Use of ship mean data for validating model and satellite flux fields during the FETCH experiment

International audienceDuring the FETCH experiment in NW Mediterranean Sea (March and April 1998), surface fluxes were measured on board a research vessel and on a moored buoy. To provide the ocean surface forcing at the mesoscale, output fields of three meteorological models (European Centre for Medium-Range Weather Forecasts (ECMWF) and Meteo-France ARPEGE and ALADIN) were gathered as well as satellite data. To evaluate the consistency of model fields, we compare ship measurements to the predicted radiative and turbulent surface fluxes and to turbulent fluxes derived from analyses using a bulk formula. We first analyze the quality of the ship bulk fluxes in terms of possible effects of the flow distortion and of intrinsic variability as function of the integration time. The airflow distortion over the ship is found to significantly affect fluxes, but the mean fluxes over the experiment remain unchanged because of compensation between positive and negative errors due to the varying apparent wind direction. Spectral analysis of the ship time series as well as comparison of fluxes obtained by using several averaging times from 1 to 90 min show that the optimal scale for computing ship fluxes is about 20 min, ensuring consistency with the derivation of bulk drag and exchange coefficient. These fluxes are then taken as reference for the comparison with models and satellites. None of the predicted radiative fluxes is consistent with ship measurements. Model turbulent fluxes differ from each other, and the main reason is the discrepancy between modeled and observed atmospheric features. Large discrepancies are found between predicted fluxes and ship fluxes in strong wind conditions due to the different flux parameterization for heat fluxes. Model bulk fluxes thus compare better to ship than predicted fluxes, particularly during the middle period of the experiment depending on the model. On the contrary, the Meteosat-derived downward radiative fluxes are comparable to the ship data, and their high time sampling allows us to describe the diurnal cycle of the solar heating. They can be used either at an instantaneous scale (comparison of the satellite flux averaged over 0.1° Â 0.1° with 10 min averaged ship fluxes) or at a larger scale (0.3° Â 0.3°, 1 hour). It is shown that the surface latent heat flux, derived from a combination of Special Sensor Microwave Imager (SSMI) brightness temperatures and sea surface temperature (SST), as obtained from IR sensors (Advanced Very High Resolution Radiometer (AVHRR), is of a quality similar or better to model bulk fluxes. INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorologyvre, Use of ship mean data for validating model and satellite flux fields during the FETCH experiment