Precipitation Chemistry in Semi-Arid Areas of Southern Africa: A Case Study of a Rural and an Industrial Site

Experimental data on the precipitation chemistry in the semi-arid savanna of South Africa is presented in this paper. A total of 901 rainwater samples were collected with automatic wet-only samplers at a rural site, Louis Trichardt, and at an industrial site, Amersfoort, from July 1986 to June 1999. The chemical composition of precipitation was analysed for seven inorganic and two organic ions, using ion chromatography. The most abundant ion was SO₄ ^{2 –} and a large proportion of theprecipitation is acidic, with 98% of samples at Amersfoort and 94% at LouisTrichardt having a pH below 5.6 (average pH of 4.4 and 4.9, respectively). This acidity results from a mixture of mineral and organic acids, with mineral acids being the primary contributors to the precipitation acidity in Amersfoort, while at Louis Trichardt, organic and mineral acids contribute equal amounts of acidity. It was found that the composition of rainwater is controlled by five sources: marine, terrigenous, nitrogenous, biomass burning and anthropogenic sources. The relative contributions of these sources at the two sites were calculated. Anthropogenic sources dominate at Amersfoort and biomass burning at Louis Trichardt. Most ions exhibit a seasonal pattern at Louis Trichardt, with the highest concentrations occurring during the austral spring as a result of agricultural activities and biomass combustion, while at Amersfoort it is less pronounced due to the dominance of relatively constant industrial emissions. The results are compared to observations from other African regions.     

Sulfur dioxide in rain clouds: Gas-liquid scavenging efficiencies and wet deposition rates in the presence of formaldehyde

The interaction of formaldehyde with SO₂ dissolved in the aqueous phase of clouds leads to the formation of hydroxymethane sulfonate. The impact of this process upon the gas-liquid equilibrium distribution of SO₂ in rain clouds and the ensuing wet SO₂ precipitation rate is explored. Model vertical SO₂ distributions are derived from observational data for three atmospheric regions: continental polluted, continental remote, and marine. The wet deposition rate for SO₂ in the polluted atmosphere increases by about a factor of two in the presence of formaldehyde compared with its absence. The effect is much stronger in the remote atmosphere leading to a potentially significant enhancement of wet SO₂ deposition. In the marine atmosphere, wet deposition of SO₂ may contribute as much as 35% to the total removal rate for SO₂ by all processes including dry deposition and chemical conversion to sulfate.     

An Overview of the Land Surface Processes Experiment (Laspex) over a Semi-Arid Region of India

To understand and quantify the land-surface-vegetation interactionwith the atmospheric boundary layer, and validate or improve upon the existing surfaceflux parameterization schemes in various weather forecast models, a LAnd SurfaceProcesses EXperiment (LASPEX), was designed and executed in the semi-arid regionof Gujarat, India during January 1997–December 1998. Micrometeorological tower observations,soil and vegetation parameters, radiation, turbulence and upper airobservations were taken continuously for two years at five sites, separated by about60–100 km from each other. Towers of 9 m height with instruments at four levels wereinstalled at sites that are agricultural fields and characterized with a variety of soilproperties, vegetation and diverse crops. An overview of the experiment is presented.Some results, such as the seasonal variation of surface energy balance and turbulence statistics,are discussed.     

7Be deposition in a high-rainfall area of New Zealand

Concentrations of natural ⁷Be in air and rainwater were monitored for one year at Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m^–3, the mean Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m^–3, the mean concentration in rainwater was 2600±1200 Bq m^–3, and the mean total deposition was estimated to be 130±99 Bq m^–2 wk^–1. Most of the ⁷Be was wet deposited and the washout ratio was independent of precipitation amount. A significant linear relationship exists between the weekly wet deposition flux and weekly precipitation at this high-rainfall site.     

