三峡库区不同城镇化梯度下河流pCO2动态及驱动因素

为研究河流溶解性CO_2(二氧化碳)对城镇化的响应,于2018年8月对三峡库区3条具有不同城镇化强度的河流——汝溪河、南河和桃花溪进行野外在线监测及取样分析,测定了样品有机质(溶解性有机碳DOC)、营养元素(溶解性总氮DTN、溶解性总磷DTP);利用Arc GIS解译各流域的土地利用组成,并利用碳酸盐平衡方程,通过p H、Alk(碱度)、水温计算不同城镇化梯度河流的p CO_2(CO_2分压).结果表明:建设用地占比从低到高依次为:汝溪河南河桃花溪,河流p CO_2依次为1790±1210、2006±3546、4094±4218μatm,与河流的城镇化梯度变化一致,河流水-气界面呈现为CO2源;DOC、DTN浓度、Alk、电导率在各河流间呈现显著性差异.DOC、DTN、DTP浓度和Alk的变化规律与河流的城镇化梯度一致.研究发现城镇化进程增加水体的p CO_2,引起区域城镇扩张进程中河流CO_2释放通量的增加,结论支持强人为活动影响增加河流碳释放的假设.     

中国平均降水和极端降水对气候变暖的响应:CMIP5模式模拟评估和预估

基于24个CMIP5全球耦合模式模拟结果,分析了中国区域年平均降水和ETCCDI强降水量(R95p)、极端强降水量(R99p)对增暖的响应.定量分析结果显示,CMIP5集合模拟的当代中国区域平均降水对增温的响应较观测偏弱,而极端降水的响应则偏强.对各子区域气温与平均降水、极端降水的关系均有一定的模拟能力,并且极端降水的模拟好于平均降水.RCP4.5和RCP8.5情景下,随着气温的升高,中国区域平均降水和极端降水均呈现一致增加的趋势,中国区域平均气温每升高1℃,平均降水增加的百分率分别为3.5%和2.4%,R95p增加百分率为11.9%和11.0%,R99p更加敏感,分别增加21.6%和22.4%.就各分区来看,当代的区域性差异较大,未来则普遍增强,并且区域性差异减小,在持续增暖背景下,中国及各分区极端降水对增暖的响应比平均降水更强,并且越强的极端降水敏感性越大.未来北方地区平均降水对增暖的响应比南方地区的要大,青藏高原和西南地区的R95p和R99p增加最显著,表明未来这些区域发生暴雨和洪涝的风险将增大.     

极端降水条件下白洋淀主淀区水化学特征及水质变化

极端降水是影响水环境质量的重要气象因素之一,随着全球气候变暖,极端降水事件的频率和量级呈显著上升的趋势,对湖泊水化学和水环境产生了深刻的影响.白洋淀是华北地区重要的湖泊型湿地和生态功能区,为查明极端降水条件下白洋淀主淀区的水化学和水质特征,本研究利用Piper三线图、Gibbs图和多元统计方法阐明了白洋淀极端降雨后的水化学特征、氢氧稳定同位素特征和水质空间变化,揭示了极端降水条件对白洋淀主淀区水化学和水质的影响.结果表明：（1）在极端降水条件下,白洋淀主淀区湖水呈弱碱性,水化学类型主要为Ca-HCO₃·SO₄型,极端降水减弱了蒸发结晶作用和人类活动等因素对白洋淀主淀区湖水水化学组成的影响.极端降水是导致白洋淀主淀区pH、电导率和总溶解性固体发生大幅度变化的原因之一.（2）极端降水条件下白洋淀主淀区湖水的δ²H和δ¹⁸O值的范围分别为-60.86‰~-35.01‰和-8.84‰~-3.45‰,其值均与水深呈显著负相关.极端降水使得白洋淀主淀区湖水的氢、氧稳定同位素贫化,但对其空间分布的影响不大,湖水氢、氧同位素关系受降水本身氢、氧同位素关系的影响更为强烈.强降水引起白洋淀的水深突然增加,导致湖水受到的蒸发分馏作用影响减弱.（3）极端降水增加了面源污染扩散的速度,导致白洋淀主淀区湖水中的氨氮、硝态氮和总磷等营养物质浓度升高,白洋淀大气降水中的氨氮也可能导致极端降水条件下主淀区湖水中氨氮浓度升高,而上游水库为了应对强降雨事件进行的调蓄放水使得白洋淀东北部水质有所改善.该研究为白洋淀的生态修复治理和雄安新区的生态系统管理提供了科学依据.     

