| Yunnan Province is a typical low-latitude plateau region w ith frequent intersection of cold and warm air.At the same time,it is affected by the mutual intersection of monsoon,subtropical high,tropical low-pressure system and cold air thus becomes an active area with strong convection activity in China.Strong convection such as hail and thunderstorms are the most important meteorological disaster in Yunnan Province.The complex terrain has obvious influence on the occurrence,development and evolution of strong convective activities.At the same time,the regional characteristics of convective monomers are also very significant.Therefore,the influence and mechanism of different topographic conditions on the evolution and evolution of strong convective activities in different regions of Yunnan are a scientific issue worthy of further study.The main purpose of this study:based on the knowledge of basic characteristics(origin.moving paths,lifespan VIL etc.),trigging mechanism and influencing factors of the strong convective activities(strong precipitation,hailstorm)in Yunnan Province under the complex weather background and topography.to study the vertical structure of radar echoes in different seasons and regions of different types and development phases,thus revealing the differences and reasons on the origin.moving paths,lifespans and developments as well as the vertical structures of radar echoes and the distribution of VIL which could indicate the vertical development of the particles within the hailstorms.In this study,based on the statistical analysis of the weather background and physical quantity characteristics of strong convection in Yunnan Province.934 cases of hailstorm processes were chosen and classified respectively according to their moving paths,life span and VIL features to study the evolution characteristics of strong convection.According to different types,seasons,and regions,vertical structures of radar echoes during the development of strong convective clouds are also been studied.As for the super-cell thunderstorms,numerical simulation has also been used to study and simulate the macroscopic and microscopic features based on the studies of previous evolution mechanism analysis.The main characteristics of strong convective weather in Yunnan Province can be seen below:(1)The circulation situation of strong convection in Yunnan Province mainly included four main weather types:shear line.summer convergence system.tropical low-pressure system(typhoon low-pressure system)and southern branch trough.Due to the influence of topography and weather systems,every convective storm differed significantly in dynamitic and thermodynamic features as well as their unstable parameter.As for the moving paths,the distances were the shortest in northvestern region with the highest altitude and the highest terrain slope,on the contrary,in the southwestern region the paths were the longest and often exceeded 100km in distance.The local type(Path?30km)and medium distance type(30km?Path ? 60km)convective activities were concentrated in the central Yunnan Province where the terrain is flat with the cold and warm air meets most frequently.The distribution of medium-long distance type(60km ? Path?100km)were mainly concentrated in southeastern and southwestern Yunnan Province.The long-distance(Path ?100km)convective cells were greatly affected by the weather situation.In central Yunnan region,the paths were mostly westward and in northestern Yunnan region they were mostly northwestward.while in other areas were mostly eastward paths which tended to disappeared as reaching the Ailao Mountains.Short lifespan type(time<90min)strong convection were concentrated in central Yunnan Province and regions to its north which mostly benefited from the summer system,low terrain slope and boundan differences between water and land surfaces.This kind of hailstorms have short moving distances,tended to move along the guiding wind field.The distribution of medium lifespan(90min?time<150min)were respectively scattered,with corresponding local type distributed in central.northeastern and northwestern Yunnan regions while medium-long path type distributed in western and southwestern Yunnan Province and long distance type distributed in southwestern Yunnan region.The distribution of long lifespan type(time?150min)convective cells were well correlated with that of the long-distance convection cells.(2)The southern branch troughs were beneficial to the increase of VIL_although the water vapor conditions in the summer systems were better,but w ith smaller diameter of the liquid particles and less the VIL.The height of the 0? layer in the central and northeastern Yunnan regions where the influence of cold front sheer was most obvious.and the phase of the particles in the strong convection were easier to transform into the solid state,which had a significant contribution to the significant increase of VIL.In April,the convective storms were mostly the medium-long distance type,while they were mainly short distance type in June and July,and the local convective cells were affected by the topography most significantly,while the long-distance convective cells were related to the weather systems.(3)The VIL peak and strong echo intensity were well correlated.In spring,the thermodynamic energy was relatively low but the conditions of the temperature layers and thermodynamics were beneficial for the formation of soft hail particles-as the condensation nucleus collided with ice crystal between-10? and-20? and super-cooled water particles between 0? and-10? to form hail drops.Thus,giving the hailstorms relatively low top height but larger echo intensity and VIL.In summer,the diameter of the hail drops was much smaller.with over all weaker intensities in different temperature layers and smaller VIL_however,top height of different intensities normally 1-2km higher than that of the spring.The single-cell hailstorms have short life spans,with an obvious characteristics of an inverted V-shape feature before and after the hail drop.Different intensity layers in multiple-cell hailstorms have higher top heights,with the layers of 35dBz and 45dBz which represents the area of large particles 1-2km higher than that of the single-cell hailstorms.while its intensities were largely the same with the single-cell hailstorms,but with much lower VIL and jump features.The generation and desolvation process in the squall line is frequent without any obvious jump characteristics,indicating the complexity of the internal structural characteristics of the squall line.