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| GOES-12 visible image of Hurricane Katrina over the central Gulf of Mexico at 1745 UTC 28 August 2005, near the time of its peak intensity of 150 kt. |
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| Vertically integrated water vapor (grayscale) and condensates associated (color scale) with Hurricane Katrina as simulated by the WRF-ARW model of 12-km grid spacing valid at 1800 UTC 28 August 2005. |
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| Ocean surface whitecaps and foam streaks in a hurricane at wind speed of ~46 m/s (courtesy of P. Black). |
Improving Parameterizations of Air-Sea Interaction for Hurricane Intensity Forecasts
Motivation
The prediction of both hurricane track and intensity relies heavily on numerical weather prediction (NWP) models. However, great uncertainties still remain in the formulation of a few key model physics components that are critical to the development of hurricanes. One of these components is the parameterization of the momentum and heat fluxes across the air-sea interface. All the flux parameterizations currently used in the operational NWP models were developed using observations taken under winds weaker than those associated with a typical hurricane. Furthermore, under hurricane conditions, the air and sea is separated by a spray-filled transition layer. Parameterizations of momentum and heat fluxes across such a layer need to take into account the effect of sea spray, but they currently do not.
Methodology
Over the past decade, research has been carried out at NOAA/ESRL to combine atmospheric observations, laboratory experiments and numerical modeling studies to develop, test and improve the parameterizations of air-sea momentum and heat fluxes under hurricane conditions.
Research Activities & Outcomes
We developed a bulk parameterization scheme of air-sea sensible and latent heat fluxes as an extension of the TOGA-COARE bulk flux model (Fairall et al. 1994), which has been refined with observations from new field campaigns (such as the CBLAST experiment) and updated theoretical understanding. The unique aspect of this scheme is that it includes the sea-spray contribution to the air-sea heat fluxes for winds greater than 25 m/s. We are currently testing and evaluating the NOAA/ESRL air-sea heat flux parameterization scheme in the operational hurricane model (HWRF) and the community weather research and forecast model (WRF-ARW). The goals of the ongoing research with both the operational and research models are (1) to determine the sensitivity of the sea-spray mediated air-sea heat flux calculation to the uncertainties in the wave dynamics and the kinematic and thermodynamic feedbacks of sea spray, and (2) to make the scheme more general by including both the spray evaporation feedback and stress reduction effects. We are also closely collaborating with researchers from NOAA and universities to evaluate the impact of the improved air-sea flux scheme on the marine boundary layer dynamics under hurricane conditions.