Field Observations And Studies Of Forecast & Warning Indicators Of Hailstorms On Both Sides Of Mt.Tian In Xinjiang

Xinjiang experiences more hail disasters than any other provinces in Northwest China,especially on both sides of Mt.Tian.Moreover,hail disasters in Xinjiang have increased in frequency,area coverage,and the extent of economic loss after entering this century such that mitigating hail disasters is facing severe challenges.It is thus imminent to carry out in-depth research on hail detection and formation mechanisms in order to help improve our capacity of artificial hail suppression operations.In view of the actual needs of artificial hail suppression,and disaster prevention and mitigation in Xinjiang,this thesis takes the Aksu region,Bortala Mongolian Autonomous Prefecture of Bortala(Bozhou for short),Kuima River Basin,and Kashgar region,which are the hardest hit by hail on both sides of Mt.Tian,as the research target areas.Using the first-hand data from the TK-2A GPS sounding rocket for the drop-in field detection of hailstorms,the comprehensive hail disaster reports,ground and high-altitude conventional soundings,multiple radar observations,and the NCEP/NCAR reanalysis and forecast data as well as other multi-source data,we studied the vertical distribution characteristics of the key convective elements for hailstorm development in order to better understand its physical mechanism and provide a scientific basis for exploring and identifying the forecast and early warning indicators of hailstorms.We also determined highly effective hailstorm forecast indicators that meet with operational requirements,and hail early-warning indicators that have high operability with certain advance times.The results so obtained have further enriched the theoretical foundation about the hail formation mechanisms in mountainous areas,and could provide new scientific and technological support for further improving our capacity to mitigate hail disasters in Xinjiang.The main research findings of this thesis are summarized as follows:(1)New physical mechanisms of hailstorm development are found from the vertical distribution characteristics of key convective elements,based on field experiments of typical hailstorm events.During a non-supercell severe hailstorm occurring in Bozhou on the north side of Mt.Tian on the evening of 23 July 2016,three TK-2A GPS sounding rockets were launched to carry out dropsonde observations,below 7 km height,i.e.,in an inflow area at the development stage,in an ascending area(at the front of the hailstorm)at the mature stage,and in a descending area at the decaying stage,respectively.The vertical measurements of meteorological variables at the different stages of a life cycle of a hailstorm were obtained for the first time on the hailstorm scale.Based on comparitive analysis of the vertical distribution characteristics and differences of convective elements such as water vapor parameters,vertical wind shear,and stability,a new understanding of the physical mechanism that is conducive to the development and enhancement of a hailstorm is obtained.The new findings related to the detection research are as follows.(i)An analysis of water vapor conditions reveals that there was a steep wet layer at 2-4 km ASL(above the sea level)in the ascending area,which played an important role in the development and maintenance of the hailstorm.A humidity peak,appeared in this layer,indicated the presence of a melting layer in the hailstorm.The methods for identifying the location and thickness of the melting layer were also given.The adiabatic liquid water content(LWC)in the cloud of the ascending area increased monotonously with height,and the precipitable water(PW)was higher than that in both the inflow and descending areas.Larger LWC and PW values were favorable for the formation of hailstones.The horizontal transport of water vapor in the inflow and updraft areas provided energy supply for hailstorm development.A salient feature of the horizontal water vapor transport in the inflow,ascending,and descending areas of the hailstorm was the shape of an upside down funnel in the vertical distribution that was narrow on the top and wide on the bottom below 3 km ASL,with the maximum inflowing water vapor content in the inflow and ascending areas near the cloud base.(ii)The strong vertical wind shear in the ascending area helped transport heat and water vapor upward,thus providing energy for hailstorm development.The vertical wind shear increased its magnitude from its moderate value in the inflow area to a large one in the ascending area,which facilitated the forward movement and development of the hailstorm.The largest storm-relative helicity in the ascending area was also conducive to the development and maintenance of the hailstorm.(iii)The nonlinear increase in convective instability with increasing pressure in the ascending area of the hailstorm provided the most favorable condition for the hailstorm development.The weakest turbulence in the ascending area also contributed to the development,maintenance,and long life cycle of the hailstorm.