Research on Water-Vapor Distribution in the Air over Qilian Mountains

Based on the re,note sensing data, the radiosonde data and precipitation data observed by weather stations, distributions of atmospheric water-vapor and cloud motion wind over the Qilian Mountains are analyzed. Moreover, on the basis of water-vapor and cloud motion wind analyses, relations of atmospheric water-vapor distribution with precipitation, atmospheric circulation, and terrain are investigated. The results show that distributions of atmospheric water-vapor and precipitation in the Qilian Mountains are affected by the westerly belt, the southerly monsoon (the South Asian monsoon and plateau monsoon), and the East Asian monsoon. In the northwest Qilian Mountains, water-vapor and precipitation are entirely affected by the westerly belt, and there is no other direction water-vapor transport except westerly water-vapor flux, hence, the northwest region is regarded as the westerly belt region. In the south and middle of the mountains, water-vapor is mainly controlled by the southerly monsoon, 37.7% of the total water-vapor is from the south, especially in summer, the southerly water-vapor flux accounts for 55.9% of the total, and furthermore the water-vapor content in the southerly flow is more than that in the westerly flow. The southerly monsoon water-vapor is influenced by the South Asian monsoon from the Indian Ocean and the plateau monsoon in the Qinghai-Tibetan Plateau, thus, the south and middle region is called southerly monsoon region. But in the northeast Qilian Mountains, the East Asian monsoon is the main climate system affecting the water-vapor. Besides west and northwest water-vapor fluxes, there are a lot of easterly water-vapor fluxes in summer. The frequency of easterly cloud motion winds in summer half year accounts for 27.1% of the total, though the frequency is not high, it is the main water-vapor source of summer precipitation in this region, therefore, the northwest region is a marginal region of the East Asian monsoon. On the other hand, atmospheric water-vapor, precipitation, and conversion rate of water-vapor into precipitation are closely related with altitudes and circulation system. Generally, there is a peak value of water-vapor content at the altitude from 3500 to 4500 m on the windward slope, but on the leeward slope,water-vapor monotonically decreases with altitude descending except for that in the East Asian monsoon region. Water-vapor on the leeward is much less than that on the windward slope, and the maximal difference in water-vapor content between the two sides may reach about 4.49 kg m-2. Either the values of water-vapor content, precipitation or the conversion rate of water-vapor into precipitation all reach their maxima in the East Asian monsoon regions, and correspondingly the peak value of water-vapor on the windward is also large and occurs at a lower altitude in comparison with other two regions.