Quality Control and Calibration of the Dual-Polarization Radar at Kwajalein, RMI
Journal of Atmospheric and Oceanic Technology
Weather radars, recording information about precipitation around the globe, will soon be significantly upgraded. Most of today's weather radars transmit and receive microwave energy with horizontal orientation only, but upgraded systems have the capability to send and receive both horizontally and vertically oriented waves. These enhanced "dualpolarimetric" (DP) radars peer into precipitation and provide information on the size, shape, phase (liquid / frozen), and concentration of the falling
... on of the falling particles (termed hydrometeors). This information is valuable for improved rain rate estimates, and for providing data on the release and absorption of heat in the atmosphere from condensation and evaporation (phase changes). The heating profiles in the atmosphere influence global circulation, and are a vital component in studies of Earth's changing climate. However, to provide the most accurate interpretation of radar data, the radar must be properly calibrated and data must be quality controlled (cleaned) to remove non-precipitation artifacts; both of which are challenging tasks for today's weather radar. The DP capability maximizes performance of these procedures using properties of the observed precipitation. Abstract The dual-polarization weather radar on Kwajalein Atoll in the Republic of the Marshall Islands (KPOL) is one of the only full-time (24/7) operational S-band dual-polarimetric (DP) radars in the tropics. Through the use of KPOL DP fields and disdrometer data from Kwajalein, quality control (QC) and reflectivity calibration techniques were developed and adapted for use in a near real-time operational environment. Data studies in light rain show that KPOL DP measurements meet or exceed quality thresholds for these applications as determined by consensus of the radar community. While the methodology for development of such applications is well documented, tuning of specific algorithms to the particular regime and observed raindrop size distributions requires a comprehensive testing and adjustment period. Presented are algorithm descriptions and results from five case studies in which QC and absolute reflectivity calibration were performed. Also described is a unique approach to calibrate the differential reflectivity field when vertically pointing scans are not available. Results show the following: 1) DP-based QC provides superior results compared to the legacy Tropical Rainfall Measuring Mission (TRMM) QC algorithm (based on height and reflectivity thresholds), 2) absolute reflectivity calibration can be performed using observations of light rain via a published differential phase-based integration technique; results are within ± 1 dB compared to independent measurements, and 3) a polarimetrically tuned reflectivity-rain rate (Z-R) application extended to six months of data shows very good agreement with independent rain gauge measurements, thereby extolling the benefit of DP-based QC and calibration of the reflectivity field.