(2)On the environmental parameter index forecasts of destructive hailstorms(i)Environmental conditions associated with 176 destructive hailstorms on both sides of Mt.Tian during the months of May-August of 2008 to 2019 are analyzed.Results reveal that the convective effective potential energy(CAPE),convective inhibition energy(CIN),lifting condensation level(LCL),and lifting index(LI),which are very sensitive to temperature and humidity at the parcel initiating level,are not significant indicators for the development of destructuve haistorms,whereas the total index(TT),K index(K),Jefferson index(JEFF),strong weather threat index(SWEAT),0 ? layer height(HGT0),the height difference between the 0°C and-20°C level(HGZ),precipitable water(PW),and the 0-6 km wind shear(SHR6)show apparent physical implications and obvious indications.Thus,the latter 8environmental parameters are used as the forecast indicators for the possible development of destructuve haistorms.(ii)An analysis of the characteristics of the above forecast indicators shows that for the Kuima River Basin,Aksu and Kashgar areas,on average,the TT,K,and SHR6 values are all similar,the JEFF and HGT0 values increase but the HGZ and PW values decrease in order,with the Aksu area having the highest mean SWEAT value.During the occurrences of destructive hailstorms on both sides of Mt.Tian,the mean TT and PW values are comparable to those in eastern China,the United States and European countries,whereas the mean SWEAT and SHR6 values are significantly lower.The mean HGZ value on both sides of Mt.Tian is significantly lower than that in eastern China.(iii)The above different characeristics of the forecast indicators are found to depend primarily on the different thermodynamic and dynamic conditions on both sides of Mt.Tian.That is,the more(less)convective unstable columns occurring on the south(north)of Mt.Tian account for the higher(smaller)mean TT,K,JEFF,and SWEAT values and the lower(higher)mean HGZ values over the areas on the southern(northern)side of the mountains.Under the conditions of comparable near-surface humidity,the stronger lifting on the northern side of Mt.Tian provides a higher mean PW value over the Kuima River Basin than that on the southern side.(iv)An environmental parameter index forecast system of destructive hailstorms is established using the NCEP/NCAR reanalysis grid data from May to August of2008-2016,and optimized with the percentile method.Results show that the forecast threshold values of TT,K,SWEAT,HGZ and PW are higher on the northern side of Mt.Tian than those on the southern side,while the opposite are true for those of JEFF,HGT0,and SHR6.An application of the above index forecast system to the months of May to August of 2017-2019 using the NCEP/NCAR forecast data indicates that the accuracy rates of trial forecasts for destructive hailstorms on both sides of Mt.Tian have reached 100%.It follows that the use of the NCEP/NCAR forecast data and the above optimized forecast indicators could play an important role in providing reasonably forcasts of destructive hailstorms on both sides of Mt.Tian in the future.(3)On the warning indicators of destructive hail clouds using radar parametersThe above-mentioned 176 hailstorms are further analyzed,but with radar observations in terms of their structural evolution and spatiotemporal characteristics.Results show that destructive hail cloud tops at 30,and 18 min prior to,and the time of hailfall are above 11,12,and 11.5 km altitudes,respectively,and their associated radar reflectivities of greater than 45 d BZ are mostly higher than 6,7,and 6.5 km,with more than 80%,87%,and 96% of their core regions exceeding 50 d BZ.The destructive hailstorms on both sides of Mt.Tian are generally more intense in June and August,with the most intense and deepest ones occurring in the Kashgar area on the south side of Mt.Tian.Damaging hailfall on the northern side of Mt.Tian is mainly produced by single-cell hailstorms from May to August.In contrast,on the southen side of Mt.Tian,May and June are the active periods of single-cell hailstorms,which are the major hail-producing thunderstorms,and damaging hailfall inn July and August is mainly produced by long-lived mesoscale convective systems.Based on the above analysis,the destructibe hail cloud warning indicators at 30,and 18 min prior to hailfall on both sides of Mt.Tian from May to August are established by using the morphological parameters of radar echoes and their height differences from the 0?level.In general,the warning indicators in the Kashgar area on the south side of Mt.Tians are significantly higher,and both sides of Mt.Tian have the highest indicators in August.Verifications against observations shows that the accuracy rate of applying the warning indicators to both sides of Mt.Tian is more than 80%,with a higher rate on the northern side of Mt.Tian.(4)Construction of an integrated forecast-warning index system for destructive hailstormsThe forecast indicators,and the warning indicators for 30 min and 18 min prior to hailfall,as mentioned under(2)and(3),respectively,are integrated to construct a forecast and nowcast indicator system for destructive hailstorms occurring on both sides of Mt.Tian.This indicator system has been used sucessfully to achieve seamless forecasts and warnings of severe hailfall on both sides of Mt.Tian from 14:00 to02:00 BST during the periods of frequent hail disasters.This was done by using the NCEP/NCAR forecast fields and radar observations at 6-min intervals,and by nesting the 0 to 30 min nowcasts in the 3 to 9 h short-term and short-range forecasts.Therefore,the construction of such an integrated forecast-warning system not only has significant theoretical value,but also provide important technical support for improving the precise operability of artificial hail suppression on both sides of Mt.Tian in Xinjiang,which would make up for some deficiencies in previous similar studies.