华南准静止锋暖区内降水的物理化学特征

介绍了一次华南准静止暖区内降水的物理化学特征的综合观测分析结果。主要有：①暖区内积云降水雨滴浓度不高，但滴径较大，谱型以多峰谱为主；②雨水ｐＨ值较低，平均４．４３，离子成分中ＮＯ３，Ｃｌ，ＳＯ４，ＮＨ＾＋４，Ｎａ＾＋的浓度较高；③本次降水清除了气溶胶总质量的４５％，对Ｃｌ，Ｆ，ＮＯ３的清除率较高；④由于气溶胶中阴离子被雨水清除的效率较高（主要是ＮＯ３）而阳离子的清除率低，致使雨水ｐＨ值下降，这可能     

Chemical Composition and Association of Size-Differentiated Aerosols at a Suburban Site in a Semi-Arid Tract of India

Size-differentiated concentrations of SPM, F, Cl, NO₃, SO₄, Na, K, Ca, Mg and NH₄ in atmospheric aerosols were measured in a suburban area of Agra city during December 1992 to March 1993. Except for NH₄, Cl and Na, all components were found to have a bimodal distribution. The fine fraction was dominated by NH₄, K, NO₃ and SO₄, while Na, Ca, Mg, F and Cl contributed to the coarse fraction. Fifty-eight percent of SO₄ and 67% of NO₃ were found in the fine mode and the coarse mode comprised 42 and 33% of SO₄ and NO₃, respectively. SO₄ was found to have a peak above the submicron range at 1.1 µm which has been attributed to secondary sulphate formation by heterogeneous oxidation of SO₂ on alkaline particles of Ca and Mg. The total aerosol was basic in nature and dominated by the soil-derived acid neutralising components (Ca, Mg and Na).     

Changes in Daily Climate Extremes of Observed Temperature and Precipitation in China

Daily precipitation for 1960–2011 and maximum/minimum temperature extremes for 1960–2008 recorded at 549 stations in China are utilized to investigate climate extreme variations.A set of indices is derived and analyzed with a main focus on the trends and variabilities of daily extreme occurrences.Results show significant increases in daily extreme warm temperatures and decreases in daily extreme cold temperatures,defined as the number of days in which daily maximum temperature(Tmax)and daily minimum temperature(Tmin)are greater than the 90th percentile and less than the10th percentile,respectively.Generally,the trend magnitudes are larger in indices derived from Tmin than those from Tmax.Trends of percentile-based precipitation indices show distinct spatial patterns with increases in heavy precipitation events,defined as the top 95th percentile of daily precipitation,in western and northeastern China and in the low reaches of the Yangtze River basin region,and slight decreases in other areas.Light precipitation,defined as the tail of the5th percentile of daily precipitation,however,decreases in most areas.The annual maximum consecutive dry days(CDD)show an increasing trend in southern China and the middle-low reach of the Yellow River basin,while the annual maximum consecutive wet days(CWD)displays a downtrend over most regions except western China.These indices vary significantly with regions and seasons.Overall,occurrences of extreme events in China are more frequent,particularly the night time extreme temperature,and landmasses in China become warmer and wetter.     

长江中下游夏季降水及其与全球热通量的关系

统计分析了1951-2002年52 a长江中下游夏季降水的年际和年代际变化特征.结果显示:长江中下游夏季降水在20世纪60、70年代处于少雨期,80年代旱涝相间,90年代处于多雨期,而且年际异常和年代际异常的配置决定了旱涝的强弱.在此基础上,分析了长江中下游夏季旱涝年的前期潜热和感热通量在年际和年代际尺度上的异常合成场,以及夏季旱涝年同期大气环流场及风场的异常合成场.结果表明:中北太平洋西部(T区)和日本以东洋面(R区)当年春季热通量的异常分布形势是长江中下游夏季旱涝的一个前期讯号.     

