Numerical Simulation And Diagnosis On Torrential Rainfall Amplification And Its Cloud Microphysics Process Of Severe Tropical Storm Bilis(0604)

The strong tropical storm Bilis(0604) caused severe disasters in Chinese mainland, which is get noticed for a long time because of unique track and distribution of wind and precipitation. Based on NCEP/NCAR1°×1°Preanalysis data, RSMC0.25°×0.25°eanalysis data and MICAPS observational data, the landfall process of Bilis is researched, especially the rainfall amplification at the edge of Guangdong, Jiangxi and Hunan provinces. Results show that Bilis is surrounded by high pressure in the landfall process, which lead that Bilis move to the west slowly; The monsoon surge onset and the Water vapor transport which supply abundant potential heat to the typhoon circulations play an important role that Bilis not weakened but continue for a long time. By using the harmonic-cosine series expansion approach, the horizontal wind field is diagnosed on the precipitation amplification, the result show that the irrotational wind change more significantly compared with the horizontal wind field during the precipitation amplification, because that irrotational wind can present strong convergent signals of the atmosphere, and which is the important condition of the cloud and rainfall.Non-hydrostatic mesoscale numerical model ARPS is used, focusing on the high-resolution numerical simulation of the landfalling and rainfall amplification process of Bilis. Detailed comparisons are taken between observations and the simulation; the high-resolution simulation data is used to perform diagnostic analysis on the precipitation amplification. Results show that the simulation reproduced the development and landfalling process of Bilis and the rainstorm amplification; the two parameters-vertical divergence flux and the moist potential vorticity agree fairly well with the precipitation distribution during the rainstorm amplification. They both have some indicative meanings to the forecast of strong precipitation. The distribution of vertical divergence flux exhibits the vertical dynamic structure that convergence and ascending motion occurs in lower troposphere, and the divergence and descending motion in middle-high level over the rainfall area. The moist potential vorticity identifies the strong signals of precipitation amplification. The reason lies in that moist isentropic surfaces tilt due to the convergence and upward transportation of water vapor, which cause the anomaly of moist potential vorticity.In a cloud-microphysical perspective, the differences of cloud microphysical processes between before and during the precipitation amplification and possible cloud microphysical cause of the rainfall amplification by using high-resolution simulation data are discussed. The results show that the cloud-microphysical characteristics between the above two periods is significantly different. With the distinct increasing of the rainfall intensity, the cloud hydrometeors contents increase markedly, especially those of the ice-phase hydrometeors including ice, snow and graupel, contributing more to the surface rainfall. The clouds develop highly and vigorously. Comparisons of conversion rates of the cloud hydrometeors between the above two periods show that the distinct increases of cloud water content caused by the distinct enhancement of water vapor condensation rate are consumed to contribute to the surface rainfall mainly in two ways. Firstly, rain water content increases significantly by accretion of cloud water by rain water, which thus contributes to the surface rainfall. Secondly, the accretion of cloud water by snow increases significantly the content of snow which is then converted to graupel by accretion of snow by graupel, and then the graupel melts into rain water which subsequently contributes to the surface rainfall amplification.