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.

The weakening effect of urbanization on tropical cyclone surface winds:An observational study for Shanghai

The observation-reanalysis difference method(OMR)and wind profilefitting method were used to analyze the influence urbanization has on the near-surface wind speed in Shanghai during tropical cyclone events.The data used include daily wind speed data from the Shanghai Meteorological Observatory Station collected from 1991 to 2020,hourly wind speed data from 100 m high wind towers from 2017 to 2019,and reanalysis data that covered the same period.The results indicated that during tropical cyclone occurrence,the wind speed of the OMR in the central urban region was significantly lower than that in the suburban area,and the OMR declined more obviously over the year,down to-8 m/s in 2020.Urbanization leads to the increase of average wind weakening rate to be higher than the maximum wind weakening rate,causing the maximum weakening rate of the central urban region to the average wind over 80%,while maximum wind speed was less than 80%.The weakening rate of both the average and maximum wind speeds during tropical cyclone impacts is greater than the rate that the annual average wind speed was reduced.A logarithmic variation was visible in the wind profile of the island station during tropical cyclone occurrence,with an average friction velocity U*of 0.9389 m/s and an average rough length Z0 of 0.4915 m.The wind speed during tropical cyclone events was higher than the three-year average wind speed within each layer.The suburban stations show a linear variation,and every hundred meters of height results in an increase of 5–6 m/s in the wind speed change rate.At 100 m of altitude,the wind speed in the suburban region is reduced by approximately 40%.