Airborne Measurements of the Turbulence Field in the Marine Atmospheric Boundary Layer Under Strong Wind Conditions: Impact of Coherent Structures on Vertical Exchanges

During winter, cold-air outbreaks take place in the north-western Mediterranean sea. They are characterized by local strong winds (Mistral or Tramontana) which bring cold and dry continental air over a warmer sea. They lead to intense air-sea interactions which can rapidly affect the structure of the marine atmospheric boundary layer (MABL). Combining large latent and sensible heat fluxes with strong wind conditions over a homogeneous surface like open sea can set up a particular organization of the turbulence field.

The HyMeX/ASICS-MED field campaign, devoted to intense sea-atmosphere exchange and deep oceanic convection analysis took place in the Gulf of Lion during the winter of 2013. The French ATR42 aircraft was operated to document the mean and turbulent structure of the MABL during strong wind conditions. The aircraft was equipped to measure fluctuations of potential temperature, water vapour mixing ratio and wind components, thus allowing the computation of turbulence parameters. The flight strategy consisted of stacked horizontal legs oriented along and across the wind direction, in order to obtain information about the isotropy of the turbulent field and thus coherent structures. Strong wind events were well-documented with 11 flights during which latent heat fluxes as high as 600 W.m-2 were measured.

The structure of the MABL is studied through a spectral analysis based on the Kristensen et al. (1989) analytical model of turbulence. Vertical profiles of characteristic length scales revealed a non-isotropic structure with a stretching of the eddies along the mean wind. This kind of organized structures plays a major role by modulating the transfers inside the MABL. They also affect the flux estimates, particularly on along-wind samples. A correction of turbulent flux estimates, taking into account the systematic and random errors due to sampling and data processing, is developed. The impact of the non-isotropic behaviour on turbulent flux estimates is then evaluated.

As a result of this study, flux-profile through the whole MABL can be corrected. This improves surface and entrainment fluxes estimates, computed from the extrapolation of the stacked levels. Those parameters, essential to characterize the air-sea coupling and its impact on oceanic convection, are not easily measured by surface platforms under strong wind conditions, and large uncertainties remain on bulk parametrizations.