Luigi Cavaleri

Dynamics and Modelling of Ocean Waves

Addresses both fundamental and applied aspects of ocean waves including the use of wave observations made from satellites. More specifically it describes the WAM model, its scientific basis, its actual implementation, and its many applications. This model has been developed by an international group (the Wave Modelling group), and is based on a detailed physical description of air/sea interactions. It is widely used for wave forecasting for meteorological and oceanographic purposes. The three sections of the volume describe the basic statistical theory and the relevant physical processes; the numerical model and its global and regional applications; and satellite observations, their interpretation and use in data assimilation. Written by leading experts, it is a comprehensive guide and reference for researchers and advanced students in physical oceanography, meteorology, fluid dynamics, coastal engineering and physics.

Statistical properties of mechanically generated surface gravity waves: a laboratory experiment in a three-dimensional wave basin

A wave basin experiment has been performed in the MARINTEK laboratories, in one of the largest existing three-dimensional wave tanks in the world. The aim of the experiment is to investigate the effects of directional energy distribution on the statistical properties of surface gravity waves. Different degrees of directionality have been considered, starting from long-crested waves up to directional distributions with a spread of ±30° at the spectral peak. Particular attention is given to the tails of the distribution function of the surface elevation, wave heights and wave crests. Comparison with a simplified model based on second-order theory is reported. The results show that for long-crested, steep and narrow-banded waves, the second-order theory underestimates the probability of occurrence of large waves. As directional effects are included, the departure from second-order theory becomes less accentuated and the surface elevation is characterized by weak deviations from Gaussian statistics.

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 Waves in the Coupled Climate System

The role waves play in modulating interactions between oceans and atmosphere is emphasized. All exchanges (e.g., momentum, energy, heat, mass, radiation fluxes) are influenced by the geometrical and physical characteristics of the ocean surface, which separates the atmospheric and oceanic boundary layers. A qualitative overview of the main relevant surface gravity wave–driven processes at the air–sea interface that may have an important role in the coupled climate system in general and the atmospheric and oceanic boundary layers in particular is provided.

Modulational instability and non-Gaussian statistics in experimental random water-wave trains

We study random, long-crested surface gravity waves in the laboratory environment. Starting with wave spectra characterized by random phases we consider the development of the modulational instability and the consequent formation of large amplitude waves. We address both dynamical and statistical interpretations of the experimental data. While it is well known that the Stokes wave nonlinearity leads to non-Gaussian statistics, we also find that the presence of the modulational instability is responsible for the departure from a Gaussian behavior, indicating that, for particular parameters in the wave spectrum, coherent unstable modes are quite prevalent, leading to the occurrence of what might be called a “rogue sea.” Statistical results are also compared with ensemble numerical simulations of the Dysthe equation.

Wave Modeling—Missing the Peaks

Abstract The paper analyzes the capability of the present wave models of properly reproducing the conditions during and at the peak of severe and extreme storms. After providing evidence that this is often not the case, the reasons for it are explored. First, the physics of waves considered in wave models is analyzed. Although much improved with respect to the past, the wind accuracy is still a relevant factor at the peak of the storms. Other factors such as wind variability and air density are considered. The classical theory of wave generation by J. W. Miles’s mechanism, with subsequent modifications, is deemed not sufficiently representative of extreme conditions. The presently used formulations for nonlinear energy transfer are found to lead to too wide distributions in frequency and direction, hence reducing the input by wind. Notwithstanding some recent improvements, the white-capping formulation still depends on parameters fitted to the bulk of the data. Hence, it is not obvious how they will perfo...

The oceanographic tower Acqua Alta — activity and prediction of sea states at Venice

Abstract The oceanographic tower Acqua Alta was constructed in early 1970 in the Adriatic Sea some 20 km from Venice. The main objective was to obtain information about the meteomarine conditions off the Venice lagoon, where the old town is threatened by ever more severe floodings. The paper describes the tower and its instrumentation, and summarizes the experience and results of activities relevant to coastal engineering. This encompasses modelling of wind field, storm surge and wind generated waves. Various types of models have been applied to and validated for the Adriatic Sea and in particular for the coastline of the Venice lagoon. This includes application of long-term statistics of extreme wave heights. Specific studies of the influence of bottom friction and wind gustiness on waves are presented as well as measurements of kinematics of wind generated waves. Further, a short discussion of white capping is included. Finally, some information about the management of the tower is given.

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 Red Sea: A Natural Laboratory for Wind and Wave Modeling

AbstractThe Red Sea is a narrow, elongated basin that is more than 2000 km long. This deceivingly simple structure offers very interesting challenges for wind and wave modeling, not easily, if ever, found elsewhere. Using standard meteorological products and local wind and wave models, this study explores how well the general and unusual wind and wave patterns of the Red Sea could be reproduced. The authors obtain the best results using two rather opposite approaches: the high-resolution Weather Research Forecasting (WRF) local model and the slightly enhanced surface winds from the global European Centre for Medium-Range Weather Forecasts model. The reasons why these two approaches produce the best results and the implications on wave modeling in the Red Sea are discussed. The unusual wind and wave patterns in the Red Sea suggest that the currently available wave model source functions may not properly represent the evolution of local fields. However, within limits, the WAVEWATCH III wave model, based on ...