|Paper title||From ASCAT winds to ASCAT surface stress information|
|Form of presentation||Poster|
Scatterometer data over the ocean are assimilated, at ECMWF and other NWP centres, in the form of ambiguous near surface ocean winds vector information. What scatterometer really measures is the surface radar backscatter that is essentially related to the directional roughness of the sea surface, which is fundamentally driven more by the surface stress caused by the relative motion between the atmospheric wind and the underlying ocean rather than the wind itself. Due to the lack of accurate in-situ surface stress measurements over the ocean to validate and calibrate scatterometer measurements, historically the scatterometer observations have been interpreted and calibrated to (equivalent neutral) wind rather than wind stress.
An ongoing EUMETSAT-funded project at ECMWF is investigating how to further increase the value of scatterometer observations in Numerical Weather Prediction (NWP), by assessing (and implementing) the assimilation of the surface stress, rather than wind components. In a coupled ocean-atmosphere data assimilation system, the ASCAT measurements assimilated as surface stress, can in principle, provide information on the atmospheric wind simultaneously constraining the ocean circulation.
An intermediate assimilation approach known as “stress equivalent winds” (following de Kloe et. al, 2017) is also being explored. This approach includes the sensitivity to air density variations.
A change in the type of assimilated observation variable required an adaptation of the observation operator and of the tangent linear and adjoint codes to enable the minimization in the 4D-Var analysis. New stress and stress equivalent wind observation operators have been developed (together with the tangent linear and adjoint) and tested and integrated into the Integrated Forecasting System (IFS). The error statistic assigned to the observations in the 4D-Var has been revised, a wind dependent error formulation has been characterized to be assigned to ASCAT surface stress observations. NWP observing system experiments assimilating ASCAT observations as either winds, stress or stress equivalent winds are being performed. Also the impact of ocean currents into the assimilation of ASCAT observations is under investigation.
The results of the study will be presented and discussed.