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Quality Control and Evaluation on Non-cloud Echo of Ka-band Cloud Radar
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摘要: 针对Ka波段毫米波云雷达观测中出现的非云回波，提出改进的质量控制方法，并利用2018年9月—2020年8月福建平和观测资料，定量评估质量控制效果及其对云-降水探测的影响。结果表明：提出的改进方法能较好改善雷达探测结果，可有效滤除非云回波。非云回波对3 km高度以下的弱云探测有重要影响，且非云回波的探测率与雷达灵敏度密切相关，整体随高度上升而下降；同时非云回波存在明显的日变化特征，午后—前半夜因湍流活动较强，非云回波的探测率也较高；后半夜—日出前因湍流活动减弱，非云回波的探测率逐渐下降。Abstract: Aiming at non-cloud echoes in Ka-band millimeter wave cloud radar observation, an improved data quality control method is proposed. Using the observation at Pinghe of Fujian from September 2018 to August 2020, the quality of the radar is quantitatively evaluated to study the actual impact of data quality control on cloud-precipitation detection. The non-cloud echoes show the characteristics of weak radar reflectivity factor (Z) and strong depolarization ratio (R) at Pinghe. But statistics show that there is difference from those of Qinghai-Tibet Plateau or Guangdong. Therefore, using the radar reflectivity factor (Z) less than -5 dBZ and linear depolarization ratio (R) greater than -22 dB as the judgment condition, and with the aid of filtering, non-cloud echoes can be effectively filtered. At the same time, a typical example is used to verify the effectiveness of the algorithm. Non-cloud echoes have a significant effect on the detection of cloud below 3 km, especially weak echoes. Non-cloud echoes account for 9.20% of all radar reflectivity factor samples, and 34.05% of all radar linear depolarization ratio samples. For the weak echo below -5 dBZ, the impact of non-cloud echoes is more significant, which accounts for 67.20% of all radar reflectivity factor sample. The detection rate of non-cloud echoes matter is closely related to the radar sensitivity, and the overall decrease with the height increaseing. The detection rate of non-cloud echoes decreases with the height increaseing. Meanwhile, non-cloud echoes have a certain relationship with the boundary layer, with an obvious diurnal change trend. From afternoon to midnight, due to strong turbulent activity, the detection rate of non-cloud echo matter is also higher, and the peak occurs at 1700 BT. From midnight to sunrise, due to the weakening of turbulent motion, the detection rate of non-cloud echo gradually decreases, and the lowest value occurs at 0400 BT. Non-cloud echoes have a significant effect on the vertical distribution of cloud precipitation. After quality control, the number of samples at the height of 0.12-2.5 km for radar reflectivity factor decrease by 17.68%, and the number of samples at the height of 0.12-4 km for radar linear depolarization ratio decrease by 14.29%.
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图 1 2018年9月—2020年10月福建平和雷达观测的非云回波和云-降水回波Z与R概率分布
Fig. 1 Probability distribution of measured Z and R of non-cloud and cloud-precipitation echoes observed by Pinghe radar in Fujian from Sep 2018 to Oct 2020
图 2 2018年9月—2020年8月福建平和3 km高度以下质量控制前后的Z和R频次分布
Fig. 2 Frequency distributions of measured Z and R below 3 km altitude before and after quality control observed by Pinghe radar in Fujian from Sep 2018 to Aug 2020
图 3 2018年9月—2020年8月福建平和非云回波的时间-高度频次分布和整层高度总频次
Fig. 3 Time-height frequency distributions of non-cloud echoes and frequency distribution of the whole levels observed by Pinghe radar in Fujian from Sep 2018 to Aug 2020
图 4 2018年9月—2020年8月福建平和非云回波质量控制前后Z和R的频次-高度分布
Fig. 4 Frequency-height distributions of measured Z and R of non-cloud echoes before and after quality control observed by Pinghe radar in Fujian from Sep 2018 to Aug 2020
图 5 2019年4月18日12:00—20:00福建平和雷达上空观测的包含低空碎积云和深对流降水云的质量控制前后回波对比
Fig. 5 Comparison of low altitude cumulus cloud and deep convective precipitation cloud echoes before and after radar quality control observed by Pinghe radar in Fujian from 1200 BT to 2000 BT on 18 Apr 2019
图 6 2019年5月19日17:00—23:00福建平和雷达上空观测的包含低空层云、积云和高空卷积云的回波质量控制前后回波对比
Fig. 6 Comparison of low altitude stratus clouds, cumulus and high altitude cirrocumulus echoes before and after quality control of observed by Pinghe radar in Fujian from 1700 BT to 2300 BT on 19 May 2019
图 7 2018年9月2日15:00—23:00福建平和雷达上空观测的包含低空积云和弱对流性降水云的质量控制前后回波对比
Fig. 7 Comparison of low altitude cumulus cloud and weak convective precipitation cloud echoes before and after quality control observed by Pinghe radar in Fujian from 1500 BT to 2300 BT on 2 Sep 2018
表 1 Ka波段毫米波云雷达主要性能参数
Table 1 Major performance parameters for Ka-band millimeter wave cloud radar
参数 数值 工作频率 35 GHz±500 MHz 波束宽度 0.4° 脉冲重复频率 5988~16666 Hz 峰值功率 20 W 时间分辨率 1 min 空间分辨率 30 m 观测资料 Z/V/σV/R 信号处理方式 FFT 
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表 2 4种探测模式主要参数
Table 2 Major parameters of 4 detection modes
参数 边界层模式 中云模式 高云模式 降水模式 脉冲宽度/μs 0.2 8 24 0.2 脉冲重复频率/Hz 16666 8333 5988 5988 驻波时间/s 0.98 1.97 1.37 1.37 相干积累数 4 2 1 1 非相干积累数 16 32 32 32 FFT点数 256 256 256 256 距离分辨率/m 30 30 30 30 有效探测高度/km 0.12~7.5 1.47~7.5 3.87~20 0.12~20 最大不模糊速度/(m·s-1) 8.93 8.93 12.83 12.83 速度分辨率/(cm·s-1) 6.98 6.98 10.02 10.02
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