A joint active/passive physical model of sea surface microwave signatures

Abstract

Active and passive microwave signatures of the ocean can only depend on the ocean wave spectrum if bound and breaking waves are neglected. However, history has not been kind to attempts to explain both radiometer brightness temperatures (Tb) and normalized radar cross sections (σo) of the sea using the same ocean wave spectrum without bound and breaking waves. In this paper, we show that if bound and breaking waves are included in physical models of radiometer and scatterometer microwave signatures of the ocean, a single wave spectrum can explain both Tb and σo to reasonable accuracy. Bound waves are the roughness produced by gently breaking, or crumpling, waves that travel near the speed of the parent wave. Bound wave modeling is based on earlier work by Plant (1997) but using additional information about the slope probability distributions of the bound waves' parents. Breaking wave and foam modeling both build on the function Λ(cb) introduced by Phillips (1985), which describes the average length of breaking wavefronts on the ocean per unit area as a function of breaker velocity, cb. We model Λ(cb) as documented in Irisov and Plant (2016) where we show that the wave spectrum completely determines Λ(cb). We thus show here that the result of including bound and breaking waves in radiometer and scatterometer models of oceanic signatures is a much closer fit to data over a wide range of microwave frequencies and incidence angles using a single wave spectrum.