青藏高原东北侧雹云单体最大垂直累积液态含水量的演变特征及其在降雹定时判断中的应用

Vertically integrated liquid water content characters of hail cells in the northeast Tibetan plateau and its application in estimation of hail-shooting time

以3D-Barnes方案插值的新一代天气雷达反射率因子等高平面资料,用垂直累积液态含水量(Vertically Integrated Liquid WaterContent,简称:VIL)的理论模式计算雹云单体在演变过程中的VIL、用MAX函数逐个提取最大VIL(简称;VIL-max),采用统计和分段函数处理技术,对2004—2005年5—8月青藏高原东北侧的16个雹云单体的VILmax的演变特征及其与地面降雹的时间关系进行详细分析。结果表明,(1)雹云单体出现降雹时所需的VILmax存在明显的时空差异,但同一雹云单体在演变过程中其VILmax均存在"爆发式增长"和"爆发式降低"现象;(2)雹云单体在首次降雹前4个资料时间间隔(22min)内其VILmax将出现2次"爆发式增长"现象,出现第1次"爆发式增长"现象时地面不会降雹,维持1—2个资料时间间隔(5—11min)后出现第2次"爆发式增长"现象时地面开始降雹;同一雹云单体再次降雹时没有第1次"爆发式增长"现象,出现"爆发式降低"现象时地面降雹停止;(3)通过个例总结并定义的雹云单体最大垂直累积液态含水量变化率(简称:GVILmax)的"正(负)峰"现象与VILmax的"爆发式增长(降低)"现象所对应的时间完全吻合,且利用GVILmax"正(负)峰"现象识别雹云单体"爆发式增长(降低)"现象具有明显的指示意义;根据雹云单体GVILmax"正(负)峰"现象与地面降雹的时间关系建立的经验公式计算降雹的时间误差在1个资料时间间隔(5—6min)内。

Hailstones are products of strong convective cells, characterized by locality and short-lived feature and hence studies of hail clouds are more or less constrained if conventional surface and upper-level observations are employed. In recent years, with advances in radar technology, the detection items and space/time resolutions have increased, the digitalized radar has now become one of the main tools in exploring hail cloud development. As early as the 1970s, a prognostic factor i.e. vertically integrated liquid water content （VIL） was proposed for studying digitalized radar echo data, and it was defined in calculation as the vertical integration of mixing ratios of the liquid water content, which was found from the empirical relation between radar measured reflectivity factor and raindrop concentration. It is well known that radar VPPI data are discontinuous from one level to another; the number of levels for elevation angles is too small, significant noises are produced in radar operation, and furthermore the reflectivity factor of the cell is non-linear so that direct vertical integration would bring about greater errors. For this reason, based on the constant level data such as the reflectiv- ity factor of new generation weather radar obtained with the 3D-Barnes interpolation scheme, the VIL during the evolutional processes of hail cells is calculated using its theoretical model, and the maximum of VIL（VILmax） is identified by use of MAX function. With statistic and piecewise function techniques, the evolutional character of VILmax and its relation with hail shooting time on the ground are detailedly analyzed for 16 hail cells in the northeast Tibetan plateau, during May - August of 2004 - 2005. Results show that there were apparently spatial-temporal differences in VILmax among different hail ceils at the hail shooting time, but similar ＂explosive increasing＂ and ＂explosive decreasing＂ phenomena around the time in the evolution process of each hail cell; two ＂explosive increasing＂ processes appeared within the four time intervals of radar data （22 min） before the first hail shooting, with no shooting on the ground after the first VILmax increasing until the occurrence of second increasing about 1 - 2 time intervals （5 - 11 rain）after the first increasing; there was no first ＂explosive increasing＂ before the second shooting in the same cell, and the hail shooting on the ground stopped with the occurrence of ＂explosive decreasing＂; the positive（negative） peak of GVILmax （which is defined as the change rate of VILmax, briefed as GVILmax） corresponded completely with the ＂explosive increasing （decreasing）＂ in time, and therefore is a good indicator for hail shooting; the time error of the empirical relation between the peaks and the time of hail shooting on the ground is within one time interval （5 - 6 min）.