Modulation of the Bulk Structure and Second-Order Statistical Properties of Ekman Layers by Buoyancy

Abstract

Parameterization of turbulence in the atmospheric boundary layers (ABL), where buoyant forces enhance or destroy turbulent kinetic energy (TKE), remains a challenging and very important problem in geophysical fluid dynamics. In order to understand the difficulties associated with understanding and modeling ABL turbulence, especially under stable conditions, this dissertation presents an in-depth analysis of changes in the bulk structure and second order statistical properties due to buoyancy, using new-generation numerical techniques for solving the fundamental equations that include direct and large-eddy simulations.

Using direct numerical simulations, changes in the mean profiles and second-order statistics due to variation in imposed surface temperature are examined. The mechanisms through which buoyancy affects turbulence and the flow are identified as primarily being related to the damping of turbulence production, rather than to direct destruction by buoyancy. Budget analyses that are crucial for researchers aiming to understand stably-stratified shear flows, and to develop higher-order closure models are performed. The results obtained shed light on what terms in the budgets are negligible and how to model the remaining important terms. Very strong stability is shown to result in global intermittency in the turbulence fields. Although this intermittency lacks simple mechanistic explanation, a method for parameterization of the duration over which these intermittent patches are seen is proposed using simple dynamical systems analysis.

Subsequently, large-eddy simulations are used to explore buoyancy modulation of large scale structures that carry TKE and fluxes in the ABL. The largest eddies are found to consist of streamwise rolls with similarities to the very large scale structures reported in wind tunnel studies. These rolls however are strongly modulated by buoyancy: they are intensified under unstable conditions and weakened, sometimes completely damped, by stable conditions. Finally, simulations with heterogeneities in surface properties affecting the real world ABL dynamics are carried out with a special emphasis on the implications for the 1.5-order turbulence closures in coarse atmospheric models. Advection is shown to become a critical term in the turbulence kinetic energy budget, and unexpected impacts on the bulk flow are identified.