Microphysical processes of a stratiform precipitation event over eastern China: analysis using micro rain radar data
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Data collected using the micro rain radar (MRR) situated in Jinan city, eastern China, were used to explore the altitudinal and temporal evolution of rainfall microphysical characteristics, and to analyze the bright band (BB) characteristics and hydrometeor classification. Specifically, a low-intensity and stable stratiform precipitation event that occurred from 0000 to 0550 UTC 15 February 2015 and featured a BB was studied. During this event, the rainfall intensity was less than 2 mm h−1 at a height of 300 m, which was above the radar site level, so the errors caused by the vertical air motion could be ignored. The freezing height from the radiosonde matched well with the top of the BB observed by the MRR. It was also found that the number of 0.5–1 mm diameter drops showed no noticeable variation below the BB. The maximum fall velocity and the maximum gradient fall velocity (GFV) of the raindrops appeared at the bottom of the BB. Meanwhile, a method that uses the GFV and reflectivity to identify the altitude and the thickness of the BB was established, with which the MRR can provide a reliable and real-time estimation of the 0°C isotherm. The droplet fall velocity was used to classify the types of snow crystals above the BB. In the first 20 min of the selected precipitation event, graupel prevailed above the BB; and at an altitude of 2000 m, graupel also dominated in the first 250 min. After 150 min, the existence of graupel and dendritic crystals with water droplets above the BB was inferred.
Key wordsdrop size distribution micro rain radar bright band microphysical processes
本文利用济南的微降水雷达数据分析降水微物理参数随时间和空间的演变, 分析零度层亮带特征, 判断零度层亮带以上粒子的相态. 选取2015年2月15日00时00分至05时50分(世界时)发生在济南地区一次小雨强、稳定的层状云降水为例来分析. 在此次降水过程中, 300 m高度上的雨强均小于2 mm h-1, 由垂直气流造成的影响可以忽略. 微降水雷达观测的零度层亮带顶的高度与探空0℃的高度有很好的一致性. 零度层亮带以下直径为0.5—1 mm的粒子数量没有明显的变化. 粒子最大下落速度和最大下落速度梯度出现在零度层亮带的底部, 提出用下落速度梯度和雷达反射率因子来判断零度层亮带的高度和厚度, 这种方法可以用来实时监测0℃层. 同时, 利用粒子的下落速度来判断零度层亮带以上粒子的相态. 在选取的降水个例的前20分钟, 零度层亮带以上霰粒子丰富; 在2000 m高度上, 前250分钟都是霰粒子占主导地位; 150分钟后, 为霰和附冻滴冰晶的混合状态.
关键词雨滴谱 微降水雷达 零度层亮带 微物理过程
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This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 41475028 and 41530427). The authors are grateful to ShandongWeather Modification Office, China, for assisting with maintaining the MRR instrument. The authors also wish to thank Dianli GONG, Dianguo ZHANG, Liming ZHOU and Qiuchen ZHANG for selecting and preparing the MRR data.
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