Spatial trends of dry spells over Peninsular Malaysia during monsoon seasons

This study aims to trace changes in the dry spells over Peninsular Malaysia based on the daily rainfall data from 36 selected rainfall stations which include four subregions, namely northwest, west, southwest, and east for the periods of 1975 to 2004. Six dry spell indices comprising of the main characteristics of dry spells, the persistency of dry events, and the frequency of the short and long duration of dry spells will be used to identify whether or not these indices have increased or decreased over Peninsular Malaysia during the monsoon seasons. The findings of this study indicate that the northwestern areas of the Peninsular could be considered as the driest area since almost all the indices of dry spells over these areas are higher than in the other regions during the northeast (NE) monsoon. Based on the individual and the field significant trends, the results of the Mann–Kendall test indicate that as the total number of dry days, the maximum duration, the mean, and the persistency of dry days are decreased, the trend of the frequency of long dry spells of at least 4 days is also found to decrease in almost all the stations over the Peninsula; however, an increasing trend is observed in the frequency of short spells in these stations during the NE monsoon season. On the other hand, during the southwest monsoon, a positive trend is observed in the characteristics of dry spells including the persistency of two dry days in many stations over the Peninsula. The frequency of longer dry periods exhibits a decreasing trend in most stations over the western areas during both monsoon seasons for the periods of 1975 to 2004.     

Spatial and temporal characteristics of wet spells in the Yangtze River Basin from 1961 to 2003

Daily precipitation records of 147 meteorological stations over the Yangtze River Basin have permitted a detailed analysis of the spatio-temporal distribution of wet spells during the period 1961–2003 by distinguishing average daily amount thresholds of 90th and 95th percentiles. The analysis are based on several time series, namely the number of the days in wet spells, the longest wet spell and the precipitation amount in wet spells. Time series trends analyses are compiled for each station by means of the Mann-Kendall test, for four sub-regions. The results show that the annual precipitation in wet spells is higher in the southeast area and the eastern Tibetan Plateau than in the other parts. The longest wet spells are found in the eastern Tibetan Plateau for both the thresholds. The indices in wet spells for most stations have no significant trends. In contrast, only some stations in eastern Tibetan Plateau and the lower Yangtze River Basin increase significantly, while some in the middle reaches show significant decreasing trends. The regional trends analysis presents a noticeable downward trend in the middle Yangtze River Basin and upward trends in the eastern Tibetan Plateau for both 90th and 95th percentiles, however, the upward trend in the lower Yangtze River Basin and downward trends in the upper Jinshajiang River Basin are not significant.     

Power-law Relationship in Describing Temporal and Spatial Precipitation Pattern in Turkey

Summary Wet and dry spell properties of monthly rainfall series at five meteorology stations in Turkey are examined by plotting successive wet and dry month duration versus their number of occurrences on the double-logarithmic paper. Straight line relationships on such graphs show that power-laws govern the pattern of successive persistent wet and dry monthly spells. Functional power law relationships between the number of dry and wet spells for a given monthly period are derived from the available monthly precipitation data. The probability statements for wet and dry period spells are obtained from the power law expressions. Comparison of power-law behaviours at five distinct sites in Turkey provides useful interpretation about the temporal and spatial rainfall pattern. As in temperate areas such as Turkey the rainfall amounts change mostly due to one-month-long dry or wet spells. Received August 29, 1995 Revised November 9, 1995     

东北地区降水日数、强度和持续时间的年代际变化

利用93站1951—2002年逐日降水资料，分析了东北地区不同强度降水事件的时空演变特征及其对旱涝的影响。结果表明:52年来，东北地区小雨事件对年降水量的贡献率呈显著增加趋势，中雨的贡献率略为减少，大雨和暴雨的贡献率变化不大；东北年总雨日减少趋势非常明显，雨日的减少主要体现在小雨日数的减少；年降水强度表现为明显的增强趋势，主要体现为小雨和暴雨强度增强；20世纪80年代中期之前多小雨事件，80年代中期之后多中雨以上强度的降水，特别是90年代中期之后多暴雨事件。在显著变暖的20世纪90年代，降水日数明显减少，但暴雨日数基本不变，强度明显增强。对东北地区降水量、降水变幅、降水事件的变化特征分析表明:该区域降水有向不均衡、极端化发展的趋势，旱涝灾害也有加重趋势。     

