【作者】 李岩瑛；

【导师】 张强；

【作者基本信息】 兰州大学 ， 气象学， 2008， 博士

【摘要】 利用祁连山区及其周围(90～104°E,32～42°N)1960～2004年55个气象站点白天08～20时、夜间20～08时和全天20～20时逐日降水资料,天气图、云图资料、典型暴雨(雪)天气个例、1951～2004年逐月74项大气环流特征量资料和典型干湿年NCEP/NCAR再分析格点资料,重点分析了祁连山区(94～104°E,36～39°N)降水不同区域时间变化特征;不同降水强度的时空分布特征及其与海拔高度的关系;暴雨(雪)时空分布气候特征及其形成机制:并采用WRF中尺度数值模式模拟地形、植被和积雪对祁连山区降水的影响。大气环流特征量与降水的关系:从高空环流形势,青藏高原低涡、青藏高压等天气系统,以及上升运动、水汽条件、风场和冷空气等干湿变化作对比诊断分析,探讨造成祁连山区降水时空分布、干湿变化和日变化的成因机制。主要结论如下:1、祁连山及周边地区季节平均降水贡献百分率分别是夏季60.1%、春季18.3%、秋季17.8%和冬季3.8%。祁连山不同区域年均降水量西部为71.4mm、东北部区189.4mm、东南部区369.6mm、东中部区377.6mm;祁连山中东南部区贡献率最大为52.9%、东中部区23.1%、西部最小为6.6%。西部和东中部区80年代降水最多,东南部区60年代降水最多;东北部区90年代呈偏多趋势,东南部区90年代呈偏少趋势,进入21世纪以来除东南部区较90年代减少外,其余均呈增多趋势,80年代后有5～7年的变化周期。不同区域季节降水量年代际变化:除夏季九十年代最多外,其余均二十一世纪初最多,秋季增多最明显,二十一世纪初较90年代增加10毫米以上。干湿变化中除春季湿年次数略多于干年外,其余季节均干年多于湿年,冬季变化最大。2、祁连山区不同区域的降水日数和强度分布,小雨和中雨日数决定了年降水量的大小,降雨日数有3年和5～7年的年际变化周期。昼夜变化中降水日数小雨白天多于夜间,但中雨以上夜间明显多于或略强于白天。得出了不同降雨强度最大雨量的海拔高度和不同季节最大降水总量出现的海拔高度。小雨日数与海拔高度较为密切,呈线性增长;中雨以上与坡向、地理位置有关,在4850米附近降雨日数最多为143天。降雨日数和总量在海拔高度4000米左右达最大峰值,而在2000米附近为次大峰值。3、祁连山暴雨夜间比白天多而强度大,主要集中在7～8月占87.7%,全天暴雨强度60年代最大,日数90年代最多占28.4%。祁连山东南部区暴雨出现最多,夜间年均日数为0.25天,全天年均为0.04天,东北部区次之,夜间年均为0.08天;暴雨强度西部最大为72.0毫米,东中部区最小为52.8毫米,2到3站局地性暴雨较多占79%。暴雨出现云系有两种:午后青藏高原对流云团发展加强北抬,高原对流云团与外来云团合并加强,概率分别为38.2%,61.8%。祁连山暴雨的水汽主要来自孟加拉湾和南海,移动路径有西、中、东三条路径,概率分别占11.1%、38.3%和50.6%,暴雨主要出现在东南部区湟水谷地和东北部区黑河流域。4、祁连山区总降雪量与中雪日数关系最密切,东北侧降雪日数最多。不同区域分布中西部雪日最少,东中部区强度最弱,其中小雪东中部区最多、中雪中南部较多较强、大到暴雪东北部区最多东南部区最强。日际分布特点是:降雪量夜间明显较多,小到中雪强度夜间较强。年变化中:西部持续增多,东中部区70年代最少,东北部区90年代最少,但西中北部均二十一世纪初最多,东南部区70年代最多,之后持续减少。降雪日数有3～4年、5～7年和12～14年的年际变化周期。暴雪出现的主要天气环流形势为北方横槽南压型和新疆冷温槽发展东移型,分别占38.1%和52.4%。暴雪均出现在山脉冬季风的迎风坡和峡谷地带。5、采用WRF中尺度模式改变祁连山东北部(36-38N,100-104E)范围内地形、植被和积雪来模拟其对祁连山区降水的影响,模拟结果表明:地形对祁连山区降水的影响范围大、强度强,水平尺度达400-500公里,强度为3-4mm以上;而积雪次之,仅对实验区内水平尺度100公里海拔3500米以上的山区局地有1mm的降雪改变量;植被的影响范围更小,无论增减仅使祁连山区海拔4000米以上水平尺度几十公里的局地雨量增加1mm。地形减半会使剖面附近垂直上升运动加强,实验区内下沉运动加强,湿度显著减小30-40%,降水减少;而改变植被和积雪仅使实验区和祁连山区近地面相对湿度有不足10%的变化。6、当夏季西太平洋副高位置偏北,面积增大时祁连山区大到暴雨日数增加,夏秋季降雨量增多。祁连山区降水偏多时,500hPa高空环流形势为西低东高型,低层700hPa青藏高原有一强辐合区,范围较大。祁连山区垂直上升运动和700hPa青藏高原低涡的日变化,造成该区降水夜多昼少。7、干湿变化成因:在湿年,200hPa青藏高压和700hPa青藏高原低涡范围大、强度强;印度季风和南海低空急流强而位置偏北;200hPa高空急流和中高纬度冷空气范围大、强度强、位置偏东,但是孟加拉湾低空西南风强度弱而位置偏南。8、祁连山区最大降水高度的出现除了受地面海拔高度的影响外,很可能与高低空两个最大相对湿度中心及相应较强的冷空气活动中心出现高度关系密切。更多还原

【Abstract】 Daily precipitation observations of 08-20h in daytime, 20-08h in nighttime, 20-20h in whole day from 55 meteorological stations during 1960-2004, weather charts, cloud maps and typical rainstorm(snowstorm) weather cases over the Qilian Mountain and its ambient areas (90-104°E, 32-42°N), 74 monthly atmospheric circulation characteristic indexes during 1951-2004 as well as the NCEP reanalysis gridded data of typical drought and wet years are primarily analyzed to study various regions’ temporal variation characteristics; the temporal and spatial distribution of the precipitation in different categories of the intensities, the frequency and intensity of the precipitation and their relationships to elevation; rainstorm (snowstorm) temporal and spatial distribution weather-climatic characteristics as well as forming mechanism; using mesoscale weather forecast model WRF the influences of terrain,accumulated snow and vegetation coverage on precipitation are simulated. The relations between atmospheric circulation characteristic indexes and precipitation, the drought and wet changes of high circulation field , weather systems such as Qing hai-Tibetan Plateau low vortex at 700hPa and Qing hai-Tibetan ridge at 200hPa, and vertical movement, atmospheric vapor,wind field and cold air are comparatively diagnosed and analyzed, so that the causes of precipitation’s emporal and spatial distribution, drought