Spatial patterns and temporal trends of daily precipitation indices in Iran

Spatial patterns of daily precipitation indices and their temporal trends over Iran are investigated using the APHRODITE gridded daily precipitation dataset for the period 1961–2004. The performance and limitations of the gridded dataset are checked against observations at ten rain-gauge stations that are representative of different climates in Iran. Results suggest that the spatial patterns of the indices reflect the role of orography and sea neighborhoods in differentiating central-southern arid and semi-arid regions from northern and western mountainous humid areas. It is also found that western Iran is impacted by the most extreme daily precipitation events occurring in the country, though the number of rainy days has its maximum in the Caspian Sea region. The time series of precipitation indices is checked for long-term trends using the least squares method and Mann-Kendall test. The maximum daily precipitation per year shows upward trends in most of Iran, though being statistically significant only in western regions. In the same regions, upward trends are also observed in the number of wet days and in the accumulated precipitation and intensity during wet days. Conversely, the contribution of precipitation events below the 75th percentile to the annual total precipitation is decreasing with time, suggesting that extreme events are responsible for the upward trend observed in the total annual precipitation and in the other indices. This tendency towards more severe/extreme precipitation events, if confirmed by other datasets and further analyses with longer records, would require the implementation of adequate water resources management plans in western Iran aimed at mitigating the increasing risk of intense precipitation and associated flash floods and soil erosion.     

Trends of precipitation extremes during 1960–2008 in Xinjiang, the Northwest China

The spatial–temporal variability of the precipitation extremes defined by eight precipitation indices based on daily precipitation dataset was analyzed using the linear regression method and the Mann–Kendall test. The results indicate that increasing trends in the precipitation amount, rainy days, and the intensity of the extreme precipitation were identified at above 70 % of the total rain stations considered in this study, with more than 30 % of them were significant, while most stations show notable decreasing trend in the annual maximum consecutive no-rain days. Significantly increasing trends of the precipitation extremes are observed mainly in the northern Xinjiang and the north of the southern Xinjiang. Most extreme precipitation indices show a potential regime shift starting from the middle of 1980s. The magnitude of the trends is compatible with their pattern of spatial stability. The generally increasing trends in precipitation extremes are found in this study.     

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.     

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.     

Assessment of climate extremes in the Eastern Mediterranean

Summary Several seasonal and annual climate extreme indices have been calculated and their trends (over 1958 to 2000) analysed to identify possible changes in temperature- and precipitation-related climate extremes over the eastern Mediterranean region. The most significant temperature trends were revealed for summer, where both minimum and maximum temperature extremes show statistically significant warming trends. Increasing trends were also identified for an index of heatwave duration. Negative trends were found for the frequency of cold nights in winter and especially in summer. Precipitation indices highlighted more regional contrasts. The western part of the study region, which comprises the central Mediterranean and is represented by Italian stations, shows significant positive trends towards intense rainfall events and greater amounts of precipitation. In contrast, the eastern half showed negative trends in all precipitation indices indicating drier conditions in recent times. Significant positive trends were revealed for the index of maximum number of consecutive dry days, especially for stations in southern regions, particularly on the islands.Current affiliation: National Observatory of Athens, Athens, Greece.     

Climatology of precipitation extremes in Estonia using the method of moving precipitation totals

A method of moving precipitation totals is described and applied for the analysis of precipitation extremes in Estonia. Numbers of extremely wet and extremely dry days and other indices of precipitation extremes were calculated using the daily precipitation data measured at 51 stations over Estonia during 1957–2009. Mean regularities of spatial and seasonal distribution were determined. Long-term changes were detected using Sen's method and Mann–Kendall test. The highest risk of heavy precipitation is in the regions of higher mean precipitation on the uplands and on the belt of higher precipitation in the western part of continental Estonia. Wet spells have their sharp maxima in July and August. The highest risk of droughts is observed in the coastal regions of West Estonia. In the coastal area, droughts appear mostly in the first half of summer, while in the eastern Estonia, they are usually observed during the second half of summer. Extreme precipitation events have become more frequent and intense. Statistically significant increasing trends were, first of all, found in the time series of winter extreme precipitation indices. In summer and autumn, trends existed in some indices, but in spring, there were no trends at all. There were no trends in time series of dryness indices in Estonia in 1957–2009.     

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.     

