Predicting L-Band Emissivity of a Wind-Roughened Sea with Foam Layers or Whitecaps and Overlying Spray, Using a Finite-Difference Time-Domain Model

Many previous laboratory, field and remote sensing studies have empirically quantified whitecap (WC) parameters such as coverage and scale, foam layer thickness, and bubble plume profiles, but only a few have simulated foam layer or WC microwave reflectivity and emissivity using analytical and numerical electro-magnetic (E-M) models. We report the development and application of a Finite-Difference Time-Domain (FDTD) E-M model to investigate the emissivity, reflectivity and detectability of WCs. The model solves Maxwell's equations directly for an arbitrary free space and dielectric configuration. It is applied to multiple dielectric layers representing foam and spray overlying a rough sea surface. The foam layer profiles are adapted from Anguelova's L-band radiative transfer model (RTM), and the rough surface is a statistical realization of the Kudryavtsev wave spectrum model. The accuracy and precision of model emissivity estimates, the roughness emissivity increment, and detectability of WCs using L-band radiometry, are assessed under various wind conditions. Possible enhancements using Monte-Carlo simulation and more deterministic simulations of active breakers with WCs of various void fractions, shapes and scales, are also considered.