and wet changes, daily changes over the Qilian Mountain are found out. The main conclusions are following:1、Different mean seasonal precipitation contribution percentages are 60.1% in summer, 18.3% in spring, 17.8% in autumn and 3.8% in winter; different regions’ mean annual precipitation are 71.4mm in west, 189.4mm in northern east, 369.6mm in southern east, 377.6mm in middle east; the most average precipitation contribution percent is 52.9% in southern east, next is 23.1% in middle east, the least is 6.6% in west. For the west and middle-east the most decadal precipitation occurred in 1980s, but for southern east the most decadal precipitation occurred in 1960s, precipitation in northern east was upward in 1990s, while precipitation in southern east was downward in 1990s. Except precipitation in southern east was less than 1990s, all of other regions’ precipitations are increasing since 21 century early, 5-7 year’s change period is obvious since 1980s. As for inter-decadal variations of variousregions’ seasonal precipitations except summer’s maximum precipitation occurred1990s, all of other seasons’ maximum precipitations occurred 2000s, autumn’sprecipitation had obviously increased most that at the beginning of 21 century it was10mm more than 1990s.In drought and wet change, all seasonal drought years aremore than wet years except spring, winter had changed most.2、In distribution of precipitation’s frequency and intensity in different categories, thefrequency (days) of flurry and middling rains is an important factor to determine themagnitude of the annual precipitation, and a 5-7 year’s inter-annual variation periodis found from the data analysis. The diurnal-nocturnal variation shows that diurnalflurry days are more than nocturnal days, but the nocturnal heavy rains are stronger.The elevation height levels of maximum total rainfall for different precipitation’sintensities and seasons are found out. The frequency of flurry days linearly increaseswith elevation heights. The relations between middling class rainfalls and the slope aswell as geographic locations are closer over the Qilian Mountain. The most day ofrainfall is 143 at 4850m of elevation. The first peak of total rain days and rainfall is at4000m of elevation or so, the next peak of total rain days and rainfall is at 2000m ofelevation or so.3、The nocturnal rainstorms are more and stronger than diurnal over the Qilian Mountain, rainstorms mainly focus between July and August which account for 87.7%, whole day’s rainstorms were the strongest in 1960s and the most in 1990s which account for 28.4%.The southern east rainstorm was the most, its nocturnal mean annual day is 0.25, and daily mean annual day is 0.04, next is northern east rainstorm whose nocturnal mean annual day is 0.08. For rainstorm intensity, the strongest rainstorm happened in west that is 72.0mm, the most weakly rainstorm happened in middle east that is 52.8mm, local rainstorms of occurred between 2 and 3 stations are the most which account for 79%.There are two groups of rainstorm clouds that one is afternoon Qinghai-Tibetan plateau convective clouds develop and move northward, another is that plateau convective cloud and foreign cloud unite and develop, whose frequencies are 38.2% and 61.8%, respectively. The water vapors of rainstorms are mainly come from Bengal gulf and South China Sea. The moving tracks are west, middle and east whose frequencies are 11.1%、38.3% and 50.6%, respectively. Rainstorms over the Qilian Mountain mostly occurred in Huangshui valley of its southeast side as well as HeiHe valley of its northern east.4、The number of