Case studySensitivity of a third generation wave model to wind and boundary condition sources and model physics: A case study from the South Atlantic Ocean off Brazil coast
Introduction
All third generation wave models are based on the wave action balance equation to simulate the directional wave spectrum. In these models, the wave spectrum at each time step is determined according to the following equation:in which represents the total time derivative (includes local rate, as well as spatial and spectral derivatives), and represents all energy sources and sinks terms. The energy transfer from wind to the waves, quadruplet nonlinear wave interaction, and white capping dissipation are the three main mechanisms controlling the wave growth and decay in deep waters. Among these three source/sink mechanisms, the white capping dissipation is the least understood term, and several formulations have been proposed to include it in a more realistic form in the third generation wave models (Cavaleri et al., 2007).
In this study, an unstructured flexible mesh was employed to study the surface wave dynamics in the South Atlantic Ocean close to the Brazilian coast, during several months in 2002 and 2003. The uncertainties in model forecasts and its sensitivity to the formulations used for simulating the physics associated with white capping were addressed by comparing the model outputs, especially significant wave height, using different model configurations with in situ measurements from an offshore buoy location. Moreover, significant wave height and wind speed measurements from satellite altimeters were employed for validation of model inputs and outputs.
Section snippets
Study area and model setup
The wave hindcasting using different parameterization for white capping and wind input terms were performed for South Atlantic Ocean off the coast of Brazil (see Fig. 1). The unstructured computational grid, covering the south-central Brazil coast and offshore, was produced using SMS software (Aquaveo, 2010), and the required bathymetry data were obtained from Nautical Charts of Brazilian Department of Hydrography and Navigation (DHN). The flexible mesh grid composed of 8608 triangles and 4481
Uncertainties in wind input
For the ease of evaluating the model uncertainties using three different parameterizations, a series of reference stations were established within the model domain and they were named P11–P41 (see Fig. 1). The model comparison using KOM, JAN and WST formulations were conducted for these reference stations. Two parallel transects were identified north and south of the buoy location with equidistant points selected from deep water into the inner continental shelf.
Performance of a wave model is
Summary and conclusion
The energy transfer from wind, quadruplet nonlinear wave interaction, and white capping dissipation are three main mechanisms controlling the wave growth and decay in deep waters. Among these three source and sink mechanisms, the white capping dissipation is the least understood term. Several formulations have been proposed for this deep water dissipation mechanism during the last two decades to improve the performance of the wave model; especially in terms of bulk wave parameters such as
Acknowledgments
Part of wind and wave data used in this study was provided by Ocean weather Inc. Dr. Andrew Cox is acknowledged for providing the wind and wave data for the study area. The authors thank Dr Elói Melo Filho (Engenharia Oceânica/FURG) for providing the wave data from the Southern Brazil coast.
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