Rainfall Measurement Error by WSR-88D Radars due to Variations in Z–R Law Parameters and the Radar Constant

Abstract

An investigation is made of the extent to which variations in Z?R (reflectivity factor?rainfall rate) relations can explain the systematically large offsets of radar-measured rainfall from rain gauge measurements as observed with some National Weather Service (NWS) WSR-88D radars. It is shown that theory predicts that the use by the NWS of the current default Z?R relation (Z = 300R1.4) should underestimate rainfall by about 25% in stratiform rain and overestimate it by about 33% in thunderstorm rain. Yet it is commonly observed that some WSR-88D radars systematically underestimate rainfall by a factor of 2 or more in stratiform rain and produce estimates of rainfall with acceptable accuracy for isolated thunderstorms. It is shown that variations in Z?R law parameters alone cannot explain these discrepancies, and it is proposed that their origin lies in a radar calibration offset. A comparison is made of reflectivity factors measured by the KGSP WSR-88D at Greer, South Carolina, with those determined from disdrometer measurements at the Clemson Atmospheric Research Laboratory. Comparisons for five storms find that the radar systematically underestimates the reflectivity factor by more than 3.5 dB. This offset of the radar-measured reflectivity factor below the disdrometer measurements accounts for the systematic factor of 2 underestimation for stratiform rain and the apparent acceptable accuracy for thunderstorms using the KGSP WSR-88D radar. The difference between the radar and disdrometer measurements of the reflectivity factor is interpreted as being due to a radar calibration offset. It is also proposed that the systematic differences observed with other WSR-88D radars may be the result of similar calibration offsets.