Impact of Surface Wave Dependent Air-Sea Fluxes on Tropical Cyclone Prediction

University Of Rhode Island, Kingston RI

Investigators

Abstract

Intellectual Merit: Present bulk parameterizations of air-sea momentum fluxes used in most hurricane research and forecast models are based on extrapolation from field measurements in much weaker wind regimes. These parameterizations predict monotonic increases of the exchange coefficients with wind speed. However, recent observational, theoretical, and experimental results demonstrate that air-sea momentum flux at high wind conditions strongly depends on the wave field and that the drag coefficient ceases to increase and may even decrease at very high wind speeds. These factors may cause current models to significantly underestimate surface winds. Recently, the Principal Investigators (PIs) have developed a coupled wave-wind (CWW) model, which provides surface momentum fluxes that are consistent with recent observations. The model explicitly calculates the wave-induced stress and resulting drag coefficient for any given wave field, even for complex seas driven by hurricanes. Numerical experiments using the CWW model clearly indicate that the behavior of momentum flux at high wind speeds is completely different from that at weak wind speeds and that the momentum flux varies significantly depending on the relative position from the storm center due to influence of ocean waves. These results strongly suggest that proper estimation of momentum flux in hurricane conditions can be only achieved by incorporating an ocean wave model and a wave boundary model into hurricane-ocean models. It is hypothesized that: (1) Surface momentum fluxes in hurricane conditions strongly depend on the ocean wave fields varying in time and space. The effect of breaking waves is significant under hurricane conditions. Momentum fluxes in hurricane conditions can be predicted accurately by using the CWW model. (2) Hurricane intensity and maximum wind predictions are significantly influenced by the parameterization of the air-sea momentum flux. In particular, the spatial/temporal variability of the drag coefficient has a major influence on predictions. Therefore, the coupling of the CWW model to a hurricane-ocean forecast model will result in a systematic improvement of the forecast skill of hurricane intensity and wind structure. To test these hypotheses, the PIs will perform four specific tasks: (1) incorporate the breaking wave effect into the CWW model, (2) develop a coupled wind-wave-ocean hurricane prediction model by combining the CWW model and the Geophysical Fluid Dynamics Laboratory/University of Rhode Island hurricane-ocean model, (3) investigate how new flux parameterizations affect hurricane intensity, track, and wind structure predictions using both idealized and actual tropical storms, and (4) compare model results with existing and newly obtained experimental data in collaboration with scientists involved in the Office of Naval Research sponsored Coupled Boundary Layers/Air-Sea Transfer (CBLAST) program and Korean Ocean Research Development Institute (KORDI). Broader impacts: This research addresses societal need to better forecast and, thereby, mitigate natural disasters caused by hurricane-generated extreme wind, waves, and rain. The project involves the education and training of a graduate student and post-doctoral scientist. The work promotes international cooperation by using hurricane-observing tower data from KORDI and in turn providing new air-sea modeling techniques to KORDI. The data from the tower will be brought to investigate the air-sea fluxes at high wind speeds and their impact on tropical cyclone predictions through the collaboration with KORDI.