On the Sensitivity of Numerical Weather Prediction to Remotely Sensed Marine Surface Wind Data: A Simulation Study

A series of observing system simulation experiments has been performed to assess the potential impact of marine surface wind data on numerical weather prediction. Care was taken to duplicate the spatial coverage and error characteristics of conventional surface, radiosonde, ship, and aircraft reports. These observations, suitably degraded to account for instrument and sampling errors, were used in a conventional analysis-forecast cycle. A series of five 72-hour forecasts were then made by using the analyzed fields as initial conditions. The forecast error growth was found to be similar to that in operational numerical forecasts. Further experiments simulated the time-continuous assimilation of remotely sensed marine surface wind or temperature sounding data in addition to the conventional data. The wind data were fabricated directly for model grid points intercepted by a Seasat-1 scatterometer (SASS) swath and were placed in the lowest active level (945 mbar) of the model. The temperature sounding experiment assimilated error-free data fabricated along actual Nimbus orbits. Forecasts were made from the resulting analysis fields, and the impact of the simulated satellite data was assessed by comparing these forecast errors with those of the control forecasts. When error-free winds were assimilated by using a localized successive correction method (SCM), the impacts in extratropical regions proved to be substantial, especially in lower tropospheric quantities such as surface pressure. In contrast, a less sophisticated assimilation method resulted in negligible impact. The assimilation of error-free sounder data (again by the SCM) gave impacts comparable to the wind data, suggesting that surface wind data alone may be as valuable as temperature soundings for numerical weather prediction. The effects of nominal SASS errors (:t:2 m/s in magnitude, +20 ø in direction) on the impacts derived from wind data were found to be small.