Spatial and temporal variations of aridity indices in Iraq

In this paper, a nonhydrostatic Weather Research and Forecasting (WRF) model has been used to simulate the extreme precipitation event of 25 November 2009, over Jeddah, Saudi Arabia. The model is integrated in three nested (27, 9, and 3 km) domains with the initial and boundary forcing derived from the NCEP reanalysis datasets. As a control experiment, the model integrated for 48 h initiated at 0000 UTC on 24 November 2009. The simulated rainfall in the control experiment depicts in well agreement with Tropical Rainfall Measurement Mission rainfall estimates in terms of intensity as well as spatio-temporal distribution. Results indicate that a strong low-level (850 hPa) wind over Jeddah and surrounding regions enhanced the moisture and temperature gradient and created a conditionally unstable atmosphere that favored the development of the mesoscale system. The influences of topography and heat exchange process in the atmosphere were investigated on the development of extreme precipitation event; two sensitivity experiments are carried out: one without topography and another without exchange of surface heating to the atmosphere. The results depict that both surface heating and topography played crucial role in determining the spatial distribution and intensity of the extreme rainfall over Jeddah. The topography favored enhanced uplift motion that further strengthened the low-level jet and hence the rainfall over Jeddah and adjacent areas. On the other hand, the absence of surface heating considerably reduced the simulated rainfall by 30% as compared to the observations.     

Spatial and temporal variations of thunderstorm activity in the Northern Caucasus

Space distribution is considered of thunderstorm activity in Northern Caucasus during 1936–2006. Area distribution is presented of numbers of days with thunderstorm and of thunderstorm duration. Time variations are analyzed of mean numbers of the days with thunderstorms and of the thunderstorm mean duration, which probably represent a part of climate changes at the area under consideration. The largest variations of thunderstorm activity occur in the western part of the Northern Caucasus, where annual number of the days with thunderstorms reaches 70. Minimum changes in the thunderstorm characteristics occur in the eastern part of the area, with annual number of the days with thunderstorms equal to 25.     

Spatial variations in the diurnal patterns of winter precipitation in India

The present study focuses on spatial patterns of diurnal variability in winter precipitation across the Indian subcontinent. Hourly precipitation data during the months of January and February from 1980 to 2002 were procured for 80 stations spread across the subcontinent. The methodology consisted of calculation of the first harmonics by harmonic analysis for three variables that included frequency, total amount, and intensity of precipitation events. There were substantial variations in the strength of the diurnal cycle prevailing over the subcontinent, with most of the peninsular region and eastern part of the northern Gangetic Plain showing relatively stronger diurnal cycles. The variance explained by the first harmonic was significant at the 90% confidence level for larger parts of the peninsular region and northern plains. The weakest patterns were observed in the interior of the continent over central India. The times of maximum for hourly frequency, total amount, and intensity of precipitation were predominantly during the latter half of the 24–hour period, during late evening hours to just before sunrise hours. This was due to the enhanced warm front lifting during the late night to early morning hours as a result of nighttime inversions in the warm sector. Further, along the foothills of the Himalayas, the times of maximum were observed during predawn hours just before sunrise, caused by the downslope movement of katabatic winds and their convergence with the cyclonic storms in the low-lying areas, leading to enhanced precipitation. Along the west coast, the times of maxima were also concentrated in the midnight to early-morning hours due to the convergence of inland moving seas breeze fronts with the northeast trades.     

