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Quality Control of S-band Polarimetric Radar Measurements for Data Assimilation
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摘要: 双偏振雷达是强对流天气分析和云微物理研究的重要探测设备，将多普勒天气雷达升级为双偏振雷达是我国未来几年强对流天气监测发展建设的重点计划，而双偏振雷达资料质量控制是其中的关键技术问题。针对国内首批业务布建的广东省S波段双偏振雷达网研发建立了一套完整的质量控制技术方案，除了重点解决非气象回波外，还考虑了非标准波束遮挡和径向高频脉动问题。在飑线、暴雨和台风3类华南季风区强天气个例的应用表明，基于模糊逻辑的水凝物分类、偏振量（零滞后互相关系数、信噪比和比差分相位）阈值检查和杂波剔除能有效剔除非气象回波，抑制异常传播波束导致的虚假回波；线性内插较好地弥补了非标准波束遮挡带来的观测缝隙；中值滤波和滑动平均既过滤了偏振观测量在雷达径向的高频脉动，又保留了主要的偏振雷达观测特征。质量控制后的气象回波约占有效观测（反射率因子大于-30 dBZ）的40%，其偏振量取值分别为反射率因子大于5 dBZ、零滞后互相关系数大于0.8和差分反射率为-0.2~4 dB。Abstract: The polarimetric radar is an important detection device whose measurements can be used for severe convective weather analysis and cloud microphysics progress research. Upgrading the traditional Doppler weather radar to polarimetric radar is a key part of severe convective weather monitoring program of China in the next few years, and the quality control of polarimetric radar measurements is key technical issue of the monitoring program. In Guangdong Province, based on the domestic and international mainstream quality control algorithms and relevant experience, a quality control system is developed for S-band polarimetric radars, to deal with the non-meteorological echo, non-standard blockage and high frequency noise in the radar radial, which have negative impacts on application of polarimetric radar measurements in data assimilation. The system is applied to the typical severe convective weather case in South China monsoon region, including a rainfall case, a severe convection case and a typhoon case in 2017. Evaluation results show that a combination of the hydrometeor classification screening based on fuzzy logic, co-polar cross-correlation coefficient (ρHV), signal-to-noise ratio (SNR) and specific differential phase (KDP) thresholding and despeckling can remove most non-meteorological echoes, and suppress virtual echo caused by anomalous propagation efficiently. Non-meteorological echoes include ground clutter, biological scatters, partial clear-air echo and radiographic noise due to anomalous propagation. A linear interpolation is employed to fill the small gap (the width of which is less than 5°) caused by non-standard blockage. A median filter and radial smooth are found effective in filtering out high frequency noise in the radar radial while maintaining polarimetric radar characteristics. After quality control, the meteorological echo is clearer and more prominent, and accounts for about 40% of valid observation which is defined by reflectivity (ZH) being larger than -30 dBZ. ZH of the meteorological echo is larger than 5 dBZ, ρHV is larger than 0.8 and less than 1.0, and the differential reflectivity (ZDR) is between -0.2 and 4 dB. Batch tests are needed to keep the quality control system stable and effective in the further work. And how to combine multiple polarimetric radar measurements to form a three-dimensional gridded product is also another important prerequisite for application of polarimetric radars measurements in the numerical model.
