The quantitative measurement of surface rainfall from scanning radar observations is dependent on the transformation of radar observables into hydrologic estimates. While some of these transformations are independent of the absolute radar calibration (for example, transforming the observed differential reflectivity into mean raindrop size estimates), many relationships are dependent on the absolute calibration of the radar.

Using the concept of up-scaling, scanning radars can be absolutely calibrated through the combination of surface disdrometers and vertically pointing profilers. The surface disdrometer measures the rain drop size distribution (DSD) at the surface. The surface reflectivity, rain rate, and mean rain drop size are estimated from this surface DSD. Through multiple precipitation events that account for time-space and precipitation regime variations, the calibration of the profiler located next to the surface disdrometer is adjusted until the profiler lowest height observations agree with the surface observations. The absolute calibration of the scanning radar is achieved by comparing the profiler and scanning radar reflectivities in commonly sampled volumes.

The NOAA Aeronomy Laboratory has deployed vertically pointing precipitation profilers operating at 1 and 3 GHz in support of the Ground Validation Program of the NASA Tropical Rainfall Measuring Mission (TRMM). The profilers and surface disdrometers were within about 35 km of either research polarmetric scanning radars or operational NEXRAD scanning radars. In this paper we show results of calibrating the scanning radars using the precipitation profilers and surface disdrometers.