The Air-sea Interface under Tropical Cyclones and Rapid Intensification of Some Storms to Major Tropical Cyclones

In 2015, several tropical cyclones in the North Pacific Ocean underwent spectacular rapid intensification. Tropical storm intensity prediction, including the problem of rapid storm intensification, is still a series challenge. Important physics of atmospheric, oceanic, and interfacial components are not yet well understood and implemented in tropical cyclone forecast models. Specific ambient environmental conditions including the ocean heat content and internal vortex dynamics (e.g., wall replacement cycle) have been considered by hurricane researchers among the factors favorable for rapid storm intensification. In this work, we focus on missing and unresolved physics of the air-sea interface, which are believed to be among the factors limiting storm intensification predictions. In a laboratory experiment and coordinated numerical simulation, Soloviev et al. (2014) found that the air-water interface under hurricane force wind may develop Kelvin-Helmholtz shear instability. The resulting two-phase environment suppresses short gravity-capillary waves and alters the aerodynamic properties of the sea surface. The unified wave-form and two-phase parameterization model shows the well-known increase of the drag coefficient (Cd) with wind speed, up to ~30 m/s. Around 60 m/s, one version of the new parameterization shows a local dip (“sweet spot”) of Cd and local peak of Ck/Cd, under assumption of constant enthalpy exchange coefficient Ck. The positive slope of the Ck/Cd wind speed dependence from approximately 40 m/s to 60 m/s would introduce asymmetry in the process of storm intensification relative to storm decline in this wind speed range, which is consistent with the Emanuel (2007) analysis. The sweet spot in Cd may thus explain rapid intensification of some storms to major tropical cyclones (the effect resembling quantum tunneling); while, the peak in Ck/Cd, the previously reported local peak of lifetime maximum intensity (bimodal distribution) in the best-track records (Kossin et al. 2013). The bimodal distribution of maximum lifetime intensity, however, can also be explained by environmental parameters of tropical cyclones alone.

References:

Emanuel, K. A Statistical Analysis of Tropical Cyclone Intensity. Mon. Weather Rev. 128, 1139–1152 (2000).

Kossin, J. P., Olander, T. L. & Kenneth R. Knapp. Trend Analysis with a New Global Record of Tropical Cyclone Intensity. J. Clim. 26, 9960–9976 (2013).

Soloviev, A., R. Lukas, M. Donelan, B. Haus, and I. Ginis. The air-sea interface and surface stress under tropical cyclones. Nature Scientific Reports 4, 5306 (2014).