Using Ground Clutter to Adjust Relative Radar Calibration at Kwajalein, RMI

The ground radar situated on Kwajalein Atoll in the Republic of the Marshall Islands serves an important role in providing reflectivity measurements for comparison with rain gauge data collected and analyzed by the Tropical Rainfall Rainfall Measuring Mission (TRMM) Ground Validation (GV) group at NASA Goddard Space Flight Center (GSFC) as well as satellite data obtained from TRMM. Radar calibration is a major source of uncertainty in radar rainfall estimation. While some techniques can mitigate the extent of this uncertainty, more detailed knowledge of the calibration is crucial to arriving at more precise rainfall estimates. The TRMM-GV group at GSFC has developed a technique that incorporates the use of a clutter mask to denote radar pixels that are sources of frequent/permanent ground clutter. Reflectivity values within these clutter areas are then identified. These pixels are used in the generation of probability distribution functions (PDFs) of reflectivity on a daily basis to assess the time evolution and stability of the calibration. By targeting the upper percentiles of these distributions, and with knowledge of radar calibration for a specific period of time obtained from independent sources, the GV team has generated a timeline of radar calibration from mid-1999 up to the present. We will show how major variations in the timeline are associated with significant events related to radar engineering issues, and how subsequent adjustments to the calibration data can have a significant impact on radar rainfall estimation as well as implications for comparisons with TRMM satellite estimates.