|Title||Assimilation of scatterometer data as equivalent-neutral wind|
|Year of Publication||2010|
|Secondary Title||Technical Memorandum|
This document describes the assimilation of scatterometer data as neutral 10m vector wind into the ECMWF 4D-Var assimilation system. For given surface stress this quantity provides the wind at 10m height for which stability effects in the surface layer (SL) have been neglected. Sofar, at ECMWF, scatterometer data has been assimilated as 10m (non-neutral) wind, i.e., the actual wind at 10m height including stability effects. Since it is believed that scatterometer data relates more closely to surface stress than wind, the usage of an observation operator that is sensitive to neutral rather than to non-neutral wind should be more accurate. Although it is straightforward to adapt the observation operator for surface wind, it emerges that the current assimilation system uses an estimation of exchange coefficients that is based on an old version of the ECMWF SL parametrization. Such coefficients are required to perform a proper vertical interpolation from the lowest model-level wind. Reason for this is that only limited model information is available at the location where the observation operator is evaluated. In this document it is described how neutral can be accessed from the actual model SL. This embraces the incorporation of the influence of the 4D-Var control vector in the minimization on this surface field. Although not further explored here, this latter extension could be applied to other diagnostic surface fields as well. Several assimilation experiments are performed at a resolution of T511 in early-delivery mode for 109 cases in the Autumn of 2009. From these it is verified that departures between scatterometer and model wind speed are slightly reduced when an observation operator for neutral wind is used. Impact on forecast skill is found to be relatively neutral, with some (not significant) positive impact over the Southern Hemisphere for the atmosphere and ocean surface waves.