三小时负变压异常指数及对强对流天气的预报意义

Three hours negative pressure anomaly index and its significance in severe convective weather forecast

低层暖平流强迫类强对流发生前,地面经常伴有低于日变化的3 h变压。结合常规地面观测资料,定义低于日变化的3 h变压异常(超过一个标准差定义为异常)指数PCR(Pressure Change Range),讨论了中国中东部地区3 h变压标准差的气候分布特征;最后以3次强对流天气过程为例说明PCR指数的预报价值和时效。结果表明,与3 h变压均值相比,中国中东部地区的3 h变压标准差的日变化较小,PCR更适合作为变压异常程度的标准。东北、华北、华东-华中区域PCR冬春季节出现站次数偏多,夏秋季节偏少;华南区域除了冬春季外,夏季也偏多,秋季偏少。PCR主要集中在低级别强度上,但PCR级别越高,越有可能出现强对流天气。东北区域出现PCR的首要原因是受东北气旋的影响,且可能有TBB≤-52℃的云系相对应;华北、华东-华中、华南出现PCR的首要原因是冷高压变性或迅速东移,没有TBB≤-52℃的云系相对应;地面倒槽中出现的PCR全部有TBB≤-52℃的云系对应。3次强对流天气过程均发生在地面倒槽中;在发生前3 h左右,地面气压场上有较明显的负PCR中心出现,强对流天气中尺度云团有向负PCR中心移动的趋势。

3 h surface pressure change that is smaller than the daily pressure variation can often be found before the occurrence of severe weather processes forced by low-level warm advection.The Pressure Change Range（PCR）is defined as 3 h pressure change that is smaller than the daily pressure variation but larger than one standard deviation calculated from long-term PCR time series based on conventional ground observations.Climatological distribution characteristics of the 3 h pressure changes over central and eastern China are analyzed in the present study.The PCR index is applies to study three severe weather processes.The result shows that the standard deviation of the 3 h pressure change has a smaller daily variation compared to the mean value of 3 h pressure change over central and eastern China.The PCR is an appropriate index for measuring the anomaly of pressure change.The PCR counts in Northeast China,North China,Central China and East China area are abnormally high in winter and spring than in summer and autumn.In South China,the PCR counts are abnormally small in autumn.PCRs are mainly concentrated over low intensity range.A high PCR value often indicates that strong convective weather might happen.Cyclonic systems are primary weather systems that lead to higher PCR in Northeast China,while the decaying or rapid eastward movement of high pressure systems are responsible for higher PCRs in other regions.Higher PCR in Northeast China possibly corresponds to clouds with TBB ≤-52℃.When the surface pressure field shows an inverted trough pattern,higher PCR always corresponds to clouds TBB ≤-52℃.For higher PCRs induced by decaying or rapid movement of high pressure system,there exist no clouds with TBB ≤-52℃.The three severe weather processes investigated in the present study all occurred over the area covered by an inverted trough.An obvious PCR center appeared about 3 h before the severe weather happened,and the mesoscale clouds moved to the PCR center during the process.