| The winter precipitation（mainly snow）in Shanxi was very little.The winter precipitation in cities all over the province accounted for only 2-3%of the annual precipitation.It was dry and cold in winter and the soil moisture was poor,which made it difficult to sow in spring in the second year or inhibited the normal development of overwintering crops.A full understanding of the microphysical structure and snowfall formation mechanism of winter snowfall cloud system in Shanxi can provide a basis for artificial snow enhancement catalytic operation.In order to deeply study the microphysical structure of snow clouds in winter in Shanxi,this paper analyzed the structural characteristics and main microphysical processes of snow clouds affected by different weather systems by using aircraft,radar and other observation data,as well as the mesoscale WRF model.The observation of typical blizzard cloud system affected by plateau trough and reflux inverted trough on 29 November 2011（case 111129）showed that block strong echo of more than 30 d BZ was embedded in the echo of large stratiform cloud of 10～20 d BZ in the strong stage of snowfall development.There were three inversion layers below 3.2 km,which inhibited the growth of riming and condensation in the cloud.The greater the inversion intensity was,the greater the inhibition effect was.The maximum ice and snow crystal number concentration and ice water content appeared near-9.3℃,which was located in the upper part of the stratified mixed snowfall cloud,mainly due to the aggregation,collision,multiplication of ice and snow crystals.The ice crystal images at-14.4～-19.7℃were mainly acicular,columnar and irregular,and it was dominated by the condensation of ice crystals.The distribution of ice and snow crystal spectrum can be well fitted by using exponential form.In order to deeply study the evolution characteristics of cloud structure of snowfall clouds at different development stages,aircraft detection was carried out at different development stages of heavy snow cloud system under the influence of westerly trough and reflux inverted trough on 29 November 2019（case 191129）.According to the observation,during the first detection the cloud top temperature was-10℃,it was detected to generate less than 200μm plate-shaped and acicular ice and snow crystals by condensation.Most snow particles below 2.0 km did not reach the ground due to the decrease of relative humidity.With the development of snow clouds,during the second detection the cloud top temperature was-12.5℃.Ice and snow crystals in the cloud were mainly plate,needle,dendrite and aggregated snow crystals.The peak value of ice and snow crystal number concentration appeared near-11℃and-2℃through condensation and aggregation growth.The maximum diameter of snowflakes near the ground was 2800μm.The phase states of the particles in the cloud retrieved by the FY-4A satellite were consistent with those measured by the aircraft.In order to further reveal the shape of snow cloud ice crystal and the distribution law of ambient temperature in the cloud,and explain its formation mechanism,the typical snowstorm cloud system affected by plateau trough and reflux inverted trough（case 111129）and the light snow cloud system affected by short wave trough and reflux system（case120303）were analysed.The type of ice and snow crystal shape was related to the height distribution of ice clouds.In case 111129,the cloud top temperature was lower than-19.5℃.In addition to needle,radial,plate and irregular ice and snow crystals,columnar cap ice and snow crystals were observed in the middle and lower layers of the cloud and columnar ice and snow crystals were observed in the upper layer of the cloud.In case 120303,the cloud top temperature was-12.6℃,and the observed ice and snow crystals mainly include needle,dendrite,plate and irregular shape.During the two snowfalls,the upper part of the cloud was dominated by condensation growth,and the middle part of the cloud was dominated by aggregation and riming growth.Case 111129 contained more liquid water,and the degree of riming was higher than case 120303.The exponential function can accurately fit the snow and ice crystal spectrum during the two snowfalls.The N0value of the snow and ice crystal spectrum fitting parameter of case 111129 was larger than that of case 120303,the variation range ofλwas one more order of magnitude.Based on the observation,case 111129 was studied by WRF model and compared with the observation.WRF3.9.1 can better simulate the distribution characteristics of snow cloud echo band,including simulated echo intensity,echo distribution,echo top height and strong echo center,and the simulated total precipitation and snowfall were slightly weaker than the observation.In terms of the vertical distribution of water condensate particles,the model can accurately simulate the vertical distribution of cloud water in the cloud,but the vertical distribution of ice crystals and snow crystals was inconsistent with the observation,which may be caused by the fact that WRF3.9.1 model did not consider the shape of ice particles and ignored the aggregation and growth process of snow particles.WRF4.3 model can better simulate the distribution of ice crystal morphology with height.The sensitivity test with no ice multiplication and doubled ice multiplication showed that doubled ice multiplication led to the increase of surface precipitation and was closer to the observed value. |
