Comparative Analysis of the Influence of the Subtropical High on the Torrential Rainfall in Sichuan Basin

Based on dynamicand thermodynamic diagnosis,a case study is performed by employing a 6 hourly and 1°×1°reanalysis data achieved at standard pressure levels for the period of 2-5 September 2006 in Sichuan basin.The case can be divided into a heavy rain formation over the subtropical high,and the torrential rainfall in Sichuan basin are affected directly by the Subtropical High when it interacts with westerly trough.Results show that a heavy rain formation over the subtropical high,dynamic effect is very important,vertical circulation cell develops and it has stronger vertical ascent,but when the subtropical high divides,a strong energy front is formed between the westerly trough and the Subtropical High.The thick layer of moist available potential energy is found to the south of the front,which is then transformed to kinetic energy by the strong ascending motion associated with slantwise vorticity development.The secondary circulation is therefore forced both dynamic effect and thermoal effect.Torrential rain then occurs along such a strong energy front with strong low-level cyclonic vorticity development and high-level divergence.Pseudo-equivalent potential temperatures are high at rain area in a heavy rain formation over the subtropical high and the torrential heavy rain at the edge of subtropical high and at rain area pseudo-equivalent potential temperature in the upper level is higher than it in the lower level,so stratification instability is in favor of the intensification of rainfall.The baroclinic disturbances of moist potential vorticity enhanced lead to the development of tilted vertical vorticity,stronger ascending motion is intensified.The water vapor of heavy rain formation over the subtropical high come form Bay of Bengal in 850hPa level.But during a heavy terrain rain process happened at the edge of subtropical high period,abundant water vapor come form Bay of Bengal and South Chian Sea in 850hPa level.The four flow intersect in the east Sichuan area in middle and low level of troposphere.Water vapor over ocean is transferred towards Sichuan area,and confluence with the north cold flow of the Qing Ling.The water vapor fluxes are converged with.