Original Articles
SHI Guangpu, SHI Yuanyuan, GUO Yuzhen, CHEN Shaoyong, LIN Shu, GUO Junting
Based on meteorological data of autumn cloud shape and surface air temperature at 29 meteorologicalstations in the Qilian Mountains and the surrounding areas during 1961-2001, the temporal variation and spatial distribution of autumn stratiform cloud arising frequency and the relationship between it and climate warming were analyzed, and the NCEP/NCAR global reanalysis data in the corresponding period were selected to analyzethe circulation of autumn stratiform cloud in this region. The results are shown as follows. (1) The autumn stratiformcloud arising frequency in the Qilian Mts. region is 8%～26%, presenting a spatial distribution of less inthe (north)west and abundant in the (south)east. (2) The autumn mean air temperature increased by 1.2℃ with arate of 0.29℃/10a in recent 41 years, and the abrupt change occurred in the mid-1980s. (3) The autumn stratiformcloud arising frequency decreased by 11% at a rate of -2.7%/10a, which was corresponding to the synchronoussignificant warming since the mid-1980s. The autumn stratiform cloud arising frequency decreased moresignificantly, and the autumn stratiform cloud arising frequency and air temperature presented an anti-phasetrend. (4) In the background of climate warming, the autumn stratiform cloud arising frequency in the Qilian Mts. region decreased, with a decreasing amplitude increasing from northwest to southeast. When the autumn average temperature increased by 1℃, the autumn stratiform cloud arising frequency decreased by 2%～10% overmost parts of the Qilian Mts. region, 2%～4% in the western and central parts, and 4%～10% in the eastern part.(5) There was a significant difference in 500hPa atmospheric circulation between abundant and less years of autumn stratiform cloud in the Qilian Mts. region. In abundant years, polar vortex extends to northern Asia, theeastern Asia deep trough is weaker, and the Ural high pressure ridge is stronger. Due to the north-northwest airflowbefore the ridge, the polar cold air flows to Northwest China in a northwest path, uninterrupted low-valuesystems develop and move towards east from Central Asia to the Qinghai-Tibet Plateau. In the meantime, southerntrough strengthened and the transportation of the warm and wet air flow from the Arabian Sea, the South China Sea and the East China Sea to the inland areas were markedly strengthened. It met with the cold air which enteredthe northern plateau region, leading to an increase in the autumn stratiform cloud arising frequency in theQilian Mts. region. In less years, the warm anticyclone range from Central Asia to Northwest China is abnormally strengthened, the East Asia deep trough is stronger, and cold air moves by east, presenting a significant winter monsoon characteristic from Asia to Western Pacific. North wind strengthened and made against the transportation of warm and wet flows from Southeast to Northwest China, thus there was less possibility for their meetingin the Qilian Mts. region, thus the autumn stratiform cloud arising frequency decreased. (6) The northward watervapor transportation from the Indian Ocean to the Bay of Bengal, the northward shift of east airflow over the subtropic Western Pacific near the South China Sea and Indo-China Peninsula, and the westerlies latitudinal watervapor transportation over the Mediterranean Sea and the Caspian Sea are the three channels for water vaportransportation which affect the autumn stratiform cloud arising frequency and then affect the autumn precipitationin the Qilian Mts. region.
