| Heavy precipitation weather has caused disasters such as urban waterlogging and landslides,and caused losses to social economy and ecological security.Coastal megacities(such as Shanghai,China)have a large population and are extremely vulnerable to flooding.Although there have been many studies on impact of coastal cities on precipitation,there are few studies focusing on the impact of coastal cities on precipitation with the background of the edge of summer subtropical high.Detailed analysis and study of the impact of cities on the thunderstorm triggering mechanism in this kind of weather background can help to understand the interaction between the urban environment and convection better,help to improve the accuracy of forecasts,and enrich the case studies of existing studies.Therefor,this article uses weather maps,satellite data,wind vector-potential temperature(V-3?)charts,Doppler radar echo charts,etc.to conduct a multi-scale analysis of local strong convective precipitation on July 5,2017 in Shanghai,to preliminary explore the impact of metropolitan on the local heavy precipitation in the afternoon.Afterwards,using the numerical simulation method,a comparative experiment with or without considering the underlying surface of the metropolitan was designed to further explore the mechanism by which the urban effect affects the development of local heavy precipitation.The main conclusions of this article are as follows:1)The multi-scale analysis of the local heavy rainfall event on July 5,2017 in Shanghai shows that before the occurrence of local heavy precipitation,Shanghai was located on the edge of the Western Pacific Subtropical High.And the low-level vapour conditions were good.The atmosphere was warm,humid and unstable.And then warm stratus clouds appeared at the upper layer,resulting in more unstable energy.In the afternoon,the two sea breeze fronts in Shanghai area converged and the atmosphere near the location where two fronts converged were lifted.At the same time,under the guidance of the southwest air current on the edge of the subtropical high,the local heavy precipitation weather was triggered.2)The metropolitan has a thermal effect on the local heavy precipitation that occurred on July 5,2017 in Shanghai.It can be seen from the analysis that the thunderstorm triggered by the double sea-breeze fronts moves to the inland cities along with the sea wind fronts.Under the weak weather background,the local circulation and the instability energy of the underlying surface of the metropolitan have caused strong convection to merge and promote thunderstorm increased.After the sea breeze front dissipated,due to the influence of the unstable energy of the underlying surface of the big city,strong convection still continued to regenerate and merge in the urban area,and under the influence of the southwest steering flow,it moved northeast.3)By comparing the control CTRL test considering the urban underlay with the sensitive CROP test without considering the urban underlay,it can be seen that the presence of the urban underlay increases the height of the atmospheric boundary layer in urban area of Shanghai and the height of convection.Comparing with cropland,the urban underlay increases surface sensitive heat flux and decreases the latent heat flux.The increase of sensitive heat flux makes the lower atmosphere heated up,and the compensating low-level airflow appears in the urban area.In addition,the underlying surface of the city increases the potential temperature of the low-level disturbance in the urban area,and increases the difference of the potential temperature between high and low layers,providing favorable stratification conditions for the occurrence and development of thunderstorms.Thus,the underlying surface of the city makes precipitation areas(especially heavy precipitation centers)more concentrated. |
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