1961—2014年我国西南地区干湿季变化特征

利用中国1961—2014年逐日降水观测等资料,分析了西南地区的干湿季变化特征。结果表明：西南地区东部和西北部最早进入湿季;干季由四川盆地、贵州南部开始。西南中东部以及南部等地的湿季长度较长,干季则与之相反。干湿季开始日期以及干湿季长度均具有明显的年代际变化特征,在1970年代中期到1980年代发生了气候突变,呈现湿季长度变短,干季变长的趋势。湿季降水呈现东南多、西北少的特征,并表现出中东部减少,西部增加的趋势;干季降水则表现为东多西少的特点,在东部呈增加,在四川等地呈减少趋势。进一步分析表明：湿季异常偏湿(干)年,开始日期易偏早(晚),结束易偏晚(早),长度偏长(短);干季开始异常偏早(晚）年,干季长度长(短),干季略偏湿(干);太平洋、印度洋海温异常影响东亚大气环流的异常是造成西南地区干湿季出现异常的主要原因。     

Trends of summer dry spells in China during the late twentieth century

Summary In the present study the trends in frequency and duration of dry spells in six sub-regions of China were analyzed for the summer-half-year season (April–September) in period 1956–2000. A dry spell was defined as a number of consecutive days without measurable precipitation. For the frequency of short dry spells (length <10 days), significant changes are observed in the North, Northeast and Southwest China. For the frequency of long dry spells (length 10 days) there are significant trends in North and Northeast China; while no remarkable trends in frequency are found in other regions. There are also significant lengthening trends in dry spell duration in North and Northeast China, resulting mainly from the long-term changes in short dry spells. No significant change is observed for the maximum length in all regions. It is found that the temporal distribution of precipitation within the rainy season would notably impact the features of dry spells. An increase in the precipitation amount does not necessarily mean a synchronous reduction in dry spell frequency and/or duration. Seasonal mean anomalies of 500hPa heights in association with the long dry spells show similar spatial patterns over the middle to high latitudes for five of the six sub-regions (with exception of the case of Southwest China), resembling a west–east direction dipole in latitudes about 30°N northwards. For the case of Southwest China the dominant feature in 500hPa heights is the negative anomalies over most middle to high latitude Asia. Among these cases there are recognizable differences, particularly, in the tropical regions in western Pacific. That would provide useful information of circulation background for understanding the climate extremes.     

Changes in precipitation extremes over arid to semiarid and subhumid Punjab,Pakistan

Asymmetrical monsoons during the recent past have resulted into spatially variable and devastating floods in South Asia. Analysis of historic precipitation extremes record may help in formulating mitigation strategies at local level. Eleven indices of precipitation extremes were evaluated using RClimDex and daily time series data for analysis period of 1981–2010 from five representative cities across Punjab province of Pakistan. The indices include consecutive dry days, consecutive wet days, number of days above daily average precipitation, number of days with precipitation ≥10 mm, number of days with precipitation ≥20 mm, very wet days, extremely wet days, simple daily intensity index, maximum 1-day precipitation quantity, maximum 5 consecutive day precipitation quantity, and annual total wet-day precipitation. Mann-Kendall test and Sen’s slope extremes were used to detect trends in indices. Droughts and excessive precipitation were dictated by elevation from mean sea level with prolonged dry spells in southern Punjab and vice versa confirming spatial trends for precipitation extremes. However, no temporal trend was observed for any of the indices. Summer in the region is the wettest season depicting contribution of monsoons during June through August toward devastating floods in the region.     

