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A wind and wave atlas for the Mediterranean Sea

Luigi Cavaleri(1)

(1) ISMAR, S.Polo 1364 , 30125 Venezia, Italy

Abstract

Long term climatological data over the sea are much in demand for a number of reasons, ranging from pure scientific knowledge to the more important applications of, e.g., safety at sea and the design of sea structures. Till 15 years ago the only practical sources of the long term data required for a meaningful statistics was the collection of visual observations done from ships at sea. Starting from 1991 two new sources of data started 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 computer power and numerical modelling led to a continuous synoptic description, typically at 6-hour interval, of the distribution of 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 characterise the basins with a complicated geometry, 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 solution lies in the complementary use of both the altimeter and the modelled data. The latter ones provide the background information, very dense in time and space, that is then calibrated using the information derived from the altimeters. This is done determining for each grid point, at 0.5 degree intervals, the series of co-located values, model and altimeter, for both wind speed and wave height. Provided the necessary reliability checks are satisfied, for each point and each parameter the best-fit slopes of the co-located data provide the correction coefficients of the model data. After some spatial averaging to avoid unrealistic local variability, long term time series of calibrated wind and wave fields have been obtained. These have been used to derive multiple long term statistics, both at synoptic and point levels. All these results have been made available in the MEDATLAS atlas of the Mediterranean Sea, available in both paper (A3) and interactive CD formats.

The results have also allowed a check of the correctness of the wind speed and wave height altimeter measurements. The direct relationship existing between these two quantities implies a consistency between their statistics. This is obviously present in the original data that are model derived, and it should be expected also in the calibrated data. On the contrary we have found that the calibrated wind speed values are lower than expected from the calibrated wave heights. As the latter ones have also been verified versus accurate buoy data, we suggest that the algorithm used to derive the altimeter wind speed provides values lower than the actual truth.

 

Workshop presentation

Full paper

 

                 Last modified: 07.10.03