at such times.With 10 Figures     

亚洲西风急流纬向非均匀性变化成因及其对东亚夏季气候的影响

利用美国NOAA（National Oceanic and Atmospheric Administration）的CMAP（Climate Prediction Center (CPC) Merged Analysis of Precipitation）月平均降水资料、NCEP/DOE（National Centers for Environmental Prediction/Design of Experiments）II的月平均再分析资料和中国气象局国家信息中心提供的中国160站逐月降水和平均气温资料,通过定义一个亚洲急流纬向非均匀性指数（I_Aja）,分析了1979～2019年夏季亚洲西风急流纬向非均匀性的年际变化特征,揭示了夏季亚洲急流纬向非均匀性变化异常的成因及其对东亚夏季降水和气温的影响。结果表明:夏季亚洲西风急流纬向非均匀性具有显著的年际变化特征,并存在6～8年和2年左右的振荡周期。当急流纬向非均匀性典型偏强（弱）年,东亚东部地区从低纬到高纬,降水异常主要呈现出偏多—偏少—偏多（偏少—偏多—偏少）的经向分布;气温则在中国西部地区和日本北部偏高（低）,贝加尔湖地区偏低（高）。引起夏季亚洲急流纬向非均匀性异常的可能原因如下:由大气非绝热加热异常而引起的热带和中纬度地区辐合/辐散运动造成的涡度源强迫,和来自西风带中波扰动能量的注入,两者共同作用形成并维持了与急流纬向非均匀性强弱变化相联系的异常环流,从而使亚洲急流东、西段强度差异增强（减弱）,进而有利于急流纬向非均匀性异常偏强（偏弱）。而上述西风带中波扰动能量的东传可能与北大西洋海表面温度异常有关。这对于深刻理解夏季亚洲急流纬向非均匀性异常的形成机理提供了有用的线索。     

CONVECTIVE ANOMALIES IN TROPICAL OCEAN AREAS AND LONG-LEAD FORECAST OF SUMMER RAINFALL IN SHANDONG

This study focuses on deep convection anomalies in tropical regions in winter-spring period and their possible influence on the following summer rainfall in Shandong province. On the basis of monthly precipitation wet and dry summers in Shandong are defined according to a precipitation index. Then monthly OLR data, observed by NOAA satellites, are used to diagnose the features of deep convection for both wet and dry summers. It is found that negative anomalies seem dominant prior to wet summers, while large areas of positive anomalies appear prior to dry summers in tropical oceans. The differences are remarkable especially in the western, middle and eastern tropical Pacific as well as in the tropical Indian Ocean. Correlative analysis confirms the relations between OLR and precipitation. Subtropical High, which plays an essential role in summer rainfall, is also connected with the deep conviction. Altogether eight EOF-CCA forecast models are established on the basis of the above study. The assessment of the models relies on the gauge observing precipitation in 1997 and 1998. The results show that models using spring OLR data appear to be more practicable than those using winter OLR data, and the models established with OLR in western Pacific and the Indian Ocean perform better than the others.     

动力延伸预报产品释用方法的改进试验

依据动力延伸预报产品释用方法中所建立的月降水距平百分率预报方程，将方程系数看成是动态系数，采用与预报月高度场相似月份的资料作为样本资料，通过改善方程系数的计算来提高月降水预报。1998～2004年的历史实况资料在安徽省的回报实验证明:在目前T63／NCC月动力延伸预报产品的准确率不能快速提高的情况下，该方法具有实际应用价值。利用T63／NCC月动力延伸产品进行安徽省月降水预报应用亦取得较好的效果。     

El Nino事件对其衰减阶段夏季中国降水季节内演变的影响及其机理

利用1979—2007年NOAA重建海温逐月资料和中国160站夏季降水资料,使用扩展奇异值分解（extended singular value decomposition,ESVD）方法,研究了冬季热带太平洋海温异常与次年夏季中国降水异常季节内演变型之间的关系,指出前冬El Nino事件是与次年夏季中国降水季节内变化相联系的最重要的热带太平洋海温异常模态。相应的降水异常季节内变化情况为：6月在长江以南为正异常,江淮流域有负异常;7月在华南沿海有负降水异常,而正异常北进到长江流域,华北地区也出现正降水异常;8月在长江南北分别为少雨和多雨。进一步研究前冬El Nino事件与次年春夏印度洋、太平洋海温异常、对流层低层风场异常以及副热带高压等的联系,结果表明：El Nio事件发生的次年春夏,热带西太平洋周边存在东负西正的海温异常分布;西太平洋反气旋异常较强;副高在6月、7月偏西偏北,但在8月迅速南退。虽然与El Nino事件相联系的6月与7月、8月的降水型不同,但是西太平洋反气旋异常带来的充沛水汽造成7月长江流域雨季多雨,8月副高迅速南退带来的又一次长江流域降水,造成了El Nino事件发生次年夏季长江流域涝而华南沿海旱的夏季平均降水异常型。     

