Northwest Pacific tropical cyclone activity and July rainfall over India

Summary In 2002, India had experienced one of the most severe droughts. The severe drought conditions were caused by the unprecedented deficient rainfall in July 2002, in which only 49% of the normal rainfall was received. One of the major circulation anomalies observed during July 2002, was the active monsoon trough over Northwest (NW) Pacific and enhanced typhoon activity over this region. The present study was designed to examine the long-term relationships between Tropical Cyclone (TC) activity over NW Pacific and monsoon rainfall over India in July. A statistically significant negative correlation between TC days over NW Pacific and July rainfall over India was observed. Spatial dependence of the relationship revealed that TCs forming over NW Pacific east of 150° E and moving northwards have an adverse effect on Indian monsoon rainfall. It was observed that TCs forming over the South China Sea and moving westwards may have a positive impact on monsoon rainfall over India. Enhanced TC activity over NW Pacific during July 2002 induced weaker monsoon circulation over the Indian region due to large-scale subsidence.     

Relationship over southern China between the summer rainfall induced by tropical cyclones and that by monsoon

本文分析华南夏季风降水(P_(SM))与热带气旋降水(P_(TC))在年际和年代际尺度上的物理联系,结果表明:在年际变化上,华南P_(SM)与P_(TC)呈显著负相关。南海-西北太平洋的气旋性涡度和相对湿度增加以及垂直风切变减弱,有利于更多的热带气旋生成,从而使得华南P_(TC)增加。同时异常增暖的赤道中太平洋SST和异常偏冷的北印度洋SST会激发南海-西太平洋异常气旋,加之中国东部-日本异常反气旋的作用,使得华南P_(SM)减少。在年代际尺度上,华南P_(SM)与P_(TC)呈显著正相关,在1990s初华南P_(SM)与P_(TC)明显增加。其中,南海生成的热带气旋对华南P_(TC)年代际增多有重要贡献。前期冬春季西太平洋持续异常偏暖的SST会通过垂直环流的作用引起热带印度洋SST增暖并持续到夏季,之后偏暖的热带印度洋SST又反馈作用于西北太平洋异常反气旋,使得华南P_(SM)增加。1990s初南海夏季风爆发年代际偏早,促使华南上空的大气显热源从5月持续增加至夏季,从而有助于东亚副热带夏季风的增强和华南P_(SM)增加。     

Water vapour transport from the tropical Atlantic and summer rainfall in tropical southern Africa

An empirical orthogonal functions analysis of the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP II Re-analyses suggests two dominant modes of variability that are linked to (a) variations in the circulation linked with the South Atlantic anticyclone (b) the intensity of the flow that penetrates from the tropical Atlantic. The second mode, referred as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subcontinent at tropical latitudes. Substantial correlations in austral summer between the Atlantic moisture flux in the tropics and rainfall over the upper lands surrounding the Congo basin suggest the potential role played by this zonal mode of water vapour transport. Composites for austral summer months when this Equatorial Westerly mode had a particularly strong expression, show an enhanced moisture input at tropical latitudes that feeds into the deep convection occurring over the Congo basin. Sustained meridional energy fluxes result in above normal rainfall east and south of the Congo belt. During years of reduced equatorial westerly moisture flux, a deficit of available humidity occurs in the southern tropics. A concomitant eastward shift of deep convection to the southwest Indian ocean and southeastern Africa, leads to below normal rainfall over the uplands surrounding the Congo basin.     

An index for tropical temperate troughs over southern Africa

Strong cases of the tropical temperate troughs (TTT) that are responsible for the most of the summer rainfall over subtropical southern Africa are analyzed. An index for identifying the TTT is introduced for the first time using anomalies of outgoing longwave radiation (OLR) and the wind. The TTT is associated with a ridge-trough-ridge wave-like structure in the lower troposphere over southern Africa and the adjoining Indian Ocean. Therefore, the index considers physical processes that occur over southern Africa, adjoining the Atlantic and Indian Oceans to depict the variability of the TTT events. Unusually strong TTT events are identified when the standard deviations of the TTT indices defined by the OLR and wind anomalies in the selected regions are above 1.5 and 0.5 respectively. After applying this criterion and filtering out consecutive events, 55 TTT events are identified during the study period of December–January–February seasons from 1980–1981 to 2009–2010. From the composite analyses of those 55 events, it is found that the TTTs evolve with suppressed (enhanced) convection over the southwest Indian Ocean adjacent to Madagascar (southern Africa). The suppressed convection is, in turn, found to be associated with the enhanced convection around Sumatra in the southeast tropical Indian Ocean. This may explain why more TTT events occur in La Niña years as compared to El Niño years. Time evolution of the canonical TTT event shows that it starts 3 days prior to the mature phase of the event, suggesting possible predictability. After reaching a matured state, the system moves east toward the Indian Ocean and decays within the subsequent couple of days. In addition, the intertropical convergence zone (ITCZ) structure changes over Southern Africa/Madagascar during the TTT event and remains similar to climatology over other regions. The results indicate that the continental part of the ITCZ intensifies prior to the TTT event and then spreads southward following the mid-latitude influence during and after the event.     