Recent changes in precipitation extremes in the Heihe River basin,Northwest China

Changes in rainfall extremes pose a serious and additional threat to water resources planning and management, natural and artificial oasis stability, and sustainable development in the fragile ecosystems of arid inland river basins. In this study, the trend and temporal variation of extreme precipitation are analyzed using daily precipitation datasets at 11 stations over the arid inland Heihe River basin in Northwest China from 1960 to 2011. Eight indices of extreme precipitation are studied. The results show statistically significant and large-magnitude increasing and decreasing trends for most indices, primarily in the Qilian Mountains and eastern Hexi Corridor. More frequent and intense rainfall extremes have occurred in the southern part of the desert area than in the northern portion. In general, the temporal variation in precipitation extremes has changed throughout the basin. Wet day precipitation and heavy precipitation days show statistically significant linear increasing trends and step changes in the Qilian Mountains and Hexi Corridor. Consecutive dry days have decreased obviously in the region in most years after approximately the late 1980s, but meanwhile very long dry spells have increased, especially in the Hexi Corridor. The probability density function indicates that very long wet spells have increased in the Qilian Mountains. The East Asian summer monsoon index and western Pacific subtropical high intensity index possess strong and significant negative and positive correlations with rainfall extremes, respectively. Changes in land surface characteristics and the increase in water vapor in the wet season have also contributed to the changes in precipitation extremes over the river basin.     

Changes in daily climate extremes in the arid area of northwestern China

There has been a paucity of information on trends in daily climate and climate extremes, especially for the arid region. We analyzed the changes in the indices of climate extremes, on the basis of daily maximum and minimum air temperature and precipitation at 59 meteorological stations in the arid region of northwest China over the period 1960–2003. Twelve indices of extreme temperature and six indices of extreme precipitation are examined. Temperature extremes show a warming trend with a large proportion of stations having statistically significant trends for all temperature indices. The regional occurrence of extreme cool days and nights has decreased by ?0.93 and ?2.36 days/decade, respectively. Over the same period, the occurrence of extreme warm days and nights has increased by 1.25 and 2.10 days/decade, respectively. The number of frost days and ice days shows a statistically significant decrease at the rate of ?3.24 and ?2.75 days/decade, respectively. The extreme temperature indices also show the increasing trend, with larger values for the index describing variations in the lowest minimum temperature. The trends of Min Tmin (Tmax) and Max Tmin (Tmax) are 0.85 (0.61) and 0.32 (0.17)?°C/decade. Most precipitation indices exhibit increasing trends across the region. On average, regional maximum 1-day precipitation, annual total wet-day precipitation, and number of heavy precipitation days and very wet days show insignificant increases. Insignificant decreasing trends are also found for consecutive dry days. The rank-sum statistic value of most temperature indices exhibits consistent or statistically significant trends across the region. The regional medians after 1986 of Min Tmin (Tmax), Max Tmin (Tmax), warm days (nights), and warm spell duration indicator show statistically more larger than medians before 1986, but the frost days, ice days, cool days (nights), and diurnal temperature range reversed. The medians of precipitation indices show insignificant change except for consecutive dry days before and after 1986.     

Spatiotemporal dry and wet spell duration distributions in southwestern Saudi Arabia

Summary Regional dry and wet spell durations within a 15 year period are mapped for the southwestern part of Saudi Arabia for summer and winter. If the number of spells is plotted versus duration a straight-line results on semi-logarithmic paper for most stations. However, a few stations exhibit two or three different slopes showing the effect of local climate. Simple regression equations are proposed for the dry or wet spell duration at any station allowing the mapping of regional variation for the entire study area. Such maps are useful tools in the assessment of the spatial and/or temporal wet and dry period duration for the area between stations. Similar maps could be developed for other parts of the world.With 10 Figures     