Summer rain events in south-east Asia: Spatial and temporal variations

During the summer monsoon period in south-east Asia marine airstreams normally prevail at the south China coast. However, when tropical cyclones approach then polluted airmasses from south-eastern China can impart high ionic concentrations and high acidity to rainwater. This is illustrated by two examples and the small-scale horizontal variations in rainwater composition are minor during these episodes. Since long-term quality-assured studies of the composition of rainwater in south-east Asia are scarce, the results at three sampling sites in Hong Kong during summer monsoon periods are compared with previous data from the same season. The results for the mid- to end-1990's show a similar trend to those for the ambient concentrations of sulphate and nitrate in aerosol in Hong Kong, which show flattened trends from 1995 to 1999. A marked increase in acidity is found in the summer monsoon period of 2004 which is attributed to the greater proportion of approaching cyclone weather systems in the dataset, reflecting both the increased local emission sources and the burgeoning economic growth of the Pearl River Delta Region. Comparison of the rainwater composition with that at other south-east Asian cities during the summer monsoon period shows that it is most acidic in Hong Kong.     

On the mechanism of winter cyclogenesis in relation to vertical axis tilt

Summary ?This work is concerned with the behavior of the vertical trough axis tilt during the development of a Mediterranean cyclone. The period covering stages of the development of the depression is from 1200 UTC 18 January to 1200 UTC 25 January 1981. Two procedures have been used to illustrate the behavior of the vertical axis tilt with the development of our case study. In the first direct procedure we trace the centers of the cyclone for each level for each day and determine its longitude. In the second indirect procedure we compute the values and signs of the product of the perturbation of temperature and meridional velocity over the computational domain. The energetics of the depression is studied and the partition of available potential energy and kinetic energy into zonal and eddy component is adopted. The energy contents and their changes in different atmospheric levels are discussed in the course of the cyclone’s development. Received April 27, 2001; Revised December 12, 2001     

中国南方地区1961-2018年冬春季低温连阴雨过程时空变化分析

基于1961-2018年1 057个地面气象站均一化气温日值数据和通过均一化检验的日降水量资料,统计得到冬春季低温连阴雨过程,分析其时空分布及变化特征。结果表明,湖南、贵州、江西三省为低温连阴雨集中发生地区,连续最长低温连阴雨超过15 d的台站集中在25°~31°N之间的区域。由经验正交函数方法分析表明,南方地区低温连阴雨日数总体上表现为一致偏多或偏少的分布型,且具有东北部与西南部反相变化、东部与西部反相变化的分布型。南方冬春季平均低温连阴雨日数总体呈减少趋势,约为-0.17 d/(10 a)。1960s末期至1990s低温连阴雨日数以偏多为主,从2000年开始以偏少为主。     

Spatial and temporal variability of winter and summer precipitation over Serbia and Montenegro

Summary The main characteristics of the spatial and temporal variability of winter and summer precipitation observed at 30 stations in Serbia and Montenegro were analysed for the period 1951–2000. The rainfall series were examined spatially by means of Empirical Orthogonal Functions (EOF) and temporally by means of the Mann-Kendall test and spectral analysis. The Alexandersson test was used to detect the inhomogeneity of the data set.The EOF analysis gave three winter and summer dominant modes of variations, which explained 89.7% and 70.4% of the variance, respectively. The time series associated with the first pattern showed a decreasing trend in winter precipitation. The spectral analysis showed a 16-year oscillation for the dominant winter pattern, around a 3-year oscillation for the dominant summer pattern, and a quasi-cycle of 2.5 years for the winter third pattern.     

近55 a中国西北地区夏季降水的时空演变特征

利用1961—2015年中国西北地区274个气象观测站点的日降水数据和再分析大气资料,采用EOF分析及累积距平等方法,研究了近55 a中国西北地区夏季降水的时空演变特征。结果表明：1）1961—2015年中国西北地区夏季降水的演变可分为三个时段,1961—1975年,该区域降水普遍偏少;1976—1996年,西北地区的东部降水偏多,西部降水偏少;1997—2015年,其东部降水偏少,而西部降水偏多。2）1976—1996年,西北地区东部降水偏多,是因为该地区夏季降水强度和降水频次明显增加,而西部降水偏少,则是该区域小雨与中雨的频次减少,降水强度偏弱造成的;1997—2015年,由于有效降水日数减少,降水强度偏弱等原因导致西北地区东部降水偏少,与此同时,西北地区的西部却因降水强度明显增强,持续降水日数和极端降水事件增加使得该区域降水呈现偏多的态势。3）降水区的转移,伴随着北半球对流层中层中纬度波列的演变,同时来自东欧与印度季风的水汽输送也对降水的异常起到了关键作用。     