MM5V3模式对IPCCA1B情景下中国地区极端事件的模拟和预估

利用MM5V3区域气候模式单向嵌套ECHAM5全球环流模式,对中国地区1978-2000年及IPCC A1B情景下2038-2070年气候分别进行了水平分辨率为50 km的模拟试验.文章首先检验了模式模拟的当代极端气候结果,在此基础上对6个极端温度指数和6个极端降水指数的未来变化进行了预估.检验结果表明:MM5V3模式对中国地区当代日最高、最低温度及强降水(大雨和暴雨)日数的空间分布和概率特征均具有一定的模拟能力,但模拟的日最高温度在大部分地区偏低,日最低温度在南方地区偏低、西北地区偏高.概率统计结果显示日最高温度向低值频段偏移,日最低温度在0℃的峰值附近明显偏高.模式对大雨和暴雨年平均日数的模拟在东部地区偏多,概率统计结果则为一致偏大.未来中国地区极端气候预估结果表明:极端高温、极端低温和相对高温在全国范围内都将升高,且线性趋势均为上升;霜日日数则为减少,并具有下降趋势;暖日日数和相对低温在青藏高原和新疆部分地区有所减少、其它地区均为增加,且线性趋势暖日日数为上升,相对低温不明显.极端降水指数的变化具有区域特征,其中单日最大降水、连续五日最大降水、最长无雨期、强降水日数、简单降水强度和极端降水总量均在江淮、华南及西南地区有所增多,而在东北及内蒙古地区有所减少,未来中国南方地区降水的极端化趋势将更加显著.极端降水指数的线性趋势除最长无雨期外其它均为上升.     

极端气候事件综合危险性等级指标构建及近60年来长江流域极端气候综合分析

本文利用经过均一化订正的长江流域共669个气象站近60年(1961—2020年)逐日观测资料,采用相对阈值和绝对阈值相结合的极值分析方法,对长江流域近60年极端高温事件、极端低温事件、极端干旱事件和极端降水事件进行识别,分析了年发生频率和线性变化趋势.在此基础上,考虑到全国极端气候事件发生情况,构建了多个极端气候事件综合危险性等级指标,比较客观地给出了长江流域极端气候事件综合危险性等级.研究结果表明,相对于全国其他地区,长江流域大部分地区极端气候综合危险性等级较高,虽然自1961年以来综合年发生频率呈现弱的线性减少趋势,但自20世纪90年代以来,长江流域极端气候事件发生的危险性相对于全国其他地区明显偏高.通过对不同极端气候事件危险性和变化规律研究,结果表明：长江流域近60年极端干旱事件年发生频率呈现线性减少趋势,与全国他其区域相比较,长江流域大部分地区极端干旱发生的危险性等级都在中级以上,说明长江流域容易发生极端干旱事件;长江流域近60年极端降水事件年发生频率呈现弱的增加趋势,危险性等级指数分析表明,高危险区主要位于长江中下游地区,湖南西部、江西大部、湖北南部等地发生极端降水事件的危险...     

