2020年黄土高原中南部一次暴雪天气过程成因及雨雪转换的低层降温机制分析  

Causes of a heavy snowfall in central-southern Loess Plateau in 2020 and its lower-level cooling mechanisms of rain-to-snow transition

作　　者：赵强[1,2] 刘瑞芳 贾蓓 屈丽玮 ZHAO Qiang;LIU Ruifang;JIA Bei;QU Liwei(Shaanxi Meteorological Observatory,Xi′an 710014,China;Severe Weather Research and Application Center,Key Laboratory of Ecological Environment in Qinling and Loess Plateau,Xi′an 710016,China;Xi′an Meteorological Service,Xi′an 710015,China)

机构地区：[1]陕西省气象台,陕西西安710014 [2]秦岭和黄土高原生态环境重点实验室灾害性天气研究与应用中心,陕西西安710016 [3]西安市气象台,陕西西安710015

出　　处：《气象与环境学报》2024年第6期10-18,共9页Journal of Meteorology and Environment

基　　金：陕西省科技厅自然科学基础研究计划(2023-JC-YB-282);中国气象局复盘总结专项(FPZJ2023-132);陕西省气象局秦岭和黄土高原生态环境气象重点实验室开放研究基金课题(2023G-1)共同资助。

摘　　要：选用地面、高空观测资料及ERA5逐小时再分析资料(0.25°×0.25°),分析2020年11月20日黄土高原入秋以来的首场大到暴雪天气过程成因及降水开始阶段导致雨雪转换的低层降温机制。结果表明:此次暴雪过程有西南和东南两条水汽供应路径;西风槽、高原槽东移合并加深,引导切变线东移,造成陕西北部水汽辐合抬升,高空急流的辐散作用使上升运动进一步加强;中层大气存在条件性对称不稳定,冷锋触发不稳定能量释放,在锋面两侧形成斜升次级环流,陕西北部位于次级环流的上升支,最大降雪强度超4 mm·h^(-1)。此次过程降水初期近地面层气温的迅速下降是雨转雪的关键因素,气温预报偏差是模式雨雪相态预报偏差的原因。雨水蒸发引起的低层异常降温利于雨雪转换,需在雨雪相态预报中关注。Using surface and upper-air observations as well as the ERA5 hourly reanalysis data(0.25°×0.25°),the study analyzes the causes of the first heavy snowfall event over the Loess Plateau on November 20,2020,and investigates the lower-level cooling mechanisms responsible for the rain-to-snow transition during the initial phase of precipitation.Results show that the heavy snowfall event is supported by two moisture transport pathways,i.e.,one from the southwest and the other from the southeast.The eastward merging and deepening of the westerly trough and plateau trough guide the eastward movement of the shear line,resulting in moisture convergence and uplift in northern Shaanxi province.The divergence of an upper-level jet stream further enhances the upward motion.Conditional symmetric instability characterizes the mid-atmosphere,and the cold front triggers the release of unstable energy,forming a slantwise secondary circulation along the front.Northern Shaanxi is located in the ascending branch of this circulation,with local maximum snowfall intensity exceeding 4 mm·h^(-1).The rapid temperature drop near the surface during the initial precipitation phase is the key factor in the rain-to-snow transition,while the temperature forecast biases are identified as the main cause of errors in model-based rain-snow phase predictions.Lower-level abnormal cooling due to evaporative cooling facilitates the rain-to-snow transition,which deserves emphasis in associated predictions.

关 键 词：锋生函数 对称不稳定 非绝热加热 蒸发潜热 

分 类 号：P458.11[天文地球—大气科学及气象学]