Study And Analysis On Climatic Characteristics Of Precipitation And Its Causes Over Qilian Mountain

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.