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Key words:
- polarimetric radar;
- quality control;
- non-meteorological echo
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图 2 2017年5月8日06：00—21:00广州偏振雷达不同仰角累计反射率因子
(a)0.5°仰角，(b)1.5°仰角，(c)2.4°仰角
Fig. 2 Accumulated reflectivity at different elevations of Guangzhou polarimetric radar from 0600 UTC to 2100 UTC on 8 May 2017
(a)0.5°, (b)1.5°, (c)2.4°
图 3 2017年5月8日10:00质量控制前后广州偏振雷达的偏振量分布
(a)原始反射率因子，(b)原始差分反射率，(c)经过非标准波束遮挡订正的反射率因子，(d)经过非标准波束遮挡订正的差分反射率，(e)经过水凝物分类筛选的反射率因子，(f)经过水凝物分类筛选的差分反射率，(g)经过阈值检查的反射率因子，(h)经过阈值检查的差分反射率，(i)经过杂波剔除的反射率因子，(j)经过杂波剔除的差分反射率，(k)经过中值滤波的反射率因子，(l)经过中值滤波的差分反射率
Fig. 3 The effect of a different quality control component on measurements from Guangzhou polarimetric radar of 0.5° elevation at 1000 UTC 8 May 2017
(a)raw reflectivity, (b)raw differential reflectivity, (c)reflectivity after non-standard blockage correction, (d)differential reflectivity after non-standard blockage correction, (e)reflectivity after hydrometeor classification filtering, (f)differential reflectivity after hydrometeor classification filtering, (g)reflectivity after thresholding, (h)differential reflectivity after thresholding, (i)reflectivity after despeckling, (j)differential reflectivity after despeckling, (k)reflectivity after median filter, (l)differential reflectivity after median filter
图 4 2017年5月8日10:00广州偏振雷达0.5°仰角水凝物分类分布
Fig. 4 Hydrometeor classification type for Guangzhou radar of 0.5° elevation at 1000 UTC 8 May 2017
图 5 中值滤波及九点平滑效果
(a)反射率因子，(b)差分反射率，(c)比差分相位
Fig. 5 The result of the median filtering and radial smooth for measurements
(a)reflectivity, (b)differential reflectivity, (c)pecific differential phase
图 6 2017年5月6日21:54广州偏振雷达0.5°仰角质量控制前后偏振量分布
(a)质量控制前反射率因子，(b)质量控制前差分反射率，(c)质量控制前比差分相位，(d)质量控制后反射率因子，(e)质量控制后差分反射率，(f)质量控制后比差分相位
Fig. 6 Measurements before and after quality control for Guangzhou polarimetric radar of 0.5° elevation at 2154 UTC 6 May 2017
(a)reflectivity before quality control, (b)differential reflectivity before quality control, (c)pecific differential phase before quality control, (d)reflectivity after quality control, (e)differential reflectivity after quality control, (f)pecific differential phase after quality control
图 7 2017年8月23日00:54阳江偏振雷达0.5°仰角质量控制前后偏振量分布
(a)质量控制前反射率因子，(b)质量控制前差分反射率，(c)质量控制前比差分相位，(d)质量控制后反射率因子，(e)质量控制后差分反射率，(f)质量控制后比差分相位
Fig. 7 Measurements before and after the quality control for Yangjiang polarimetric radar of 0.5° elevation at 0054 UTC 23 Aug 2017
(a)reflectivity before quality control, (b)differential reflectivity before quality control, (c)specific differential phase before quality control, (d)reflectivity after quality control, (e)differential reflectivity after quality control, (f)specific differential phase after quality control
图 8 不同个例的ρHV与ZH散点分布
(a)2017年5月6日21:54广州偏振雷达0.5°仰角，(b)2017年5月8日10:00广州偏振雷达0.5°仰角，(c)2017年8月23日00:54阳江偏振雷达0.5°仰角
Fig. 8 The scatter plot of ZH-ρHV during quality control for different case
(a)Guangzhou radar of 0.5° elevation at 2154 UTC 6 May 2017, (b)Guangzhou radar of 0.5° elevation at 1000 UTC 8 May 2017, (c)Yangjiang radar of 0.5° elevation at 0054 UTC 23 Aug 2017
图 9 通过质量控制的ZDR与ZH频数统计分布
(a)2017年5月6日21:54广州偏振雷达0.5°仰角，(b)2017年5月8日10:00广州偏振雷达0.5°仰角，(c)2017年8月23日00:54阳江偏振雷达0.5°仰角
Fig. 9 Frequency distribution of ZDR and ZH after quality control
(a)Guangzhou radar of 0.5° elevation at 2154 UTC 6 May 2017,
(b)Guangzhou radar of 0.5° elevation at 1000 UTC 8 May 2017,
(c)Yangjiang radar of 0.5° elevation at 0054 UTC 23 Aug 2017表 1 2017年5月8日10:00广州偏振雷达资料阈值检查剔除率(单位：%)
Table 1 The rejection rate for various ρHV, SNR and KDP thresholds for Guangzhou polarimetric radar at 1000 UTC 8 May 2017(unit:%)
仰角/(°) ρHV检查 SNR检查 KDP检查 ZH ZDR KDP ZH ZDR KDP KDP 0.5 7.09 3.16 17.30 5.58 5.55 5.44 27.29 1.5 5.34 1.75 15.35 6.69 6.67 6.52 28.19 2.4 7.51 3.15 18.20 5.66 5.64 5.51 25.77 3.3 6.59 2.45 16.59 6.22 6.21 6.02 22.37 4.3 5.92 2.42 12.56 7.75 7.74 7.57 25.59 6.0 6.93 3.51 13.74 9.14 9.14 8.88 36.33 9.9 8.78 5.34 20.46 9.33 9.20 8.61 38.52 14.6 13.06 9.83 29.64 10.58 10.49 9.57 44.04 19.5 28.00 23.92 39.50 11.43 11.35 10.32 59.74 
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