基于REOF的两种卫星降水产品(IMERG和MSWEP)在金沙江流域降水分区尺度精度评估

高时空分辨率降水产品的精度评估是卫星降水用于水文气象干旱等研究的前提。本研究提出在降水分区尺度下评估IMERG和MSWEP两种卫星降水产品的精度,并与不分区尺度(即流域尺度)进行比较。首先采用旋转经验正交函数(REOF)对金沙江流域(JSB)进行降水分区,通过贡献率得出8个分区较为适合。然后识别降水的空间分布特征,发现2种降水产品都可以很好地捕捉降水呈现出的从上游到下游逐渐增加的趋势。最后在日尺度、降水发生概率和极端降水探测能力3个方面对降水产品在分区尺度和不分区尺度的性能进行评估。结果表明,在日尺度上,MSWEP的精度在多数降水分区优于IMERG,被推荐5次(1、3、6、7和8区),集中在流域的中游。同时流域尺度也推荐MSWEP。在降水事件发生概率方面,MSWEP能再现不同等级降水强度的概率密度分布,但过高估计0.1~1 mm/d降水事件的发生概率;而IMERG过高估计小于0.1 mm/d降水事件的概率。在极端降水探测能力方面,流域尺度的KGE值都是正值,且IMERG优于MSWEP,但分区尺度上,KGE值在部分降水分区中存在负值,表明IMERG和MSWEP均不能很好地探测出该区的极端降水事件。本研究成果表明降水分区尺度是必需的,能够更加精细地评估降水产品。研究结果可为具有类似气候条件的卫星降水评估提供参考。     

滇池、抚仙湖、阳宗海长期水位变化(1988-2015年)及驱动因子

水位变化影响湖泊水质、水量和生态系统功能,是研究湖泊演变的重要内容,但目前针对滇中高原湖群水位变化特征还少见系统报道.本文选择滇池、抚仙湖、阳宗海3个滇中高原湖泊作为研究对象,基于1988-2015年实测水位数据和Mann-Kendall趋势检验法评估了3个湖泊水位变化特征;运用RClimDex模型获得了流域极端降水指标,结合其他指标构建了基于极端气象因子的湖泊水位驱动力指标体系;采用主成分-多元回归模型,解析了极端降水、蒸发等气象因子对滇中高原湖泊水位变化的贡献.结果表明：①滇池、抚仙湖、阳宗海水位年际波动不突出.滇池的年平均水位总体略呈上升趋势,年均上升0.025 m.阳宗海和抚仙湖水位无明显变化.②滇中高原湖泊流域的极端降水指数年际变化趋势不明显.滇池的蒸发量呈明显减小趋势,年均减小21.05 mm.抚仙湖蒸发量呈明显增加趋势,平均每年增加5.52 mm.阳宗海蒸发量的变化不明显.③气象指标可解释滇池水位变化的49.7%,滇池水位变化受气候变化和人类活动的综合影响;阳宗海和抚仙湖水位变化主要受气象条件控制,蒸发量、综合降水指标和连续降水指标对阳宗海水位变化的解释率高达93.3%;综合降水指标和干旱状况指标可以解释抚仙湖水位变化的64.5%.极端降水指标对解释高原湖泊水位变化具有重要作用.     

太湖平原高度城镇化下降雨特征时空变化研究

随着人类活动和气候变化影响的加剧,降雨特征变化引发的洪水问题日益突出,探讨其时空演变特征对保障高度城镇化地区洪涝安全有较大理论和实际意义.本研究以高度城镇化的太湖平原地区为例,选取降雨强度、降雨历时、降雨集中程度和降雨峰值程度为降雨类型特征指标,以高分辨率短历时加权集合降水资料MSWEP （multi-source weighted-ensemble precipitation）为基础探讨了快速城市化发展下（1979—2016年）不同降雨历时-强度-类型的时空演变规律及其演变机制.研究结果表明：①研究区的降雨频次呈现出强度小、历时短、雨量集中于中期和高峰型降雨发生的频次高的特征;同时历时较短、集中于前期和后期的降雨频次趋于增加,而历时较长、集中于中期且中峰型降雨的频次趋于减少.②城镇化对降雨雨型的影响分析发现,高城镇化水平地区较低城镇化水平地区的降雨量更大,其中降雨强度大于25 mm/d的大雨和暴雨事件更易发生;并且更倾向于发生前期型和高峰型的极端分布降雨.随着城镇化发展研究区更易于遭受因降雨量过于集中而导致的雨涝灾害.     

