83rd Annual

Wednesday, 12 February 2003
Evaluating the Quikscat/Seawinds radar for measuring rainrate over the oceans using collocations with NEXRAD and TRMM
David E. Weissman, Hofstra University, Hempstead, NY; and M. A. Bourassa and J. Tongue
Poster PDF (248.5 kB)
Recent studies have demonstrated that the SeaWinds radar has the potential to operate as a meteorological weather radar, from space. This research program seeks to enable the SeaWinds scatterometer (as well as future Ku-band scatterometers) to convert its measurement of radar cross section into rainrate estimates over the earth's oceans, with the same spatial resolution with which it can make wind vector measurements.One unique benefit of the Scatterometer is its global reach. Its swath width is 1400 km and its latitude range is 80o. This exceeds the coverage of TRMM and the reach of coastal NEXRAD stations.

Measurements indicate that the radar backscatter from the rain volume in the atmosphere can exceed the sea surface radar cross section (at 14 GHz) in many situations. The relative magnitudes of received radar backscatter depend on the volumetric rainrate, the rain column height and surface wind speed. Algorithms for inverting the RCS are being developed using methods of calibration and validation of the rainrate with TRMM and NEXRAD, and with winds from buoys. The dual polarization of the SeaWinds instrument (H-pol and V-pol) enables the application of a highly sensitive technique for measuring rainrate, which is commonly used in radar meteorology. Comparing the collocated V-pol and H-pol radar cross section data, it is seen that for rainrates greater than 5 mm/hr, the H-pol magnitudes tend to exceed the V-pol. This is the differential reflectivity, caused by the increasing, significant oblateness of raindrops at the higher rainrates. The size of this observed differential reflectivity and its dependence on rainrate is in agreement with the theoretical results in the literature.

A broad set of collocations among TRMM, NEXRAD and QuikSCAT is being analyzed to compare their absolute rainrate estimates, and to conduct extensive statistical analysis of these rainrates in their 0.25o and 0.50o LAT/LON cells, and their spatial distribution. Other physical variables are being examined using complementary sensors and component analysis. The approach is to determine what additional variables are affecting the each sensors response, so that the volumetric rainrate can be separated and extracted.

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