Satellite Observations For Calibration of Ground Radar Networks
Abstract
Calibration differences between weather service ground radars is one source of error that can lead to bias in quantitative precipitation estimates. In the U.S., calibration differences among Weather Service Radar-1988 Doppler (WSR-88D) radars are know to vary by up to several decibels in reflectivity. Such differences have been shown to cause significant radar-to-radar observation differences, and can lead to significant error in precipitation estimates. The calibration of 21 WSR-88D radars in the southeast U.S. was assessed using methods developed for NASA's Global Precipitation Mission (GPM) Validation Network (VN) prototype. The VN performs geometric matching of Precipitation Radar (PR) data from the Tropical Rainfall Measuring Mission (TRMM) satellite to ground radars. The VN geometric matching method averages PR reflectivity (both raw and attenuation corrected) and rain rate, and ground radar (GR) reflectivity at the geometric intersection of the PR rays with the individual GR elevation sweeps. The algorithm thus averages the minimum PR and GR sample volumes needed to ''matchup'' the spatially coincident PR and ground radar data types. This geometric matching method has been demonstrated to out-perform gridding techniques by providing better estimates of GR-to-PR bias. TRMM PR data were used as the calibration reference because analyses of the PR performance estimated the instrument calibration to be stable and accurate to within less than 1dBZ (3-sigma). The calibration accuracy of the 21 WSR-88D radars was assessed for the period of record from August 2006 to July 2011. For purposes of calibration assessments, the data were restricted to PR-GR match-up volumes >750m above the bright band in stratiform rain areas where PR radar attenuation is not at issue. Based on space and ground radar matchups, most WSR-88D radars were found to have a mean PR-GR bias of less than 1 dBZ. Several adjacent WSR-88D sites near or along the Gulf Coast between Louisiana and Florida (KLCH, KLIX, KSHV, KTBW, and KTLH) exhibit a PR-GR bias of -1 dBZ or lower, indicating a positive calibration offset of the WSR-88D radar. This set of Gulf Coast radars, KEVX and KMOB excepted, seem to be well calibrated to one another, but run ''hot'' compared to the other WSR-88D sites in the VN subset. Additional analyses were performed to examine the temporal variability of mean PR-GR reflectivity differences to identify ground radar calibration drift. Several sites (KAMX, KBYX, KCRP, and KMLB) show consistent, small biases with respect to the PR, indicating a stable calibration of these WSR-88D systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H41G1135S
- Keywords:
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- 1895 HYDROLOGY / Instruments and techniques: monitoring;
- 3354 ATMOSPHERIC PROCESSES / Precipitation