Momentum and heat fluxes across the air-sea interface during CBLAST-low

Direct covariance measurements of the turbulent Reynolds stress and direct measurements of the vertical gradient of the Reynolds-averaged horizontal velocity were obtained simultaneously on both sides of the air-sea interface during the low-wind component of the Coupled Boundary Layers and Air-Sea Transfer (CBLAST-low) program.

The measurements were obtained at the Martha’s Vineyard Coastal Observatory (MVCO), a site exposed to the open sea, from an air-sea interaction tower, which spans the 15-m water column and extends 22 m into the atmosphere. The air-side measurements were obtained by a vertical array of fixed sonic anemometers, which determine the Reynolds stress, and a set of sonic anemometers mounted on a vertical profiler, which determines the vertical gradient of the Reynolds-averaged velocity. Similarly, the water-side measurements were obtained by a horizontal array of acoustic Doppler velocimeters (ADVs), which determine the Reynolds stress, and an acoustic Doppler current profiler (ADCP), which determines the vertical gradient of the Reynolds-averaged velocity. The horizontal array of water-side ADVs was designed to permit use of spatial and temporal filtering techniques to separate the relatively small turbulent velocity fluctuations from the much larger wave-induced velocity fluctuations, which is essential in order to produce meaningful estimates of the turbulent Reynolds stress.

The analysis focuses on (1) comparison of Reynolds stresses measured on the air and water sides of the air-sea interface and (2) evaluation of the relationship between the Reynolds stress and the Reynolds-averaged velocity gradient. The air-side and water-side Reynolds stresses should be equal, except for relatively small effects of acceleration and pressure gradient, which can be quantified by means of supplementary CBLAST-low measurements. Observed good agreement between air- and water-side stresses indicates that the stress estimates (particularly the water-side estimates) are meaningful, and that, in particular, they resolve the spatial and temporal scales that are primarily responsible for the vertical transport of momentum. To our knowledge, a successful test of agreement between measured air-side and water-side Reynolds stresses has not previously been obtained in an observational program. The relationship between the Reynolds stresses and the velocity gradients are examined on both sides of the air-sea interface in light of Monin-Obukhov similarly theory, recent models of effects of near-surface pressure-velocity correlations in the atmospheric boundary layer, and recent models of effects of wave breaking and Langmuir circulations in the oceanic surface layer.