我国干旱半干旱区降水场及与北极海冰的关系

采用复经验正交函数方法对我国干旱半干旱区４５个代表站，近４０年来的月降水距平场划分出３种主要的分布型，分析了它们的分布特征，同时还揭示了北极海冰状况与干旱半干旱区夏季降水的密切关系，并初步分析了海冰与极涡，西太平洋副热带高压等大气环流的关联，以了解其可能的天气学影响过程。     

中国北方干旱半干旱区降水的多年代际变化特征及其与太平洋年代际振荡的关系

基于最新版本的全球降水气候中心(Global Precipitation Climatology Centre Version 7,GPCC_V7)资料与欧洲中期数值预报中心20世纪再分析资料(ERA-20C)融合的百年尺度逐月降水资料(1901~2012年),运用集合经验模态分解方法(EEMD)、合成分析等方法系统分析了我国北方干旱半干旱区降水多年代际变化特征及与太平洋年代际振荡(PDO)之间的相互关系。结果表明:北方干旱半干旱区大多数区域降水都具有50~60年的平均变化周期,而PDO对大多数地区降水多年代际变化特征具有明显的调制作用;其中新疆北部和内蒙古北部的降水与PDO呈现出显著的正相关,而河套东西部地区的降水则与PDO的变化呈现显著负相关。进一步分析表明,当PDO为暖相位时,径向环流增强使得北冰洋水汽南下,当遇到低空北上的阿拉伯海域暖湿气流时,会造成新疆中南部的降水增多;另一方面,PDO暖相位时赤道西太平洋及印度洋区域通过对流加热的作用激发了太平洋—日本/东亚—太平洋(PJ/EAP)遥相关型的产生,这有利于渤海湾暖湿水汽输送至干旱半干旱区北部区域,增大降水概率;同时,当偏北和偏西气流在河套北部区域相遇时会形成降水中心。当PDO位于冷位相时,结论则反之。     

位涡诊断在黄土高原强对流风暴预报中的应用

利用位涡理论,对2004年6月15~16日宁夏、内蒙、陕西、山西和河南出现大范围的强对流风暴和局地冰雹天气过程作了诊断分析。个例分析发现,干位涡空间结构表现为:从风暴区下游到风暴区形成随高度向西倾斜的大值正位涡柱,风暴区形成对流层高层大值正位涡中心和对流层中低层伴有位涡梯度增强的位涡等值线密集区的叠置。对流层低层干位涡场特征表现为,风暴区形成干位涡等值线密集区和风场切变的耦合。对流层低层湿位涡场特征表现为,风暴区形成湿位涡正压项小于0对流不稳定舌和湿斜压中心以及湿位涡斜压项等值线密集区的耦合。风暴发生前,对流层中层500hPa河套生成经向位涡等值线密集区,500hPa蒙古地区强偏北气流中同时出现正位涡扰动和指向河套的正位涡平流,对黄土高原大范围强对流风暴的发生有指示意义。     

闽北一次初春强降水的成因分析

利用常规观测资料、NCEP 1°×1°再分析资料、新一代天气雷达资料,对2012年3月6日-7日福建省北部地区暴雨成因进行诊断分析。结果表明:此次降水过程在高空西风槽南下带来的冷空气与西南暖湿气流交汇的背景下产生,暴雨区位于低空西南急流北侧、低层切变南侧、地面冷锋附近;强降水落区位于层结不稳定的湿区中,低层辐合、高层辐散有利于对流发展;干冷空气的侵入时高层高值位涡库向北向下伸展,促使中低层气旋涡度发展,从而导致强降水的发生;雷达回波分析表明,低层暖平流、高层冷平流、区域上空辐合形势都有利于对流性降水的产生。     

A Case Study of the Impacts of Dust Aerosols on Surface Atmospheric Variables and Energy Budgets in a Semi-Arid Region of China

The authors present a case study investigating the impacts of dust aerosols on surface atmospheric variables and energy budgets in a semi-arid region of China. Enhanced observational meteorological data, radiative fluxes, near-surface heat fluxes, and concentrations of dust aerosols were collected from Tongyu station, one of the reference sites of the International Coordinated Energy and Water Cycle Observations Project (CEOP), during a typical dust storm event in June 2006. A comprehensive analysis of these data show that in this semi-arid area, higher wind velocities and a continuously reduced air pressure were identified during the dust storm period. Dust storm events are usually associated with low relative humidity weather conditions, which result in low latent heat flux values. Dust aerosols suspended in the air decrease the net radiation, mainly by reducing the direct solar radiation reaching the land surface. This reduction in net radiation results in a decrease in soil temperatures at a depth of 2 cm. The combination of increased air temperature and decreased soil temperature strengthens the energy exchange of the atmosphere-earth system, increasing the surface sensible heat flux. After the dust storm event, the atmosphere was dominated by higher pressures and was relatively wet and cold. Net radiation and latent heat flux show an evident increase, while the surface sensible heat flux shows a clear decrease.     

