Dynamic synthetic analysis of circulation field of tropical cyclones affecting Shanghai

Eleven tropical cyclones(TCs)affected Shanghai and crossed the same latitude as Shanghai from 2007 to 2018.According to similar tracks from best-track data,TCs that cause significant precipitation in Shanghai can be divided into three types:landfall TCs,nearshore northward TCs,and western TCs.Based on ERA5 reanalysis data,the dynamic synthesis method was used to synthesize TC circulation situations to compare thermal,dynamic,water vapor,and stability conditions within TC circulations during the period when they affected Shanghai.The conclusions are as follows.1)When the three TC types are at the same latitude as Shanghai,they are all in the divergent field in the upper troposphere.For the landfall type,the subtropical high at 500 hPa is stronger and farther north than usual,and there is a high-pressure dam on the north side of the TCs.2)The warm advection of the three TC types at 925 hPa is located in the northern quadrant of the TCs.The dynamic and water vapor conditions are good in the north-western quadrant of landfall and western TCs,and more favorable in the eastern quadrant of nearshore northward TCs.3)The favorable effects of all three types on precipitation in Shanghai come from the boundary layer.Water vapor,upward motion,and instability conditions of landfall TCs are superior to the other two TC types.The best water vapor,dynamic,and convective instability conditions are at the northern boundary in Shanghai during landfall TCs,and the main sources of water vapor in Shanghai come from the eastern and northern boundaries.During nearshore northward TCs,the main contribution to precipitation is from the eastern boundary,while better dynamic and water vapor conditions come from the western and northern boundaries during western TCs.The above findings provide scientific and technical support for operational forecasting precipitation from TCs affecting mega-cities.

Comparative analysis of dry intrusion in the different position of pre-TC squall line on typhoon Lekima(1909) and Matsa(0509)

Using conventional observation data,FY2 satellite data,typhoon data and NCEP reanalysis data,the different position of Pre-TC squall line accompanying the similar tracks with"Lekima"(No.1909)and"Matsa"(No.0509)is studied.The article analyzes the circulation background and trigger mechanism generated by the two pre-TC squall lines,and uses various physical quantities such as low-level vertical wind shear,humidity,wind field,wet potential vorticity,and vertical vorticity.Also,try to study the dynamic mechanism of dry intrusion during the process using the Slantwise Vorticity Development(SVD)theory.The results show that:the high-altitude system and the ground trigger mechanism produced are significantly different.On the high-altitude system,the"Lekima"squall line is generated between the continental high pressure and the typhoon with the northeast jet.The"Matsa"squall line occurred between the typhoon and the subtropical high,and near the typhoon inverted trough,which was generated with the strong easterlies.On the ground trigger mechanism,the"Lekima"squall line is generated at the convergence line under the intersection of northeast and northwest air currents.The"Matsa"squall line is generated in the ground inverted trough moves westward and meridional degree increases.During the two pre-TC squall lines,low-level vertical wind shear values are above 16 m·s-1.The dry air intrusion in the lower layer came from the upper and middle troposphere in the vertical direction,and in the east from the west along the latitude in the horizontal direction.Theoretical analysis base on the development of wet potential vorticity,vertical vorticity,and inclined vorticity indicate that the downward transmission of the high potential vorticity band on the upper troposphere,the level of dry invasion,and the air mixing of equivalent potential temperature at high and low levels play an important role in the generation and development of the preTC squall line.The slantwise vorticity has a good corresponding relationship with the vertical vorticity over the pre-TC squall line.The two development processes of the pre-TC squall line can be better explained by using the SVD theory.