Seasonal variation of cloud activity and atmospheric profiles over the eastern part of the Tibetan Plateau

Abstract

Cumulus activity over the Tibetan Plateau has a clear pattern of seasonal progression. Activity occurs with significant frequency in the eastern part of the plateau from April to mid‐May. From mid‐May to mid‐June the frequency of cumulus activity drastically decreases and then increases after mid‐June. Causes of such definite seasonal progression are revealed by in situ radiosonde observation data. In early spring the vertical profile of potential temperature is almost uniform from the surface to 7000 m above sea level around noon, and atmospheric stratification is vertically neutral for dry convection. Under such a condition, dry convection easily occurs and develops cumulus cloud that is frequently observed. From mid‐May to mid‐June the atmosphere has conditionally unstable stratification, but the atmosphere is relatively dry and hardly saturated and cumulus activity occurs less frequently. Though the atmosphere is also conditionally unstable after mid‐June, the increase in total precipitable water content allows easy atmospheric saturation and cumulus activity recommences. At the same time, satellite data and reanalysis data show, in the eastern part of the Tibetan Plateau, a consistent correspondence of the seasonal variation of cumulus activity, total precipitable water content, and vertical instability of the atmosphere. Results of synoptic‐scale analysis indicate the enhancement of cumulus convection in the northern part of India and the Tibetan High cause a moisture increase over the Tibetan Plateau and leads to the recommencement of frequent cumulus activity over the plateau.