SeaWinds validation with research vessels

Abstract

The accuracy of the SeaWinds scatterometer's vector winds is assessed through comparison with research vessel observations. Factors that contribute to uncertainty in scatterometer winds are isolated and examined as functions of wind speed. For SeaWinds on QuikSCAT, ambiguity selection is found to be near perfect for surface wind speed (w) > 8 m s−1; however, ambiguity selection errors cause directional uncertainty to exceed 20° for w < ∼5 m s−1. These average uncertainties for wind speed and direction are found to be 0.45 m s−1 and 5° for the QSCAT‐1 model function and 0.3 m s−1 and 3° for the Ku‐2000 model function. The QuikSCAT winds are examined as vectors through two new approaches. The first is a method for determining vector correlations that considers uncertainty in the comparison data set. The second approach is a wind speed‐dependent model for the uncertainty in the magnitude of vector errors. For the QSCAT‐1 (Ku‐2000) model function this approach shows ambiguity selection dominates uncertainty for 2.5 < w < 5.5 m s−1 (0.6 < w < 5.5 m s−1), uncertainty in wind speed dominates for w < 2.5 m s−1 and 5.5 < w < 7.5 m s−1 (w < 0.6 m s−1 and 5.5 < w < 18 m s−1), and uncertainty in wind direction (for correctly selected ambiguities) dominates for w > 7.5 m s−1 (w > 18 m s−1). This approach also shows that spatial variability in the wind direction, related to inexact spatial co‐location, is likely to dominate rms differences between scatterometer wind vectors and in situ comparison measurements for w > 4.5 m s−1. The techniques used herein are applicable to any validation effort with uncertainty in the comparison data set or with inexact co‐location.