Spatial and temporal variability of precipitation indices during 1961–2010 in Hunan Province,central south China

Based on daily precipitation records at 75 meteorological stations in Hunan Province, central south China, the spatial and temporal variability of precipitation indices is analyzed during 1961–2010. For precipitation extremes, most of precipitation indices suggest that both the amount and the intensity of extreme precipitation are increasing, especially the mean precipitation amount on a wet day, showing a significant positive trend. Meanwhile, both of the monthly rainfall heterogeneity and the contribution of the days with the greatest rainfall show an upward trend. When it comes to rainfall erosivity, most of this province is characterized by high values of annual rainfall erosivity. Although the directions of trends in annual rainfall erosivity at most stations are upward, only 6 of the 75 stations have significant trends. Furthermore, the spatial and temporal variation of dryness/wetness has been assessed by the standardized precipitation index (SPI). The principal component analysis (PCA) was applied to the SPI series computed on 24-month time scales. The results demonstrated a noticeable spatial variability with three subregions characterized by different trends: a remarkable wet tendency prevails in the central and southern areas, while the northern areas are dominated by a remarkable dry tendency.     

Extreme climate events in China: IPCC-AR4 model evaluation and projection

Observations from 550 surface stations in China during 1961–2000 are used to evaluate the skill of seven global coupled climate models in simulating extreme temperature and precipitation indices. It is found that the models have certain abilities to simulate both the spatial distributions of extreme climate indices and their trends in the observed period. The models’ abilities are higher overall for extreme temperature indices than for extreme precipitation indices. The well-simulated temperature indices are frost days (Fd), heat wave duration index (HWDI) and annual extreme temperature range (ETR). The well-simulated precipitation indices are the fraction of annual precipitation total due to events exceeding the 95th percentile (R95T) and simple daily intensity index (SDII). In a general manner, the multi-model ensemble has the best skill. For the projections of the extreme temperature indices, trends over the twenty-first century and changes at the end of the twenty-first century go into the same direction. Both frost days and annual extreme temperature range show decreasing trends, while growing season length, heat wave duration and warm nights show increasing trends. The increases are especially manifested in the Tibetan Plateau and in Southwest China. For extreme precipitation indices, the end of the twenty-first century is expected to have more frequent and more intense extreme precipitation. This is particularly visible in the middle and lower reaches of the Yangtze River, in the Southeast coastal region, in the west part of Northwest China, and in the Tibetan Plateau. In the meanwhile, accompanying the decrease in the maximum number of consecutive dry days in Northeast and Northwest, drought situation will reduce in these regions.     

Trends and periodicity of daily temperature and precipitation extremes during 1960–2013 in Hunan Province,central south China

In this study, the trends and periodicity in climate extremes are examined in Hunan Province over the period 1960–2013 on the basis of 27 extreme climate indices calculated from daily temperature and precipitation records at 89 meteorological stations. The results show that in the whole province, temperature extremes exhibit a warming trend with more than 50% stations being statistically significant for 7 out of 16 temperature indices, and the nighttime temperature increases faster than the daytime temperature at the annual scale. The changes in most extreme temperature indices show strongly coherent spatial patterns. Moreover, the change rates of almost all temperature indices in north Hunan are greater than those of other regions. However, the statistically significant changes in indices of extreme precipitation are observed at fewer stations than in extreme temperature indices, forming less spatially coherent patterns. Positive trends in indices of extreme precipitation show that the amount and intensity of extreme precipitation events are generally increasing in both annual and seasonal scales, whereas the significant downward trend in consecutive wet days indicates that the precipitation becomes more even over the study period. Analysis of changes in probability distributions of extreme indices for 1960–1986 and 1987–2013 also demonstrates a remarkable shift toward warmer condition and increasing tendency in the amount and intensity of extreme precipitation during the past decades. The variations in extreme climate indices exhibit inconstant frequencies in the wavelet power spectrum. Among the 16 temperature indices, 2 of them show significant 1-year periodic oscillation and 7 of them exhibit significant 4-year cycle during some certain periods. However, significant periodic oscillations can be found in all of the precipitation indices. Wet-day precipitation and three absolute precipitation indices show significant 1-year cycle and other seven provide significant power at the 4-year period, which are mainly found during 1970–1980 and after 1992.     

Variability and trends in daily precipitation extremes on the northern and southern slopes of the central Himalaya

This study focuses on the precipitation extremes recorded on the northern and southern slopes of the central Himalaya, especially those documented at higher altitudes. Daily precipitation data recorded over a 35-year period at nine meteorological stations in the region were studied. We used the precipitation extreme indices delineated by the Expert Team on Climate Change Detection and Indices (ETCCDI). The spatial and temporal variations in these precipitation extremes were calculated. When regional patterns were investigated to detect any anomalies, only 1 of the 10 precipitation extreme indices from the southern slopes of the central Himalaya showed a statistically significant trend; none from the northern slopes of the central Himalaya highlighted a statistically significant trend. On the southern slopes, all indices increased, apart from the maximum 1-day precipitation (RX1) and simple daily precipitation intensity (SDII) indices. Indices such as the consecutive dry days (CDDs) and RX1 indices exhibited similar increases on both the northern and southern slopes of the central Himalaya. These results suggest that increases in precipitation have been accompanied by an increasing frequency of extremes over the southern central Himalaya. Nonetheless, no relation could be established between the precipitation extreme indices and circulation indices for higher altitudes.

