Characteristics of the Summertime Boundary Layer and Atmospheric Vertical Structure over the Sichuan Basin

The Sichuan Basin (SCB), located east of the Tibetan Plateau, is surrounded by mountains. This study examines the influence of terrain and weather events on the diurnal cycle of the planetary boundary layer (PBL) and the vertical structure of the atmosphere over the SCB, by utilizing field data obtained mainly from Wenjiang Station (western SCB) for the period 20 June-19 July 2008. The results show inversion layers of specific humidity and prevailing wind directions in the lower troposphere from the southwest and northeast. Migration of the 500 hPa trough to the western SCB results in the formation of low-level jets and cyclonic circulation, induced by terrain obstruction of northward airflow entering the basin from its southeast corner. Migration of the 500 hPa trough to the west (east) of Wenjiang Station results in an abruptly decreased (increased) surface downward short-wave radiation flux, which is associated with an increased (decreased) cloud fraction. In turn, this results in decreased (increased) upward sensible and latent heat fluxes, and a decrease (increase) in the elevation of the mixing layer (ML). Rapid increase in the ML height depends not only on the intensity of nocturnal temperature inversions, but also on the stability of the lower troposphere. At Wenjiang Station, the period prior to rainfall events is marked by increasing water vapor, instability and wind speed, and the wind direction changes from westerly to easterly. Subsequent to rainfall events, the PBL is rapidly restored to its normal state. The PBL is an important source of water vapor for rainfall. An increase in water vapor in the lower troposphere during night-time contributes to nocturnal rainfall. Basin terrain may play an important role in the formation of the southwest China vortex.