基于TRMM/PR的长江下游地基雷达一致性订正

Consistent Correction to Ground-based Radars in the Lower Reachers of the Yangtze Based on TRMM/PR Observations

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。

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.