QX气象Meteorological Monthly1000-0526气象编辑部中国北京qx-44-10-132510.7519/j.issn.1000-0526.2018.10.009P412, P457论文Articles基于CloudSat卫星观测的贵州冻雨形成机制分析Analysis on Mechanisms of Freezing Rain in Guizhou Province Using CloudSat Data杨寅YANGYin
The observation data of satellite CloudSat are used to analyze the mechanisms of freezing rain in Guizhou Province. The results show that, for the freezing rain under ice-phased mechanism, reflectivity of cloud profile radar, ice water content and temperature profile of CloudSat data could describe the process of ice-phased particles melting into liquid clearly. Bright bands of reflectivity at the 0℃ layer of cloud profile radar are the direct reflection of the melting process, and ice water content data could represent the melting process in melting layer as well. For the freezing rain under the super-cooled warm rain mechanism, CloudSat data could depict the falling process of precipitation particles kept in the super-cooled liquid in environment of temperature below 0℃ vertically. This research proves that the Guizhou freezing rain has warm rain mechanism with melting layer based on observation of cloud-physics. With the existence of temperature inversion in vertical temperature profile, precipitation particles exist in the liquid state in melting layer but remain super-cooled in environment of temperature below 0℃. The super-cooled liquid precipitation particles could be heated when falling into melting layer, getting frosted into super-cooled droplets in freezing layer below, colliding with surface objects and forming freezing rain with temperature below 0℃ at last. Thus, melting layer could not be used to distinguish the formation mechanism of freezing rain.
贵州冻雨形成机制分析CloudSat卫星观测资料Guizhou Provincefreezing rainanalysis of mechanismCloudSat data国家重点研发计划2016YFC0402702安徽省自然科学基金青年项目1708085QD89国家自然科学基金项目51509043国家自然科学基金项目41775111国家气象中心水文气象预报团队项目共同资助国家重点研发计划(2016YFC0402702)、安徽省自然科学基金青年项目(1708085QD89)、国家自然科学基金项目(51509043和41775111)及国家气象中心水文气象预报团队项目共同资助引言
很早就有学者对冻雨形成机制开展研究,Bocchieri(1980)分析一些冻雨天气的探空数据,发现整层大气都在0℃以下,提出这种冻雨机制为“过冷暖雨”。Stewart(1985)研究美国东南部的冻雨和降雪时,发现暖层(>0℃)能将雪花完全融化,液滴落到低层冷层中冷却但未被再次冻结时,在低于0℃的地表上将很有可能观测到冻雨,这种理论常被称为融化过程或“冰相融化机制”。Huffman and Norman(1988)总结了冻雨的两种机制,一种是冰相机制:大气温度垂直结构呈上下冷、中间暖,上层降落的固态冰晶或雪花在暖层内融化为液态,当液滴到达下层冷层时再冷却到0℃以下,待下落碰到地物或地面后立即冻结;另一种为暖雨机制:大气整层气温小于0℃,雨滴以过冷却水的形式降落到地面发生冻结。后来的研究多沿用该结论,认为存在融化过程的冻雨为冰相机制;在一致略低温度环境(< 0℃)下,雨滴以过冷水形式降落到地面冻结的冻雨为暖雨机制,并通常使用融化层来区分。
对“过冷暖雨”机制(单层模式)和存在融化层的暖雨机制(二层模式),其理论基础是冰核在温度高于-10℃的环境中很少活化,云顶温度高于-10℃的云大部分情况都是由过冷却水构成的(Hobbs and Rangno, 1985),这意味着即使存在融化层,如果融化层上方的云所处环境温度略低于0℃,云中降水粒子以过冷水形式存在,其下落经过融化层时升温变为普通液态水但不存在融化过程。欧建军等(2011)认为当有暖层而云顶温度大于-10℃时,冻雨也有可能是暖雨机制。刘朝茹等(2015)根据地面和探空常规观测资料分析,也认为暖雨机制在融化层存在的情况仍可能出现。黎惠金等(2011)利用中尺度模式模拟了2008年南方地区一次冻雨,发现其属于暖雨机制,但温度垂直结构存在着融化层。张昕等(2015)模拟了2008年初一次冻雨过程,发现高空固体降水粒子稀少,中低空存在大量液态粒子,同时900~600 hPa存在逆温层和融化层。陶玥等(2012)利用中尺度云分辨模式模拟了2008年一次贵州冻雨过程,发现冻雨是在“暖—冷”温度层结下的暖雨机制。以上研究从理论和常规观测资料分析,以及数值模拟的角度提出融化层存在情况下的冻雨也可能是暖雨机制,但对于大气存在融化层的情况下,降水粒子以液态形式存在,并不存在“融冰”过程的分析和模拟尚缺乏云物理实际观测的直接佐证。
CloudSat是NASA地球系统科学探路者卫星计划的一颗卫星,其搭载的云廓线雷达(CPR)可以观测到云内液态水和冰水的垂直廓线。马占山等(2008)发现CPR雷达反射率和云冰含量可以真实地反映云垂直结构和水凝物分布规律;陈英英等(2011)发现CloudSat资料的温度廓线与探空数据吻合;目前已经有不少学者使用CloudSat资料研究云垂直结构(李积明等,2009;汪会等,2011;Luo et al, 2009)。
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