Fetch- and Duration-Limited Nature of Surface Wave Growth inside Tropical Cyclones and Microwave Remote Sensing of Hurricane Wind Speed Using Dominant Wave Parameters

Wednesday, 20 April 2016: 2:45 PM
Ponce de Leon B (The Condado Hilton Plaza)
Paul A. Hwang, NRL, Washington, DC; and X. Li, B. Zhang, and E. Walsh
Manuscript (1.4 MB)

Handout (4.8 MB)

One of the most difficult issues in microwave wind sensing is signal saturation in high winds. The problem is especially critical for the active radar systems that respond to short and intermediate scale ocean surface roughness, because surface wind stress driving the roughness may not increase with wind speed monotonically.

Despite the much more complex wind forcing conditions, the dimensionless growth curves obtained with the wind-wave triplets (reference wind velocity, significant wave height and dominant wave period) inside hurricanes (except near the eye region) are comparable to the reference similarity counterparts constructed with the wind-wave triplets collected in field experiments conducted under ideal quasi-steady fetch-limited conditions. In dimensionless terms, the youngest waves are in the back quarter of the hurricane. In the northern hemisphere, the dimensionless frequency decreases systematically counterclockwise (CCW) and the most mature waves are in the left hand sector. The air-sea energy exchange can be estimated with the wave growth function. Based on the calculation of the wind input or energy dissipation in the wave field, a conservative estimate of the air-sea energy exchange over the coverage area of a category 1 to 2 hurricane is about 5 terawatts.

Making use of the fetch- and duration-limited nature of wave development inside hurricanes, a technique of hurricane wind retrieval using the dominant wave properties is introduced in this paper. Because of the importance of nonlinear wave-wave interaction mechanism in the dynamics of dominant waves, the spectral peak downshifts monotonically with increasing winds. Wind retrieval using the dominant wave parameters thus produces monotonic results despite surface wind stress saturation.

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