Location
MC 5417
Candidate
Perseus Sepahi| Applied Mathematics, University of Waterloo
Title
Stratification and Vertical Turbulence Structure in the Atmospheric Boundary Layer: A Large-Eddy Simulation Study
Abstract
The stably stratified atmospheric boundary layer (SBL) forms over land at night as the surface cools, and the buoyant suppression of turbulence in this regime makes it one of the most difficult parts of the atmosphere to observe, simulate and parameterise. This thesis investigates how the vertical structure and intensity of boundary-layer turbulence respond to increasing surface cooling, and how that response depends on the strength of the mechanical forcing, using large-eddy simulation (LES) with the Advanced Research version of the Weather Research and Forecasting model (WRF). Eight idealised simulations cross two wind speeds — the strongly forced A-cases (Vg = 16.5 m s⁻¹) and the weakly forced D-cases (Vg = 8.25 m s⁻¹) — with four surface cooling rates (0, 0.25, 0.50 and 1.00 K h⁻¹), of which the 0.50 and 1.00 K h⁻¹ cases double and quadruple the reference forcing and constitute the extension contributed by this work. All other elements of the configuration — domain, resolution, numerics, subgrid closure, Monin–Obukhov surface exchange and run protocol — are held fixed, so that differences between cases isolate the response of the boundary layer to its two external controls.
The strongly forced A-cases respond monotonically to the fourfold increase in cooling, consistent with the classical quasi-steady stable boundary layer: the surface sensible heat flux strengthens from approximately 28 to 51 W m⁻² in magnitude, the surface friction velocity decreases, the boundary layer contracts from approximately 486 to 278 m as the low-level jet strengthens and descends, the interior gradient Richardson number remains close to its critical value, and the vertical-velocity spectra retain the surface-attached scaling of continuous turbulence at every cooling rate. The weakly forced D-cases depart from this regime. The surface friction velocity collapses while the surface sensible heat flux saturates at approximately 10 W m⁻² in magnitude despite the fourfold increase in imposed cooling, the signature of the maximum sustainable heat flux — so that the cooling that cannot be mixed upward accumulates in a shallow, strongly stratified layer adjacent to the surface, while the flow aloft decouples from the surface and is no longer dominated by continuous turbulence.