The Effect of Sea Spray Evaporation on Tropical Cyclone Boundary Layer Structure and Intensity

Abstract

Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts of spray in the lower part of the atmospheric boundary layer. The effects that the evaporation of this sea spray may have on the transfer of energy between the ocean and the atmosphere, and consequent effects on the boundary layer structure, cumulus convection, and the evolution of the tropical cyclone, are largely unknown. In this study, a high-resolution tropical cyclone model with explicit cloud microphysics, developed by Y. Wang, has been used to study these potential effects. The sea spray evaporation is incorporated into the model by two bulk parameterization schemes with quite different properties. The numerical results show that inclusion of the Fairall et al. sea spray parameterization increases the direct sensible heat flux from the ocean by about 70%, but has little effect on the direct latent heat flux. Sea spray itself causes a sensible heat flux of only about 6% of the direct sensible heat flux, while it contributes a latent heat flux by evaporation of sea spray droplets by 60%?70% of the direct latent heat flux. As a result, the total enthalpy flux with sea spray evaporation increases by about 20%, while the net contribution by sea spray is only about 1.5% of the total enthalpy flux. Consistent with this, the intensity of the model tropical cyclone is moderately increased by 8% in the maximum wind speed by the introduction of sea spray. The lower atmosphere becomes cooler and moister due to the evaporation of sea spray, which is supported by the available observations. The cooling in the surface layer further modifies the boundary layer structure and the activity of convection, especially in the near-core region where the highest concentration of sea spray exists. On the other hand, with the Andreas and DeCosmo parameterization scheme, the intensity of the model tropical cyclone is increased by 25% in maximum wind speed. This dramatic increase in the model tropical cyclone intensity is due to both the large net sensible heat flux and the latent heat flux associated with the effect of sea spray by this parameterization scheme. The net upward sensible heat flux warms the air near the surface and results in a near-isothermal surface layer in the near-core environment under the tropical cyclone. Such a structure, however, is not supported by the available observations, which the authors argue is not physically realistic. The radically different results with this scheme are due to the unusual way that the feedbacks between direct and spray-mediated fluxes are handled within the parameterization.