Dual Polarimetric Quality Control for NASA's
Precipitation Measurement Missions Ground Validation Program
Jason L. Pippitt, and David A. Marks
NASA Goddard Space Flight Center, Greenbelt, and Science Systems and Applications, Inc.,
David B. Wolff
NASA Wallops Flight Facility
Wallops Island, Virginia
The recent upgrade of the National Weather Service WSR-88D radar network to dual polarization (DP) and the availability of research DP radars, such as NASA Polarimetric (NPOL) and Kwajalein Polarimetric (KPOL), allows NASA's Precipitation Measuring Missions Ground Validation program (PMM-GV) to capture unique polarimetric data to foster improved understanding of precipitation microphysics, and provide essential input for development of precipitation retrieval algorithms. The quality control (QC) of these data sets is a critical first step in this process. The developed and applied DPQC algorithm is modular, physically based, and employs NASA's Radar Software Library (RSL) using the IDL programming language (RSL-in-IDL). The modular functions and procedures were written such that the algorithm can easily be used with other polarimetric radars via passing of an RSL “radar” structure, and includes tunable parameters applied using keywords specified at execution time. RSL-in-IDL allows the user to easily view and manipulate the radar structure by volume, sweep, ray, and range. The DPQC algorithm can then output the quality controlled radar structure into Universal Format for downstream product generation.
Significant QC challenges include mitigation of ground clutter, sea clutter, biological targets, multiple trip echo, and anomalous propagation (AP) from both density gradients and nocturnal atmospheric decoupling. DP values commonly fall outside the established QC thresholds near the melting level, resulting in unwarranted data removal. To account for this problem, hourly model soundings are used to estimate the melting level, and DPQC is only applied to data 1-km below this level. Primary fields and calculations for QC threshold tests include co-polar cross correlation ρHV, differential reflectivity ZDR, standard deviation of differential phase σ(ΦDP), and specific differential phase KDP. The cross-polar correlation is very useful in distinguishing rain from non-rain. The ZDR threshold test is an excellent identifier for ground clutter, AP, biological targets, and chaff. The σ(ΦDP) threshold test is also useful in the detection of AP, ground clutter, and multiple trip echo. The DPQC code can be modified with add on functions to improve our operational QC product. A sector of non-precip echo can be targeted and removed, ΦDP thresholds can remove spikes, and velocity data can remove trees in RHI scans. Unique threshold tests can also be created to target specific non-precip echo such as nocturnal AP.
DPQC is being applied to data from the GPM-GV Validation Network, and from GPM field campaigns. In support of the GPM-GV Wallops Precipitation Science Research Facility, DPQC is applied to radar data from NPOL, as well as WSR-88D sites at Dover, DE (KDOX), and Wakefield, VA (KAKQ).