Organized Turbulence in a Cold-Air Outbreak: Evaluating a Large-Eddy Simulation with Respect to Airborne Measurements

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.

A case‐study of the coupled ocean–atmosphere response to an oceanic diurnal warm layer

“…The turbulent surface fluxes can also be fixed, as in the Barbados Oceanographic and Meteorological Experiment (BOMEX) case-study investigating marine shallow cumulus convection (Siebesma et al, 2003). More recently, case-studies of cold air outbreaks have been performed with a prescribed time-varying SST in order to represent the transport of a cold and dry air mass over a warmer sea (e.g., Roode et al, 2019 in the CONSTRAIN case;Brilouet et al, 2020 in a HyMeX 1 case). A Lagrangian approach has also been proposed by Sandu and Stevens (2011) or Lemarié et al (2020), with an air mass advected over a changing SST, mimicking the presence of an SST front.…”

Sampling error in aircraft flux measurements based on a high-resolution large eddy simulation of the marine boundary layer

“…In recent years, it has become possible to use large eddy simulation (LES) models to explicitly resolve turbulent fluctuations of mass, energy, and motion in the planetary boundary layer at fairly fine scales, leaving only a small fraction of the total turbulent exchange to subgrid-scale parameterizations, especially at levels far above the surface. Model turbulence fields can be directly compared with aircraft measurements in the same environment, as was done, for example, by Brilouet et al (2020) for a cold-air outbreak over the northwest Mediterranean, who found that the LES successfully reproduced convective structures observed by the aircraft.…”

A Ship-Based Characterization of Coherent Boundary-Layer Structures Over the Lifecycle of a Marine Cold-Air Outbreak

“…The longitudinal axis of rolls tends to align with a small angle, α, to the mean wind direction. This is, for example, discussed by Atkinson and Zhang (1996), and evident in large-eddy simulations by Brilouet et al (2020). Cross-wind movement of the ship can introduce an increased α, relative to wd a .…”

“…Organized convection, which results in coherent structures in the wind, temperature, and moisture fields, was found to contribute an essential part to these turbulent heat fluxes (e.g., LeMone 1973;Chou and Ferguson 1991;Brilouet et al 2020). Mesoscale shallow convection patterns, such as roll vortices, open cellular convection, and closed cellular convection, are generally affiliated with marine CAO events (Atkinson and Zhang 1996).…”