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

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.