Relationship between Topography and Daytime Cloud Activity around Tibetan Plateau

The relationship between topography and daytime cloud activity over the Tibetan Plateau and surrounding areas was examined by using Geostationary Meteorological Satellite (GMS) Visible (VIS) and Infrared (IR) images during the premonsoon, and monsoon periods in 1998. Previous studies using IR images have already confirmed the strong diurnal variation of convective activity over the Tibetan Plateau. This study relies primarily on VIS images to analyze daytime cloud distribution because VIS images are more adaptable to low-level clouds and they offer better spatial resolution than IR images. IR images are used to judge whether cloud tops are high or low. High-level clouds are prominent over the large-scale (100-300 km) mountain ranges, but fewer clouds are observed in the major valleys in the plateau during premonsoon and monsoon periods at 15 Local Solar Time (LST), when the amount of cloud cover is at its maximum from 09 LST to 15 LST. However, the relationship between cloud distribution and topography is not always clear when the horizontal scale of the topography is less than 100 km. Less cloud coverage was observed over the plateau in the morning during the premonsoon period. On the contrary, clouds frequently appeared over the southeastern part of the plateau in the morning during the monsoon period. Low-level clouds often cover the southern slope of the Himalayas, and the frequency of cloud coverage exceeds 75% at 15 LST in the monsoon period. While the cloud tops are growing high elevation in the afternoon over the plateau, low-level clouds cover the southern slope of the Himalayas through the daytime (09-15 LST).