The Effects of Sea Spray on Tropical Cyclone Intensity Using an Idealized Model

Under high-wind conditions, whitecaps and breaking waves eject sea spray droplets into the atmosphere. The spray droplets are initially the same temperature and salinity as the ocean surface. The smaller spray drops evaporate and contribute little to the net enthalpy flux. Larger drops however, fall back to the ocean before they completely evaporate. These re-entrant spray drops contribute a net sea-to-air enthalpy flux through two simultaneous mechanisms. First, the drop transfers sensible heat to the environment as it cools to the air temperature. Second, the drop evaporates since it is located in a sub-saturated environment. This process releases latent heat and cools the drop below the air temperature to its quasi-steady temperature.

The goal of this study is to investigate the effects of spray on the structure of the tropical cyclone boundary layer as well as how spray modifies the net sea-to-air enthalpy flux, and ultimately, tropical cyclone intensity. To do so, an axisymmetric control hurricane was created from the output of a real-data simulation of Hurricane Floyd (1999) using the NCAR-PSU MM5V3.4 non-hydrostatic model. Simulations of the idealized hurricane were conducted using a version of the Fairall et al. (1994) sea spray parameterization which has been modified to also account for spray drag. The simulations were run using varying spray source functions and with and without horizontal spray drag. Results indicate that the inclusion of sea spray greatly effects the net sensible and latent heat fluxes, as well as horizontal wind speed. The intensity of the idealized hurricane varied significantly depending upon both the amount of sea spray and horizontal spray drag effects. Also, when sea spray effects were included in the modeled BL, the near-surface temperature was 1K warmer than the near-surface temperature of the modeled BL without spray.