middling snowfall days is the closest to the total snowfall, the most snowfall days occurred in northern east. In various regions’ distribution the fewest snowfall days occurred in west, the least intensity occurred in middle east, but the most flurry days occurred in middle east, middling snowfall is more and stronger in middle and south, heavy snow and snowstorm are the most in northern east, the strongest in southern east. Nocturnal snow days are more obviously, nocturnal flurry and middling snowfall are stronger than diurnal. In annual variation, the annual snowfall was increasing continuously in west, the least occurred in 1970s of middle east and in 1990s of northern east, but the most occurred at the beginning of 21 century of west, northern east and middle east, the peak occurred in 1970s of southern east, then was decreasing continuously. For snowfall days, 3-4 year, 5-7 year and 12-14 year’s change periods are obvious in inter-annual variation. The primary weather calculation backgrounds of occurring snowstorm are two types: one is northern horizontal trough of pressing southward, another is Xinjiang cold trough of developing and moving eastward, account for 38.1% and 52.4%, respectively. All snowstorms occurred in the Mountain windward slopes of winter monsoon and gorge terrain.5、Using meso-scale weather forecast model (WRF), the influences of changing terrain, perpetual snow and vegetation in northwest of Qilian Mountain (36-38N, 100-104E) on precipitation are simulated. The results show that changing terrains have obvious influences on precipitation of Qilian Mountain, and the width area of coverage is 400-500 km, and influencing precipitation intensity is above 3-4mnm. The second important factor is perpetual snow, only affecting 1mm of local precipitation variation above 3500m mountain in experiment area whose scale is 100 km. That of vegetation is the least. Whether vegetation increases or decreases, it can increase 1mm precipitation above 4000m local Qilian Mountain whose scale is about several tens of kilometers. When the northeast of Qilian Mountain terrain is halved, vertical ascending motion become strong nearby section, and sinking motion in experiment area become strong. So the relative humidity decreases by 30-40%, and precipitation decreases. But changing perpetual snow and vegetation only result less than 10% change of surface relative humidity in experiment area and Qilian Mountain.6、When the west Pacific subtropical high moves to the north side and enlarges, the days of heavy rain and rainstorm, and the total rainfall of summer and autumn increase over the Qilian Mountain. When there is relatively more rainfall amount, circulation field at 500hPa is high in east and low in west, there is large and strong low vortex at 700hPa over Qing hai-Tibetan Plateau. The daily changes of vertical raising movement and Qing hai-Tibetan Plateau low vortex at 700hPa lead to the more precipitation in nighttime than daytime.7、Analysis on causes of drought and wet changes: in wet years, Qing hai-Tibetan Plateau low vortex at 700hPa and Qing hai-Tibetan high at 200hPa are relative large and stronger, the monsoons of India and South China sea are also strong and on north side, high air Jet stream at 200hPa and cold air of middle-high latitude are also large, stronger and on east side, but low south-westerly Jet over Bengal gulf is weak and on south side.8、Except for impacted by near surface elevation, the height of peak total rainfall is possibly determined by the heights of occurring two maximum relative humidity centers and their two cold air centers in high air.更多还原