近50a三峡库区汛期极端降水事件的时空变化

利用三峡库区35个台站1961-2010年汛期(5-9月)的逐日降水量资料,首先定义不同台站的极端降水量阈值,统计各站近50 a逐年汛期极端降水事件的发生频次,进而分析其时空变化特征.结果表明:三峡库区汛期极端降水事件发生频次的最主要空间模态是主体一致性,同时存在东西和南北相反变化的差异.三峡库区汛期极端降水事件发生频次具有较大的空间差异,可分为具有不同变化特点的5个主要异常区.滑动t检验表明,三峡库区西南部区代表站巴南的极端降水事件在1974年后发生了一次由偏多转为偏少的突变,北部区代表站北碚在1981年后和1993年后分别发生了由偏少转为偏多和由偏多到偏少的突变,中部区代表站武隆在1984年后发生了一次由偏多转为偏少的突变.结合最大熵谱和功率谱分析表明,近50 a来各分区汛期极端降水事件发生频次的周期振荡不太一致,三峡库区东北部区代表站宜昌、北部区代表站北碚和中部区代表站武隆分别存在5、2.4和8.3 a的显著周期.     

中国日极端降水和趋势

利用中国2000多站50年以上的日降水资料,提取各站6个极端降水指数,即历年最大值(top-1)、历年10个最大值(top-10)累积量、历年总降水量、历年夏季top-1、历年夏季top-10累积量、历年夏季总降水量,分别形成各站各极端降水指数时间序列.统计了历年全国及6个子区域整个研究资料时段内出现top-1和top-10值的总站次,分别形成历年全国及6个子区域极端降水总站次时间序列.各极端降水指数时间序列的趋势分析表明,中国各站top-1和top-10累积量时间序列趋势分布存在较为一致的3个明显趋势区域,即中国东南正趋势区、西北正趋势区和华北负趋势区,但各时间序列的趋势分布区域特征和趋势幅度增减程度不尽相同;正负趋势明显地区的大多数站点趋势增(减)幅度比(趋势幅度与平均值的比值)达10~30%,部分站点达30%以上.top-1和top-10降水全国年总站次时间序列表明了一致的线性增加趋势,平均每十年分别增加2.4站和15站,但年际差别较大;极端降水年总站次表现为三段极端降水多发期,分别出现在20世纪60年代初、90年代中后期和21世纪初,6个区域极端降水年总站次趋势特征与全国年总站次不尽相同,极端降水全国年总站次出现较多的年份在区域上表现不一样.与降水有关的环流形势要素的时间序列趋势分析表明,华北及上游蒙古高原地区呈现明显的位势高度增加趋势,而低层从中国南部沿海地区到华北地区呈现一条南风风速减弱的趋势带和华北地区水汽含量减少的趋势区,这些趋势特征有利于华北地区极端降水的减少.经验回归水平分析表明,50年一遇重现水平从南部沿海的400~600mm减少到西北地区的50mm以下,相比20年一遇重现水平提升率平均达6.8%,但比降水最大值可小一倍以上.     

Spatio-temporal characteristics of extreme precipitation events during 1951–2011 in Shandong,China and possible connection to the large scale atmospheric circulation

Analyses of the spatio-temporal variability of precipitation extremes defined by eleven extreme precipitation indices in Shandong were conducted by utilizing the methods of linear regression, ensemble empirical mode decomposition (EEMD) and Mann–Kendall test. The results revealed that statistically significant decreasing trends existed for almost all extreme precipitation indices except for the consecutive dry days (CDD) and simple daily intensity index. A periodicity of 10–15 years for precipitation extremes is detected by EEMD analysis. Greatest 5-day total rainfall (RX5day), very wet days (R95p) and annual total wet-day precipitation (PRCPTOT) experienced decreasing trends in the region stretching from the southeast coast to the west, while the spatial distribution of the decreasing trends for other indices was more complicated. Moreover, the frequency of occurrence in precipitation extremes at Changdao station, surrounded by the sea in the northeast region, increased in contrast to surrounding stations. This may suggest a possible effect from the local marine environment on extreme precipitation. In addition, the stations with statistically significant positive trends for CDD were mainly located in mid-west Shandong and along the southeast coast, where the extreme precipitation and total rainfall were, on the contrary, characterized by decreasing trends. These results indicate that drought or severe drought events have become more frequent in those regions. Analysis of large-scale atmospheric circulation changes indicates that a strengthening anticyclonic circulation and increasing geopotential height as well as decreasing strength of monsoonal flow in recent decades may have contributed to the variations in extreme precipitation in Shandong.     

