Abstract
Cold-air outbreaks (CAO) lead to intense air–sea interactions, the appropriate representation of which are fundamental for climate modelling and numerical weather forecasting. We analyze a CAO event with low-level wind speeds of approximately 25 m s$$^{-1}$$ observed in the north-western Mediterranean Sea. The marine atmospheric boundary layer (MABL) was sampled with an aircraft equipped for turbulence measurements, revealing the organization of the MABL flow in coherent structures oriented along the mean wind direction, which was then simulated in two steps. First, a one-dimensional simulation enabled the determination of the forcing terms (particularly horizontal advection) required to adequately reproduce the vertical structure of the MABL flow. These terms were computed from a limited-area forecast model in operation during the entire field campaign. Then, a large-eddy simulation (LES) was performed during the well-established phase of the CAO event. The LES output is validated with respect to airborne data, not only with respect to the mean wind-speed and thermodynamic profiles, but also the turbulence statistics and coherent structures. The validated LES results enable description of the turbulent field as well as the coherent structures. The main discrepancy is a considerable underestimation of the simulated evaporation (computed with a parametrization of the turbulent surface fluxes), and hence of the moisture fluctuations throughout the boundary layer. Several possible explanations may explain this underestimation. The structure of the boundary layer is nonetheless well reproduced by the LES model, including the organized structures and their characteristic scales, such as the structure wavelength, orientation, and aspect ratio, which closely agree with observations. A conditional-sampling analysis enables determination of the contribution of the coherent structures to the vertical exchange. Although they occupy a limited fractional area, organized structures are the primary contributors to the turbulent exchange.
Highlights
As oceans cover more than 70% of the Earth’s surface, the study of mass and energy transfer between the ocean and the atmosphere is fundamental for climate issues and to short- and medium-term weather forecasting
The turbulence characteristics of the marine atmospheric boundary layer (MABL) in high wind speeds have been analyzed using both airborne measurements collected during the HyMeX field campaign in the north-western Mediterranean basin and a large-eddy simulation (LES) model
In Brilouet et al (2017), a statistical approach on an ensemble of Cold-air outbreaks (CAO) events identified the organization of the turbulence field into longitudinal rolls
Summary
As oceans cover more than 70% of the Earth’s surface, the study of mass and energy transfer between the ocean and the atmosphere is fundamental for climate issues and to short- and medium-term weather forecasting. Inside the MABL, mass and energy exchanges between the surface and the free atmosphere are made through turbulence processes. We note the parametrization developed by Moon et al (2007) for hurricane conditions, and the work of Andreas et al (2015), who introduced a new approach to take into account sea spray. These flux formulations agree quite well under moderate wind speeds, but uncertainties become important at low or high wind speeds or under strong stratification (Brunke et al 2003)
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