     

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。     

Change-points in climate extremes in the Zhujiang River Basin, South China, 1961–2007

In this paper, change-points in time series of annual extremes in temperature and precipitation in the Zhujiang River Basin are analyzed with the CUSUM test. The data cover the period 1961–2007 for 192 meteorological stations. Annual indicators are analyzed: mean temperature, maximum temperature, warm days, total precipitation, 5-day maximum precipitation, and dry days. Significant change-points (1986/87, 1997/98, 1968/69, and 2003/04) are detected in the time series of most of the indicators. The change-point in 1986/87 is investigated in more detail. Most stations with this change-point in temperature indicators are located in the eastern and coastal areas of the basin. Stations with this change-point in dry days are located in the western area. The means and trends of the temperature indicators increase in the entire basin after 1986/87. The highest magnitudes can be found at the coast and delta. Decreasing (increasing) tendencies in total and 5-day maximum precipitation (dry days) are mostly observed in the western and central regions. The detected change-points can be explained by changes in the indices of the Western Pacific subtropical high and the East Asian summer monsoon as well as by change-points in wind directions. In years when the indices simultaneously increase and decrease (indices taking reverse directions to negative and positive) higher annual temperatures and lower annual precipitation occur in the Zhujiang River Basin. The high station density and data quality are very useful for spatially assessing change-points of climatic extreme events. The relation of the change points to large-scale oscillation can provide valuable data for planning adaptation measures against climate risks, e.g. for flood control, disaster preparedness, and water resource management.     

Rainfall and rainy days trend in Iran

In this study, long-term annual and monthly trends in rainfall amount, number of rainy days and maximum precipitation in 24?h are investigated based on the data collected at 33 synoptic stations in Iran. The statistical significance of trend and climate variability is assessed by the Mann-Kendall test. The Linear trend analysis and the Mann-Kendall test indicate that there are no significant linear trends in monthly rainfall at most of the synoptic stations in Iran. However, the maximum number of stations with negative trends have been observed in April (29 station), and then in May (21 stations) and February (21 stations) and with positive trends in December (26 stations) and July (24 stations). The significant linear trends, with a significant level of 0.05, in annual rainfall have been noticed only at five stations. The monthly number of rainy days does not show any significant linear trend for most areas in Iran. The maximum number of stations with monthly negative trends in rainy days has also been observed in April with the minimum in December. In April, out of 24 stations with negative trends, 12 stations have a significant negative trend. Contrary to that, in October there is no significant linear trend. Most stations have positive trends in annual number of rainy days. Also, the monthly maximum precipitation in 24?h does not show any significant linear trend for most areas in Iran. The maximum number of stations with monthly negative trends in maximum precipitation has also been observed in February with the minimum in December. In spite of that, there are almost no significant precipitation variations in Iran during the last 50-odd years, the tendency of decreasing rainfall amount in April and increasing rainfall amount in December and July could indicate an eventual climate change in this area in the future.     

中国降水极值变化趋势检测

利用中国２９６个分布均匀的测站的逐日降水资料,研究了中国过去４５ａ中降水量、降水频率、降水强度等方面的极值变化趋势。结果表明,总体上讲,中国年降水量、１日和３日最大降水量以及不同级别的强降水总量没有发现明显的极端化倾向,但伴随着降水日数极端偏多的区域范围越来越小的变化趋势,平均降水强度极端偏高的区域范围表现为扩大的趋势。中国降水极值变化还反映出明显的区域性特点。在中国东部,平均降水强度极值出现的范围趋于扩大。如华北地区在年降水量明显趋于减少的同时,年降水量极端偏多的范围减少,１日和３日最大降水量、日降水≥５０ｍｍ和１００ｍｍ的暴雨日数极端偏多的情况也趋于减少,而平均降水强度极值显著增加。在年降水明显趋于增多的西北西部地区,降水日数的极值变化趋势不明显,但年降水量、１日和３日最大降水量以及日降水≥１０ｍｍ的降水总量极端偏多的区域范围均反映出趋于增加的变化趋势。     

