Simulation of Mountain Forced Turbulence and Boundary Layer Interactions

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This research involves a numerical modeling study centered on understanding how mountains force turbulence through internal wave breaking, wake effects, and interaction with the atmospheric boundary layer. The central goal is to determine the role of turbulence in terrain forced flows with an emphasis on (i) internal wave breakdown, (ii) boundary layer interaction, and (iii) the direct, local forcing of turbulent eddies by terrain features. Research is planned that expands on previous efforts and focuses on the role of the upstream boundary layer and formation of turbulence associated with mountain-forced flows. The main objective of this study is to better understand how turbulence and boundary layer dynamics modify terrain-induced circulations. The Principal Investigator (PI) will simulate flow over an idealized two-dimensional mountain for a range of mountain scales, flow conditions, and bottom boundary conditions. Specific objectives are to: 1) Determine the role of upstream boundary layer depth on mountain wave structure; 2) Examine how resolved turbulence at the wave reflection layer interacts with mountain wave dynamics; 3) Assess the role of surface convective heating and drag on mountain wave characteristics; 4) Examine the role of turbulence in the formation of lee side wind storms and the effects of steep terrain on flow separation and rotor formation; 5) Evaluate parameterizations of mountain wave drag.Objectives will be achieved by applying a large-eddy simulation model that can simulate both terrain features and a fully turbulent boundary layer. Simulations will be conducted for a range of flow conditions that include low level wave breaking, critical layers, rotor formation, and the effects of surface roughness and surface heating. Results from this study will aid in the development of improved mountain drag parameterizations by providing quantitative relationships between momentum flux and flow parameters (e.g. Froude numbers), surface boundary forcing, and boundary layer depth. Intellectual Merit: The intellectual merit of this research is based on the significant impact mountain-forced turbulence has in aviation, weather forecasting, and accurate prediction of the atmospheric general circulation. The research should lead to an improved understanding of how mountains force turbulence throughout the troposphere and how surface boundary layer forcing affects mountain-induced internal waves and turbulence. Broader Impacts: This study has potential for broad impacts by improving predictive capability for mountain forced windstorms and local turbulence that is of importance for aircraft operations. On longer time scales, understanding how mountains force mixing and transport momentum will lead to better representation of mountain dynamics in climate prediction models. Participation by a graduate student for the duration of the project is planned and results will be published in recognized peer-reviewed journals. Elements of this research will be incorporated into the graduate curriculum as well as in public learning efforts.