Influences of sea spray and wave drag on midlatitude storm structure and intensity

A coupled atmosphere –wave – sea spray model system is used to evaluate the combined impacts of spray evaporation and wave drag on midlatitude storms. Our focus is on the role of air-sea fluxes on storm intensity and development, and related impacts on the structure of the atmospheric boundary layer. The composite model system consists of the Canadian Mesoscale Compressible Community (MC2) atmospheric model coupled to the operational wave model WaveWatchIII (WW3), and a recent bulk parameterization for heat fluxes due to sea spray. Case studies are extratropical hurricane Earl (1998) and two intense winter storms from 2000 and 2002, hereafter denoted Superbomb and Bomb, respectively. Results show that sea spray tends to intensify storms, whereas wave-related drag tends to de-intensify. The mechanisms by which spray and wave-related drag can influence storm intensity are quite different. When wind speeds are high and sea surface temperatures (SSTs) warm, spray can significantly increase the surface heat fluxes. By comparison, momentum fluxes related to wave-drag are important over regions of the storm where young, newly generated waves are prevalent, for example during the rapid-development phase of the storm, and decreases in areas where the storm waves reach maturity. We show that the collective influence of spray and waves on storm intensity depends on their occurrence in the early stages of a storm's rapid intensification phase, and their spatial distribution with respect to the storm center. Moreover, for the case of Superbomb, we use a potential vorticity framework to show relative importance of these surface flux impacts compared to baroclinic processes.