The seasonal and intraseasonal variability of diurnal cloud activity over the Tibetan Plateau

Seasonal variation of diurnal cloud activity (abbreviated DCA) over the Tibetan Plateau throughout the year is examined using 3-hourly geostationary meteorological satellite (GMS) data for 6-years (1989-1994). The DCA shows two distinct variance maxima in the seasonal cycle. One is in spring (pre-monsoon season), and the other is in the summer monsoon season. The DCA begins in late January, and reaches its maximum from March through April. The active DCA extends over almost the whole of the plateau, especially over the southern part (around 30degreesN, 90degreesE) and the zonally oriented belt between 35degreesN, 80degreesE and 31degreesN, 102degreesE. A short interval between the two active DCAs is found around late May to early June. The DCA starts again over the southeastern region of the plateau (centered around 30degreesN, 101degreesE) in June and moves up to the southern region. From July to August, the DCA is most active over the southern region (around 30degreesN, 86degreesE). After September, the active DCA retreats to the southeastern region. In both spring and summer, the amplitude of the DCA fluctuates on intra-seasonal time scales. In the active period of the DCA in spring, corresponding to the meandering of the upper-level mid-latitude westerly to the south of the plateau with a trough, the cold air mass at the upper-level is clearly seen over the plateau and weak wind speed is observed through the plateau troposphere. This atmospheric structure and a heating of the lower atmosphere during daytime are likely to be responsible for the enhancement of the DCA in spring. In contrast, during summer monsoon season, the increase of humidity and temperature are identified at the lower atmosphere over the plateau, associated with a humid and warm air intrusion from the South Asian monsoon area into the lower atmosphere over the plateau and precipitation due to active convection. This indicates an increase of the instability for moist convection. These features of the atmospheric circulation and the surface heating during daytime induce active moist convection. Corresponding to the enhanced DCA over the southern part of the plateau, the center of the Tibetan high is located over there. A possible mechanism for the intraseasonal variability of the DCA associated with that of the upper-level atmospheric circulation is also discussed.