A wind and wave atlas for the Mediterranean Sea
S.Polo 1364 ,
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.