副高断裂前后四川盆地一次暴雨过程的比较分析

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

利用NCEP1°×1°6 h再分析资料,对副热带高压与西风槽典型环流形势配合下发生的一次四川区域性暴雨过程的不同阶段进行对比分析。结果表明,前期暴雨天气过程,其动力条件占到了主导地位,具有明显的经向垂直环流圈和垂直上升运动支,而在副高断裂后较强冷空气作用下,在副高边缘发生的区域性暴雨过程受西风带槽前的能量锋区影响,动力强迫作用和热力强迫作用激发的次级环流,进一步加强了四川盆地垂直运动的发展;冷空气作用前期的暴雨过程和冷空气进入后副高边缘发生的区域性暴雨过程中暴雨区域内的假相当位温均强于高层,大气处于对流性不稳定层结状态,对四川盆地暴雨的增强也起了不可忽视的作用,但由副高控制到副高逐渐断裂,湿位涡的斜压扰动是逐渐增强的过程,导致倾斜垂直涡度发展,激发更为强烈的上升运动;副高与西风槽环流形势相配合的暖区暴雨过程水汽主要来自中低层孟加拉湾;而副高断裂后发生在副高边缘的区域性暴雨过程,水汽主要来自850hPa层南海和孟加拉湾,从对流层中到低层,四川盆地东部恰恰是冷暖气流的交汇处,偏南气流将海上充沛的水汽输送到盆地东部,为暴雨的发生提供充足的水汽条件,并与对流层低层秦岭附近的东北冷气流交汇。

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.