天山北坡一次特大暴雨过程诊断分析

利用常规观测、风云卫星、多普勒天气雷达、CMORPH卫星降水量融合资料和NCEP/NCAR（0.25°×0.25°）再分析资料,对2016年6月16-17日新疆西部一次罕见暴雨过程进行中尺度分析。结果表明：（1）该暴雨过程具有累计雨量大、暴雨强度强、局地日雨量破极值、短时强降水范围广等特点。暴雨区位于200 hPa高空西南急流出口区左侧、500 hPa偏南气流及700 hPa切变线附近。较强的CAPE和K指数对该暴雨有很好的指示意义。（2）该暴雨过程发生在低层辐合、高层辐散、低层较湿的有利背景下。强正涡度、强辐合和强上升运动不断将水汽和能量向上输送,为暴雨的产生提供有利的环境条件。（3）中亚地区中尺度雨团在发展演变过程中,逐渐形成西南-东北向带状多中心雨带,中心依次到达伊犁北部沿山地区,和原有的中尺度雨团共同作用,造成暴雨天气过程。中尺度对流云团不断产生于中亚地区,在东移过程中不断发展加强依次到达暴雨区,致使暴雨区不断产生短时强降水。（4）暴雨过程两个时段的中尺度对流系统存在明显差异,第一时段主要为孤立中尺度对流系统,造成伊宁博尔博松站成为暴雨中心并出现最强短时强降水的直接系统是风场特征明显的中γ尺度对流单体并在暴雨区维持少动。第二时段为CR达50 dBz、DVIL达4 g·m^-3,长度达70 km、宽度达10km且呈准南北态的线状中尺度对流系统,其在向东移动过程中造成多站依次出现短时强降水天气。

     

我国月降水频数的时空特征

本文利用谱分析、因子分析及因子转动等方法对我国1951—1980年期间46个站逐月的月雨日资料进行分析。 对各站每年月雨日序列的波谱分析表明,我国大部分地区年周期的(第1谐波)振动在与其余波动比较中占绝对优势,显著地区是华北、东北、西南及华南地区,振幅最大值出现在7—8月份。 对全国各站年雨日振幅的逐年变化资料进一步作主分量分析及因子分析,对其中前六个主要因子作极大方差转动,转动后的新因子对应不同的特征气候区域,它们除具有逐年增加的气候趋势外,还具有准两年及22年左右的周期振动特征。     

A universal spectrum for interannual variability of monsoon rainfall over India

Continuous periodogram analyses of 115 years (1871-1985) summer monsoon rainfall over the Indian region show that the power spectra follow the universal and unique inverse power law form of the statistical normal distribution with the percentage contribution to total variance representing the eddy probability corresponding to the normalized standard deviation equal to [(log L/log T50) – 1] where L is the period length in years and T50 the period up to which the cumulative percentage contribution to total variance is equal to 50. The above results are con-sistent with a recently developed non-deterministic cell dynamical model for atmospheric flows. The implications of the above result for prediction of interannual variability of rainfall is discussed.     

