A STUDY OF NUMERICAL MODELLING ON THE DEVELOPMENT OF DEPRESSIONS IN THE SOUTH CHINA SEA

The comparison analyses between two tropical depressions in the South China Sea are completed by numerical experiments with a limited area model developed in Guangzhou Institute of Tropical and Oceanic Meteorology. One of the depressions develops and finally becomes a typhoon within 24 hours of the analyzing period (defined as DVD hereafter),and the other not (defined as UNDVD) The analyses show that their initial structures of temperature, humidity, pressure. wind and stability are obviously different from each other. There are a very deep wet layer and a clear warm-cored structure in the mid-lower troposphere in the depression area in the DVD case, but with the warm core in the upper rather than lower troposphere arid a very shallow wet layer in the lower troposphere in the depression area in the UNDVD case. The developing and non-developing processes are successfully simulated by the model, showing that the CISK mechanism plays the most important role in the development of SCSDs (Tropical Depressions in the South China Sea). Several numerical experiments show that the structures of humidity and temperature in the depression area have important effect on the development of SCSDs. It is favourable to the development of SCSDs when a very deep wet layer exists in the mid-lower troposphere or a warm-cored structure exists in the mid-lower troposphere instead of in upper troposphere, and conversely, it is unfavourable to the development of SCSDs when the wet layer is very shallow in the lower troposphere or the warm-cored structure is in the upper troposphere rather than in mid-lower troposphere. The structures of stability for each case are also analysed, which shows evident differences between the two cases, with a deeper instability layer in the DVD case and a shallower one in the UNDVD case. Finally, the sensitivity of the development of the SCSDs to the vertical structures of humidity and temperature in the depression area is discussed.