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Retrieval of Air Vertical Velocity and Droplet Size Distribution in Squall Line Precipitation Using C-FMCW Radar
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摘要: 垂直指向探测的C波段调频连续波雷达具有高灵敏度和高时空分辨率等特点, 以2016年5月广东两次飑线降水为例, 结合同址K波段微雨雷达和地面激光雨滴谱仪, 探究C波段调频连续波雷达两种反演大气垂直速度(Va)和雨滴谱的方法:粒子平均下落末速度(Vt)-反射率因子(Ze)关系法(简称经验关系法)和小粒子示踪法(简称示踪法)。结果表明:经验关系法和示踪法反演的上升和下沉气流的时空分布基本一致;当地面雨强R≤1 mm·h-1, 经验关系法反演的雨滴谱与雨滴谱仪观测结果更接近;当1<R≤10 mm·h-1时, 两种方法反演的雨滴谱均与雨滴谱仪观测及微雨雷达产品较吻合;当R>10 mm·h-1时, 两种方法反演的中雨滴数浓度与雨滴谱仪观测结果接近, 但大雨滴数浓度较低;从各物理量时序变化看, 经验关系法反演结果更接近雨滴谱仪观测结果。Abstract: The zenith C-band frequency modulation continuous wave(C-FMCW) radar has good detection capability with high temporary-spatial resolution and large dynamic range. The Doppler spectral density data of two squall lines precipitation cases at Longmen of Guangdong are utilized to retrieve the air vertical velocity (Va) in clouds and droplet size distribution (DSD). The empirical relation method (checking relationship between mean particle falling velocity (Vt) and reflectivity factor) and the small-particle-trace method are explored to retrieve the air vertical velocity in clouds. And then the droplet size distribution is retrieved from the translated Doppler spectral density by a velocity-diameter relation. The retrieved DSD of two squall lines are then compared and validated with the observation of K-band micro rain radar and second-generation Parsivel disdrometer. The retrieved Va by the empirical relation method is slightly smaller for strong monomer than that by the small-particle-trace method and slightly larger for weak convective precipitation, but Vt is the opposite. The absolute value of Vt negative velocity by the empirical relation method and the small-particle-trace method corresponds to the moment of large particles and heavy rain observed by Parsivel disdrometer, indicating that Va of two methods is basically reliable. The comparison in DSD retrieval show that the number of small droplets observed by radar is higher, but Parsivel disdrometer may underestimate it. The results of the empirical relation method are closer to micro rain radar and Parsivel disdrometer when rain rate is below 1 mm·h-1. The medium droplets obtained by radar retrieval are consistent with Parsivel disdrometer measurements, but the concentration of large droplets is low when rain rate is stronger than 10 mm·h-1. The retrieval results of both methods are close to Parsivel disdrometer and micro rain radar when rain rate is between 1 mm·h-1 and 10 mm·h-1. The strong convection makes droplets rupture severer in the peak area of heavy precipitation in the squall line, resulting in smaller mass-weighted mean diameter (Dm) and larger generalized intercept parameter Nw of the empirical relation method and the small-particle-trace method retrieval. For weak convective precipitation at the back of the squall line, the value of empirical relation method is quite close to Parsivel disdrometer. Under different rain rate, μ value of C-FMCW radar is less than 10 and the fluctuation is smaller, indicating that the results of C-FMCW radar is even more reliable than Parsivel disdrometer and micro rain radar.
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图 1 2016年5月15日飑线后部弱对流降水观测及反演结果
(a)C-FMCW雷达反射率因子Ze, (b)经验关系法反演的大气垂直速度Va, (c)示踪法反演的大气垂直速度Va, (d)经验关系法反演的粒子群平均下落末速度Vt, (e)示踪法反演的粒子群平均下落末速度Vt, (f)雨滴谱仪观测的雨滴谱和雨强R
Fig. 1 Weak convective precipitation after the squall line passing on 15 May 2016
(a)C-FMCW reflectivity factor(Ze), (b)air vertical velocity(Va) retrieved by the empirical relation method, (c)air vertical velocity retrieved(Va) by the small-particle-trace method, (d)mean particle falling velocity(Vt) retrieved by the empirical relation method, (e)mean particle falling velocity(Vt) retrieved by the small-particle-trace method, (f)droplet size distribution and rain rate(R) measured by disdrometer
图 2 2016年5月15日不同雨强下的平均雨滴谱
(a)0<R≤0.2 mm·h-1，(b)0.2<R≤1 mm·h-1，(c)R>1 mm·h-1
Fig. 2 Mean droplet size distribution under three rain rate conditions on 15 May 2016
(a)0<R≤0.2 mm·h-1, (b)0.2<R≤1 mm·h-1, (c)R>1 mm·h-1
图 3 2016年5月15日19:43—21:20 3个设备物理量对比
Fig. 3 Comparison of physical parameters for three instruments from 1943 BT to 2120 BT on 15 May 2016
图 4 2016年5月6日飑线过境强对流降水观测及反演结果
(a)C-FMCW雷达的反射率因子Ze，(b)经验关系法反演的大气垂直速度Va，(c)示踪法反演的大气垂直速度Va，(d)经验关系法反演的粒子群平均下落末速度Vt，(e)示踪法反演的粒子群平均下落末速度Vt，(f)雨滴谱仪观测的雨滴谱和雨强R
Fig. 4 Strong convective precipitation of the squall line passing on 6 May 2016
(a)C-FMCW reflectivity factor(Ze), (b)air vertical velocity(Va) retrieved by the empirical relation method, (c)air vertical velocity(Va) retrieved by the small-particle-trace method, (d)mean particle falling velocity(Vt) retrieved by the empirical relation method, (e)mean particle falling velocity(Vt) retrieved by the small-particle-trace method, (f)droplet size distribution and rain rate(R) measured by disdrometer
图 5 2016年5月6日不同雨强下的平均雨滴谱
(a)0<R≤1 mm·h-1，(b)1<R≤10 mm·h-1，(c)R>10 mm·h-1
Fig. 5 Mean droplet size distribution under three rain rate conditions on 6 May 2016
(a)0<R≤1 mm·h-1, (b)1<R≤10 mm·h-1, (c)R>10 mm·h-1
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