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Consistent Correction to Ground-based Radars in the Lower Reachers of the Yangtze Based on TRMM/PR Observations
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摘要: 我国有近200部地基多普勒天气雷达，已经积累了近20年的观测数据，这些数据对雷达气候学研究非常重要。但由于不同雷达的标定误差不同，雷达之间存在观测值不一致性的现象（与美国的地基雷达类似），有的反射率因子差异超过了3 dB。这种不一致影响了多雷达联合降水估计的精度和雷达组网临近预报的效果。为此，采用筛选比较法对地基雷达与TRMM/PR（Tropical Rainfall Measuring Mission/Precipitation Radar）进行空间匹配和异常数据剔除，以TRMM/PR为参照计算并订正地基雷达偏差。对2013年5—9月长江下游7部S波段雷达数据订正后，结果表明：订正后7部雷达之间的平均反射率因子差异从1.8 dB降至0.5 dB，任意两部雷达之间的差异均小于1.0 dB，多雷达的观测一致性和空间连续性有明显改善。与传统的几何匹配法比较，筛选比较法订正结果相对稳定，不存在过量订正的问题。Abstract: There are almost 200 ground-based operational Doppler weather radars in China's new generation weather radar network, accumulating a large amount of radar data continuously for nearly 20 years. These historical data from weather radars are essential for researches related to China's radar climatology. However, reflectivity factors of two adjacent radars are often found inconsistent in observation overlap area, which is similar to the WSR-88D (Weather Surveillance Radar-1988 Doppler) in US. Some reflectivity differences are reported more than 3 dB. The fundamental cause for this problem is that different radar has different calibration error which will cause a reflectivity factor bias. Reflectivity differences of two adjacent radars will certainly reduce accuracy and precision of multiple radars joint Quantitative Precipitation Estimation and Nowcasting. Therefore, a ground-based radar correction method based on TRMM/PR (Tropical Rainfall Measuring Mission/Precipitation Radar) observations is proposed, which is named as Selective Comparison Method. Ground-based radar and TRMM/PR data are spatially matched and the abnormal values are gradually eliminated to extract an Optimal Matchup Datasets with relatively high correlation coefficient in the new method. Then the ground-based radar bias is calculated and corrected with the Optimal Matchup Datasets. The Selective Comparison Method is applied for consistency correction to seven S-band radars in the lower reaches of the Yangtze from May to September in 2013. Results show that annual reflectivity factor biases of three radars are greater than 1.5 dB among seven ground-based radars in the research area. These biases lead to some differences in adjacent radar observations and bring about a significant spatial discontinuity in multiple radar reflectivity fields. After correction, the average reflectivity differences of seven radars significantly decrease from 1.8 dB to 0.5 dB. Furthermore, all reflectivity differences between two adjacent radars are less than 1.0 dB. The reflectivity factor consistency and spatial continuity of multiple radars are greatly improved. Comparing to the traditional Geometric Matchup Method, the Selective Comparison Method performs better, and it overcomes the problem that some radars may be over-corrected. This correction method is only suitable for the quality control of historical ground-based radar data because TRMM/PR stopped its observations in 2014. Further improvements are still needed to extend this method to GPM/DPR to achieve real-time radar data processing in the future.
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图 1 长江下游7部地基雷达地理位置
(虚线为雷达150 km的探测范围)
Fig. 1 Seven ground-based radars in the lower reaches of the Yangtze
(dashed lines represent radar coverages of 150 km)
图 2 2013年5—9月地基雷达与TRMM/PR的有效配对事件
Fig. 2 Available matching events between ground-based radar and TRMM/PR from May to Sep in 2013
图 3 地基雷达与TRMM/PR数据在不同距离上的相关系数(a)和差异标准差(b)
Fig. 3 Correlation coefficient(a) and standard deviation(b) of reflectivity difference between ground-based radar and TRMM/PR at different distances
图 4 地基雷达与TRMM/PR数据在不同高度层上的相关系数(a)和差异标准差(b)
Fig. 4 Correlation coefficient(a) and standard deviation(b) of reflectivity difference between ground-based radar and TRMM/PR at different levels
图 5 第1步到第5步处理后配对数据的相关系数(a)和差异标准差(b)
Fig. 5 Correlation coefficient(a) and standard deviation(b) of difference between ground-based radar and TRMM/PR from the first step to the fifth step
图 6 筛选前(a)、筛选后(b)和订正后(c)的配对数据散点
Fig. 6 Scatter plot of matchup data before selection(a), after selection(b) and after correction(c)
图 7 地基雷达反射率因子年平均偏差(a)和月平均偏差(b)
Fig. 7 Annual(a) and monthly(b) average reflectivity biases of ground-based radars
图 8 相邻地基雷达年平均反射率因子差异
Fig. 8 Annual average reflectivity differences of adjacent ground-based radars
图 9 相邻地基雷达月平均反射率因子差异
(a)订正前，(b)GM_ICE订正后，(c)GM_WATER订正后，(d)筛选比较法订正后
Fig. 9 Monthly average reflectivity differences of adjacent ground-based radars
(a)uncorrected, (b)corrected by GM_ICE, (c)corrected by GM_WATER, (d)corrected by Selective Comparison Method
表 1 地基雷达与TRMM/PR观测差异
Table 1 Differences between ground-based radar and TRMM/PR observations
参数 地基雷达 TRMM/PR 波段(波长) S波段(10 cm) Ku波段(2.2 cm) 300 km×300 km区域扫描时间/min 6 2 扫描方式 从下而上 从上而下 衰减影响 可忽略 严重 地物影响 严重 小 有效照射体积 1 km×1°×1° 5 km×5 km×0.25 km 最小雷达反射率因子/dBZ -10 18.5 
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