Evidence of Energy and Momentum Flux from Swell to Wind

Abstract

Measurements of pressure near the surface in conditions of wind sea and swell are reported. Swell, or waves that overrun the wind, produces an upward flux of energy and momentum from waves to the wind and corresponding attenuation of the swell waves. The estimates of growth of wind sea are consistent with existing parameterizations. The attenuation of swell in the field is considerably smaller than existing measurements in the laboratory.

Observations of Air‐Sea Momentum Flux Variability Across the Inner Shelf

“…In a mean sense, the parameterized values underestimate the drag by O(2–4) times, though there was considerable spread (generally ±100%) about these means. Note that the entire CLASI data set comes from waves oblique (following) to the wind, which over the open ocean is typically associated with reduced drag (Kahma et al, ; Potter, ), but the measurements given here reveal higher ‐than‐expected C D . These findings would suggest that the mechanism governing drag reduction may be overwhelmed in the nearshore environment.…”

“…For example, in light wind conditions (U < 2 m/s) it is evident that the net momentum flux may transition from downward to upward, that is, from waves to air (Grachev & Fairall, 2001;Högström et al, 2018). This leads to a negative drag coefficient (Kahma et al, 2016). In these conditions (i.e., low wind speed and fast waves), the waves can disrupt the turbulence spectrum (Drennan, Kahma, & Donelan, 1999) and force U∕ z to be nonlogarithmic (Donelan, 1990;Smedman et al, 1999).…”

Effects of Swell Waves on Atmospheric Boundary Layer Turbulence: A Low Wind Field Study

“…Data from the Baltic Sea Swell Experiment also show that the drag coefficient is systematically lower than Rough Evaporation Duct experiment (Högström et al, ; Högström et al, ). In addition, the experiment of Kahma et al () gives a direct evidence to demonstrate that an upward momentum can be generated by swell waves.…”

The Role of Roughness and Stability on the Momentum Flux in the Marine Atmospheric Surface Layer: A Study on the Southwestern Atlantic Ocean

“…From the analysis of 187 high-frequency sampled segments of temperature and wind velocity, carefully selected from three ship campaigns of the Air-Sea Interaction at Brazil-Malvinas Confluence project, we found a particular behavior of the drag coefficient, with a negative trend for a calm wind speed up to 10 m/s when the significant wave height was lower than 2.5 m, and a continuous decrease of the drag coefficient with increasing wind speed for significant wave height higher than 2.5 m. The results suggest that waves act as roughness elements during high wave conditions, inducing a zero-plane displacement in the order of 0.1 to 1 m as an indication for a wave-induced roughness layer. In addition, the analysis of the turbulent kinetic energy (TKE) budget indicates the occurrence of upward TKE transport mainly during stable conditions, while the general patterns of transport and dissipation of TKE are similar to observations taken over land surfaces.Many studies show that the turbulent flow above the sea surface is locally modulated by the momentum transferred from waves (Donelan et al, 1997;Kahma et al, 2016) and the heat fluxes caused by air-sea temperature differences (Messager & Swart, 2016;Small et al, 2008;Spall, 2007), which drives the MASL stability. At the same time, the SST and waves are also modulated by the downward momentum flux from the atmosphere (Donelan et al, 2012;Gaube et al, 2015;Putrasahan et al, 2013).…”

“…Many studies show that the turbulent flow above the sea surface is locally modulated by the momentum transferred from waves (Donelan et al, 1997;Kahma et al, 2016) and the heat fluxes caused by air-sea temperature differences (Messager & Swart, 2016;Small et al, 2008;Spall, 2007), which drives the MASL stability. At the same time, the SST and waves are also modulated by the downward momentum flux from the atmosphere (Donelan et al, 2012;Gaube et al, 2015;Putrasahan et al, 2013).…”