Estimating Air-Sea Energy and Momentum Exchanges inside Tropical Cyclones Using the Fetch- and Duration-Limited Wave Growth Properties

For wind-generated waves, the wind-wave triplets (reference wind speed, significant wave height and spectral peak wave period) are intimately connected through the fetch- or duration-limited wave growth functions. The full set of the wind-wave triplets can be obtained knowing only one of the three variables combined with the fetch (duration) information using the fetch-limited (duration-limited) wave growth functions. The air-sea energy and momentum exchange rates are functions of the wind-wave triplets and can be quantified with the wind-wave growth functions. Previous studies have shown that wave development inside hurricanes follows the same growth functions established for steady wind forcing conditions to a large degree. Here we present the analysis of wind-wave triplets collected inside Hurricanes Bonnie 1998 and Ivan 2004 at category 2 to 4 stages using the airborne scanning radar altimeter (SRA) combined with the NOAA HRD hurricane wind velocity product. The energy and momentum exchange rates are calculated from the measured wind-wave triplets along several (between 6 and 11) transects radiating from the hurricane center. These results reveal the complex azimuthal and radial variations of air-sea exchanges inside the hurricane. Using the wind-wave triplets from the 4 hurricane scenes, a parameterization model is formulated for the effective fetches in different sectors of the hurricane. The fetch formulas are applied to the 2D hurricane wind fields from post analysis to investigate the detailed spatial and temporal evolution of the wave field and the associated energy and momentum exchange rates inside a hurricane. The spatial distribution of energy and momentum exchanges show considerable asymmetry. Referenced to the hurricane heading, the exchanges on the right half plane of the hurricane are much stronger than those on the left half plane, the right-to-left ratio of about 3:1 is not uncommon.