热带环流异常与我国夏季降水分布的关系

本文利用经验正交函数（EOF）分解方法，给出我国夏季雨带分型的定量指标，它们分别代表三种典型雨型。文章分析了夏季不同雨型年的热带环流特征，讨论了热带风场与前三个特征向量所对应的时间系数之间的相关场分布。结果表明:夏季南方类雨型与赤道中、东太平洋地区850 hPa（200 hPa）纬向风成正（负）相关；北方类雨型与澳大利亚附近高低空纬向风密切相关；中间类雨型则对印度洋地区的环流异常响应最为敏感。     

华北地区的降水特征及趋势估计

华北地区位于干旱和半干旱地区。气候降水是该区水资源的主要来源之一,也是影响该区水资源周期性变化的主要因素之一。华北及其北、中、南三个分区的年降水距平曲线变化趋势３具有相似性,而且此四个地区连续出现正距平的年数不超过４年,华北及其北、中部连续出现负距平的年数不超过５年,南部不超过６年,华北、黄淮和东北地区东部与印度次大陆大地区夏季降水距平之间存在正相关关系,同时又与澳洲大部分地区冬季降水距平有负相关     

东北5月降水在21世纪初的年代际变化及可能成因

基于1961—2015年东北地区的台站降水观测资料及全球环流和海温再分析资料,利用统计分析、物理量诊断等方法,探讨了东北5月降水的年代际变化,及其与大气环流和海温外强迫的关系。研究发现,东北5月降水具有和东北盛夏降水明显不同的年代际变化特征,在20世纪80年代中期至90年代初处于年代际偏少阶段,而在21世纪初转变为年代际偏多阶段。东北5月降水在21世纪初的年代际变化主要由5月东北亚低压强弱的年代际变化造成,在21世纪初,东北亚低压相对于气候态明显偏强,有利于东北降水偏多;而在20世纪80年代初至90年代初,东北亚低压减弱为较浅的低槽,导致东北降水偏少。来自北大西洋的欧亚大陆位势高度异常波列引起东北亚上空的垂直运动异常,导致了东北亚低压的上述年代际变化。5月热带北大西洋海温异常很可能是激发上述波列进而造成东北亚低压和东北5月降水在21世纪初年代际变化的外强迫信号。     

Effects of the southern oscillation on the probability for climatic categories of monthly rainfall, in a semi-arid region in the southern mid-latitudes

In many regions of the world, planning agricultural and water management activities is usually done based on probabilities for monthly rainfall, taking on values on specified intervals of values. These intervals of monthly rainfall amounts are commonly grouped into three categories: drought, normal rainfall, and abundant rainfall. Changes in the probabilities for occurrence of monthly rainfall amounts within these climatic rainfall categories will influence the decisions farmers and water managers will take (for example, crops to cultivate, flood preparedness, and operations of water reservoirs). This research explores the changes produced by the SO (Southern Oscillation) on the probability that the areal average of monthly rainfall (AAvMR) takes on values belonging to specified climatic rainfall categories. The semi-arid region under study is a major agricultural region in central Argentina; weather effects on agriculture in this region influence the world market of several crops. The evolution of the Southern Oscillation was divided into three phases: LSOI (low Southern Oscillation index phase, that includes ENSO events), NSOI (neutral SOI phase), and HSOI (high SOI phase that includes La Niña–SO events). The following are the criteria defining the three phases of the SO: (1) low SOI (ENSO), where the five-month moving average of the SO index, SOI, is less than −0.5 standard deviation during at least five consecutive months, and is equal to or less than −1 standard deviation during at least one month; (2) high SOI (La Niña–SO), where the SOI is greater than 0.5 standard deviation during at least five consecutive months, and is equal to or greater than 1 standard deviation during at least one month; and (3) neutral SOI (transition between extremes), where the SOI does not correspond to low SOI nor to high SOI. It was found that the Southern Oscillation influences the probability distribution of monthly rainfall only in four months of the year. Findings show that monthly rainfall has a complex response to the evolution of the SO. The response is not restricted to higher probability for occurrence of abundant rainfall or drought categories during low SOI (ENSO) or high SOI (La Niña–SO) episodes, respectively. The LSOI (ENSO) phase influences the AAvMR in several ways: depending on the month, it increases or decreases the probability of the abundant rainfall category. LSOI (ENSO) also increases or decreases, depending on the month, the probability of the normal rainfall category. It also decreases the probability that AAvMR takes on values in the drought category. A similar kind of complex response of monthly rainfall amounts occurs when the active phase is the HSOI (La Niña–SO). The responses are: (1) the probability of the category `drought' increases only in three months of the year, (2) increase or decrease of the probability of the normal rainfall category, depending on the month, and (3) decrease of the probability of the abundant rainfall category. Finally, the effects of NSOI (neutral phase of the SO) are not negligible. Depending on the month, NSOI episodes increase or decrease the probability of drought, or abundant rainfall, or normal rainfall categories.     

