Rainfall Measurements During Hurricane Agnes by Three Overlapping Radars

Abstract

As part of the International Field Year for the Great Lakes, rainfall measurements were made for hurricane Agnes over the Lake Ontario watershed by two C-band radars and one S-band radar. Two dense networks of raingages were located in radar overlap areas. One of the networks contained a raindrop disdrometer for measuring the number and size of drops. Comparison between radar and gage rainfall measurements show there is considerable time and space variability in the accuracy of the radar rainfall estimates. Because of the high correlation between radars measuring rain over the same area, it is concluded that this observed variability is due to meteorological causes rather than radar measurement errors. The Z-R relationship as measured by the disdrometer remained constant throughout the storm; thus, it is improbable that the variability in the radar accuracies resulted from changes in the Z-R relationship. Some of the larger radar overestimates could be attributed to evaporation below the radar beam. Because of precipitation growth below the beam, incomplete beam filling, and interception of the freezing level, the radar underestimated the rainfall for radar ranges where the beam was greater than 6000 ft above the earth. For areas where the beam height was less than 6000 ft, 90% of the radar estimates were within a factor of 1.3 of the gage measurements.