Estimates of surface fluxes from global operational numerical weather predictions

Abstract

The success of coupled models of the ocean-atmosphere system depends in part on their ability to estimate the momentum, heat and water vapour fluxes at the interface accurately. Their accuracy as now calculated by atmospheric numerical forecast models with relevant variables at the sea surface given is uncertain. The achievement of an acceptable accuracy in this simpler situation is an essential preliminary to a solution of the more difficult interactive problem. The ISCU/WMO Joint Scientific Committee (JSC) for the World Climate Research Programme (WCRP) considered how best to obtain surface flux values primarily for running large-scale ocean models. They concluded that the most promising approach was to extract them from numerical prediction models whose capabilities in terms of assimilating observations of many diverse kinds and of quality controlling meteorological information, have reached a high level of development and are being improved continuously. The committee requested its Working Group on Numerical Experimentation (WGNE) to investigate and report on the accuracies now achieved. In the absence of measurements of surface fluxes suitable for validation, the WGNE decided that the first step was to compare surface fluxes as estimated by state-of-theart global prediction models. The ECMWF (European Centre for Medium-Range Weather Forecasts) and Meteorological Office models were considered suitable, and have the advantage of employing different approaches to data assimilation, model configuration and the parametrization of physical processes. This report will outline the results obtained by the two models for a first comparison period, May-June 1987.