云贵高原两个国家基准站降水特性比较

为更好地理解和认识小尺度地形对降水特性的影响,利用位于云贵高原地区相近的两个国家基准站太华山和昆明站2006—2018年雨季(5—10月)小时降水资料,统计分析了两站降水精细化的时空特征.结果表明,两站的海拔高度差约500 m、站距约5 km,暖季降水量差异不大,但降水的精细特征却存在明显差异,主要表现为:(1)两站的降水量和平均降水强度年际差异不明显,但太华山站多数年份的降水频次远多于昆明站;(2)降水日变化上,太华山站在11—20时的累积降水量要高于昆明站;两站降水频次均具有双峰型特征,但在03—09时和11—17时太华山站的降水频次要明显高于昆明站,00—13时和21—23时昆明站的平均降水强度高于太华山站.(3)两站的降水事件特征不同,太华山站的降水事件次数和累积降水量都明显多于昆明站,主要由持续时间在6 h以上的降水事件贡献.(4)两站降水事件主要为共有降水事件,降水特性差异也主要由共有降水事件造成.太华山站先开始(结束)降水的共有降水事件次数比昆明站多(少),持续时间(降水频次)比昆明站长(多),短、长时降水事件的降水量(降水频次)比昆明站大(多),平均降水持续时间比昆明站多0.36 h.(5)两站单独降水事件占总降水事件的39.9％,太华山站的单独降水事件数是昆明站的1.83倍,而且平均持续时间长于昆明站.     

Intense rainfall events over the west coast of India

Summary The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day⁻¹ at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.     

The bi-decadal rainfall cycle,Southern Annular Mode and tropical cyclones over the Limpopo River Basin,southern Africa

The association between bi-decadal rainfall variability over southern Africa and the rainfall contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a potential means towards understanding decadal-scale variability over parts of the region. A multi-decadal period is considered, focusing on the anomalous tropospheric patterns that induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet sub-period was also characterized by a larger contribution to rainfall by tropical cyclones and depressions. The findings suggest that a broadening of the Hadley circulation underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered tropospheric steering flow, relative vorticity and moisture contents spatially during the sub-period of 8 years. These circulation modulations induced enhanced potential for tropical systems from the SWIO to cause precipitation over the Limpopo River Basin. The same patterns are also conducive to increasing rainfall over the larger subcontinent, therefore explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by tropical cyclonic systems from the SWIO. An overview of regional circulation anomlies during alternating near-decadal wet and dry epochs is given. The regional circulation anomalies are also explained in hemispheric context, specifically in relation to the Southern Annular Mode, towards understanding variation over other parts of the Southern Hemisphere at this time scale.     

New statistical models for long-range forecasting of southwest monsoon rainfall over India

The India Meteorological Department (IMD) has been issuing long-range forecasts (LRF) based on statistical methods for the southwest monsoon rainfall over India (ISMR) for more than 100 years. Many statistical and dynamical models including the operational models of IMD failed to predict the recent deficient monsoon years of 2002 and 2004. In this paper, we report the improved results of new experimental statistical models developed for LRF of southwest monsoon seasonal (June–September) rainfall. These models were developed to facilitate the IMD’s present two-stage operational forecast strategy. Models based on the ensemble multiple linear regression (EMR) and projection pursuit regression (PPR) techniques were developed to forecast the ISMR. These models used new methods of predictor selection and model development. After carrying out a detailed analysis of various global climate data sets; two predictor sets, each consisting of six predictors were selected. Our model performance was evaluated for the period from 1981 to 2004 by sliding the model training period with a window length of 23 years. The new models showed better performance in their hindcast, compared to the model based on climatology. The Heidke scores for the three category forecasts during the verification period by the first stage models based on EMR and PPR methods were 0.5 and 0.44, respectively, and those of June models were 0.63 and 0.38, respectively. Root mean square error of these models during the verification period (1981–2004) varied between 4.56 and 6.75% from long period average (LPA) as against 10.0% from the LPA of the model based on climatology alone. These models were able to provide correct forecasts of the recent two deficient monsoon rainfall events (2002 and 2004). The experimental forecasts for the 2005 southwest monsoon season based on these models were also found to be accurate.     

Interdecadal change in the relationship of southern China summer rainfall with tropical Indo-Pacific SST

The present study investigates the interdecadal change in the relationship between southern China (SC) summer rainfall and tropical Indo-Pacific sea surface temperature (SST). It is found that the pattern of tropical Indo-Pacific SST anomalies associated with SC summer rainfall variability tends to be opposite between the 1950–1960s and the 1980-1990s. Above-normal SC rainfall corresponds to warmer SST in the tropical southeastern Indian Ocean (SEIO) and cooler SST in the equatorial central Pacific (ECP) during the 1950–1960s but opposite SST anomalies in these regions during the 1980–1990s. A pronounced difference is also found in anomalous atmospheric circulation linking SEIO SST and SC rainfall between the two periods. In the 1950–1960s, two anomalous vertical circulations are present between ascent over SEIO and ascent over SC, with a common branch of descent over the South China Sea that is accompanied by an anomalous low-level anticyclone. In the 1980–1990s, however, a single anomalous vertical circulation directly connects ascent over SC to descent over SEIO. The change in the rainfall–SST relationship is likely related to a change in the magnitude of SEIO SST forcing and a change in the atmospheric response to the SST forcing due to different mean states. A larger SEIO SST forcing coupled with a stronger and more extensive western North Pacific subtropical high in recent decades induce circulation anomalies reaching higher latitudes, influencing SC directly. Present analysis shows that the SEIO and ECP SST anomalies can contribute to SC summer rainfall variability both independently and in concert. In comparison, there are more cases of concerted contributions due to the co-variability between the Indian and Pacific Ocean SSTs.     

