An Evaluation of the Constant Flux Layer in the Atmospheric Flow Above the Wavy Air‐Sea Interface.

The constant flux layer assumption simplifies the problem of atmospheric surface layer (ASL) dynamics and is an underlying assumption of Monin‐Obukhov similarity theory (MOST), which is ubiquitously applied to model interfacial exchange and atmospheric turbulence. Within the marine environment, the measurements necessary to confirm the local ASL as a constant flux layer are rarely available, namely: direct observations of the near‐surface flux gradients. Recently, the Research Platform FLIP was deployed with a meteorological mast that resolved the momentum and heat flux gradients from 3 to 16 m above the ocean surface. Here, we present an assessment of the prevalence of the constant flux layer within the ASL, using an approach that accounted for the role of wave‐coherent motion in air and evaluated each observed flux gradien. Our analysis revealed that only 30%–40% of momentum flux gradients were approximately constant; for the heat fluxes, this increased to 50%–60%. For low winds, the stationarity of local turbulence was critical to the constant flux layer's validity, but resulted in excising a large proportion of the observed profiles. Under moderate wind speeds, swell‐wind alignment was associated with momentum flux divergence. Our findings suggest that the constant flux layer, as it is conventionally defined, is not generally valid over the ocean. This holds significant implications for measuring air‐sea fluxes from single point sources and marine applications of MOST. Plain Language Summary: Our ability to quantify the exchange of energy and material (e.g., gas) between the atmosphere and ocean has greatly improved over the second half 20th and into the beginning of the 21st centuries. While there have been significant technological and methodological advancements within the community of researchers studying this problem, a central theory to the physical framework we use to conduct the majority of studies has not been adequately validated or assessed using observations over the ocean. The constant flux layer model (or assumption) greatly simplifies the physical problem of studying the atmosphere near the ocean surface, but the data necessary to validate this theory are rarely collected. A recent field campaign deployed a unique ocean‐going platform that enabled us to conduct this much needed evaluation. We found strong evidence suggesting that the constant flux layer model is only valid within the general marine environment at most 50%–60% of the time. We also found that the prevalence of this theory's validity differed between the exchange (i.e., flux) of kinetic energy and heat, two critical parameters controlling the atmosphere‐ocean system. Our findings suggest that the simplified physics we rely on to study air‐sea exchange needs to be critically re‐evaluated. Key Points: Profiles of eddy covariance fluxes were used to evaluate the prevalence of the constant flux layer above ocean wavesOnly one third of momentum flux gradients were deemed "constant"; non‐divergence was substatially higher for the heat fluxesFlux divergence was strongly linked with turbulence non‐stationarity, swell‐wind alignment, and moderate‐strong stability conditions [ABSTRACT FROM AUTHOR]