(4)In central and eastern Yunnan Province.the VIL.intensities of different temperature layers in hailstorms were significantly larger,with much more obvious features of a three-stage jumps.In the western Yunnan Province,the echoes intensity of the hailstorm processes were stronger but without obvious jump features,and the inverted V-shape on the change of height are not obvious.The heights of different echo intensities were about 2km lower and more dispersed,and the VILs were larger than the hailstorms in central and eastern Yunnan Province.In the northwestern Yunnan Province,it is dominated by local convections.Thus,the echo intensities at different heights were significantly weaker than that of the central and western Yunnan Province.but with more obvious jump features before hail drops.Also,the VILs were obviously smaller but the step-like jump was more significant.In the northeastern Yunnan Province.the echo intensities,heights,and VIL of the strong convection process are larger,with significant inverted V-shape features on their changes.and showing good correlation between the intensity-at 0? layer and the maximum echo intensity.(5)The lightning activities in strong convective hailstorms were domin ated by negative CG lightning.And as the length of lifespan increased,the proportion of positive CG lightning decreased continuously and showed different degrees of step-jump increase both before and after the hail drop.In short lifespan type hailstorms,the increment of particles reached maximum diameter 30 min before hail drop,but the increasing gradient of long lifespan hailstorms was not obvious,the strong updraft in the convective cloud and the main growth time of the particles is 30min before hail drops.(6)The analysis of vertical structural characteristics and echo evolutions in hail cases showed that dry and cold air intrusion and low-level convergence were important determinants of the occurrence of hailstorms and other convective weathers.The dry and cold air intrusion.on the one hand.reduced the height of the internal temperature layer within the strong convections,which made the convective growth zone of mixed-phase particles to thickening.which were fav orable for the accumulation and growth of hail particles and the accumulation of electric charge in the hailstorm clouds.On the other hand it also enhanced the vertical movement within the convective clouds which w as conducive to the maintenance of the uplift and sinking airflows as well as the grow th of larger hail particles.(7)A Comparison had been made between the real-time data and numerical simulation of super-cell storms in two different w eather backgrounds,the results showed that the simulation effect of WRF strong convection model was promising.For the typhoon tropical low-pressure systems,the key mechanism for the emergence of super-cell hailstorms was dry and cold air intrusion and low-level convergence,which caused the positive feedback effect of rising and sinking motion inside the storms,and as the pressure field increased.causing the cold pool and gust front to appear on the ground.While in the strong convective process in the warm and humid environments in front of the southern branch troughs,the warm and humid inflow and the intrusion of dry and cold air could make the precipitation and evaporation and sinking movement form a positive feedback effect,the key mechanism for the hailstorms to sustain was the strong vertical wind shear caused by the combination of downdraft movement caused by thunderstorm and the inflow of the rear side.The major differences between the super-cell storms in these two kinds of w eather backgrounds were mainly reflected in the difference of hail size and CG lightning activity in the storm caused by inflow.In the hailstorm case of 23rd Aug.2017.the inflow came from the deep warm and humid airflow in the rear side exit area of jet stream,cooperating with the low-level dry and cold air in the front side and the cold air mass in the middle and high levels of the rear side penetrating into the lower layer,causing intense rising motions and the height of its development.Due to the high particle's mixing ratio and the number concentration,the larger concentration of cerium particles and ice crystal particles collided and accumulated negative electrical charges,resulting intensive negative CG lightning density in the super-cell hailstorm.As the strong ascending motion cam ing many ice crystals in the high-level divergence to the rear side of the anvil site,positive CG lightning appeared from the maturity stage of the super-cell hailstorm.While in the case of 17th Apr.2018,the inflow w as the warm and humid air flow in front of the trough,and the intrusion of dn and cold air on the rear side formed the ascending motion inside the super-cell.The height of the developed convection was relatively low,with no obvious divergence after the ascending airflow reached the top.Moreover,the ice crystals were concentrated above the growth zone of graupel particles at the top of the hailstorm.Due to the collisions and separations of multi-phase particles in the super-cooled water zone.negative electrical charges were accumulated,while no positive CG lightning had been observed in this case. |
PDF Full Text Request