Sea Spray Transport Intensifying Tropical Cyclones

The forecasts of tropical cyclone (TC) intensity have been of importance in the atmospheric sciences because of the potential severity of resulting disasters. In particular, it has been recently recognized that the rate of occurrence of rapid intensification events becomes almost doubled in the last 10 years (Ito 2016). Therefore, it is important to develop the sophisticated numerical model that helps improving TC intensity forecasts. In this work, the horizontal transport of sea spray droplets is proposed as a previously unknown mechanism that intensifies TCs. Conventional models assume that surface evaporation depends on local physical variables. However, sea spray droplets can remain suspended in the air for several minutes before evaporating under humid and violent wind conditions. Therefore, spray-mediated evaporation could occur about a few kilometers away from the spray genesis location according to a boundary layer inflow toward the TC center. This is favorable for inner updrafts that lead to intensification through quasi-conservation of absolute angular momentum. To verify this hypothesis, ensemble simulations were conducted using a coupled model that can consider both droplet flight duration and evaporation timescale. The minimum sea level pressure is 922 hPa when the transport of sea spray droplets is incorporated, but is higher (929 hPa) when the transport is neglected, and higher still (944 hPa) without sea spray droplets. Further analysis revealed that the transport of sea spray droplets induced the inner updraft and enhanced inflow toward the TC center as expected. It suggests that the previous exclusion of sea spray transport in numerical models may partly account for failures in predicting violent TC intensities.