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Retrieval and Experiments of Atmospheric Vertical Motions in Convective Precipitation Clouds
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摘要: 为深入认识对流降水云结构及动力特征, 基于降水频段调频连续波5520 MHz垂直指向雷达(VPR-CFMCW), 使用地面至15 km高度的反射率因子及径向速度, 建立对流降水云中大气垂直运动的反演方法, 分析对流垂直结构及大气垂直运动随高度分布的演变特征。对在广东龙门测站探测的2019年4月20—22日前汛期4次对流降水进行反演试验发现, 对流降水前大气上升运动对降水云反射率因子及地面降水有正贡献, 深厚对流具有倾斜性, 会导致垂直剖面在某些时刻呈分层结构;对流降水整层以下沉运动为主导, 高层大气上升运动与下沉运动交替出现, 低层大气下沉运动占比最高, 大气上升运动在6 km高度以上占比有所增加;大气垂直速度在高层较大、在低层较小, 超过10 m·s-1的强上升运动与下沉运动多出现在6 km高度以上, 4~6 km高度垂直运动变化较大, 4 km高度以下的平均下沉运动小于5 m·s-1, 上升运动约为2 m·s-1。Abstract: Detecting the vertical motions of the atmosphere in convective clouds is difficult. The cost of aircraft detection is expensive, limited by maximum flight altitude, high detection risk, low frequency, etc. In recent years, remote sensing instruments are applied with the development of detection technology. Ground-based vertical pointing radar has become a reliable way to obtain atmospheric vertical motions, through which the cloud structure and dynamic characteristics of convective precipitation clouds can be obtained, and the distribution and evolution characteristics of the intensity and proportion of atmospheric vertical movement during the mature stage of convection can be monitored in detail. Based on the vertical structure detection data of precipitation clouds from ground to 15 km height by a vertical pointing radar with 5520 MHz C-band Frequency Modulation Continuous Wave (VPR-CFMCW) technology, the vertical motions in the convective precipitation cloud are retrieved, and the vertical structure of convection and evolution characteristics of vertical motions at different heights are analyzed. The VPR-CFMCW is used to carry out the atmospheric vertical motion retrieval experiments on 4 convective precipitation events at the Longmen Station in Guangdong Province during pre-monsoon from 20 April to 22 April in 2019. It is found that the updrafts of the atmosphere before convective precipitation have a positive contribution to the intensity of reflectivity and surface precipitation afterwards. The deep convection is inclined, which causes the vertical section to show a layered structure at certain moments. Convective precipitation is dominated by downdrafts of the entire level, updrafts and downdrafts of the upper-level atmosphere appear alternately, when downdrafts account for the highest proportion in the lower-level, and the updrafts account for an increased proportion above 6 km height. The intensity of atmospheric vertical motions is strong in the upper-level, as strong updrafts and downdrafts exceeding 10 m·s-1 mostly appear above 6 km height. The vertical motions vary greatly at 4-6 km height. The average downdraft speed is less than 5 m·s-1 and the average updraft speed is around 2 m·s-1 under 4 km height. The development of ground-based vertical pointing radar can improve the understanding of vertical structure evolution and dynamic characteristics of convective precipitation clouds.
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图 1 2019年4月20—22日4个个例的VPR-CFMCW数据
(a)反射率因子, (b)径向速度, (c)速度谱宽
Fig. 1 VPR-CFMCW data of four convections events from 20 Apr to 22 Apr in 2019
(a)reflectivity, (b)radial velocity, (c)velocity spectral width
图 2 Z-Vt关系
(a)液态区拟合结果, (b)不同关系比较
Fig. 2 Z-Vt relationship
(a)the fitting result of rain region, (b)the comparison of different relationships
图 3 4个个例对流核反射率因子的累积频率随高度变化
(红线为亮带平均高度，黑色粗实线为最大频率廓线)
Fig. 3 Cumulative frequency altitude diagrams of four events convective core reflectivity
(the red line denotes the average height of the bright band, the bold solid line denotes profile of frequency)
图 4 个例1对流阶段
(a)反射率因子，(b)径向速度，(c)大气垂直运动Wair (黑色椭圆为强烈上升运动，红色椭圆为强烈下沉运动)，(d)分钟降水量
Fig. 4 The convective period of event 1
(a)reflectivity, (b)radial velocity, (c)atmospheric vertical motions Wair (the black (red) ellipse denotes strong ascent (descent)), (d)minute rainfall
图 8 滤除低速度值(|Wair|≤1 m·s-1)前后4个个例对流核的大气垂直运动
(实线表示平均值，虚线表示上升气流第5百分位数，点线表示下沉气流第95百分位数) (a)滤除前全部样本，(b)滤除后的上升气流样本，(c)滤除后的下沉气流样本
Fig. 8 The atmospheric vertical motions in convective core of four events before and after filtering the low velocity value(|Wair|≤1 m·s-1)
(the solid line denotes the average, the dashed line denotes the 5th percentile of updraft, the dotted line denotes the 95th percentile of downdraft) (a)all samples before the filtering,(b)updraft samples after the filtering, (c)downdraft samples after the filtering
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