| With the development of urbanization,the urban underlying surface changes.The change of the urban underlying surface will affect local weather and climate.Shanghai,as one typical representative of the rapid development of urbanization in our country,has experienced a remarkable change.At the same time,China is also one of the countries affected by typhoons most.Every year,many typhoons invade our country and cause property losses.As the eastern coastal city in our country,Shanghai suffers the impact of typhoons every year.However,existing studies seldom pay attention to the impact of urbanization development on weather systems such as typhoons.Therefore,studies on the impact of urbanization development in Shanghai on typhoon wind fields are carried out.In this paper,wind speed data of National Meteorological observation Station from 1991 to 2020,wind speed data and reanalysis data of tower station from 2017 to 2019 were used,and urban and suburban comparison method,observation and reanalysis comparison method,wind profile fitting,numerical simulation experiment and other methods were adopted.The influence of the change of urban underlying surface on near surface wind speed and typhoon wind speed in Shanghai area and its possible causes are analyzed.The main conclusions are as follows:（1）In the past 30 years,due to the influence of urbanization,the daily mean wind speed,maximum wind speed,maximum frequency wind speed and wind speed variation near the surface in Shanghai urban area have shown an obvious decreasing trend,especially in summer:The average daily wind speed（maximum wind speed）near the surface of the central urban station decreased by 0953m/s（1.62m/s）every 10years,which was much greater than the 0.375m/s（0.579m/s）of the suburban station,and the urbanization contribution rate was more than 60%.Under the influence of urbanization,the wind profile within 100m near the ground in the urban area shows a significantly different structure from that of island station.The wind profile of island station conforms to logarithmic law,roughness length is 0.3-0.35m and friction velocity is 0.45-0.5m/s.The wind profile of the urban area showed a linear change with height,and the growth rate of wind speed per 100 meters was below 3m/s,which showed a decreasing trend year by year during the study period.The higher the wind speed in the upper air,the faster the wind near the ground increases with the height.The growth rate of wind speed per 100 meters of strong winds above grade 6 can reach 10m/s.（2）In the period of typhoon influence,urbanization makes the weakening rate of urban areas to the mean wind and maximum wind speed increase continuously,which can reach about 80%,which is significantly higher than the urban weakening rate of the annual mean wind speed.During the typhoon period,the wind profile in the low degree urbanization area also presents logarithmic changes.The friction velocity U*is about two times larger than the annual average wind friction velocity,and the rough length Z0has little difference.In moderately urbanized areas,the change rate is greater than the annual mean wind speed,and the wind speed increases faster with height.（3）Based on the numerical simulation and underlying surface sensitivity test of Typhoon Lekima（2019）,the changes of typhoon wind speed in the urban and no-city experiments are compared.The results show that the simulated path of Typhoon Lekima can be well improved in the experiment after improving the underlying surface of the city,in the experiment without urban underlying surface,the wind speed in urban areas is overestimated significantly,and the area with weak land wind speed is smaller.Different degrees of urbanization have different effects on the range and height of wind speed,the city’s ability to reduce wind speed increases as the wind speed increases.The higher the degree of urbanization of the underlying surface,the greater the friction velocity of the urban area,and the closer to the coastline,the greater the friction velocity.Under the influence of typhoon,the contribution rate of the city to the friction velocity is 8%,and in the non-typhoon period,the contribution rate is 0.7%.The existence of urban underlying surface will lead to the increase of urban heat flux,and the higher the degree of urbanization,the greater the heat flux,the heat flux in urban areas will be weakened by typhoons,and the higher the degree of urbanization,the stronger the ability to resist weakening.In this paper,long-term surface observation data,reanalysis data and WRF numerical model are combined to deepen the understanding of the influence of urbanization process on near-surface wind speed change in Shanghai and the influence of cities on typhoon wind speed during typhoon,which provides a certain reference for future refined wind field forecast. |
