Analysis of Surface Normalized Radar Cross Section for the Improvement of the TRMM Precipitation Radar Algorithm
PART I : Over the Ocean

Abstract

The Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) standard algorithm 2A21 produces the surface normalized radar cross section (NRCS) values under no rain conditions as functions of the incidence angle and surface physical parameters which affect surface scattering. These NRCS values are used as the reference values to calculate the path integrated attenuation (PIA) values in the surface reference technique (SRT) to perform the rain attenuation correction. This paper (Part I) shows the NRCS values as functions of incidence angle and wind speed over the ocean and tries to improve the methods to calculate reference NRCS values over ocean, and the following paper (Part II) concerns NRCS over the land.
We analyze statistically the ocean surface NRCS as a function of incidence angle over the eleven years from 1998 to 2008. The relation between the NRCS and the incidence angle is approximated better by the two quadratic curves than the single quadratic curve. This approximation will improve the across-track hybrid method used to calculate the reference NRCS in the present PR 2A21 version 6 algorithm. The present PR 2A21 version 6 algorithm also uses the temporal reference method to calculate the reference NRCS only as a function of incidence angle over the ocean. We introduce a new equation which expresses the NRCS not only by the incidence angle but also by the ocean surface wind speed. This equation is expected to improve the reference NRCS in the temporal reference method over the ocean. The relation between the NRCS and the relative wind direction is also studied by comparing the TRMM PR NRCS and the wind direction observed by Seawinds on the QuikSCAT. Best-fit approximation curve shows the slight dependence of the NRCS on the relative wind direction.