The quantitative measurement of surface rainfall from scanning radar observations is dependent on the transformation of radar observables into hydrologic estimates. The most common transformation is the Z-R transformation that converts the measured radar reflectivity into surface rainfall estimates. While there are many Z-R relationships used by radar meteorologists around the globe, the utility of this transformation is dependent on the absolute calibration of the scanning 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 which is used to estimate the surface reflectivity, rain rate, and mean rain drop size. Vertically pointing profilers observe the reflectivity vertical structure of the precipitating cloud systems that pass directly overhead. Using simultaneous surface disdrometer and profiler observations, the profiler is calibrated to the surface disdrometer reflectivity. The absolute calibration of the scanning radar is then 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.