Recovery of the Elusive Moment of Spaceborne Doppler Radars: the “Expli-SyT” Approach

This article presents a novel method to retrieve accurate Doppler measurements from spaceborne radars like the one of EarthCARE (Earth Cloud Aerosol Radiation Explorer developed by the European and Japanese space agencies). The method is based on a explicit “Expli-SyT” formulation of the Doppler broadening in terms of the distributions of reflectivity and mean velocity in the footprint. Two practical implementations are described, an analytical implementation based on local Taylor expansions ofthe observations, and a Wiener-deconvolution approach. The results obtained show that the measured mean Doppler, and, the spectral width can be retrieved accurately particularly if the radar measurements are oversampled in range and the noisiness of the measurements is limited or mitigated by filtering. Moreover, the main contributor to the broadening is the component that depends only on the reflectivity distribution. This component is expected to be accurately measured by radars like EarthCARE owing to the spectral broadening. The newly enabled retrievals give access to higher-order Doppler information such as the Doppler entropy. Such higher-order information can be used to better characterize atmospheric dynamics and cloud micro-physics.