Increasing atmospheric temperature in the upper troposphere and cumulus convection over the eastern part of the Tibetan Plateau in the pre-monsoon season of 2004

Abstract

In the spring of 2004, we implemented intensive field observations in the eastern part of the Tibetan Plateau. In this study, we use the field observation data, as well as satellite observation data and reanalysis data, to examine increases in diurnal potential temperature and variations in atmospheric conditions over the eastern part of the Tibetan Plateau. The results of intensive radiosonde observations reveal a significant temperature increase attaining to the tropopause in April. The depth of the mixing layer, estimated from the vertical profile of potential temperatures, was much smaller than the height of the temperature increase. This result indicates that the temperature rise was not solely caused by thermally induced dry convection. During the observation period, strong cloud activity was observed. Cloud-top heights derived from GOES-9 and radiosonde observations indicate that active cumulus convection coincides with the temperature rise in the upper troposphere. For small increases in temperature, lower cloud-top heights are observed. Numerical simulations were also conducted using a regional atmospheric circulation model and radiosonde observation data as initial conditions. The results of the numerical simulations with wet and dry conditions and ideal wind direction (no meridional wind component) reveal that in the wet case air is more efficiently warmed in the upper troposphere with cloud activity. By introducing the observed wind direction as an initial condition, the simulation predicts strong cloud convection over a wide region of the model domain and a greater increase in potential temperature; this result is comparable to observed data. In previous studies, sensitive heat transfer by thermally induced dry convection was thought to be the main cause of pre-monsoon atmospheric heating over the Tibetan Plateau; however, the results of the observation analysis and numerical simulations described in the present study suggest that strong cloud convection plays an important role, even during the period prior to the rainy season. © 2007, Meteorological Society of Japan.