Non uniform beam filling correction for the Doppler velocity measured by the WIVERN conically scanning radar

Existing and planned spaceborne radar missions offer unique three-dimensional views of clouds, precipitation, and convection, enhancing our understanding of Earth’s water and energy cycles. Doppler and wide swath capabilities are now being considered as additional features to enhance radar capabilities. One such system is the ESA Wind Velocity Radar Nephoscope (WIVERN), currently in phase A studies; its payload consists of one conically-scanning Doppler W-band radar in low-Earth orbit, designed to measure the profile of the line-of-sight (LoS) wind speed and the vertical structure of hydrometeor content. These observations are crucial for improving Numerical Weather Prediction models and evaluating cloud and precipitation processes in next-generation Earth System Models. A significant error source in WIVERN’s LoS Doppler velocities is the Non-Uniform Beam Filling (NUBF) error, introduced by reflectivity field inhomogeneities within the backscattering volume coupled with the rapid motion of the satellite and with the Doppler velocity changes introduced by wind and sedimentation velocity variability. The satellite motion related NUBF errors have been extensively studied for nadir-looking Doppler radars, like EarthCARE. This work proposes a methodology to reduce such error which is applicable to WIVERN’s conically-scanning configuration. Depending on the antenna pointing with respect to the satellite velocity, the correction is proportional to spatial reflectivity gradients in directions which are not necessarily sampled by the scanning pattern. End-to-end simulations of WIVERN for different storms show that satellite motion NUBF errors depend on the antenna scanning-angle, are unbiased and generally have a standard deviation below 1 m/s. The satellite motion NUBF correction can reduce this error by approximately 40%, though its effectiveness decreases when accounting for the reflectivity measurements noisiness. Errors associated to the coupling between NUBF and Doppler velocity gradients are smaller, but the proposed mitigation schemes are less effective due to the Doppler velocity and reflectivity measurements combined noisiness.