An Algorithm to Detect Spurious Differential Phase with Large Oscillation in the Range Direction

The differential phase measured by a polarimetric weather radar is readily contaminated by noises. Nonmeteorological echoes and the differential backscatter phase induce gate-to-gate fluctuations in the range direction. Several methods have been designed to remove these short-wavelength noises. There is also spurious differential phase with the wavelength of oscillation greatly larger than gate spacing, which is caused by factors such as nonuniform beam filling and multiple-trip echoes. This large oscillation of the differential phase in the range direction can result in artifactually large specific differential phases of both signs and cannot be removed effectively by existing methods.

This study has developed an algorithm to detect and remove spurious differential phase with large oscillation observed in rain. The algorithm is a simple threshold filter based on an objective function, which is designed according straightforwardly to the relationship between the specific differential phase and radar reflectivity. The function makes large oscillatory noises be amplified and easily detected.

The ability of the algorithm is tested by using the data from the measurements of the world’s first shipborne polarimetric radar on board the research vessel Mirai, which conducted an intensive observation in tropical rainy regions around the Island of Sumatra in Indonesia from November 23 to December 17, 2015. The evaluation of the algorithm is performed for a variety of precipitation events evolving over both open oceans and coastal regions with complex topography. It turns out that the algorithm can effectively identify and remove spurious differential phase with large oscillation, thus facilitating the quality control of both the differential phase and the specific differential phase. The efficacy of the algorithm in improving the accuracy of quantitative precipitation estimation by using the specific differential phase is also undergoing an investigation.