Gansu Province Hail Formation Mechanism Of Weather Characteristics And Radar Recognition Technology Research

Gansu Province is located in the junction of Qinghai-Xizang, Mongolia and Loess plateaus with complex terrain and rare vegetation cover. Ternperature difference is large there, and strong convective weather formed easily. It is one of regions with more hail occurring in China. Hail falling connected with local condition significantly, and the complex rule in different temporal-spatial scale makes it more difficult to observe hail cloud and carry out experimental study. Based on hail observations from 79 meteorological stations in Gansu Province during 1973-2002, combined with radar data and sounding observations on hail days from Yongdeng and Minxian since 1971, as well as five years' lightning data from lightning location net (2000-2004), the climatic features, synoptic causes and characteristics of hail weather formation, and temporal-spatial distribution of hail cloud were investigated; the air flow and temperature structures in hail cloud, and the feature of inversion layer in hail cloud were analyzed; the radar echo features of typical hail cloud were summarized; the temporal-spatial evolution of lightning in hail cloud, the temporal evolutions of total frequency of lightning and frequencies of cloud lightning as well as cloud-ground lightning, and the correlation between radar echo intensity and lightning frequency were revealed; the technology for identifying hail cloud by radar automatically in Gansu was discussed finally.It is showed that in more hail years in Gansu Province, the summer circulation at the level of 500 hPa presented positive anomaly over Xinjiang and negative anomaly over North China, and the northwest air flow was stronger over Northwest China, with cold air moving to south frequently; but in less hail years the situation was inverse. Hail occurs mainly in July and June in Gansu. mean annual hail days are 6-13 days. The total distribution feature of hail days is more in plateau and mountain regions, and less in valleys, basins and deserts. Hail occurs mostly at 14:00 to 16:00 in a day. and some difference exists with latitude and terrain. Because Minxian county is located in the cast border of Gannan Plateau and there are many high mountains in its west, north and northwest, it has abundant moisture on underlying surface, when northwest cold air encountered high mountains, it was held up and lifted, and lee wave triggered by this air flow promoted convective activity in the wave crest, so the quasi-stationary hail zone formed and hail disaster is more serious there.We found that before hail falling there was a local convergence wind field in hail areas, but when hail was over, the convergence wind field was also broken. In hail cloud, firstly the velocity of vertical updraft increased with altitude, then at the middle and upper parts of hail cloud the velocity reached the maximum, after that it decreased quickly. The updraft entered hail cloud from its right bottom of moving frontage, then passed through the cloud, and left the cloud from its top moving frontage. While the downdraft entered the hail cloud from its back and middle part and left from the bottom of hail cloud.Under the pattern of northwest air flow, on hail days pseudo-equivalent potential temperature (θ_se) presented negative departure in the whole troposphere, andθ_se decreased quickly from surface to 700 hPa but began to increase gradually from 500 hPa, while the monthly mean value ofθ_se increased from 600 hPa, andθ_se decreased from surface to 500 hPa on hail days in July and August, and the rate ofθ_se decrease is bigger than that in June, all these showed that convective instability was stronger in July and August. Under the pattern of southwest air flow,θ_se on hail days decreased from surface to 500 hPa and remarkably at the level of 700 hPa with the decreasing rate of 4℃/30 hPa, and it began to increase from 500 hPa, while the surfaceθ_se on hail days is 9℃higher than that on no hail days. Compared with the meanθ_se in June,θ_se On hail days is higher below 500 hPa and over 300 hPa, so hail fall under the pattern of southwest air flow resulted from warm and moist stream in the middle and low layers of troposphere, and from the change of 0θ_se with altitude we can know that convective instability under the pattern of southwest air flow was stronger than that under the pattern of northwest air flow.The temperature at the height of hail cloud echo top is 2-6℃lower than that of thundercloud and it is remarkable(6.1℃) in June, while the temperature at the height of strong hail cloud echo(with a statistical mean value of 55 dBZ) top is 10-16℃lower than that of thundercloud, and it is 16.3℃lower in July. The mean height of thundercloud and weak hail cloud echoes top can reach the inversion layer near the top of clouds but can't overtop it. While the mass of hail cloud with middle intensity (with a statistical mean value of 45dBZ) is much larger than those of thundercloud or weak hail cloud because of strong updraft, and the height of its echo top can reach 600-800 m over inversion layer. The height of strong hail cloud echo top can extend into the inversion layer. The thickness of negative temperature area is different obviously with different intensity of hail cloud, and they are 3 300 m, 4 460 m and 5 600 m, respectively for the weak hail cloud, the middle intensity hail cloud and strong hail cloud. Through further investigation we find that when strong temperature falling occurs at upper level, it is of advantage to hail formation, and the thicker the negative temperature area, the stronger the hail falling.The density, frequency and polarity of lightning are correlated nearly with the development of convective cloud cluster, and the center of lightning density corresponds well to the areas of hail fall, torrential rain and flooding. By further analysis we find that the three centers with lightning density beyond 400 times per 225 km² connect reasonably with local terrain and climatic distribution of hail. The two of four hail sources of Gansu Province are located in the northeast border of Qinghai-Xizang Plateau and Maxian Mountain region, while the center of the highest lightning density is Situated in Maxian, Taibai and Nanping mountains region where hail occurs frequently, and Weiyuan, longxi and Dingxi counties in this center are the severest hail disaster regions which are also the areas with the densest operation sites for hail suppression in Gansu Province. The location of the northeast center for the highest lightning density is special with a direction from northeast to southeast, but the local landform inclines approximately from southeast to northwest; the altitude ranges from I 500 to 2 400 m; Jingyuan county is a small basin where the Yellow River passes by, and the Zuli River whose sources is Huining county flows into the Yellow River there, and as a result the local mountainous landform and the underlying surface with different moisture and heat provide advantaged conditions for convective weather formation and development, and more lightning occurs there. The lightning center in Lingxia is located in the area of Taizi and Jishi mountains in the northeast border of Qinghai-Xizang Plateau. The center of the highest lightning density occurs in front of hail falling area, and five-minute frequency of lightning presents regular variation in the course of hail cloud development. If the lightning frequency is over twenty times during 4-97 minutes before hail falling, this can be one of indexes for hail cloud identification by lightning, and that the proportion of positive cloud -ground lightning to total cloud- ground flash is over 15% also can be an index for hail cloud identification by lightning.