Statistical modeling of CMIP5 projected changes in extreme wet spells over China in the late 21st century

The observed intensity, frequency, and duration (IFD) of summer wet spells, defined here as extreme events with one or more consecutive days in which daily precipitation exceeds a given threshold (the 95th percentile), and their future changes in RCP4.5 and RCP8.5 in the late 21st century over China, are investigated by using the wet spell model (WSM) and by extending the point process approach to extreme value analysis. Wet spell intensity is modeled by a conditional generalized Pareto distribution, frequency by a Poisson distribution, and duration by a geometric distribution, respectively. The WSM is able to realistically model summer extreme rainfall spells during 1961–2005, as verified with observations at 553 stations throughout China. To minimize the impact of systematic biases over China in the global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5), five best GCMs are selected based on their performance to reproduce observed wet spell IFD and average precipitation during the historical period. Furthermore, a quantile–quantile scaling correction procedure is proposed and applied to produce ensemble projections of wet spell IFD and corresponding probability distributions. The results show that in the late 21st century, most of China will experience more extreme rainfall and less low-intensity rainfall. The intensity and frequency of wet spells are projected to increase considerably, while the duration of wet spells will increase but to a much less extent. The IFD changes in RCP8.5 are in general much larger than those in RCP4.5.     

Spatial and temporal analysis of dry spells in Croatia

Systematic statistical analysis of dry day sequences, which are defined according to 0.1, 1, 5 and 10 mm of precipitation-per-day thresholds, is performed on seasonal and yearly basis. The data analysed come from 25 Croatian meteorological stations and cover the period 1961–2000. Climatological features of the mean and maximum dry spell durations, as well as the frequency of long dry spells (>20 days) are discussed. The results affirm the three main climatological regions in Croatia, with the highlands exhibiting shorter dry spells than the mainland, and the coastal region exhibiting longer dry spells. The prevailing positive trend of both mean and maximal durations is detected during winter and spring seasons, while negative trend dominate in autumn for all thresholds. Positive field significant trends of mean dry spell duration with 5 and 10 mm thresholds are found during spring and the same is valid for annual maximum dry spell duration with a 10 mm threshold. It is found that the Discrete Autoregressive Moving Average (DARMA(1,1)) model can be used to estimate the probabilities of dry spells in Croatia that are up to 20–30 days long.     

On regional dynamical downscaling for the assessment and projection of temperature and precipitation extremes across Tasmania, Australia

The ability of an ensemble of six GCMs, downscaled to a 0.1° lat/lon grid using the Conformal Cubic Atmospheric Model over Tasmania, Australia, to simulate observed extreme temperature and precipitation climatologies and statewide trends is assessed for 1961–2009 using a suite of extreme indices. The downscaled simulations have high skill in reproducing extreme temperatures, with the majority of models reproducing the statewide averaged sign and magnitude of recent observed trends of increasing warm days and warm nights and decreasing frost days. The warm spell duration index is however underestimated, while variance is generally overrepresented in the extreme temperature range across most regions. The simulations show a lower level of skill in modelling the amplitude of the extreme precipitation indices such as very wet days, but simulate the observed spatial patterns and variability. In general, simulations of dry extreme precipitation indices are underestimated in dryer areas and wet extremes indices are underestimated in wetter areas. Using two SRES emissions scenarios, the simulations indicate a significant increase in warm nights compared to a slightly more moderate increase in warm days, and an increase in maximum 1- and 5-day precipitation intensities interspersed with longer consecutive dry spells across Tasmania during the twenty-first century.     

