| Snow cloud system of winter mountain is one of important research components in cloud and precipitation physics.The ski game of 2022 Beijing Winter Olympics will be held in Haituo mountain.Thorough understanding the vertical structure and formation mechanism of snow system can provide basic theory for artificial snow enhancement and is important to increase the mountain snowfall river storage.As the development of atmospheric observational technology and numerical model in recent years,many experts have done a lot of investigations in winter snow cloud formation and development mechanism,and had a better understanding in macro-and micro-physical characteristics of snow clouds,which have provided a good foundation for us to carry out the winter snow cloud observation and simulation study in Beijing.The researches of winter snow cloud system in Beijing are insufficient,moreover,the comprehensive observational and numerical simulation studies in vertical dynamical and microphysical structure of winter snow system in Haituo Mountain in Beijing is almost absent.The data of wind profiler radar,cloud radar and microwave radiometer data after quality controlled have high vertical precisions and spatial and temporal resolutions,which can detect snow cloud system continuously.In this paper,by using comprehensive observational data such as automatic meteorological station,wind profiler radar,cloud radar,microwave radiometers,airborne particle measuring system and disdrometer,the macro and micro physical characteristics of winter snow clouds in Beijing Haituo mountain have been obtained.On the basis of reasonable validation of cloud-resolving model,we have acquired the water content distributions and micro-physical processes and analyzed the transformation of micro physical characteristics and specific process in winter snow,so as we can understand the mechanism of winter snow in Beijing Haituo mountain and provide scientific evidence in weather modification,and also can provide guidances in simulation and forecast of snow process.The main conclusions are follow,(1)The weather system that causing the snowfall in Beijing Haituo mountain mainly includes low trough,low vortex and reflow weather.Most snowfall occurred after the enhancement of southwest wind and terrain lifting.When snowfall occurred,the visibility droped rapidly.When northwest wind controlled the whole atmospheric layer,the snow could stop.The snowfall often lasted for a long time,75% of all snowfall cases more than 5 hours,about 40% of all snowfall cases lasted for 5-10 hours,35% in all cases was larger than 10 hours;mainly snowfall cases occured at night,the max snow rate most appeared at the beginning of snow process,accounting for 65%.(2)The cloud radar was used for continuous observation of snow process on March 23 to 24,2017 in Beijing Haituo mountain.The radar echo was corrected by the method of bin-by-bin correction method,and the attenuation correction was proved to be effective when compared with the weather radar,the data of cloud radar after correction can obtain the precise vertical feature of macrostructure and evolution characteristics of snow clouds.The echo of snow clouds was weak,max echo < 20 DBZ,the top of echo < 7 km,the height of strong echo area < 3 km.There was a positive correlation between the surface snowfall and the low-level strong echo,the time of low-level stronger echo corresponds to larger snowfall.The updraft appeared before the snowfall,the spectral width value was large;when the snowfall started,the updraft disappeared,the spectral width value was small,and the LDR value was also small,it may because the snowfall particles are small,it may also because the phase of various particles in snow clouds were relatively single,and there was almost no mixed phase in snow clouds.(3)The wind field derived from Wind profile radar shows that the snow was occurred at the time when the southwest wind appeared in the middle and lower layers.When the southwest wind went stronger,the snowfall went bigger.The snowfall stopped after the southwest wind disappeared.It can explained that the southwest wind in the middle and lower layers provides favorable water vapor conditions for the occurrence and development of snowfall.The change of vertical velocity could reflect the occurrence and evolution of the whole snowfall.The microwave radiometer data showed that the liquid water content of the snowfall was low this time,and the liquid water would be consumed during the snowfall frozen process.The time when the snowfall intensity was high corresponded to the time that the liquid water content was consumed.(4)The cloud system observed by aircraft in the northwest area of Changping and Yanqing of northern Beijing was mainly stratified cloud system with a large horizontal range and long duration.Both liquid water content(LWC)and ice water content(IWC)in the cloud were low,and the total concentration of particles in larger scale was also low.The particle spectra at all altitudes observed by aircraft basically showed a Gamma distribution.The number concentration of small-size particle of cloud water was higher,while the number concentration of large-size ice particle was lower.As the altitude decreased,the concentration of small particles in cloud water gradually decreased,and the diameter corresponding to the spectrum peak increased,which may caused by the aggregation process of particles.(5)Similar to the particle spectrum observed by aircraft,the snowfall particle spectrum observed by the ground particle spectrometer also showed a Gamma distribution.The concentration of small particles was high,and snowfall particles with a diameter of D>4mm was rare.The peak value and variation trend of snowfall particle spectrum were closely related to the snowfall intensity.The snowfall process was mainly composed of small size graupel or ice phase particles(D<1.5mm),the max spectral width <3mm.Water vapor supply and dynamics conditions were insufficient.The ice crystals grew only by deposition and aggregation without vigorous development and large snowflakes.The cloud that produced snowfall tended to be stratiform cloud types.(6)The mesoscale WRF model can simulate the basic characteristics of the snow process,and can be used for the detailed study of snowfall.The height distribution of particles in the cloud is: ice crystal > snow > cloud water and graupel > rain.The content of hydrometeors is: snow > cloud water and graupel > ice > rain.The change in the whole process is: snow > cloud water > graupel > ice crystal > rain.The terrain of Haituo mountain tended to force the uplift of airflow and intensified the snowfall on the windward slope.However,this snowfall was located at the edge of the main system,and the supercooled cloud water was less.The snow was mainly formed by the process of ice crystal deposition and snow-ice automatic transformation,and the rime process was small,the overall snowfall process was weak. |
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