Identifying and Correcting for Residual Biases in the ASCAT Wind Speed Product

The Advanced SCATterometer (ASCAT) instrument is a real-aperture radar aboard the MetOP satellites. ASCAT operates at C-band and measures backscatter from the ocean surface. This backscatter data allows the estimation of near-surface wind speeds over the ocean, which is essential for understanding weather patterns. However, the variance of radar backscatter is not completely explained by that of the near surface wind speed, and there are other environmental factors that can also perturb the backscatter. The ASCAT wind speed retrieval algorithm attempts to correct for the other factors, and it is largely successful. We focus here on two environmental factors, precipitation and air-sea temperature difference, which are found to correlate strongly with residual biases in the ASCAT wind speed product. Physically, precipitation rate causes a bias due to an increase in the ocean surface roughness due to raindrops enhancing centimeter-scale ring wave growth, and additional backscatter off raindrops. Air-sea temperature difference increases vertical mixing at the surface, which in turn changes the fundamental relationship between wind speed and wave growth. A correction algorithm is developed to remove the dependence of bias on these two environmental variables. With the corrections applied, ASCAT’s mean global wind speed bias is reduced and regional variations in the bias are narrowed. As one example of the impact of the biases, they are estimated to cause a 2.1% average global error in latent heat flux transferred from the ocean to the atmosphere. Reducing these biases enhances the reliability of climate models and contributes to better forecasts of weather and climate phenomena, demonstrating the boarder impact and importance of refining satellite-derived wind speed measurements.