Dry spell characteristics over India based on IMD and APHRODITE datasets

Selected characteristics of dry spells and associated trends over India during the 1951–2007 period is studied using two gridded datasets: the Indian Meteorological Department (IMD) and the Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of the water resources (APHRODITE) datasets. Two precipitation thresholds, 1 and 3 mm, are used to define a dry day (and therefore dry spells) in this study. Comparison of the spatial patterns of the dry spell characteristics (mean number of dry days, mean number of dry spells, mean and maximum duration of dry spells) for the annual and summer monsoon period obtained with both datasets agree overall, except for the northernmost part of India. The number of dry days obtained with APHRODITE is larger for this region compared to IMD, which is consistent with the smaller precipitation for the region in APHRODITE. These differences are also visible in the spatial patterns of mean and maximum dry spell durations. Analysis of field significance associated with trends, at the level of 34 predefined meteorological subdivisions over the mainland, suggests better agreement between the two datasets in positive trends associated with number of dry days for the annual and summer monsoon period, for both thresholds. Important differences between the two datasets are noted in the field significance associated with the negative trends. While negative trends in annual maximum duration of dry spells appear field significant for the desert regions according to both datasets, they are found field significant for two regions (Punjab and South Interior Karnataka) for the monsoon period for both datasets. This study, in addition to providing information on the spatial and temporal patterns associated with dry spell characteristics, also allows identification of regions and characteristics where the two datasets agree/disagree.     

干湿持续期随机模拟

该文应用数据建模技术, 实现干湿期随机建模。主要包括:利用历史气象资料, 从中采集干湿期数据; 应用实测数据, 创建干湿期经验分布函数; 应用Monte Carlo方法和经验分布参数, 随机生成干湿期序列, 通过和Markov链模型输出的对比分析, 讨论生成序列的统计误差, 测试结果显示, 与两状态Markov链方法相比, 所建模型性能更好。     

Regional trends in recent precipitation indices in China

Summary Regional characteristics of recent precipitation indices in China were analyzed from a daily rainfall dataset based on 494 stations during 1961 to 2000. Some indices such as precipitation percentiles, precipitation intensity, and precipitation persistence were used and their inter-decadal differences were shown in this study. Over the last 40 years, precipitation indices in China showed increasing and decreasing trends separated into three main regions. A decreasing trend of annual precipitation and summer precipitation was observed from the southern part of northeast China to the mid-low Yellow River valley and the upper Yangtze River valley. This region also showed a decreasing trend in precipitation intensity and a decreasing trend in the frequency of persistent wet days. On the other hand, increasing trends in precipitation intensity were found in the Xinjiang region (northwest China), the northern part of northeast China, and southeast China, mainly to the south of the mid-low Yangtze River. The indices of persistent wet days and strong rainfall have contributed to the increasing frequency of floods in southeast China and the Xinjiang region in the last two decades. Persistent dry days and weakening rainfall have resulted in the increasing frequency of drought along the Yellow River valley including North China. Regional precipitation characteristics and trends in precipitation indices indicate the climate state variations in the last four decades. A warm-wet climate state was found in northwest China and in the northern part of northeast China. A warm-dry climate state extends from the southern part of northeast China to the Yellow River valley, while a cool-wet summer was found in southeast China, particularly in the mid-low Yangtze River valley over the last two decades.     

1980-2020年山西省极端降水的研究

基于1980-2020年山西省109个气象观测站点的逐日降水资料,选取10个极端降水指数,采用气候倾向率、相关分析、因子分析、R/S预测方法等方法,对山西省极端降水进行了时空分布的研究,以期为山西省的气候变化、生态环境保护、防灾减灾、气象服务工作提供参考依据,结果表明：(1)从时间尺度来看,1980-2020年期间,山西省极端降水的强度和极值都有明显增加,连续干旱日数和连续湿日日数呈下降趋势,其余均表现出不同程度的增加,其中年总降水量增加幅度最明显;从空间尺度来看,年总降水量、降水强度、降水频率、极值均为从西北向东南逐渐增多,空间差异较明显;从各站点的空间分布来看,北部和中部地区的极端事件增加最显著,北部地区的干旱日数仍以增加趋势为主,连续湿日日数气候倾向率的空间差异较大,中部地区站点显著增加,南北部以减少趋势为主;(2)基于相关分析方法表明各极端降水指数(除干旱日数外)与年总降水量都有很好的相关关系,强降水量和极强降水量对年总降水量的贡献值呈现出增加趋势;采用因子分析方法提取了3个公共因子,方差贡献率累计达到了87％,可以看出极端降水强度和降水量指数在对极端降水方面影响较大;利用R/S分析法可以得到年总降水量、中雨日数、大雨日数、最大5日降水量这几个指数未来呈现弱减少趋势,而干旱日数仍为减少趋势,连续湿日日数为持续弱增加趋势。总体看来,山西省极端降水近年来呈现出增加趋势,在空间分布有明显差异。     

