Revised Parameterization of Air–Sea Exchanges in High Winds for Operational Numerical Prediction: Impact on Tropical Cyclone Track, Intensity, and Rapid Intensification

Abstract In high-wind conditions, sea spray, in conjunction with a generally decreasing drag coefficient for increasing winds, greatly modulates surface heat and momentum fluxes. It has been suggested that the process can be particularly important for the prediction of tropical cyclones (TCs), yet its robust application in operational forecast systems has remained elusive. A sea spray inclusion scheme and a modified algorithm for momentum exchange have been implemented in the Australian Bureau of Meteorology’s current operational TC model. Forecasts for a limited sample of TCs demonstrate that the revised parameterizations improve initialized and forecast intensities, while mostly maintaining track prediction skill. TC Yasi (2011) has been studied for impacts of the revised parameterization on rapid intensification (RI). Compared with the conventional bulk air–sea exchange parameterization, the revised version simulates a cooler and moister region near the surface in the eyewall/eye region, adjusts the RI evolution by an earlier and stronger subsidence in the eye, and simulates a stronger radial pulsating of the eye and eyewall convection on relatively short time scales. The inclusion of the new scheme enhances RI features characterized by eyewall ascent, radial convergence, and inertial stability inside the radius of azimuthal-mean maximum wind over low- to midlevels, and by a ringlike radial distribution of relative vorticity above the boundary layer. In addition, it allows a higher maximum intensity wind speed based on Emanuel’s maximum potential intensity theory. It is shown that, as expected, this is mainly because of a larger ratio of enthalpy and momentum exchange coefficients.

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Microphysics of the air-sea coupling at high winds and its role in the dynamics and thermodynamics of severe sea storms

10.5194/egusphere-egu2020-7149 ◽

2020 ◽

Author(s):

Yuliya Troitskaya ◽

Alexander Kandaurov ◽

Daniil Sergeev ◽

Olga Ermakova ◽

Dmitrii Kozlov ◽

...

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Water Surface ◽

Synergetic Effect ◽

Ocean Surface ◽

Small Scale ◽

Momentum Exchange ◽

Momentum Fluxes ◽

The Impact ◽

High Winds

Showing the record strengths and growth-rates, a number of recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea coupling. Especially challenging is the nature and effect of the very small-scale phenomena, the sea-spray and foam, supposed to strongly affecting the momentum- and heat- air-sea fluxes at strong winds. This talk will focus on our progress in understanding and describing these "micro-scale" processes, their physical properties, the spray and foam mediated air-sea fluxes and the impact on the development of marine storms.The starting points for this study were two laboratory experiments. The first one was designed for investigation of the spray generation mechanisms at high winds. We found out 3 dominant spray generating mechanisms: stretching liquid ligaments, bursting bubbles, splashing of the falling droplets and "bag-breakup". We investigated the efficiency spray-production mechanisms and developed the empirical statistics of the numbers of the spray generating events of each type. Basing on the "white-cap method" we found out the dependence of the spray-generating events on the wind fetch. The main attention was paid to the "bag-breakup" mechanism. Here we studied in detail the statistics of spray produced from one "bag-breakup" event. Basing on these developments, we estimated heat and momentum fluxes from the spray-generating events of different types and found out the dominant role of the "bag-breakup" mechanism.To estimate the direct heat and momentum fluxes from the ocean surface to the atmosphere, we studied in the special experiment the foam impact on the short-wave part of the surface waves and the heat momentum exchange in the atmospheric boundary layer at high winds. Based on these results, we suggest a simple model for the aerodynamic and temperature roughness and the eddy viscosity in the turbulent boundary layer over a fractionally foam-covered water surface.The synergetic effect of foam at the water surface and spray in the marine atmospheric boundary layer on ocean surface resistance at high winds is estimated so as to be able to explain the observed peculiarities of the air-sea fluxes at stormy conditions. Calculations within the nonhydrostatic axisymmetric model show, that the "microphysics" of the air-sea coupling significantly accelerate development of the ocean storm.This work was supported by RFBR grant 19-05-00249 and RSF grant 19-17-00209.

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