Using ASCAT Scatterometer Winds to Evaluate Relative Biases in the QuikSCAT-Derived Wind Vorticity

Abstract

Relative wind vorticity ω (hereafter vorticity) is a crucial parameter to understand the spatial features of the wind field. In the Mediterranean Sea, which is the area where this study is focused, these are particularly interesting because they are often the effects of the interaction between the airflow and the orography. Wind vorticity has been derived from both Quick Scatterometer (QuikSCAT) and Advanced Scatterometer (ASCAT) 12.5-km scatterometer data downloaded from the National Aeronautics and Space Administration Physical Oceanography Distributed Active Archive Center data archive, and compared in the period from March to November 2009. The monthly mean fields of the vorticity ω show discrepancies which need to be understood. This paper thus aims to understand the nature of these differences, to make the two vorticity data sets compatible and usable as a common data set. Results have been provided in terms of the relative bias in vorticity 〈Δω〉, which is the mean difference between the ASCAT ωA and QuikSCAT ωQ monthly mean vorticities averaged over the entire Mediterranean Basin and the entire study period. This difference 〈Δω〉 = 0.093 ·10-5 ±0.05 ·10-5s-1) is mainly due to a relative vorticity bias in the cyclonic component of ω, rather than in the anticyclonic component, whose bias is four times smaller. This bias does not depend significantly on the variable accuracy of the wind speed and direction across the QuikSCAT swath. This study led us to define and analyze the so-called vorticity noise, which is present particularly in the QuikSCAT-derived vorticity, to understand if, and how, it can contribute to the relative bias in vorticity. The contribution of this kind of noise on ω has been found relevant only for the cyclonic vorticity of ωQ. By applying a cyclonic denoising to each swath of QuikSCAT, 〈Δω〉 = -0.016 ·10-5 ±0.05 ·10-5s-1 is obtained, drastically reduced with respect to the initial value. This may be considered the typical bias over the Mediterranean Sea between ωA and (ωQ derived from the 12.5-km data, after applying the cyclonic denoising to QuikSCAT vorticity fields.