Formation of mesoscale convective systems over the eastern Tibetan Plateau affected by plateau-scale heating contrasts

This study revealed the formation processes of mesoscale convective systems (MCSs) over the eastern Tibetan Plateau (TP), associated with the establishment of subplateauscale convergences affected by the longitudinal surface wetness gradient. Composites of reanalysis data and satellite infrared images showed that large MCSs occurred in the afternoon under the condition of the eastward expansion of upper tropospheric anticyclone with the enhancement of near-surface low pressure in the western plateau. The low-level convergence was systematically formed through the eastward propagation of a thermally induced cyclonic circulation formed in the north central plateau before the day of the MCS genesis. A numerical model successively simulated the MCS occurrence processes with the diurnal evolution of convergences, namely, (1) the formation of horizontal wind shear between southwesterlies and northeasterlies over the strong heated land-surface causing a thermal low in the northwest, (2) the eastward propagation of the vorticity due to intensification of upper westerlies in the night, and (3) the MCS genesis by low-level convergences behind the migrated vortex with a convective instability condition over the eastern wet land surface. Numerical sensitivity experiments confirmed that eliminating either the western surface sensible heat flux or the eastern surface latent heat flux prevented the development of MCS. Namely, the enhanced southeast-northwest gradation of the plateau-scale soil moisture distribution could effectively form the MCSs in the eastern plateau during the monsoon season.