Luciana Bertotti

Accuracy of the modelled wind and wave fields in enclosed seas

The meteorological model of the European Centre for Medium-RangeWeather Forecasts, run with different resolutions, has been used to explore, with a number of numerical experiments, the underestimate of wind speeds and wave heights found in enclosed basins. Comparisons have been made between the results from the different runs, and also against satellite and buoy data. It is found that the error depends on fetch, i.e. on the distance from the closest land from which the wind is blowing. Large errors are found at short fetches (order of 100 km), gradually decreasing with the distance from the coast. The error is larger and more persistent for waves. Increasing the resolution leads to an improvement of the results. However, the bias does not disappear at the highest resolution we have used (about 25 km). Experiments with the single-point version of the meteorological model do not suggest that a slow development of the marine boundary layer is the main reason for the underestimate of the wind speeds.With respect to the mean orography, the use of envelope orography leads to a substantial increase of the marine wind speeds in the area affected by land.

In Search of the Correct Wind and Wave Fields in a Minor Basin

Abstract The authors analyze the accuracy of the surface wind of the Adriatic Sea from a global model. They find it to be substantially underestimated and propose a calibration by a suitable enhancement of the strength of the fields. The reasons for the underestimate are discussed.

Wind wave cast in the Mediterranean Sea

An advanced, third-generation wave model has been repeatedly applied to the Mediterranean Sea. We have studied the accuracy of the results and the factors that control it. The grid resolution, when less than 0.5°, is shown to have in general negligible effect in the open sea. The related accuracy of description of the coastal border can have a relevant effect on the wave field on the side and on the wake of the coastal details, for a distance of a few grid points. The difficulty of correctly evaluating the wind in the Mediterranean basin is dramatically shown by comparing the results obtained using wind fields with different resolution. The crucial role of the orography in shaping the wind fields and the need for higher-resolution models is established. It is found that owing to the strong dependence of wave height on wind speed, for resolution coarser than 70 km the loss of accuracy of the wind fields, even if meteorologically acceptable, makes them useless for an efficient evaluation of the wave conditions. The exception, explained by the local absence of relevant mountain ridges, is the case of a southerly wind in the eastern Mediterranean. The accuracy of wave forecasting is obtained by comparing analysis and forecast fields for a very severe storm. In connection with the preferential west to east flow of most of the Mediterranean storms, and with the scarcity of information on the Atlantic Ocean, the equality of the meteorological forecast rapidly deteriorates beyond 1 or 2 days of forecast. This leads to an even faster deterioration of the wave forecast, whose practical limits in the western Mediterranean can at present be established at 1 day.

The modelled wind climatology of the Adriatic Sea

SummaryA 29 year wind hindcast has been used to investigate the climatology of the Adriatic Sea. First we characterize the basin in terms of its monthly and annual mean distributions. Then we consider the evolution in time of the average conditions and look for the possible existence of trends. The analysis of the daily mean values over the whole period shows the existence of preferential periods of the year for atmospheric activity. Different areas of the basin have been characterized with respect to combined wind speed and direction and to the frequency of the dominant storms. Finally, we discuss the accuracy of the results.

Long term wind hindcast in the Adriatic sea

SummaryWe have carried out a long term hindcast of the wind fields on the Adriatic Sea. After discussing the possible approaches to the problem and the final choice, we give a detailed description of the used model. We describe the organization of the input data. The results are discussed and compared with the available measured data. We have estimated the extremes statistics for one location in the Northern Adriatic Sea. Finally we discuss the accuracy of the overall results.

The Gorbush Storm in the Mediterranean Sea: Atmospheric and Wave Simulation

Abstract The storm that disrupted the meeting between President Gorbachev and President Bush in Malta between 2 and 3 December 1989 is analyzed. The meteorological situation that caused the storm was simulated with two European Centre for Medium Range Weather Forecasts (ECMWF) atmospheric numerical models—a global model (resolution T106) and a limited-area model (resolution T333)—and its effect on the sea was simulated with an advanced wave model (WAM). Verification of the wind and wave forecast shows that to obtain a realistic forecast of wave height and distribution it is necessary to increase the horizontal resolution of the model, whose grid size must be no more than 40 km.

Nettuno: Analysis of a Wind and Wave Forecast System for the Mediterranean Sea

AbstractNettuno is a wind and wave forecast system for the Mediterranean Sea. It has been operational since 2009 producing twice-daily high-resolution forecasts for the next 72 h. The authors have carried out a detailed analysis of the results, both in space and time, using scatterometer and altimeter data from four different satellites. The findings suggest that there are appreciable differences in the measurements from the different instruments. Within the overall positive results, there is also evidence of differences in Nettuno performance for the various subbasins. The related geographical distributions in Nettuno performance are consistent with the various satellite instruments used in the comparisons. The extensive system of buoys around Italy is used to highlight the difficulties involved in a correct modeling of wave heights in Italys coastal areas.

Effect of Reduced Diffusion on Surface Wind and Wave Fields

Horizontal diffusion is used in meteorological models to reduce noise in the shorter spatial scales and to increase numerical stability. In turn, this affects the surface wind distribution. A series of tests on real situations in the Mediterranean Sea has been done to explore the practical consequences on wind and wave fields. The results indicate a substantial reduction of the peak values, particularly in areas with strong spatial gradients, and a general smoothing of the fields, more evident where these are dominated by the local orography.

ACCURACY OF WIND AND WAVE EVALUATION IN COASTAL REGIONS

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Waving in the rain

We consider the effect of rain on wind wave generation and dissipation. Rain falling on a wavy surface may have a marked tendency to dampen the shorter waves in the tail of the spectrum, the related range increasing with the rain rate. Historical and sailors’ reports suggest that this leads to calmer wave conditions, certainly so for the action of breakers. We have explored this situation using a fully coupled meteorological-wave model system, adding an artificial rain rate-dependent damping of the tail. Contrarily to direct marine experience, the experimental results show higher wind speeds and wave heights. A solid indication of the truth is achieved with the direct comparison between operational model (where rain effect is ignored) and measured data. These strongly support the sailors’ claims of less severe wave conditions under heavy rain. This leads to a keen analysis of the overall process, in particular on the role of the tail of the spectrum in modulating the wind input and the white-capping, and how this is presently modeled in operational activity. We suggest that some revision is due and that the relationship between white-capping and generation by wind is deeper and more implicative than presently generally assumed.