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

Abstract

To take into account the change of surface roughness length induced by ocean waves, a coupled atmospheric and ocean wave model system is developed. A two-way coupling is done between a mesoscale atmospheric model, MC2, and an oceanic wave model, a regional version of WAM Cycle-4. 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 coupled atmospheric and ocean wave system was applied to four intense storms in the western Atlantic, to examine the impact of the two-way interaction using real midlatitude storm cases. In Part I, the atmospheric forecasts resulting from this two-way coupling are discussed for these different synoptic cases. The two approaches are evaluated by comparing atmospheric outputs obtained from the coupled and uncoupled systems against buoy observations. The authors conclude that, at least for short-term forecasts, the effects of sea-state-dependent surface roughness on the evolution and synoptic-scale aspects of the storm are rather weak, while small beneficial effects are noted on surface variables. The most significant changes are a reduction of about 10% of the surface winds associated with enhanced surface roughness lengths in areas of younger and rougher seas. The coupling also modifies the sea surface fluxes by as much as 30%, but these are not located in the active areas of the storm. Consequently, precipitation only increases slightly and does not affect the storm development. The impact on the ocean wave forecasts is discussed in Part II.