Simulation Of Local Circulation Characteristics, Causes And Urbanization Effects In The Sichuan Basin In Winter

In order to study the response mechanism of local circulation in the basin to urbanization in winter,this paper uses the ERA-Interim wind and temperature field reanalysis data from the last 30 years from 1990 to January 2019 firstly to decompose the horizontal wind field into flow and climb The components of the current are discussed in terms of the thermal and dynamical causes of the local circulation in the Sichuan Basin in winter.Then,the three phases of urbanization development since the21 st century(2005,2010,and 2015)are combined,and the data are analyzed using NCEP to compare and analyze the characteristics of the day and night differences of the local circulation in the basin under the urbanization process in winter are also discussed.Finally,the introduction of the mesoscale numerical forecasting model WRF,which fixes the initial field(January 1 ? 4,2016)and the boundary field,and replaces the actual land use types in different urbanization periods,revealing the local circulation of the Sichuan Basin(hereinafter referred to briefly as basin)to the city.The response mechanisms are as follows:(1)There is a valley wind circulation in the Basin in winter.A vertical meridional profile shows a sinking center on the plateau in the middle layer of the troposphere and a basin,forming two opposite circulation circles,which cool down at night.It converges with the eastern wind in the basin,which is consistent with the path of the mountain wind at night.The temperature difference between the day and night in the basin is the thermal reason for the formation of valley winds.The orbiting and creeping motions are the dynamic reasons.(2)The creeping flow in the Sichuan Basin dominates in winter.Under urbanization,the horizontal wind speed blocked by the urban construction surface decreases,the bypass and creeping flow weakens,and the vertical wind strengthens.A regional urban dry island is formed in the center of the basin,and humidity at night The height of the boundary layer does not change significantly;the absolute vorticity in the basin is positive,and the upper and middle tropospheres are larger than the lower ones.Under urbanization,the absolute vorticity increases,and the high-level convergence is enhanced at night,which is conducive to sinking motion.(3)In this case(January 1?4,2016),the model can well reproduce the day and night valley winds in the basin according to the diurnal variation characteristics of near-surface temperature,which indicates that temperature is the thermal reason for the formation of valley winds.Urbanization reduces the city's downwind horizontal wind speed,deepening the roughness of the lower layer promotes the strengthening of vertical movement,and the increament at night is greater than during the day,and the intensity is weaker than that of the upper layer,which is because the effect of the urban heat island at night inhibits the strengthening of the low-level updraft,and the vertical motion of the low-level increases weakly.At the same time,the urbanization construction caused the height of the boundary layer increased,the low-level floor to turn into an impervious underlying surface,resulting in reduced humidity and the formation of urban dry islands,and the response to the factorized field of the urbanized boundary layer was more intense during the day.In addition,the change in surface energy during the day is more significant,in which the peaks of sensible and latent heat fluxes appear at 13:00 in the afternoon,and the day and night in urbanized key areas generally increase in a consistent manner,while Chengdu shows decrease significantly in the afternoon;The short-wave radiation of the sun increases during the day,while the long-wave radiation of the night decreases.On the contrary,it is because the key areas of urbanization in this case analysis period are mainly cloudy and cloudy,and there are fewer sunny days,indicating that the "dry island" effect of the city is cloudy in winter It is also an important factor that affects the radiation status of the city.The day in Chengdu is mainly sunny,and the result is consistent with that.