青藏高原黑碳气溶胶传输及沉降的季节特征模拟分析

根据全球气溶胶气候模式GEM-AQ/EC的1995~2004年模拟,分析了青藏高原大气黑碳气溶胶的来源、传输及沉降季节特征。研究表明:青藏高原黑碳气溶胶主要来自自由对流层和大气边界层的输送。相对于自由对流层的黑碳输送,紧邻青藏高原的南亚、东亚以及东南亚大气边界层的输送更有效,它形成了青藏高原由北向南、自西往东黑碳气溶胶浓度和沉降明显递增的基本分布形态。横跨欧亚大陆自由对流层的黑碳气溶胶由西向东向青藏高原的输送全年不变,夏季输送路径最北但强度最弱,冬季路径最南而强度最强。大气边界层黑碳气溶胶的输送受控于亚洲季风环流变化,来自南亚的黑碳气溶胶在春季越过孟加拉湾传输进入高原东南部,夏季则可翻越喜马拉雅山抵达青藏高原南部腹地;同时我国中部排放的黑碳气溶胶也在东亚夏季风向北扩展中驱动它从东向西往青藏高原东北部传输。从秋季到冬季,随着夏季风撤退,南亚黑碳源区向青藏高原传输衰退,东亚冬季风的反气旋性环流的南侧及西南侧的偏东风携带秋季我国东南部源区和冬季东南亚源区黑碳气溶胶向青藏高原东南部传输。受青藏高原明显的暖湿季和干冷季气候影响,干湿沉降分别主导了青藏高原冬季和夏季黑碳沉降,夏季青藏高原黑碳气溶胶沉降总量大多超过8~10 kg·km-2,在高原东北部的最高值超过40 kg·km-2。冬季青藏高原黑碳气溶胶沉降量最低,大部地区黑碳沉降低于5 kg·km-2。青藏高原黑碳沉降的冬夏季节相差约为2~8倍。     

春末环贝加尔湖地区环流的年际和年代际变化及与中国温度和降水的关系

本文利用1951~2011年的再分析资料及观测资料,探讨了春末环贝加尔湖地区环流的年际和年代际变化特征及其与中国温度、降水的关系,主要结论如下:(1)春末贝加尔湖脊(简称贝脊)在1951~1979年间较弱,但在1980~1999年期间显著增强。进入21世纪后,春末贝脊呈显著减弱的趋势;(2)强(弱)贝脊使我国北方地区的温度显著升高(降低),使西南地区的温度降低(升高)。贝脊的强弱变化,对北方地区温度的影响在1951~1979年期间最为显著,对西南地区温度的影响在1980~2011年期间显著。在1951~1999年期间,贝脊的强弱变化与长江下游的降水量存在明显的反位相变化关系。进入21世纪后,贝脊的强弱变化与西南地区降水量之间存在较为显著的正相关关系(99%置信度);(3)强贝脊与由北大西洋/欧洲上空向东传播的定常行星波密切相关,而弱贝脊主要与欧亚大陆次极区纬向伸展的行星尺度脊相联系。在不同的年代际时段,与贝脊强弱变化相联系的北半球异常环流特征也存在一定的差异;(4)5月份的强贝脊环流以前期的北大西洋涛动、东大西洋型以及弱东亚大槽环流为前兆信号。同年3月乌拉尔山气旋式异常环流为5月份贝脊弱的前兆信号。     

