Laboratory Wave and Stress Measurements Quantify the Aerodynamic Sheltering in Extreme Winds

Abstract

In strong winds, air flow detaches from the ocean surface in the lee of wave crests and creates a low-pressure zone on the wave’s leeward face. The pressure difference between the wave’s rear and front face modulates the momentum input from wind to waves. Numerical wave models parameterize this effect using a so-called sheltering coefficient. However, its value and dependence on wind speed are not well understood, particularly with background swell waves. To bridge this gap, we conducted laboratory experiments with winds up to Category 4 hurricane force blown over various mechanically generated wave conditions (pure wind sea, mixed waves with directional spreading, and monochromatic unidirectional waves) and measured the wind, waves, and stress at a sufficient frequency to resolve wind-wave variability over the long-wave phase. We analyze the results in the context of Jeffreys’s sheltering theory and find two regimes: (a) from low-to-moderate wind forcing (10 m s−1 < U10 < 33 m s−1), the aerodynamic sheltering increases with wind speed, consistent with previous studies; (b) in hurricane conditions (U10 > 33 m s−1), the aerodynamic sheltering decreases with wind at a rate depending on wave state. Further, we isolate the short wind waves from the longer paddle waves and find that the aerodynamic sheltering by longer waves leads to a phase-dependent variability of the short wind-waves’ local steepness, which is evidenced by the sheltering coefficient’s value derived from wind and wave measurements. Our results emphasize the need for further measurements of aerodynamic sheltering and improving its representation in models.