Fitting the distribution of dry and wet spells with alternative probability models

The development of the rainfall occurrence model is greatly important not only for data-generation purposes, but also in providing informative resources for future advancements in water-related sectors, such as water resource management and the hydrological and agricultural sectors. Various kinds of probability models had been introduced to a sequence of dry (wet) days by previous researchers in the field. Based on the probability models developed previously, the present study is aimed to propose three types of mixture distributions, namely, the mixture of two log series distributions (LSD), the mixture of the log series Poisson distribution (MLPD), and the mixture of the log series and geometric distributions (MLGD), as the alternative probability models to describe the distribution of dry (wet) spells in daily rainfall events. In order to test the performance of the proposed new models with the other nine existing probability models, 54 data sets which had been published by several authors were reanalyzed in this study. Also, the new data sets of daily observations from the six selected rainfall stations in Peninsular Malaysia for the period 1975–2004 were used. In determining the best fitting distribution to describe the observed distribution of dry (wet) spells, a Chi-square goodness-of-fit test was considered. The results revealed that the new method proposed that MLGD and MLPD showed a better fit as more than half of the data sets successfully fitted the distribution of dry and wet spells. However, the existing models, such as the truncated negative binomial and the modified LSD, were also among the successful probability models to represent the sequence of dry (wet) days in daily rainfall occurrence.     

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.     

Spatial and temporal characteristics of wet spells in Greece

Summary This study examines the characteristics of wet spells in Greece, using daily rainfall gauge data, over a 40-year period (1958–1997). The longest wet spells, computed for two different thresholds (0.1mm and 1mm), were observed in Western Greece and Crete, whereas the shortest ones were found in the central and south Aegean. A detailed analysis of the wet spells, dividing them into three classes, shows that on an annual basis their highest frequencies were observed in Western and Northwestern Greece. The seasonal results are also quite similar. The trends and the variability of the mean length of the wet spells were also analyzed. Negative trends were found in the case of the year-to-year analysis and during winter. Finally, this study presents an evaluation of two theoretical distribution models; the second order Markov Chains (MC2) and the Negative Binomial Distribution (NBD), by their adjustment to the empirical data. Both models can be used in the future for an estimation of the wet spells in the area under study.     

Spatial and temporal analysis of dry spells in Greece

Summary ?A spatio-temporal analysis of the dry spells that occur in the Greek area is carried out for an extended period of 40 years (1958–1997). The dry spells can be defined as a number of consecutive days with no rain. The number of days defines the length of the dry spells. The longest spells are identified in central (Cyclades) and the south-east Aegean Sea whereas dry spells with the minimum length are shown over the north-west of the Greek area that reflects the significance of the latitude and the topography. Negative Binomial Distribution and Markov Chains of second order have been used to fit the duration of the dry spells of different lengths. The study of the seasonal and annual distribution of the frequency of occurrence of dry spells revealed that the dry spells in Greece depict a seasonal character, while medium and long sequences are associated with the duration and hazards of drought. Received February 20, 2002; revised September 5, 2002; accepted October 6, 2002     

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.     

Intraseasonal wind anomalies related to wet and dry spells during the “long” and “short” rainy seasons in Kenya

Summary The largest part of Kenya exhibits two major rainy seasons, the March–May «long rains» and the October–December «short rains», both related to the passage of the ITCZ, but differing in the amount of rainfall recorded and its interannual variability. In order to investigate whether these differences also apply at intraseasonal time-scales, daily rainfall data for the peak month of each rainy season (April and November) were collected for 7 consecutive years (1982–1988). The network comprises 68 stations, from which a classification of the spatial patterns of daily rainfall anomalies has been performed. Wind anomalies corresponding to the various rainfall types and to specific regional rainfall departures were determined using four pilot balloon stations and one radiosonde station. They revealed that there exist significant differences between upper-air circulation anomalies exhibited in the «long» and «short» rainy seasons, especially as far as rain spells in the Eastern Highlands are concerned. In that region, easterly anomalies in the «short rains» period are associated with an increase in rainfall. During the «long rains», enhanced easterlies more generally coincide with an overall drop of convection in the country. In Western Kenya, wet conditions are more systematically associated to westerly wind anomalies.With 12 Figures     

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.     

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.     

Extreme value analysis of wet and dry periods in Sicily

Summary For assessing risk of highly unusual events extreme value statistics needs to be applied, which plays an important role in engineering practices for water resources design and management. In hydrology, the typical application of extreme value theory concerns floods in river basins or landslides. The present paper is, instead, focused on the analysis of extreme wet and dry periods in a sample area (Sicily). First, we have studied monthly precipitation extremes both using the annual maximum and partial duration methods, and return times have been estimated by standard statistical techniques. Next, we studied the extremes of the Standardized Precipitation Index (SPI), which has been proposed as an indicator for monitoring wet and dry conditions. We found considerable differences both in the return periods and in the time location of the extremes. From our study it appears that the SPI better describes wet and dry periods than the precipitation does. Maps of return times for extreme conditions in Sicily are also presented, which cluster the territory into areas of different extreme return periods. Finally, the occurrence of extremes in Sicily has been related to large-scale atmospheric circulation.     

Spatio-temporal variability of dry and wet periods in eastern China

Summary An analysis, based on rain gauge observations, of the time-space variability of dry and wet periods during the last fifty years in eastern China is presented. The Standardized Precipitation Index (SPI) is used to assess the climatic conditions of the area, and principal component analysis (PCA) is applied to capture the pattern of co-variability of the index at different stations. Results suggest that the northern part of eastern China is experiencing dry conditions more frequently from the 1970s onwards indicated by a negative trend in the SPI time series. Long-term fluctuations characterize the SPI signal and contribute to the power spectrum variance at periods ranging from interdecadal to interannual time scales, that is respectively, 24 years and from 16 to 4–3.7 years. These periodic components provide a useful resource for long-term predictability of dry and wet periods in eastern China.     