Trends in precipitation extremes over the Yellow River basin in North China: Changing properties and causes

The Mann–Kendall test, composite analysis, and 68 high‐quality meteorological stations were used to explore the spatiotemporal variations and causes of precipitation extremes over the Yellow River basin (YRB) during the period of 1960–2011. Results showed that (a) the YRB is characterized by decreases of most precipitation indices, excluding the simple daily intensity index, which has increasing trends in most locations, suggesting that the intensity of rainfall and the probability of occurrence of droughts have increased during the last decades. (b) Trends of extreme precipitation show mixed patterns in the lower reach of the YRB, where drought–flood disasters have increased. The increases in heavy rainfall and decreases in consecutive wet days in recent years over the northwestern portions of the YRB indicate that the intensity and frequency of above‐normal precipitation have been trending upward in domains. In the central‐south YRB, the maximum 1‐day precipitation (RX1day) and precipitation on extremely wet days (R99p) have significantly increased, whereas the number of consecutive dry days has declined; these trends suggest that the intensity of precipitation extremes has increased in those regions, although the frequency of extreme and total rainfall has decreased. (c) The spatial distributions of seasonal trends in RX1day and maximum 5‐day precipitation (RX5day) exhibited less spatial coherence, and winter is becoming the wettest season regionwide, particularly over the central‐south YRB. (d) There were multiple and overlapping cycles of variability for most precipitation indices, indicating variations of time and frequency. (e) Elevation is intimately correlated with precipitation indices, and a weakening East Asian summer monsoon during 1986–2011 compared to that in 1960–1985 may have played an important role in the declines in most indices over the YRB. Therefore, the combined effects from local and teleconnection forcing factors have collectively influenced the variations in precipitation extremes across the YRB. This study may provide valuable evidence for the effective management of water resources and the conduct of agricultural activities at the basin scale.     

Changes in the spatial scale of Beijing UHI and urban development

The seasonal and interannual variations of Beijing urban heat island (UHI) are investigated in this paper using the temperature data from 1960 to 2000 at 20 meteorological stations in the Beijing region, and then the relationship between the intensity and spatial scale of UHI and Beijing urbanization indices is analyzed and discussed. Main conclusions are the followings. First, Beijing UHI shows obvious seasonal variations, and it is strongest in winter, next in spring and autumn, and least in summer. The seasonal variation of the UHI mainly occurs in the urban area. The UHI intensity at the center of Beijing is more than 0.8℃ in winter, and only 0.5℃ in summer. Second, the intensity of Beijing HUI exhibits a clear interannual warming trend with its mean growth rate (MGR) being 0.3088℃/10 a. The MGR of HUI is largest in winter, next in spring and autumn, and least in summer, and the urban temperature increase makes a major contribution to the growth of HUI intensity. Third, since the Reform and Opening, the urbanization indices have grown several ten times or even one hundred times, the intensity of HUI has increased dramatically, and its spatial scale also expanded distinctively along with the expansion of urban architectural complexes. Fourth, the interannual variation of urbanization indices is very similar with that of HUI intensity, and their linear correlation coefficients are significant at a more than 0.001 confidence level.     