Spatial and temporal variations of methanesulfonic acid and non sea salt sulfate in Antarctic ice

A simultaneous glaciochemical study of methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄ ^-) has been conducted on the Antarctic plateau (South Pole, Vostok) and in more coastal regions. The objective was to investigate marine sulfur emissions in very remote areas. Firstly, our data suggest that MSA and nss-SO₄ present in antarctic ice are mainly marine in origin and that DMS emissions have been significantly modulated by short term (eg. El Nino Southern Oscillation events) as well as long term climatic changes in the past. Secondly, our study of spatial variations of these two sulfur species seems to indicate that the atmosphere of coastal antarctic regions are mainly supplied by local DMS emissions whereas the atmosphere of the high plateau is also influenced by DMS emissions from more temperate marine latitudes. Thirdly, our study of the partitioning between MSA and nss-SO₄ suggest that the temperature could have been an important parameter controlling the final composition of the high southern latitude atmosphere over the last climatic cycle; colder temperature favoring the formation of MSA. However, our data also support a possible role played by changes in the transport pattern of marine air to the high antarctic plateau.     

东北春季极端低温的变化特征及其与大西洋海温的关系

利用1958—2007年全球海温、位势高度场月平均资料和我国东北地区64个测站的春季地面气温资料,分析了我国东北地区春季极端低温的时空变化特征及其与大气环流和海温异常的关系。结果表明：近50a来,我国东北春季极端低温事件频数整体呈减少趋势;极端低温频数主要存在全区一致变化型和南北反相变化型两种模态;极端低温事件频数与北极涛动存在显著负相关关系,同时在极端低温多年,极涡较弱,乌拉尔山阻塞高压偏强,东北冷涡较强,反之亦然;影响我国东北地区春季极端低温的关键海区为中北大西洋海区,极端低温多年,中北大西洋海温较常年偏冷,反之则中北大西洋海温较常年偏暖。这对于预测东北春季极端低温事件的年际变化具有很好的指示意义。数值试验结果表明,中北大西洋海温异常可激发欧亚波列,致使中国东北温度异常。     

1961年-2012年山西雾霾的时空变化特征及成因分析

利用1961年-2012年山西逐日天气现象、能见度、相对湿度和日平均气温资料,采用Kendall-tau方法和相关分析法研究山西雾霾日数的时空变化特征及成因。结果表明：雾多发区在中南部,北部雾日较少。霾、烟幕日数高值区出现在以大同、太原、临汾为中心线的带状区域。季节分布来看,轻雾、雾日数峰值出现在8、9月份,谷值在5月份出现;霾和烟幕日数的峰值出现在12、1月份,谷值在8、9月份出现。近50余年以来,山西雾霾日数呈现增多趋势,雾日增加趋势较弱,60、70年代为增多趋势,进入21世纪则为减少趋势;轻雾和霾日数均为显著单调上升趋势;烟幕日数也为显著增多趋势,但表现为抛物线型,90年代后期以前为增多,之后转为下降趋势。山西霾和烟幕日数与E1Nino事件有很好的对应关系,E1Nino事件发生年往往霾和烟幕日数较多,赤道中东太平洋的海温异常通过海气相互作用,引起东亚地区上空的大气环流异常,形成利于霾和烟幕出现的天气条件。山西冬季气温偏高往往导致霾和烟幕天气的增多,气候变暖对霾和烟幕天气的影响不容置疑。     

Climate impact of the European winter blocking episodes from the NCEP/NCAR Reanalyses