A Regression-based Assessment of the Predictability of New Zealand Climate Anomalies

Summary Estimates of the predictability of New Zealand monthly and seasonal temperature and rainfall anomalies are calculated using a cross-validated linear regression procedure. Predictors are indices of the large scale circulation, sea-surface temperatures, the Southern Oscillation Index and persistence. Statistical significance is estimated through a series of Monte Carlo trials. No significant forecast relationships are found for rainfall anomalies at either the monthly or seasonal time scale. Temperature forecasts are however considered to exhibit significant skill, with variance reductions of the order of 10–20% in independent trials. Temperature anomalies are most skilfully predicted over the North Island, and skill is greatest in Spring and Summer in most areas. At the monthly time scale, predictors local to the New Zealand region account for most of the forecast skill, while at the seasonal time scale, skill depends strongly upon “remote” predictors defined over regions of the southern hemisphere distant from New Zealand. Indices of meridional flow over the Tasman Sea/New Zealand region are found to be useful predictors, especially for monthly forecasts, perhaps as a proxy for atmospherically-forced sea surface temperature anomalies. Sea surface temperature anomalies to the west of New Zealand and in the tropical Indian Ocean are also useful, especially for seasonal predictions. Forecast skill is more reliably estimated at the monthly time scale than at the seasonal time scale, as a result of the larger sample size of monthly mean data. While long-term mean levels of skill may be estimated reliably over the whole data set, statistically significant decadal-scale variations are found in the predictability of temperature anomalies. Therefore, even if long-term forecast skill levels are reliably estimated, it may be impossible to predict the short-term skill of operational seasonal climate forecasts. Implications for operational climate predictions in mid-latitudes are discussed. Received July 18, 1997 Revised April 2, 1998     

冬季各月北太平洋涛动的年际变化及其与我国降水异常的联系

利用ERA-Interim的海平面气压(Sea Level Pressure,SLP)再分析资料和中国160站的降水观测资料,分析了冬季各月(当年12月、次年1月和2月)北太平洋涛动(North Pacific Oscillation,NPO)的年际变化特征,及其与我国同期降水异常之间的联系。结果表明:1)冬季各月NPO指数的年际变化较为显著,但各月NPO指数年际变化之间的相关性较差,1979—2012年冬季12月与1月NPO指数年际变化之间的相关系数为0.09,而1月与2月NPO指数的相关系数仅为-0.003,均没有通过信度检验。2)1月和12月NPO指数年际变化与同期我国黄淮流域降水异常之间存在明显的正相关,而2月NPO指数年际变化与同期我国华北降水异常之间为明显的负相关。3)当1月(12月)NPO指数增加1个标准差时,我国黄淮流域降水量比多年平均值增加约50%(40%);而当2月NPO指数增加1个标准差时,我国华北降水量比多年平均值减少约30%。     

Simulation of rainfall anomalies leading to the 2005 drought in Amazonia using the CLARIS LPB regional climate models

The meteorological characteristics of the drought of 2005 in Amazonia, one of the most severe in the last 100 years were assessed using a suite of seven regional models obtained from the CLARIS LPB project. The models were forced with the ERA-Interim reanalyses as boundary conditions. We used a combination of rainfall and temperature observations and the low-level circulation and evaporation fields from the reanalyses to determine the climatic and meteorological characteristics of this particular drought. The models reproduce in some degree the observed annual cycle of precipitation and the geographical distribution of negative rainfall anomalies during the summer months of 2005. With respect to the evolution of rainfall during 2004–2006, some of the models were able to simulate the negative rainfall departures during early summer of 2005 (December 2004 to February 2005). The interannual variability of rainfall anomalies for both austral summer and fall over northern and southern Amazonia show a large spread among models, with some of them capable of reproducing the 2005 observed negative rainfall departures (four out of seven models in southern Amazonia during DJF). In comparison, all models simulated the observed southern Amazonia negative rainfall and positive air temperature anomalies during the El Nino-related drought in 1998. The spatial structure of the simulated rainfall and temperature anomalies in DJF and MAM 2005 shows biases that are different among models. While some models simulated the observed negative rainfall anomalies over parts of western and southern Amazonia during DJF, others simulated positive rainfall departures over central Amazonia. The simulated circulation patterns indicate a weaker northeasterly flow from the tropical North Atlantic into Amazonia, and reduced flows from southern Amazonia into the La Plata basin in DJF, which is consistent with observations. In general, we can say that in some degree the regional models are able to capture the response to the forcing from the tropical Atlantic during the drought of 2005 in Amazonia. Moreover, extreme climatic conditions in response to anomalous low-level circulation features are also well captured, since the boundary conditions come from reanalysis and the models are largely constrained by the information provided at the boundaries. The analysis of the 2005 drought suggests that when the forcing leading to extreme anomalous conditions is associated with both local and non-local mechanisms (soil moisture feedbacks and remote SST anomalies, respectively) the models are not fully capable of representing these feedbacks and hence, the associated anomalies. The reason may be a deficient reproduction of the land–atmosphere interactions.