Effects of Sea Spray on the Simulated Tropical Cyclone Development: Dependence on Surface Drag Coefficient Parameterization

Wave breaking under strong wind conditions in tropical cyclones (TCs) can generate sea spray droplets, which, during their suspension in air, release sensible heat due to the air-sea temperature difference while absorb sensible heat from the environment when they evaporate and release latent heat to the environment. Since the spray mass flux is a function of surface drag coefficient (C-D), the effect of spray on TC evolution should depends on C-D parameterization, while this has not been addressed so far. This study examines the effects of sea spray on the simulated TC evolution with two different C-D parameterizations (the Weather Research and Forecasting (WRF) default scheme and the Donelan scheme). Results show that during the primary intensification stage, the TC with spray effect becomes stronger than that without spray when the WRF C-D scheme is used, but becomes weaker when the Donelan C-D scheme is used. This occurs because C-D is maximum outside the radius of maximum wind (RMW) with the Donelan scheme, which produces relatively large spray-mediated latent heat flux outside the RMW, which is unfavorable for TC intensification. The difference is enlarged by a feedback between spray and TC intensification involving the inertial stability and surface friction-induced radial inflow. However, in the mature stage, the simulated TCs with spray become stronger no matter which C-D scheme is used. In addition, the spray effect on the TC inner-core size evolution also weakly depends on the drag parameterization. When C-D is relatively greater outside the RMW, the inclusion of the spray effect would lead to the inner-core size increase. Plain Language Summary Wave breaking under strong wind conditions, such as in tropical cyclones (TCs), can generate abundant sea-spray droplets, which, during their suspension in air, release sensible heat due to the air-sea temperature difference while absorb sensible heat from the environment when they evaporate and release latent heat to the environment. This will mediate the air-sea enthalpy transfer and affect the TC intensification. As the spray mass flux is closely related with sea surface drag coefficient (C-D), we investigated how the spray effects on TC intensity evolution depend on the C-D scheme used in idealized numerical simulations. Two C-D schemes were used to perform four numerical experiments. Results show that the sea spray effect on TC intensity evolution depends on the C-D scheme used and the stages of the TC lifetime, largely due to the different wind speed dependence of C-D and its effect on the radial distribution of the spray-mediated latent flux. However, the finding demonstrates that caution should be given to surface drag parameterization when the sea spray effects on TC evolution are studied using numerical models. It is also suggested that efforts to measure spray properties under TC conditions should be conducted to validate/improve spray parameterization in the future.