青藏高原地区季风特征及与我国气候异常的联系

利用1951~2000年NCEP/ NCAR 600 hPa逐日再分析资料, 计算候时间尺度能够代表青藏高原地区季风特征的高度场指数序列, 研究青藏高原高度场指数的基本特征及其年代际变化趋势。结果表明:青藏高原地区600 hPa夏季为低压, 冬季为高压, 夏季低压形成的时间呈提早的趋势, 夏季低压强度也呈增强趋势; 青藏高原高度场指数年变化与高原雨日的年变化基本相似。夏季青藏高原高度场指数与同期我国160站的降水和气温相关表明:与降水相关分布从华北到华南呈“-+-”东西向带状分布; 冬季高原高压强度指数与同期气温均为正相关, 青藏高原东侧边缘区和华南地区正相关最为显著。     

Observed changes in daily temperature and precipitation indices for southern Québec, 1960–2005

Abstract

Trends and variations in daily temperature and precipitation indices in southern Québec are examined for the period 1960–2005. The indices are based on daily temperature and daily precipitation which have been recently adjusted at 53 climatological stations. The adjustments were made for site relocation, changes in observing programs, known instrument changes and measurement program deficiencies. The results show that the surface air temperature has increased in southern Québec over 1960–2005. Significant warming is evident in the western, southern and central parts of the province but the increasing trends become smaller toward the east. The warming is greater during the winter although many significant increasing trends are found in the summer. The analysis of the temperature extremes strongly indicates the occurrence of more nights with extreme high temperatures in all seasons. The temperature indices also suggest an increase in the number of thaw/frost days during the winter (days with maximum temperature above 0°C and minimum temperature below 0°C), a decrease in the length of the frost season, an increase in the length of the growing season, a decrease in heating degree days and an increase in cooling degree days. The precipitation indices show an increase in the annual total rainfall although many stations indicate decreasing trends during the summer. The number of days with rain has increased over the region whereas the number of days with snow and the total snow amounts have decreased over the past 46 years.     

Assessment of the changes in extreme vulnerability over East Asia due to global warming

A number of indices have been employed to describe weather extremes on the basis of climate regimes and public concerns. In this study, we combined these traditional indices into four groups according to whether they relate to warm (T_warm), cold (T_cold), wet (P_wet), or dry (P_dry) extremes. Analysis of the combined indices calculated for the daily temperatures and precipitation at 750 meteorological stations in Korea, China, and Japan for 1960s?C2000s shows increasing trends in T_warm and P_dry events and decreasing trends in T_cold events in recent decades, particularly in the northern part of East Asia. A notable regional variation is an increase in the P_wet events in the Korean Peninsula. We applied the same analysis to a 200-year global climate model simulation for 1900?C2099 using the National Center for Atmospheric Research-Community Climate System Model 3. During the 20th century, the changes in T_warm and T_cold calculated from the model data are largely consistent with those calculated from the observations, especially in northern East Asia. The model projections for the 21st century indicate statistically significant increasing T_warm and decreasing T_cold trends in extreme events over the region. Results obtained from historical archives and model simulations using our combined weather extreme indices suggest that northern East Asia will be subject to increased warm and dry extremes and the Korea Peninsula will experience more wet extremes.     

Changes of precipitation and extremes and the possible effect of urbanization in the Beijing metropolitan region during 1960–2012 based on homogenized observations

Daily precipitation series at 15 stations in the Beijing metropolitan region (BMR) during 1960-2012 were homogenized using the multiple analysis of series for homogenization method, with additional adjustments based on analysis of empirical cumulative density function (ECDF) regarding climate extremes. The cumulative density functions of daily precipitation series, the trends of annual and seasonal precipitation, and summer extreme events during 1960-2012 in the original and final adjusted series at Beijing station were comparatively analyzed to show the necessity and efficiency of the new method. Results indicate that the ECDF adjustments can improve the homogeneity of high-order moments of daily series and the estimation of climate trends in extremes. The linear trends of the regional-mean annual and seasonal (spring, summer, autumn, and winter) precipitation series are -10.16, 4.97, -20.04, 5.02, and -0.11 mm (10 yr)-1, respectively. The trends over the BMR increase consistently for spring/autumn and decrease for the whole year/summer; however, the trends for winter decrease in southern parts and increase in northern parts. Urbanization affects local trends of precipitation amount, frequency, and intensity and their geographical patterns. For the urban-influenced sites, urbanization tends to slow down the magnitude of decrease in the precipitation and extreme amount series by approximately -10.4% and -6.0%, respectively; enhance the magnitude of decrease in precipitation frequency series by approximately 5.7%; reduce that of extremes by approximately -8.9%; and promote the decreasing trends in the summer intensity series of both precipitation and extremes by approximately 6.8% and 51.5%, respectively.