Dynamics of Atmospheric Boundary Layers: Large Eddy Simulations and Reduced Analytical Models

Abstract

Real-world atmospheric and oceanic boundary layers (ABL) involve many inherent complexities, the understanding and modeling of which manifestly exceeds our current capabilities. Previous studies largely focused on the “textbook ABL”, which is (quasi) steady and barotropic. However, it is evident that the “real-world ABL”, even over flat terrain, rarely meets such simplifying assumptions.

The present thesis aims to illustrate and model four complicating features of ABLs that have been overlooked thus far despite their ubiquity: 1) unsteady pressure gradients in neutral ABLs (Chapters ‎2 and ‎3), 2) interacting effects of unsteady pressure gradients and static stability in diabatic ABLs (Chapter ‎4), 3) time-variable buoyancy fluxes (Chapter ‎5) , and 4) impacts of baroclinicity in neutral and diabatic ABLs (Chapter ‎6).

State-of-the-art large-eddy simulations will be used as a tool to explain the underlying physics and to validate analytical models we develop for these features. Chapter ‎2 focuses on the turbulence equilibrium: when the forcing time scale is comparable to the turbulence time scale, the turbulence is shown to be out of equilibrium, and the velocity profiles depart from the log-law; However, for longer, and surprisingly for shorter forcing times, quasi-equilibrium is maintained.

In Chapter ‎3, a reduced analytical model, based on the Navier-Stokes equations, will be introduced and shown to be analogous to a damped oscillator where inertial, Coriolis, and friction forces mirror the mass, spring, and damper, respectively. When a steady buoyancy (stable or unstable) is superposed on the unsteady pressure gradient, the same model structure can be maintained, but the damping term, corresponding to friction forces and vertical coupling, needs to account for stability. However, for the reverse case with variable buoyancy flux and stability, the model needs to be extended to allow time-variable damper coefficient. These extensions of the analytical model are presented respectively in Chapters ‎4 and ‎5.

Chapter ‎6 investigates the interacting effects of baroclinicity (direction and strength) and stability on ABLs. Cold advection and positive shear increased the friction velocity, the low-level jet elevation and strength while warm advection and negative shear acted opposite. Finally, Chapter ‎7 provides a synthesis and a future outlook.