An Evaluation of the Constant Flux Layer in the Atmospheric Flow above the Wavy Air-Sea Interface

Abstract

The constant flux layer assumption simplifies the problem of atmospheric surface layer (ASL) dynamics and is an underlying assumption of Monin‐Obukhov Similarity Theory, 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 findings of a study assessing the prevalence of the constant flux layer within the ASL, using an approach that accounts for wave‐coherent turbulence, defines the wave boundary layer height, and empirically quantifies the observed flux divergence. Our analysis revealed that only 30‐40% of momentum flux gradients were approximately constant; for the heat fluxes, this increased to 50‐60%. 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. Swell‐wind alignment was associated with momentum flux profile divergence under moderate wind speeds. In conjunction, our findings suggest that the constant flux layer, as it is conventionally defined, is not generally valid within the marine ASL. This holds significant implications for measuring air‐sea fluxes from single point sources and the application of Monin‐Obukhov similarity theory over the ocean.