Why Are There More Summer Afternoon Low Clouds Over The Tibetan Plateau Compared To Eastern China?

In this study, we analyze the relationships between summer afternoon low cloud cover and environmental conditions over the Tibetan Plateau (TP). Using in situ measurements, satellite data, and reanalysis, and based on theoretical analysis, we find that there is stronger thermal turbulence, lower temperature, and higher frequency of low cloud formation for the same surface relative humidity over the eastern and central TP compared with eastern China. With the same sensible heat flux, decreased air density enhances buoyancy flux, which increases the planetary boundary layer height and moisture vertical transport. At the same time, with the same near-surface relative humidity, lower temperature over the TP decreases the lifting condensation level, which increases the probability of the air parcel reaching this level. Compared to the low-elevation region in eastern China, these two mechanisms enhance low cloud occurrence in the afternoon over the TP.Plain Language Summary Previous studies suggested that planetary boundary layer processes play an important role in the formation of more low clouds in the afternoon over the Tibetan Plateau (TP). However, detailed mechanisms for the differences in low cloud occurrence in the afternoon over the TP versus eastern China (low-elevation region) remain unclear. Using in situ measurements, satellite data, and reanalysis, and based on theoretical analysis, we first quantify the more frequent low cloud occurrence over the TP, and then propose two mechanisms: (i) compared to eastern China, in the afternoon over the TP, the lower near-surface temperature results in a lower height where condensation occurs (given the same relative humidity), and (ii) the lower air density (given the same surface sensible heat flux) results in a larger buoyancy flux, leading to a deeper boundary layer. These two mechanisms work together to increase cloud formation in the afternoon over the TP.Key PointsThere are more summer afternoon low clouds over the Tibetan Plateau (TP) than over eastern China as a result of two mechanisms Compared to eastern China, with the same relative humidity, lower temperature over the TP results in a lower lifting condensation level With the same surface sensible heat flux, lower air density over the TP results in a larger buoyancy flux and a deeper boundary layer