The assimilation of ambiguous scatterometer winds using a variational technique: method and forecast impact


The Met.Office
Année de publication

The European Remote Sensing (ERS) satellite series carry a scatterometer instrument that emits radar beams at the earth's surface in three different azimuthal directions. Ocean surface wind vectors can be derived from the backscatter measurements and this technique has the advantage that cloud and rain only weakly affect the backscatter at the wavelengths used. However due to uncertainties in both the geophysical model (which relates the observed backscatter to wind vectors) and the measurements themselves normally two wind vectors can be retrieved from each observation. We have now developed a route in which each wind pair is presented to the variational assimilation system via a dual-wind cost function. This takes advantage of the variation approach by using a cost function which contains the properties of the observations. Impact trials using the new scatterometer scheme show a positive improvement in the short-range forecasts in both the Tropics and the Southern Hemisphere. The forecast improvement has been measured by both using conventional validation sources and also by using independent global observations of ocean windspeed from the ERS Radar Altimeter.

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