Observations of the flux-gradient relationship at the air-sea interface: preliminary results

The wind blows and waves form in response. This general process forms the basis for the widespread use of Monin-Obukhov similarity theory (MOST) in the parameterisation of turbulent exchange between the atmospheric boundary layer (ABL) and the ocean. There are a multitude of conditions under which the assumptions required for the application of MOST fail, however. While the wind forcing in the ABL is highly localised, the local wave field can be strongly impacted by non-local wind forcing. During low wind conditions, waves excited non-locally have the capacity to force motions within the ABL. This wave-forcing on the ABL presents conditions under which the application of MOST fails, particularly within the first ten meters. The deviations from MOST in this case are, however, rarely documented in great detail.

A novel and comprehensive field campaign was designed to both qualify and quantify the flux-gradient relationship in the atmospheric boundary layer during low-wind large-swell conditions. We present here the preliminary results from this field campaign at Belle-Île-en-mer. The dataset includes measurements of mean wind and temperature profiles, high frequency momentum and heat fluxes, and global ABL and wave conditions.

Above the sea surface, the mean wind profile (wind speed and wind direction) was remotely sensed at 3~km offshore at a very fine vertical resolution thanks to two scanning LiDARs and the temperature profile was measured by a flying drone. Fluxes at the surface were recorded at high frequency by equipment mounted on a dedicated trimaran. Global ABL conditions were monitored using a profiling LiDAR, a microwave-radiometer and atmospheric soundings. We provide a first look into the measured flux-gradient relationships and highlight cases during which deviations from MOST are observed.