ANALYSIS OF RAINSTORMS ASSOCIATED WITH SIMILAR TRACK TROPICAL CYCLONES HAITANG (0505) AND BILIS (0604)

It is generally thought that the influence of comparable track typhoons is approximately similar, but in fact their wind and especially their rainstorm distribution are often very different. Therefore, a contrastive analysis of rainstorms by tropical cyclones (TCs) Haitang (0505) and Bilis (0604), which are of a similar track, is designed to help understand the mechanism of the TC rainstorm and to improve forecasting skills. The daily rainfall of TC Haitang (0505) and Bilis (0604) is diagnosed and compared. The result indicates that these two TCs have similar precipitation distribution before landfall but different precipitation characteristics after landfall. Using NCEP/GFS analysis data, the synoptic situation is analyzed; water vapor transportation is discussed regarding the calculated water vapor flux and divergence. The results show that the heavy rainfall in the Zhejiang and Fujian Provinces associated with Haitang (0505) and Bilis (0604) before landfall results from a peripheral easterly wind, a combination of the tropical cyclone and the terrain. After landfall and moving far inland of the storm, the precipitation of Haitang is caused by water vapor convergence carried by its own circulation; it is much weaker than that in the coastal area. One of the important contributing factors to heavy rainstorms in southeast Zhejiang is a southeast jet stream, which is maintained over the southeast coast. In contrast, the South China Sea monsoon circulation transports large amounts of water vapor into Bilis – when a water-vapor transport belt south of the tropical cyclone significantly strengthens – which strengthens the transport. Then, it causes water vapor flux to converge on the south side of Bilis and diverge on the north side. Precipitation is much stronger on the south side than that on the north side. After Bilis travels far inland, the cold air guided by a north trough travels into the TC and remarkably enhances precipitation. In summary, combining vertical wind shear with water vapor transportation is a good way to predict rainstorms associated with landing tropical cyclones.

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 heavy rainfall area between landfalling typhoon LUPIT (2109) and typhoon LISA (9610)

Based on the ERA5 reanalysis data and the surface observations from automatic weather stations, a comparative analysis has been conducted toinvestigate the differences in heavy rainfall distributions caused by two landfalling tropical cyclones (TCs): LUPIT (2109) and LISA (9610). Thetwo TCs have similar tracks, intensity and landing points, but show different asymmetric features in their rainstorm location relative to their tracks.The results indicate that the TC rainfall differences are mainly caused by different rainstorm formation mechanisms. The wind shear contributesmost to the rainstorm of LISA, while land-sea contrast and topographical effect are the main factors of LUPIT rainstorm. Under the influence ofstrong environmental vertical wind shear and the weak cold air invasion from the west, the circulation center of LISA tilts westward with height,which cooperates with the low-level water vapor convergence and vertical ascending movement on the western side of the TC center to jointlycause the heavy rainstorm to the west of LISA center. In contrast, LUPIT has weak environmental vertical wind shear and no obvious structuretilting with height. Topographic effect plays a crucial role in causing the heavy rainstorm on the north of TC center. The southeasterly jet isblocked by the Taimu Mountain in the northeastern Fujian Province, and the strong ascending motion caused by the terrain-induced convergenceappears to the north of LUPIT center. In addition, the moisture convergence is more pronounced in the north and weaker in the south. Theintrusion of weak cold air from the east to the coastal areas of central-northern Fujian, and the moisture-convergence distribution, jointly cause theheavy rainstorm to the north of LUPIT.

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.