The Sensitivity of GNSS Reflectometry-Derived Ocean Winds to Wind Speed Persistence

Global Navigation Satellite System Reflectometry (GNSS-R) is a form of bistatic radar that relies on GNSS navigation signals and measures their forward (specular) scattering by the Earth surface. Modifications to the strength of the navigation signal by the scattering process contain information about the surface roughness. Over the ocean, surface roughness is related to near surface wind speed, which can be estimated from the scattered signal strength. Near surface ocean wind data products are routinely produced by the existing constellations of GNSS-R satellites (CYGNSS, Spire, Triton, FY-3, and Muon). The wind speed is typically estimated using a Geophysical Model Function (GMF) that relates the wind and normalized bistatic radar cross section (NBRCS). Notably, contributions to the NBRCS are made across a range of wavelengths of the surface roughness spectrum that includes both smaller capillary waves and longer swell waves. Stress at the ocean surface caused by wind imparts energy into the surface roughness at capillary-scale wavelengths and turbulent energy cascade transfers the energy from shorter to longer wavelengths. After a sufficient length of time, the complete roughness spectrum will reach an equilibrium referred to as a fully developed sea. Prior work has shown that the time scale of this energy cascade is of order 1 hour. If the wind has changed in the ~1 hour prior to a measurement of the NBRCS, the capillary waves will react to the change more quickly and the swell waves and the relationship between NBRCS and the wind at the time of the measurement may not be consistent with that assumed by the GMF. In this presentation, we examine the dependence of GNSS-R wind speed retrieval bias on wind persistence (defined as the change in wind speed in the recent past). There is found to be a very strong dependence, with the bias increasing with increasing change in wind speed in the recent (~1 hour) past, prior to the GNSS-R measurement of NBRCS. This sensitivity to wind persistence is not generally present with other remotely sensed ocean wind products such as radar scatterometers and passive microwave imagers.