Microphysics of the Air-Sea interface and Rapid Intensification and Decay of Tropical Cyclones: Volume of Fluid to Discrete Phase Method and Newly Available Laboratory and Open Ocean Observations

Microphysics of the air-sea interface can be a factor in mass, momentum, and enthalpy exchange in tropical cyclones (Sroka and Emanuel 2022). The NOAA Saildrone observations in Hurricane Sam 2021 have for the first time provided high resolution imagery of the air-sea interface under hurricane conditions. This imagery is consistent with the previous observations in the University of Miami (UM) SUSTAIN facility. Both laboratory and open ocean data sets indicate development of short waves that resemble Kelvin-Helmholtz type interfacial waves (KH). The KH waves at the air-sea interface develop and break within tens of milliseconds. KH at the air-sea interface is an example of the process that is stable on average (Miles 1959) but unstable to fluctuations (Farrell and Ioannou 2008). The KH at an interface with a very large density difference, like air and water, is strongly asymmetric with most of the action taking place on the side of the liquid with lower density (Hoepffner et al. 2011, Soloviev et al, 2017). As a result, KH is an effective mechanism for sea spray production between whitecaps, while bubbles are mostly produced within whitecaps occupying only ~4% of the sea surface in hurricanes (Holthuijsen et al. 2012). As a result, KH is an effective mechanism for sea spray production. Notably, KH at the air-sea interface starts at a wind speed above 33-35 m/s, which curiously coincides with the transition from a tropical storm to a hurricane (Koga 1981; Soloviev and Lukas 2010). Here we apply a Volume of Fluid to Discrete Phase Method (VOF to DPM) multiphase model with a mesh adaptation implemented by Vanderplow et al. (2020). This model is consistent with the lab gas exchange data obtained by Krall et al. (2019) at the UM and Kyoto University high wind speed facilities and with field gas exchange measurements by McNeil and D’Asaro (2007) in a hurricane. The VOF to DPM multiphase model, which discerns between near-spherical and non-spherical spray components, has been applied to estimate the sea spray generation function under tropical cyclone conditions, directly at the air-sea interface. For major tropical cyclones (Cat. 3-5), non-spherical spray appears to dynamically dominate over near-spherical spray, which is an indication that the influence of surface tension is diminished and effectively there is no longer the separation between the air and water phases. The diminishing effect of surface tension is an indication of a transition to a different regime of the air-sea interaction that can be specific to major tropical cyclones. The possible change of the regime of air-sea interaction in major tropical cyclones is consistent with Lee et al. (2022) using different considerations. The consequences for the drag coefficient Cd, enthalpy exchange coefficient Ck, and Ck/Cd may include rapid intensification or decay of tropical cyclones.

References:

Farrell, B. F. and Ioannou, P. J., 2008. The Stochastic Parametric Mechanism for Growth of Wind-Driven Surface Water Waves. J. Phys. Oceanogr. 38, 862–879.

Holthuijsen, L. H., Powell, M. D., & Pietrzak, D., 2012. Wind and waves in extreme hurricanes. J. Geophys. Res. 117, C09003.

Hoepffner, J., Blumenthal, R., & Zaleski, S., 2011. Self‐similar wave produced by local perturbation of the Kelvin‐Helmholtz shear‐layer instability. Phys. Rev. Lett. 106, 104502‐1– 104502‐4.

Krall, K. E., Smith, A. W., Takagaki, N., and Jähne, B., 2019. Air–sea gas exchange at wind speeds up to 85 m s−1. Ocean Sci. 15, 1783–1799.

Koga, M., 1981. Direct production of droplets from breaking wind-waves-Its observation by a multi-colored overlapping exposure technique. Tellus 33, 552–563.

Lee, W, Kim S-H, Moon I-J, Bell MM and Ginis I., 2022. New parameterization of air-sea exchange coefficients and its impact on intensity prediction under major tropical cyclones. Front. Mar. Sci 9:1046511, doi: 10.3389/fmars.2022.1046511.

McNeil, C.L., and E.A. D'Asaro, 2007. Parameterization of air-sea gas exchange at extreme wind speeds. Journal of Marine Systems 66, 110-121.

Miles, J. W., 1959. On the generation of surface waves by shear flows. Part 3. Kelvin-Helmholtz Instability. J. Fluid Mech. 6, 583–598.

Soloviev, A. and Lukas, R., 2010. Effects of bubbles and spray on air-sea exchange in hurricane conditions. Bound.-Layer Meteorol. 136, 365–376.

Soloviev, A. V., Lukas, R., Donelan, M. A., Haus, B. K., & Ginis, I., 2017. Is the state of the air‐sea interface a factor in rapid intensification and rapid decline of tropical cyclones? J. Geophys. Res. Oceans 122(12), 10174-10183.

Sroka, S., and Emanuel, K., 2022. Sensitivity of sea-surface enthalpy and momentum fluxes to sea spray microphysics. Journal of Geophysical Research: Oceans 127, e2021JC017774.

Vanderplow, B., Soloviev, A.V., Dean, C.W., Haus, B.K., Lukas, R., Sami, M., Ginis, I., 2020. Potential effect of bio-surfactants on sea spray generation in tropical cyclone conditions. Sci Rep 10, 19057.