面雨量计算方法及其在海河流域的应用

基于1961—2018年汉江流域57个国家气象站日降水观测资料,利用线性倾向估计、滑动平均等方法研究了汉江流域5个子流域极端面雨量时空分布特征,并对年最大极端面雨量（P_max）的重现期进行了拟合,结果表明：（1）汉江流域的年极端面雨量日数（D）与年极端5 d累积面雨量日数（D_5d）年均分布从上游向下游逐渐减少,年极端面雨量（R）与年极端5 d累积面雨量强度（I_5d）表现为两头多、中间少。（2）D和R年均分布在石泉以上、石泉安康和丹皇区间表现出一定的正相关,在安康丹江口和皇庄以下子流域表现出一定的反相关。（3）D、R、D_5d、I_5d等年代际分布特征较为相似,石泉以上、石泉安康、安康丹江口3个子流域呈双峰分布,皇庄以下子流域呈单峰分布,丹皇区间年代际特征不明显。（4）汉江各子流域D、R、D_5d、I_5d与年最大极端面雨量（P_max）等指数峰（谷）值出现的时间重叠性较高,表明同一子流域强降水发生频次较高（低）的年份,降水的持续性和极端性均相对较高（低）,但同一年所有子流域出现持续性极端降水事件的概率不大。（5）汉江流域P_max呈逐年增大趋势。重现期在50 a以前P_max增速较快,之后增速变缓。皇庄以下子流域不同重现期内P_max及其增幅最强。

     

2021年\

2021年,我国天气形势复杂,气候异常性显著,极端事件频发。本文简要回顾了我国主汛期和华西秋雨的异常性特征,并对河南和湖北4次高影响天气过程展开了初步分析。结果表明：(1)2021年4—10月全国平均降水较常年同期偏多7.8%,其中河南、河北和山西等部分地区偏多1倍以上。(2) 在外强迫因子“拉尼娜”异常海温的影响下,副热带高压外围水汽和北方冷空气异常活跃,导致华西秋雨降水量打破1961年以来历史记录。(3) 河南“21.7”特大暴雨发生在台风、低涡、切变线和辐合线共同作用的背景下,大量来自西北太平洋的水汽,在低空急流与边界层急流耦合、地形的动力阻挡抬升和热力抬升共同作用下,强降水雨带出现在山前。(4) 在有利的多尺度环流背景下,地形与低层风场辐合带的相互作用触发了对流,并使得降水维持在狭长山谷地形区域内,引发随州“8.12”极端强降水。(5)中γ尺度的强对流单体的合并加强导致了武汉两次对流大风事件,中层突然增强的水平风引发了下沉运动,下落后造成地面局地强冷池和地面极值大风。

     

Spatial and temporal variability of consecutive dry and wet days in Greece

The objective of this study is to find out the spatial and temporal variability of the dry and wet spells in Greece, during the period 1958–2007. The meteorological data with respect to daily precipitation totals were acquired from 27 meteorological stations of the Hellenic National Meteorological Service, which are uniformly distributed over the country. The dry spells concern consecutive dry days (CDD); the largest number of consecutive days with daily precipitation amount less than 1 mm, within a year. The wet spells concern consecutive wet days (CWD); the largest number of consecutive days with daily precipitation amount more than or equal to 1 mm, within a year, as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI), jointly sponsored by the Commission for Climatology (CCl) of the World Meteorological Organization's (WMO) World Climate Data and Monitoring Programme (WCDMP), the Climate Variability and Predictability (CLIVAR) Programme of the World Climate Research Programme (WCRP) and the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology (JCOMM).As results from the analysis, the spatial distributions of the mean annual CDD and the mean annual CWD along with their trends, within the examined period, are presented. The findings indicate that CDD obtain maxima in the Cyclades Islands and the southeastern Aegean Sea, while minima are found in the northwestern Greece. On the contrary, the longest CWD are observed in western Greece and western part of Crete Island and the shortest in the eastern continental Greece and in the majority of the Aegean Sea. On an annual basis, the temporal variability of CWD shows statistically significant (confidence level of 95%) negative trends, mainly in the western region of Greece, while insignificant positive trends for CDD appear almost all over the country with emphasis in the southeastern region. Finally, in order to interpret the drier and wetter periods within the examined period, the 850 hPa and the 500 hPa geopotential height (m) composites of the anomalies from 1958–1996 climatological normal (clino), are analysed using the National Centers for Environmental Prediction (NCEP) reanalysis data.     

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     

Modelling surface energy fluxes and temperatures in dry and wet bare soils

Abstract

A physically‐based numerical model was developed to estimate the temporal course of the surface energy flux densities and the soil temperatures in dry and wet bare soils. Aerodynamic heat, vapour and momentum transfer theory was used to calculate the sensible and latent heat flux densities at the surface under diabatic and adiabatic conditions. A finite‐difference solution of the differential equation describing one‐dimensional heat transfer was used to calculate the surface soil heat flux density and soil profile temperatures. The surface temperature was determined iteratively by the simultaneous solution of equations describing radiative, heat and momentum transfer at the surface. The model was tested with measurements from energy balance studies conducted on a dry, sandy soil and a wet, silt loam soil, and was found to predict accurately the surface energy fluxes and soil temperatures over three‐day periods under conditions of potential and negligible evaporation. The sensitivity of the model to uncertainties in the aerodynamic roughness lengths for momentum (z₀) and heat (z_T) is reported. Values for z₀ and Z₀/Z_T of 0.5 mm and 3.0, respectively, resulted in the best agreement between modelled and measured values of the fluxes and temperatures for both soils.