近46年青藏高原干湿气候区动态变化研究

利用青藏高原62个气象站1961～2006年逐日气象资料, 用世界粮农组织 (FAO) 在1998年推荐的、并唯一承认的Penman-Menteith模式计算潜在蒸散量; 比较了降水量、积温降水比、气温降水比、蒸散降水比和降水蒸散比5种湿润度指标在青藏高原的适用性, 用常规统计方法和墨西哥帽小波变换分析青藏高原各气候区干湿状况及其界线的动态变化。结果表明: 5种指标中, 用降水蒸散比得到的青藏高原湿润、半湿润、半干旱、干旱和极端干旱气候区的分区结果比较合理; 近46年来青藏高原大部分地区湿润度和每个气候区的平均湿润度均呈增加趋势, 半干旱和半湿润气候区的界线呈向西北推进趋势, 气候在向暖湿方向发展。     

Precipitation changes in wet and dry seasons over the 20th century simulated by two versions of the FGOALS model

Seasonal precipitation changes over the globe during the 20 th century simulated by two versions of the Flexible Global Ocean–Atmosphere–Land System(FGOALS) model are assessed. The two model versions differ in terms of their AGCM component, but the remaining parts of the system are almost identical. Both models reasonably reproduce the mean-state features of the timings of the wet and dry seasons and related precipitation amounts, with pattern correlation coefficients of 0.65–0.84 with observations. Globally averaged seasonal precipitation changes are analyzed. The results show that wet seasons get wetter and the annual range(precipitation difference between wet and dry seasons) increases during the 20 th century in the two models, with positive trends covering most parts of the globe, which is consistent with observations. However,both models show a moistening dry season, which is opposite to observations. Analysis of the globally averaged moisture budget in the historical climate simulations of the two models shows little change in the horizontal moisture advection in both the wet and dry seasons. The globally averaged seasonal precipitation changes are mainly dominated by the changes in evaporation and vertical moisture advection. Evaporation and vertical moisture advection combine to make wet seasons wetter and enhance the annual range. In the dry season, the opposite change of evaporation and vertical moisture advection leads to an insignificant change in precipitation. Vertical moisture advection is the most important term that determines the changes in precipitation, wherein the thermodynamic component is dominant and the dynamic component tends to offset the effect of the thermodynamic component.     

全球干旱/半干旱区年代尺度干湿变化研究的进展及思考

干旱/半干旱区气候变化研究一直是广泛关注的前沿科学问题,尤其是气候干湿变化规律及未来的发展趋势。过去大量的研究揭示了全球不同干旱/半干旱区的干湿变化事实和机理,取得了一系列重要进展,IPCC第6次评估报告明确指出未来全球干旱化将加剧,但也存在诸多问题没有得到一致的认识。本文将对全球变暖背景下有关干旱/半干旱区年代尺度干湿变化,特别是年代尺度干旱研究进行梳理,系统评述当前相关研究的现状并提出干旱/半干旱区研究所面临的关键科学问题。     

A Study of DMS Oxidation in the Tropics: Comparison of Christmas Island Field Observations of DMS, SO2, and DMSO with Model Simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO₂ and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO₂, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO₂ data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO₃. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×10⁹ molecs cm^-2 s^-1. The dominant sourceof BL SO₂ was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO₂ was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10^-5 s^-1. In contrast to the DMSand SO₂ simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.     

基于CCSM4.0长期积分试验评估不同辐射强迫对中国干旱半干旱区降水的影响

本文基于气候系统模式CESM4.0长期积分试验,分析评估了工业革命前（1850年）及当前（2000年）两种辐射强迫作用（分别为太阳活动和温室气体）对中国北方干旱半干旱区降水的影响。结果表明,模拟结果与观测之间尽管存在一定的偏差,但仍能再现降水气候态的空间分布以及季节变化特征;两种辐射强迫下的降水长期变化均无明显趋势,但二者的差异却呈现出70~100年的准周期振荡;由人类活动引起的当前辐射强迫作用对降水的多年际变率幅度有一定影响,造成极端强降水事件出现的概率增多,而由太阳活动引起的辐射强迫作用主要对降水多年代际周期具有一定的调制作用。进一步分析表明,两种辐射强迫下中国北方干旱半干旱区降水年多年代际变率的主要模态基本一致,但人类活动引起的辐射强迫作用会影响降水多年代际变率与热带海温异常的相互作用的强度,从而改变降水多年代际变率的幅度。