A WIND AND WAVE ATLAS FOR THE MEDITERRANEAN SEA Luigi Cavaleri and Mauro Sclavo Institute of Marine Sciences – CNR, San Polo 1364, 30125 Venice, Italy ABSTRACT We describe the procedure followed and the data used for the preparation of an atlas of the wind and wave conditions in the Mediterranean Sea. The two basic sources of data have been the altimeters on board of the ERS-1/2 and TOPEX/Poseidon satellites and the operational wind and wave results of the European Centre for Medium-Range Weather Forecasts. The data have been combined providing a ten year data-set from which both overall and point by point statistics have been derived. The results are critically analysed and suggest an underestimate of the altimeter derived wind speeds in the Mediterranean Sea. 1. INTRODUCTION Long-term climatological data over the sea are much in demand for a number of reasons, ranging from pure scientific knowledge to the important applications of, e.g., safety at sea and the design of sea structures. Till 15 years ago the only source of long-term data available for a meaningful statistics was the collection of visual observations done from ships at sea. Starting from 1991 two new sources of data began to be available. The launch of ERS-1, and soon after of TOPEX/Poseidon, offered an unprecedented continuous flow of wind speed and wave height measurements. At the same time the improvements in computing power and numerical modelling led to a continuous synoptic description of the wind and wave characteristics at sea. These two sources offered a wealth of data and today quite reliable statistics exist in the open oceans. However, the situation is less favourable in the inner seas. Here the global meteorological models exhibit a steady underestimate of the wind speeds that in turn leads to an underestimate of the associated modelled wave heights. On the other hand the strong spatial gradients that characterises the basins with a complicated orography, as in our case the Mediterranean Sea, imply that the spatial resolution of the altimeter ground tracks is not sufficient to provide the necessary details. The obvious solution lies in the combined use of the two sources. The atlas here presented has been commissioned by the French, Greek and Italian Navies via the W.E.A.O. Research Cell. It is the product of several institutions from the three countries. A full description is found in the introductory part of the atlas itself [1], available both in paper and CD interactive formats from the authors. The atlas is based on wind and wave model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) models, appropriately calibrated by means of satellite altimeter measurements. In so doing the systematic space-time coverage, a unique feature of numerical models, is fully exploited and, at the same time, the quality of the data and of the presented results is significantly improved by using the most up-to-date and reliable measurement technique able to cover large sea areas: satellite remote sensing. 2. THE AVAILABLE DATA 2.1. Satellite Data Wind and wave data are provided by altimeters (wind speed U and significant wave height H s ) and a scatterometer (wind speed and direction). We have made use of the data from ERS-1/2 and TOPEX/Poseidon, available respectively since 1991 and 1992. The altimeter data, made available by Meteo- France, provide point estimates, at about seven kilometre intervals, along the ground track of the satellite. Scatterometer data are available along a wide lateral swath with respect to the ground track, as averages within 25 x 25 or 50 x 50 square kilometre areas. Having been used with data assimilation, hence considered in the ECMWF analysis, the scatterometer data could not be considered for this project. A quality control procedure has been applied to the data, resulting in the elimination of some spurious data. The significant wave height and the wind speed from TOPEX altimeter have been corrected according to relations deduced from comparisons with buoy measurements [2], [3]. SWH cor .=1.052 SWH gdr - 0.094 U10 cor. = 0.87 U10 gdr + 0.68 For TOPEX data, a linear time dependent correction has been applied to the Ku band data in order to correct a