Airborne Doppler radar velocity measurements of precipitation seen in ocean surface reflection

Airborne Doppler radar measurements over the ocean reveal mirror images of precipitation echoes in reflection from the ocean surface at their virtual positions “below” the surface. Because the detected vertical component of the precipitation motion changes sign on reflection, the velocity field in the reflection appears shifted by amounts depending on the mean reflectivity-weighted hydrometeor fall speed, wind speed, and viewing angle. The shift of the zero-velocity isotach, for example, from the upwind side of the aircraft in the direct echoes to the downwind side in the mirror image is readily observed. With two views of each precipitation volume, one direct and the other in mirror image, one may solve for both a horizontal wind component and the net mean particle fall speed. These components may thereby be mapped in an appreciable volume. Measurements of divergence may be made by either flying or scanning the beam in a circle. The availability of mean fall speed with radar reflectivity provides a promising two-parameter system for improved measurements of rainfall rate. The airborne radar also provides measurements of the normalized radar backscatter cross section at all angles from nadir to glancing incidence, thus providing a variety of estimates of ocean surface properties and winds. The availability of direct Doppler wind measurements using echoes from either precipitation or chaff provides a much needed “ground truth” system for the radar used as a scatterometer or for corresponding observations from space.