A comprehensive multivariable characterisation of the climatic impacts of winter blocking and strong zonal-flow (non-blocking) episodes over the Euro-Atlantic sector is presented here, using a 40-year (1958–97) consistent dataset from NCEP/NCAR. Anomaly fields of surface or low troposphere climate variables are then interpreted based on large-scale physical mechanisms, namely, the anomalous mean flow (characterised by the 500 hPa geopotential height and the surface wind) and the anomalous eddy activity (characterised by the surface vorticity and cyclonic activity). It is shown that the lower troposphere (850 hPa) temperature patterns are mainly controlled by the advection of heat by the anomalous mean flow. However, at the surface level, the anomaly patterns obtained for maximum and minimum temperatures present important asymmetries, associated with a different control mechanism, namely the modulation of shortwave and longwave radiation by cloud cover variations. It is shown that blocking and non-blocking episodes are typically associated with important meridional shifts in the location of maximum activity of transient eddies. The influence of persistent anomaly events in precipitable water is strongly related to the corresponding anomaly fields of lower troposphere temperature. The precipitation rate, however, appears to be essentially controlled by the surface vorticity field and preferred locations of associated cyclones.     

1955—2006冬半年中国极端低温的时空变化特征

采用1955—2006年全国275个台站冬半年逐日最低气温资料,对中国冬半年极端低温的时空特征和变化趋势进行了分析。结果表明：在全国范围内冬半年极端低温频数变化一致,均呈减小趋势,1983年前后发生了明显的突变;近52 a来最显著的减小趋势位于华北、长江中下游及江南地区,最不显著的减小趋势位于河套地区,较弱的减小趋势位于四川盆地和云贵高原地区;中国冬半年年极端低温事件频数的空间差异明显,各区域极端低温频数突变的年份均不一致,各区域突变前后的极端低温频数存在较大差异,但近年来,各区域冬半年极端低温频数变化均趋于稳定。     

1960—2015年中国冬半年极端降温过程事件的时空演变特征

利用1960-2015年中国2 474个站点的逐日最低气温资料,采用REOF方法将中国分为7个区域,通过对每个区域内所有站点降温阈值进行平均,得到了7个降温阈值。当某区域内测站单日降温幅度超过区域平均阈值时,认为该测站发生一次极端降温事件。7个区域降温平均阈值各异,总体呈北大南小特征,最大降温-10.6℃,最小-7.6℃。根据上述定义,研究了中国冬半年极端降温过程事件的时空演变特征。结果表明：极端降温事件发生频数呈北多南少的空间分布。北部地区存在多个频发中心,在42°N和35°N附近存在两个高频发生带。南部地区频数呈一定的带状分布,在25°N附近发生该事件的频率亦高于南部其他地区。近56 a来,极端降温事件频数变化总体呈减少趋势,前期较明显,中后期趋于稳定。各年代频数的空间分布基本一致,但前期总体表现为北部增多。而在1990s则转变为南负北正的空间分布,后期中部及沿海地区发生了趋势符号的年代际改变。进一步分析表明,各区域极端降温频数突变的年份均不一致且突变前后的频数存在较大差异。这些结果可为深刻认识极端低温事件的变化规律和气候预测提供线索。     

1961-2012年江苏省霾日时空变化特征及影响因素分析

利用1961—2012年江苏省69个地面气象站观测资料和2012年苏州市大气气溶胶观测资料,在对霾日进行判识和筛选的基础上,分析江苏省霾日的时空变化特征及霾与气象条件和污染物的关系。结果表明：1961—2012年江苏省各站年总霾日数均呈上升趋势,85％的台站呈极显著上升趋势;江苏省年平均霾日数呈显著上升趋势,其中2011—2012年呈急剧上升趋势;1980年前霾日的空间分布差异不明显,1980年后,沿江和苏南地区为霾的高发区,东部沿海大部地区霾日较少。霾天气主要发生在冬季和春季,以12月和1月发生最多。降水少和风速小有利于霾天气的发生;除SO2外,PM10、PM2.5和NO2等污染物浓度随着霾等级的增加而增大,其中PM2.5浓度增大明显。