城市化对石家庄站日气温变化的影响

利用1962—2011年逐日平均、最低和最高气温资料,对比分析了石家庄站和藁城站平均、最低和最高气温的概率分布特征.石家庄站是我国少有的自建站以来从未迁址的城市气象站,而附近的藁城站可近似看作乡村气象站.结果表明:城市化致使石家庄站1962—2011年平均、最低和最高气温的概率密度分布向高温方向偏移,其中对最低气温分布的影响尤其明显,对最高气温分布的影响很小;受城市化影响,石家庄站最低气温概率密度分布的高温部分增温比低温部分增温更加明显,最低气温分布形状更加扁平;相对于1962—1986年,1987—2011年石家庄站平均、最低和最高气温概率密度分布均向高温方向偏移,其中最低气温偏移最为明显,并导致1962—2011年整个分析时期最低气温分布出现非正态性;城市化对石家庄站气温分布的影响在冬、春季比夏、秋季更显著,最显著的城市化影响出现在冬季最低气温上;石家庄站基于最低气温的极端气温指数趋势受城市化影响严重,冷夜日数和暖夜日数的城市化影响分别为-1.13d/10a、1.48d/10a,但基于最高气温的冷昼和暖昼日数等极端气温指数变化趋势受城市化影响不明显.出现这种现象的主要原因是城市化对最低、最高气温分布的影响存在差异.     

Impacts of different threshold definition methods on analyzing temporal-spatial features of extreme precipitation in the Pearl River Basin

Extreme precipitation event is rare and mostly occurs on a relatively small local scale, which presents marked uncertainties when analyzing its characteristics. Using daily precipitation data covering 1959–2009 from 62 stations over the Pearl River Basin, the percentile method (PM) and the absolute critical value method (ACVM) are applied to define extreme precipitation thresholds (EPT), and their different impacts on the spatial–temporal distribution of extreme precipitation event were analyzed in this study. The findings of this study show: (1) Using the K-means clustering algorithm in terms of precipitation indices and the topography, longitude and latitude of each station, the whole basin is divided into eight precipitation zones. (2) The extreme indices, including extreme precipitation frequency, extreme precipitation proportion and proportion of extremely n-day precipitation, calculated by PM are markedly higher than those calculated by ACVM during five decades, which is particularly obvious in the low precipitation area such as the west-northern of the basin since more daily precipitation events are treated as extreme precipitation in this region if EPT is defined by PM. (3) The spatial distributions of extreme frequencies respectively calculated by these two methods are quite different across the basin. The spatial distribution of extreme frequencies calculated by ACVM shows a high-value center in the southeast coastal areas and a low-value center in the northwest mountain areas. However, the extreme frequencies calculated by PM distribute evenly over the basin, which is obviously inconsistent with the empirical results, an area with heavy precipitation usually has a high extreme precipitation frequency, and vice versa.     

城市化对北京气象站极端气温指数趋势变化的影响

利用5个乡村气象站和北京气象站(简称北京站)1960～2008年日最高、最低气温资料,比较分析了北京地区城市和乡村极端气温指数年、季节的时间变化以及城市化对北京站各极端气温指数趋势变化的影响.结果表明:1960～2008年北京站霜冻日数、冷夜日数、冷昼日数和平均日较差均显著减少,暖夜日数、暖昼日数、平均最高气温和平均最...     

Inconsistent linear trends in Senegalese rainfall indices from 1950 to 2007

Abstract

Daily precipitation data from 31 Senegalese stations spanning the period from 1950 to 2007 were used to examine the inter-annual variations of seven rainfall indices: the annual mean precipitation (MEAN); the annual standard deviation of daily precipitation (STD); the frequency of wet days (Prcp1); the maximum number of consecutive dry days (CDD); the maximum 3-day rainfall total (R3D); the wet day precipitation intensity (SDII); and the 90th percentile of rain-day precipitation (Prec90p). The indices were spatially averaged over three agro-climatic regions in Senegal. Trends in the time series of the averaged indices were assessed using both visual examination and a modified version of the Mann-Kendall (MM-K) test. Initially negative significant trends in all seven indices suggest gradually drier conditions over the three agro-climatic regions between 1950 and 1980. In contrast, no significant trends, or even positive significant trends, were observed from the mid-1980s to 2007. The MM-K test was applied to all available data (1950–2007) and the period from 1971 to 2000. While several indices were found to have significant trends towards drier conditions for the 1950–2007 period, only PRCP1 showed a positive significant trend for the 1971–2000 period. The MM-K test did not detect a significant trend for the other indices. It was found that the rainfall deficit and therefore drought is no longer intensifying, and that the region may even become wetter. However, the period covered by the observations is still too short to resolve the question of whether there is now a trend towards wetter conditions.

