LMFL Fluid Mechanics Webinar: J. G. Brasseur

LMFL Fluid Mechanics Webinar series 2021 https://lmfl.cnrs.fr/en Speaker: James G. Brasseur Title: The Dramatic Transition in Atmospheric Boundary Layer Turbulence Structure from Neutral towards Convective, and the Development of Large-scale Rolls Abstract: The “neutral” boundary layer is driven entirely by mean shear. The rough-surface daytime atmospheric boundary layer (ABL), however, is driven both by mesoscale winds and surface heating, and the scale and structure of ABL turbulence eddies depend on the relative contributions of buoyancy-driven vertical motions and shear-generated horizontal fluctuations. This balance is characterized by the global stability parameter -zi/L, the ratio of boundary layer depth zi and Obukhov length scale L (negative). Nevertheless, the canonical daytime ABL is commonly modeled as neutrally stable (-zi/L = 0), a model that we show does not properly characterize daytime ABL turbulence. Using carefully designed large-eddy simulation, we study the transition between the fully shear-driven neutral ABL with strong surface “streak” structure, and the unstable "moderately convective" ABL characterized by very-large-scale roll structure. We discover that the addition of extremely low levels of surface heat flux to a previously neutral ABL suddenly creates a new, and very different, ABL structure that initiates at a critical stability state -zi/L ≈ 0.4. My hope is that the description of this critical state will excite theoreticians and experimentalists to explore the underlying mechanisms that, we show, initiates a new receptivity to buoyancy—a receptivity where, in the supercritical states, further increases in surface heating stimulate strong interactions between shear-driven surface layer streaks and convectively-driven thermals in the mixed layer, leading to a “maximally coherent” large-scale roll structure at -zi/L ≈ 1. Acknowledgments: This work was a collaboration with Dr. Balaji Jayaraman, currently Research Consultant and Scientist, SciAI LLC, USA, and was supported by DOE/EERE. The study was recently published in JFM (doi:10.1017/jfm.2021.3).