On the relations of the rainfall variability and distribution with the mean rainfall over India

Summary Along with averages, rainfall variability and distribution are important climatological information. In this study, using 114 years (1871–1984) data of 306 stations, it is demonstrated that the variability and spatial distribution of annual, summer monsoon and monthly rainfall are highly dependent upon the respective period mean rainfall variation over India. The magnitude of three selected absolute measures of variability, e.g. standard deviation, absolute mean deviation and mean absolute interannual variability is found to increase linearly with mean rainfall.In order to describe the relation between the rainfall frequency distribution and the mean rainfall, a linear regression between the rainfall amount expected with a specified exceedance/non-exceedance probability and the mean rainfall amount is presented. Highly significant linear curves for a large number of probabilities specified in an average probability diagram clearly demonstrate the dependence of the rainfall frequency distribution on mean rainfall over India.With 8 Figures     

Statistical analysis of annual maximum rainfall in North-East India: an application of LH-moments

An attempt has been made to determine the best fitting distribution to describe the annual series of maximum daily rainfall data for the period 1966 to 2007 of nine distantly located stations in North East India. The LH-moments of order zero (L) to order four (L₄) are used to estimate the parameters of three extreme value distributions viz. generalized extreme value distribution (GEV), generalized logistic distribution (GLD), and generalized Pareto distribution (GPD). The performances of the distributions are assessed by evaluating the relative bias (RBIAS) and relative root mean square error (RRMSE) of quantile estimates through Monte Carlo simulations. Then, the boxplot is used to show the location of the median and the associated dispersion of the data. Finally, it can be revealed from the results of boxplots that zero level of LH-moments of the generalized Pareto distribution would be appropriate to the majority of the stations for describing the annual maximum rainfall series in North East India.     

Homogeneous zones of heavy rainfall of 1-day duration over India

Summary The highest recorded 24-hour rainfall totals from 1875 to 1982 for about 300 stations were plotted and isohyets drawn to delineate the homogeneous zones of heavy rainfall. The isohyetal pattern indicated an unsteady increase from less than 20 cm in the far west and far north to over 50 cm on or near the coasts. At a few inland stations outstanding amounts have been recorded but these are randomly distributed in space and time. Besides these, there existed a densely gauged area in the Central Peninsula between latitudes 8 °N to 21 °N within which more than 20 cm of rain in 24 hours have never been recorded. The areas of heavy rainfall of one day duration include the entire Indian region except the far western extremity, the northern area bordered by the Himalayas and the central peninsula. The correlation coefficient between the highest rainfall and elevation indicated no significant relationship.With 2 Figures     

Homogeneous clusters over India using probability density function of daily rainfall

The Indian landmass has been divided into homogeneous clusters by applying the cluster analysis to the probability density function of a century-long time series of daily summer monsoon (June through September) rainfall at 357 grids over India, each of approximately 100 km × 100 km. The analysis gives five clusters over Indian landmass; only cluster 5 happened to be the contiguous region and all other clusters are dispersed away which confirms the erratic behavior of daily rainfall over India. The area averaged seasonal rainfall over cluster 5 has a very strong relationship with Indian summer monsoon rainfall; also, the rainfall variability over this region is modulated by the most important mode of climate system, i.e., El Nino Southern Oscillation (ENSO). This cluster could be considered as the representative of the entire Indian landmass to examine monsoon variability. The two-sample Kolmogorov-Smirnov test supports that the cumulative distribution functions of daily rainfall over cluster 5 and India as a whole do not differ significantly. The clustering algorithm is also applied to two time epochs 1901–1975 and 1976–2010 to examine the possible changes in clusters in a recent warming period. The clusters are drastically different in two time periods. They are more dispersed in recent period implying the more erroneous distribution of daily rainfall in recent period.