Recovering the Elusive Spectral Width From Spaceborne Doppler Profiling Radar Measurements: The "ExpliSyT" Approach.

This article presents a novel method to retrieve spectral widths from measurements of spaceborne Doppler profiling radars (SDPRs) operating in low-Earth orbit. The proposed Explicit Sy-Tanelli (ExpliSyT) approach is based on a formulation of the Doppler broadening in terms of the distributions of reflectivity and velocity in the footprint. This formulation makes it possible to estimate the broadening and remove it from the spectral-width measurement. Two implementations of the ExpliSyT correction are described: 1) a numerical implementation that uses Wiener's deconvolution and 2) an analytical implementation based on Taylor expansions of the radar observations. The ExpliSyT corrections enable retrievals of higher-order Doppler information, such as posterior power spectra. Results are shown for simulations of the radar of the Earth Cloud Aerosol Radiation Explorer (EarthCARE developed by the European and Japanese space agencies) and a Displaced Phase Center Antenna (DPCA) configuration, for which the ExpliSyT corrections perform significantly better. A novel canonical Doppler resampling diagram is introduced to explain the correlation between nonuniform beamfilling (NUBF) and spectral broadening. This tool shows that: 1) the NUBF of reflectivity modulates the spacecraft-induced spectral broadening and 2) the NUBF of velocity can either broaden or shrink the Doppler spectrum. This spectral shrinkage, although important, is not typically thought of when dealing with spaceborne Doppler radars. The ability to estimate spectral widths accurately from space paves the way for future microphysical and dynamical characterizations of hydrometeors in shear or turbulence.