Abstract
Numerical weather prediction (NWP) and buoy ocean surface winds show some systematic differences with satellite scatterometer and radiometer wind measurements, both in statistical results and in local geographical regions. It is possible to rescale these reference winds to remove certain aspects of these systematic differences. Space-borne ocean surface winds actually measure ocean surface roughness, which is related more directly to stress. Air mass density is relevant in the air–sea momentum transfer as captured in the stress vector. Therefore, apart from the already common “neutral wind correction” for atmospheric stratification, also a “mass density wind correction” is investigated here to obtain a better correspondence between satellite stress measurements and buoy or NWP winds. The bicorrected winds are called stress-equivalent winds. Stress-equivalent winds do not strongly depend on the drag formulation used and provide a rather direct standard for comparison and assimilation in user applications. This paper presents details on how this correction is performed and first results that show the benefits of this correction mainly in the extratropical regions.
Highlights
S ATELLITE wind scatterometer instruments are active remote sensing instruments emitting microwave radiation and detecting its scattering (σ0) from the ocean surface [1], [2]
In the current paper a simplified version of the stress calculation is used in which wind surface stress is taken to be proportional to the square of the neutral wind speed u210n, the local air mass density ρ and a drag coefficient CD that contains all remaining complexities (it may depend on sea state, sea surface temperature (SST), and on the surface wind itself, etc.)
The results presented in this paper are based on the Ocean and Sea Ice Satellite Application Facility (OSI SAF) operational near-real-time (NRT) wind product of the ASCAT instrument on Metop-A produced by KNMI [18]
Summary
S ATELLITE wind scatterometer instruments are active remote sensing instruments emitting microwave radiation and detecting its scattering (σ0) from the ocean surface [1], [2]. Because of the complexity of the relation between the ocean-modulated scattering and wind speed and direction this GMF is derived empirically, by comparing a large amount of scatterometer data with corresponding buoy and Numerical weather prediction (NWP) model data, combined with additional measurements like aircraft campaign data and symmetry considerations. This effect is corrected for by converting reference buoy or NWP wind data to equivalent neutral winds and to use these to derive the GMF. At a given wind speed cold air will generate more impact on the ocean surface than warm air would, due to the higher air mass density It would be physically more consistent when air mass density was taken into account when comparing scatterometer or radiometer winds to buoys or NWP model winds.
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