南海夏季风爆发与华南前汛期锋面降水气候平均的联系

利用1958-2000年NCEP/NCAR再分析日平均资料、中国气象局气候中心常规地面观测日降水资料,从气候平均角度诊断分析了南海夏季风爆发和撤退前后大气结构特征及其与南亚季风的差异,探讨华南前汛期锋面降水对南海夏季风爆发的可能影响。结果表明:①季节转换期间南海地区大气热力结构、动力结构的配置具有与孟加拉湾和南亚地区明显不同的特征,大气低层(850 hPa以下)温度梯度的逆转(由负变正)发生在西南季风爆发之后。②850hPa西风建立在南海大气低层(850 hPa以下)经向温度梯度为弱负值的时候,是受热成风约束的结果。③季节转换期间南海地区大气热力结构、动力结构的配置具有独特性,是由于东亚地区独特的地理位置,受来源于中纬度冷空气影响的缘故。④随着华南降水强度加强,对流释放潜热加热了中高层大气,有利于南海经向温度梯度的逆转,从而在热成风关系约束下使高层南亚高压的北移,因此华南前汛期第一阶段锋面降水是南海夏季风爆发的有利因素。

Relationship between Onset of South China Sea Summer Monsoon and South China Frontal Precipitation in the First Raining Period on Climatological Mean

The daily NCEP/NCAR reanalyzed data and daily rainfall data from National Climate Center of China Meteorology Administrator for the period of 1958-2000 were employed to diagnose the dynamic and thermodynamic characteristics associated with the onset and retreat of South China Sea summer monsoon, as well as the differences compared with South Asian monsoon on climatological mean. Possible impacts of precipitation in the yearly first raining period in South China on the onset of South China Sea summer monsoon are explored. The conclusions are as follow: (1) The allocation of atmospheric dynamic and thermodynamic characteristics over the South China Sea is distinctively different from that in the Bay of Bengal and South Asia during seasonal transitions. Tropospheric temperature meridional gradients under 850 hPa reverse (from negative to positive) later than the onset of southwest monsoon in South China Sea. (2) Under the constraint of thermal wind relation, 850hPa westerly onsets when the tropospheric temperature meridional gradient at 925hPa and 1000hPa is still negative. (3) During seasonal transitions, the allocation of atmospheric dynamic and thermodynamic characteristics over the South China Sea is distinctive owing to the unique geographic position and the impact of cold air from middle latitudes. (4)With reinforcing rainfall in South China, convection latent heat on the middle upper troposphere favors the reverse of the temperature meridional gradient. South Asia high in the upper troposphere consequently moves northward under the constraint of thermal wind relation. Therefore, South China frontal raining in the yearly first raining period is a beneficial factor for the onset of South China Sea summer monsoon.