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双偏振雷达对一次水凝物相态演变过程的分析
A Process of Hydrometeor Phase Change with Dual-polarimetric Radar
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摘要: 联合利用3 GHz双偏振雷达RHI探测数据和温度廓线数据, 建立了云粒子相态反演的模糊逻辑算法, 算法采用Beta型成员函数, 成员变量包括:水平反射率因子、线性退偏比、差分反射率及温度0℃,-40℃对应高度, 反演出的相态包括毛毛雨、雨、低密度干冰晶、高密度干冰晶、湿冰晶、干霰、湿霰、小冰雹、大冰雹、雨夹雪和液态云滴等11种, 并利用雷达的连续探测数据对一次层状云降水过程中水凝物相态的演变情况进行了分析, 得到如下结果:初始阶段层状云相态呈现分层结构, 从上至下依次为高密度干冰晶、湿冰晶和液态云滴; 初始阶段云体中的回波大值区核心区域为大的冰相粒子, 其余部分为液态粒子; 在初始到成熟阶段演变中, 回波大值区上部液态粒子逐步向冰相转化; 消散阶段云中零度层亮带逐步消失, 零度层以上云粒子结构呈现高密度干冰晶包裹湿冰晶的情况。关键词:双偏振雷达; 模糊逻辑; 水凝物相态反演; 层状云降水过程; 水凝物相态演变Abstract: The phase of hydrometeor is one of the most important microphysics characteristics of cloud. The development of dual-polarimetric weather radar makes the retrieval of the hydrometeor phases possible theoretically, which has been one of the hottest applications of the dual-polarimetric radar. The fuzzy logic has been extensively used in the classification of hydrometeor now, and become the dominant technique in this field. Th rough continuously studying in statistics with more and more in situ measurements, the parameters in fuzzy logic algorithm have become relatively steady for individual dual-polarimetric radar in operational use. The evolution of hydrometeor phase in the cloud process is an important aspect to the research of water microphysical circular in the cloud-precipitation system, and plays great role in many meteorological fields, such as weather modification, aviation security, weather model, and so on. How ever the studies on the changing of the hydrometeor phase with time series of radar data are relatively immature, publications in which are seldom seen. A fuzzy logic system for classifying hydrometeors based on the combination of polarimetric radar measurements and conventional observation data is described, and a Beta membership function is utilized for the fuzzification, the parameters of which are also given based on the former statistics achievements for the S-band radar.The input variables include radar reflectivity, LDR, ZDR and the height of 0℃ and -40℃ layer, and the output types are drizzle, rain, low-density dry ice crystal, highdensity dry ice crystal, wet ice crystal, dry graupel, wet graupel, small hail, large hail, sleet, and cloud droplet. Then a case study on an evolution of the hydrometeor phase in a stratiform cloud precipitation process is analyzed based on the CAM Ra radar and RAOBs data, which takes place at Chilbolton the UK summer morning, and lasts approximate 39 minutes. The whole process is divided into three phases including the initial phase, mature phase and the dissipating phase, for each phase a analysis on the changing of the hydrometeor type is given based on the classified results of all the radar observations in it, and the results show that in the initial phase stratiform cloud has a layered structure of hydrometeor types including high-density dry ice, wet ice crystal and liquid droplet from top to bottom; the core of the large-echo region is filled by large ice crystals, and the other area in the large-echo region is filled by liquid hydrometeors in initial phase; from initial phase to mature phase liquid hydrometeors on the top of the large-echo region have a trend of freezing; in the dissipating phase the 0℃ layer bright band disappears gradually, on the top of which a wet ice crystals are wrapped by high-density dry crystal.
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图 2 各参量对应不同相态的成员函数示意图 (a)LDR, (b)Zhh, (c)Zdr, (d)H
Fig. 2 Membership functions of 4 input variables for its 10 fuzzy sets (a)LDR, (b)Zhh, (c)Zdr, (d)H
图 3 发展阶段的雷达反射率因子 (a) 和相态分类结果 (b)
Fig. 3 CAMRa Zhh(a) and classification results (b) during initial phase
图 4 成熟阶段的雷达反射率因子 (a) 和相态分类结果 (b)
Fig. 4 CAMRa Zhh(a) and classification results (b) during mature phase
图 5 消散阶段的雷达反射率因子 (a) 和相态分类结果 (b)
Fig. 5 CAMRa Zhh(a) and classification results (b) during dissipating phase
表 1 CAMRa雷达性能
Table 1 CAMRa characteristics
表 2 4个输入参量对应10种相态的成员函数参数
Table 2 Membership function values of 4 input variables for 10 types of hydrometeors
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