The application of a present-day climatic model to the late quaternary in southern Africa

A present-day climatic model is presented in which extended wet spells of near-decadal duration and dry spells of similar length are explained on the basis of surface and upper tropospheric circulation variations. Wet spells are shown to be the result of increased tropical atmospheric disturbances and tropical-temperate interaction, and to be linked to variations in the Walker Circulation. Conversely, dry spells are shown to result from diminished tropical activity over southern Africa, equatorward movement of westerly storm tracks and temperate perturbations in the westerlies.The present-day analogue is compared to preliminary spatial reconstructions of the climate of southern Africa over the last twenty-five millennia and is shown to have wide applicability in the explanation of the late-Quaternary palaeoclimates of the subcontinent. In particular, it is argued that the Last Glacial Maximum was associated with northward-displaced circulation conditions similar to those of present-day dry spells over the summer rainfall region, whereas the extensive moist conditions that prevailed for several thousand years after 9000 BP were analogous to present-day wet spell conditions with little apparent displacement of major circulation features.     

Climatology and trends of wet spells in China

Summary Climatological features and variations of wet spells, especially their trends over China, are investigated using a dataset of 594 meteorological stations across China from 1951 to 2003. The results show that the lower the latitude is, the longer the annual duration of wet spells is. The mean annual precipitation from wet spells is higher in southeastern coastal areas and much lower in western and northern China. The longest wet spells are found in Southwest China and the eastern Tibetan Plateau. The maximum daily precipitation of wet spells decreases from the southeast to the northwest, with the highest in southeastern coastal areas and the lowest in western China. The trends of wet spells exhibit striking regional differences. In most areas of western China, the annual number of days in wet spells has slightly increased, but significantly decreased over North China, Central China and Southwest China. The annual precipitation amount from wet spells displays significant downward trends in North China, eastern Northeast China and the eastern part of Southwest China, but upward trends in the eastern Tibetan Plateau and some southeastern coastal areas. Two clearly-contrasting regions in climatic changes of wet spells are the mid-lower reaches of the Yellow River and the eastern Tibetan Plateau, characterized by a decrease of about 24 days and an increase of about 6 days in annual wet spell days from 1953 to 2003, respectively.     

基于EOF的江西省秋季降水时空分布特征

基于1980—2020年秋季江西省83个气象观测站逐月降水数据,利用EOF方法分析了该地区秋季降水的时空分布特征。结果表明,江西省秋季降水场主要有4种类型,分别为全区型、北湿(干)南干(湿)型、西湿(干)东干(湿)型、中心湿(干)南北干(湿)型,累计贡献率为86.7%。1980—2020年,全区型和中心湿(干)南北干(湿)型降水呈增加趋势,而北湿(干)南干(湿)型和西湿(干)东干(湿)型降水呈下降趋势。其中全区型降水分布的年份占比75.6%,主要受大尺度大气环流的影响。北湿（干）南干（湿）型降水分布的年份占比17.1%,这是由于赣北地区受地形抬升作用,降水较多,而中南部在背风坡,降水较少,同时秋季赣北处于副热带高压边缘,且受到台风外围的影响,易发生降水,使得南北降水呈反相位变化。