Examination of the Impact of a Coupled Atmospheric and Ocean Wave System. Part II: Ocean Wave Aspects

Abstract

A coupled atmospheric and ocean wave system has been developed to study the impact of changes of surface roughness length induced by ocean waves. A two-way coupling between a mesoscale atmospheric model, MC2, and an oceanic wave model, a regional version of WAM Cycle-4, was designed to ensure consistency in the treatment of the atmospheric boundary layer parameterizations between the two models. Two different approaches, based on the wave age of Smith et al. and the wave-induced stress of Janssen, are used to compute a coupling parameter, called the Charnock parameter, expressed as the nondimensional surface roughness length. The coupling between the two models is accomplished by the use of this parameter, which is a function of sea state, instead of the constant value obtained from empirical studies using the well-known Charnock relation. The impacts on the atmospheric forecasts are discussed in Part I. In Part II, the ocean wave forecasts resulting from this two-way coupling are discussed for four different real cases. The two approaches are evaluated by comparing ocean wave model outputs obtained from the coupled and uncoupled systems against buoy observations. The coupling has some beneficial impact, especially in areas of extreme sea states. The significant wave heights are reduced in the coupled runs and generally show better agreement with the buoy observations. The impact of the coupling also exhibits some dependence on the intensity of the cyclone development, with larger changes occurring in the case of rapidly deepening storms.