Comments on "On Surf Zone Fluid Dynamics"

We commented on the empirical adjustment term recently introduced by Mellor. The purpose of the adjustment term is to include the effect of concentrating momentum at the surface, and the adjustment significantly improves the comparisons between modeled and measured velocity profiles. We found that the concentrating momentum from the adjustment term is largely overestimated relative to the commonly used parameterization approaches of concentrating momentum. The overestimated concentrating momentum leads to stronger velocity shear at the surface, which is partially canceled out by the vertical mixing caused by wave breaking. The model results also agree well with the measured velocity profiles if the fractions of the adjustment and the vertical mixing are simultaneously reduced. We also discussed an alternative method that includes the vertical radiation stress gradient term. The method exhibits no empiricism or uncertainty under the given wave conditions.