Editor Z.W. Kundzewicz; Associate editor K. Hamed     

Impacts of Urbanization and Station-relocation on Surface Air Temperature Series in Anhui Province, China

Since the 1990s, many meteorological stations in China have passively “entered” cities, which has led to frequent relocation and discontinuity in observational records at many stations. To study the impacts of urbanization on surface air temperature series, 52 meteorological stations in Anhui Province were chosen based firstly on a homogeneity test of the time series, and then their surrounding underlying surfaces during different decades were identified utilizing Landsat Multispectral Scanner images from the 1970s, Landsat Thematic Mapper images from 1980s and 1990s, and Enhanced Thematic Mapper images after 2000, to determine whether or not the station “entered” city, and then these stations were categorized into three groups: urban, suburban, and rural using Landsat-measured land use/land cover (LULC) around the station. Finally, variations in annual mean air temperature (T _mean), maximum air temperature (T _max), and minimum air temperature (T _min) were analyzed in urban-type stations and compared to their surrounding rural-type stations. The results showed that, in Anhui Province over the past two decades, many rural stations experienced urbanization and changed into urban or suburban locations. This process is referred as the “city-entering” phenomena of stations. Consequently, many of the latest stations were relocated and moved to currently rural and suburban areas, which significantly influenced the continuity of observational records and the homogeneity of long-term trends. Based on homogeneous data series, the averaged annual T _mean, T _max, and T _min over Anhui Province increased at a rate of 0.407, 0.383 and 0.432 °C decade^?1 from 1970 to 2008. The strongest effect of urbanization on annual T _mean, T _max, and T _min trends occurred at urban stations, with corresponding contributions of 35.824, 14.286, and 45.161 % to total warming, respectively. This work provides convincing evidences that (1) urban expansion has important impacts on the evaluation of regional climate change, (2) high spatial resolution images of Landsat are very useful for selecting reference climate stations for evaluating the potential urban bias in the surface air temperature data in certain regions of the continents, and (3) meteorological observation adjustments of station-relocation-induced inhomogeneities are essential for the study of regional or global climate change.     

Daily extreme precipitation and trends over China

Based on daily precipitation data of more than 2000 Chinese stations and more than 50 yr, we constructed time series of extreme precipitation based on six different indices for each station: annual and summer maximum(top-1) precipitation,accumulated amount of 10 precipitation maxima(annual, summer; top-10), and total annual and summer precipitation.Furthermore, we constructed the time series of the total number of stations based on the total number of stations with top-1 and top-10 annual extreme precipitation for the whole data period, the whole country, and six subregions, respectively. Analysis of these time series indicate three regions with distinct trends of extreme precipitation:(1) a positive trend region in Southeast China,(2) a positive trend region in Northwest China, and(3) a negative trend region in North China. Increasing(decreasing)ratios of 10–30% or even 30% were observed in these three regions. The national total number of stations with top-1 and top-10 precipitation extremes increased respectively by 2.4 and 15 stations per decade on average but with great inter-annual variations.There have been three periods with highly frequent precipitation extremes since 1960:(1) early 1960 s,(2) middle and late 1990 s,and(3) early 21 st century. There are significant regional differences in trends of regional total number of stations with top-1 and top-10 precipitation. The most significant increase was observed over Northwest China. During the same period, there are significant changes in the atmospheric variables that favor the decrease of extreme precipitation over North China: an increase in the geopotential height over North China and its upstream regions, a decrease in the low-level meridional wind from South China coast to North China, and the corresponding low moisture content in North China. The extreme precipitation values with a50-year empirical return period are 400–600 mm at the South China coastal regions and gradually decrease to less than 50 mm in Northwest China. The mean increase rate in comparison with 20-year empirical return levels is 6.8%. The historical maximum precipitation is more than twice the 50-year return levels.