Investigation Of Circle Situations And Macro And Micro Physical Processes Of Precipitation Fogs In Nanjing Winter

Fog is one of the disaster weather in winter in China.Study the formation of fog circulation system and macro and microcosmic physical process is to improve the accurate prediction and is the basis of disaster prevention and mitigation.Based on the conventional sounding data and comprehensive observations of fog during the December of 2007 in the surface layer in Nanjing University of Information Science and Technology,combined with NCEP Reanalysis data and 30a month mean December data from NCAR and AO ENSO indexes from NOAA,this paper analyzed the multi-scale features of fog in Nanjing,as well as the evolution of the macro-and microphysical properties of the precipitation fog in December 2007 in Nanjing.Furthermore,a typical case was investigated by using almost all available observational data and high-resolution modeling results of meso-scale model Weather Research and Forecast model(WRF).Through the above research,we come to the following conclusions.1.The six precipitation fogs we studied all occurred in December 2007,while,December 2007 is the month of the most fog days in the past 30 years in Nanjing.It is diagnostically studied on the climate scale with the findings as follows.Positive anomalies of Arctic Oscillation(AO)were favorable to the increase of the atmospheric stability.Two days in advance,water vapor increased over the 20 °N area on the coast of China to act as a source of water vapor for the heavy fog.A Southwest-China cyclone transferred the water vapor.Running dual-variable Empirical Orthogonal Function(EOF)for the surface temperature and atmospheric precipitable amount in East Asia shows that the first EOF mode reflects seasonal change from early winter to prime-time winter,which is conducive to reduce the surface temperature on the large scale,and the second mode reveals the northward advancement of large-scale warm and humid airflow.With the enhancement of the subtropical high,sustained humid weather occurred in Nanjing causing the frequent appearance of fog.2.Comprehensive observations of fog were conducted during winter 2007 in Nanjing University of Information Science and Technology,including the measurements of visibility,rain-drop spectrum,fog,and aerosol spectrums.Salient synoptic-scale features were identified using conventional meteorological observations and NCEP reanalyses.We studied the synoptic situations and macro-micro evolutionary characteristics of six precipitation fog events that occurred in Nanjing during December 2007.The results show that:A.The six events were all mainly controlled by the synoptic system and were in the form of fog compared with light rain.The minimum visibility in these events was more than 250m.in addition,the precipitation fog mostly occurred under the influence of an easterly airstream,and generally weak precipitation occurred first,when the air was close to saturation.An invasion of cold air from the north and vapor condensation finally caused the formation of precipitation fog.B.An inversion layer was always present near the surface before most precipitation fogs,and the T2m-Tsurface changed from positive to negative as inversion layer disappeared during fogs.However,the observation of inversions below 900 hPa was difficult.Altocumulus(Ac)or Altostratus(As)or Stratocumulus(Sc)and Fractonimbus(Fn)occurred in the middle and lower layer before precipitation fogs,while the height of low clouds was 0.3?2.5km.During these processes,the layer below 600 hPa was nearly saturated,most of events occurred with Fn and the height of low clouds markedly decreased.After the fog,the nearly saturated layer may still have been present.In addition,there was significant wind shear near 900 hPa before precipitation fogs.C.The number and concentration of larger and smaller fog droplets increased markedly in the early stages of precipitation fog formation.Aitken mode particles are more efficiently scavenged through nucleation by precipitation fogs in Nanjing,and the reduction in Aitken particles is closely related to wind direction and speed.Because of the influence of a stronger north wind,the concentration of aerosol particles decreased significantly.3.In this study,the Weather Research and Forecasting-Advanced Research WRF(WRF-ARW)Model for version 3.5 was used to simulate a typical precipitation fog that occurred on 27 December.The results show that:two weak signal(weak precipitation and fog)in this process both can be simulated by the meso-scale model WRF.The single station simulation shows that the temperature and cumulative precipitation in urbanization model are closer to the measured compared with the without-urbanization.But with/without-urbanization did not play an important role in the simulation of precipitation fog.That is to say,underlying surface is not the major factor which led to this event.The starting time of simulated precipitation fog is 2 hours in advance than observation and both of which lasted for 4 hours.The simulated LWC(Liquid Water Content)high-value regions were consistent with the low-value area of visibility.Besides,the clouds always appeared below 3km in altitude.And from the LWC(Liquid Water Content),precipitation as well as the spatial-temporal evolution of temperature and humidity stratification,we can deduce that this process is resulted in precipitation which caused the settlement of stratiform cloud.A more intensive simulation in low altitude of vertical layer shows that:A thin temperature inversion layer existed in the near surface before the fog formation.And there was weak radiation cooling effect before sunrise.Then after the sunrise,the heat exchange of turbulence is not so significant and the temperature changed little.The atmosphere transport water vapor to the ground before the precipitation fog.But after the event occurred,this kind of transmission reversed,the vapor of ground was conveyed back to the atmosphere,causing the atmosphere supersaturated,finally leading to the fog formation.