Spatial-temporal Distribution Characteristics And Numerical Simulation Of Summer Dense Fog In Jiangsu

Based on the consecutive observational data per ten minute from 70 surface stations on the National Automatic Weather Observational Net during from 2013 to 2016 in Jiangsu Province,the spatial and temporal distribution characteristics of summer dense fog and its meteorological influence factors are analyzed.Meanwhile,the synoptic situation at both high and low altitude during the process of summer dense fog are concluded.From the above study,the Weather Research and Forecast Model for Version 3.8.1(WRF),a mesoscale numerical forecasting model with the high spatial and temporal resolutions is used to simulate a summer dense fog event,and the observation data is used to verify the results.At the same time,according to the physical quantities based on the WRF model output,the mechanism of summer fog formation and elimination is diagnosed and analyzed from the aspects of water vapor conditions,inversion layer,turbulent mixing and radiative cooling.The main conclusions are as follows:The results of statistical analysis show that:(1)The heavy fog is mainly formed under the condition that the temperature is less than 29 °C,the wind speed is lower than 3 m/s and prevailing in the easterly wind.The low temperature and high humidity during the Meiyu period are the reasons for summer heavy fog with a higher frequency(42.4%)in June.(2)The diurnal variation of summer heavy fog is significantly different from the diurnal variations in autumn and winter.The heavy fog mainly appears from 0:00 a.m.to 6:00 a.m.and dissipates from 54:00 a.m.to 8:00 a.m,and the duration is mainly within 6 hours.(3)The frequency of heavy fog presents a general decrease trend from the northeastern coastal region to the southwestern inland because the cooling rate in the north of the Huaihe River of Jiangsu is higher than that in the southern Jiangsu.(4)The weak rainfall of 6-24 hours before the fog formation makes the near-surface layer to reach condensation,after that,the weather clears with the stable atmosphere statically is beneficial to the appearance of summer heavy fog.The results of weather pattern classification show that:(1)There are six kinds of surface weather situations in Jiangsu during the summer dense fog,which are the low pressure inverted groove,high pressure rear,high pressure rear pressure equalization,high pressure front equalization,high pressure into the sea rear and northeast low pressure westward extension control.There is no obvious control of high and low pressure in summer when dense fog occurs.The pressure field is uniform and the ground wind speed is small.Frontal fog occurs mainly when the surface weather situation is low-pressure inverted groove type,advection fog occurs mainly when the surface weather situation is high-pressure rear type,radiation fog appears mainly when the surface weather situation is high-pressure front type and high-pressure rear type or low-pressure westward extension control type.(2)The 500 hPa high-altitude weather situation in Jiangsu during summer dense fog mainly includes four weather situations: low trough front type,low trough transit type,northeast low trough rear type and northeast low trough front type.Frontal fog occurs mainly when the 500 hPa high-altitude weather situation is low trough front type.Advection fog appears mainly when the 500 hPa high-altitude weather situation is low trough front type or low trough transit type.Radiation fog occurs mainly when the 500 hPa high-altitude weather situation is northeast low trough rear type or northeast low trough front type.The results of numerical simulation show that:(1)During the process of this summer dense fog,influenced by the northwest cold advection,Jiangsu area is in the pressure-sharing field controlled by high pressure for a long time,and the surface wind speed is weak,which is an important weather condition for the fog formation.(2)The numerical simulation results of fog area,surface meteorological elements and visibility are basically consistent with the actual situation.The WRF model can accurately simulate the fog generation and disappearance process and visibility intensity.(3)Pre-precipitation is the main source of water vapor during the fog process.Pre-precipitation region is in good agreement with the post-fog region.The divergence of surface water vapor accelerates the dissipation of the fog.The inversion layer near the stratum(below 500m)provides a stable condition for the formation and development of fog.The mixing of turbulence plays an important role in the formation and development of fog.Radiation cooling at night is an important cause of fog formation,and shortwave radiation warming after sunrise is the direct cause of fog rapid attenuation and dissipation.