Atmospheric conditions and increasing temperature over the Tibetan plateau during early spring and the pre-monsoon season in 2008

Abstract

An intensive field observation was implemented for the Tibetan Plateau from early spring to the mature season of the Asian summer monsoon in 2008. Atmospheric conditions in the early spring and the pre-monsoon season are investigated in detail using radiosonde observation data. In early spring, atmospheric profiles show unstable stratification, which can result in strong dry convection. In the mixing layer developed by convection, the potential temperature (PT) increases significantly during the day near the surface. On the other hand, there is no clear tendency in the variation in PT above the mixing layer. In the pre-monsoon season, there is a clear increasing trend of PT at each level from the surface to the upper troposphere. The largest heating amount and trend are observed in the upper troposphere. Since cumulus activities become less frequent in the pre-monsoon season, the upper tropospheric heating in this season cannot be caused by latent heat release, as noted in past studies. On the other hand, the profiles of PT show quite stable stratification and the mixing layer is shallow in this period. Under such a condition, the remarkable heating in the upper layers cannot be due to the development of the mixing layer. In past studies, a dry convection induced by strong surface heating is thought to be a possible cause of atmospheric heating over the Tibetan Plateau in the pre-monsoon season. However, the observation results reveal that such a locally induced convection is not enough for the significant heating. In addition, the temporal variations of PT at the eastern and western part of the plateau show some similarity. This similarity and the characteristics in dry and wet convections denote some synoptic scale processes may have important roles in the heating over the Tibetan Plateau in the pre-monsoon season. © 2012, Meteorological Society of Japan.