Abstract
Abstract. To simulate winds and water levels, numerical weather prediction (NWP) and storm surge models generally use the traditional bulk relation for wind stress, which is characterized by a wind drag coefficient. A still commonly used drag coefficient in those models, some of them were developed in the past, is based on a relation, according to which the magnitude of the coefficient is either constant or increases monotonically with increasing surface wind speed (Bender, 2007; Kim et al., 2008; Kohno and Higaki, 2006). The NWP and surge models are often tuned independently from each other in order to obtain good results. Observations have indicated that the magnitude of the drag coefficient levels off at a wind speed of about 30 m s−1, and then decreases with further increase of the wind speed. Above a wind speed of approximately 30 m s−1, the stress above the air-sea interface starts to saturate. To represent the reducing and levelling off of the drag coefficient, the original Charnock drag formulation has been extended with a correction term. In line with the above, the Delft3D storm surge model is tested using both Charnock's and improved Makin's wind drag parameterization to evaluate the improvements on the storm surge model results, with and without inclusion of the wave effects. The effect of waves on storm surge is included by simultaneously simulating waves with the SWAN model on identical model grids in a coupled mode. However, the results presented here will focus on the storm surge results that include the wave effects. The runs were carried out in the Gulf of Mexico for Katrina and Ivan hurricane events. The storm surge model was initially forced with H*wind data (Powell et al., 2010) to test the effect of the Makin's wind drag parameterization on the storm surge model separately. The computed wind, water levels and waves are subsequently compared with observation data. Based on the good results obtained, we conclude that, for a good reproduction of the storm surges under hurricane conditions, Makin's new drag parameterization is favourable above the traditional Charnock relation. Furthermore, we are encouraged by these results to continue the studies and establish the effect of improved Makin's wind drag parameterization in the wave model. The results from this study will be used to evaluate the relevance of extending the present towards implementation of a similar wind drag parameterization in the SWAN wave model, in line with our aim to apply a consistent wind drag formulation throughout the entire storm surge modelling approach.
Highlights
Storm surge is largely determined by a combination of inverse barometric effects and air-sea interactions
With respect to the drag parameterization, we observe that Charnock’s parameterization tends to overestimate the surge levels considerably, especially when the computed surge level is large (>1 m), which can be directly associated with periods of strong wind conditions
This can be expected as the values of the wind drag coefficient based on Charnock’s parameterization is significantly larger in this high wind speed regime
Summary
Storm surge is largely determined by a combination of inverse barometric effects and air-sea interactions. The transfer of momentum from the atmosphere into the ocean represents the dominant process. This transfer of momentum results in the generation of the surge and waves which interact. For the computation of the surface momentum flux, traditionally NWP and storm surge models use a common bulk relation, which involves a formulation for the drag coefficient CD. The magnitude of this coefficient increases to a good approximation linearly with increasing wind speed (Charnock, 1955; Large et al, 1981; Smith et al, 1975).
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