Air-sea fluxes in numerical modelling of Tropical Cyclones: effects of surface drag and spray route on storm track, intensity and rapid intensification process

At high wind speeds in a Tropical Cyclone (TC), the transfer of sensible and latent heat flux via the spray route becomes a significant contributor to the total flux at the air-sea interface. It is also no longer valid at high winds that drag coefficient increases with an increasing surface wind speed that is recognised at normal wind speeds. This change in mechanism governing air-sea exchanges at high winds lead to amendments to the bulk algorithms currently used for the parameterisation of the surface layer. Algorithms for spray route and drag coefficient at high wind speeds are implemented into the atmospheric surface layer scheme in ACCESS-TC (the operational TC forecast model for Australia).

Statistical analysis of a large number of TC forecasts over Pacific Ocean demonstrates that the algorithms enhance the initial TC positioning and the subsequent intensity forecasts. A case study of rapid intensification (RI) during the devastating Coral Sea TC that impacted north-eastern Australia (YASI, 2011) is presented. Impacts of the simulated surface exchange processes to RI process are discussed, including the ratio of the surface exchange coefficient for enthalpy and the drag coefficient on which a constraint exists for intense TCs as proposed from the previous theoretical studies.