Collaborative Research: Understanding Interactions between Mesoscale and Microscale Flows in the Stable Boundary Layer over Shallow Terrain

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

Atmospheric flows under unstable conditions (e.g., sunny days) are better understood than that under stable conditions which often occur at night. If the terrain is not perfectly flat or the surface is vegetated, flows become even more difficult to understand. This uncertainty means nighttime weather forecasts can lack accuracy. This is particularly true in slightly sloping topography as seen through much of the central US. This project will answer fundamental physics questions that exist for stable conditions over much of the planet including: How do plant, terrain, and elevation changes impact atmospheric flows? How do the impacted atmospheric flows interact with flows from other regions? What special flows (like down gully cold air flow; flows colliding) exist in gently sloped areas? This project will use previously collected experimental data to deduce empirical relationships defining when, where, and why these phenomena are likely to occur. This improvement of current theoretical frameworks will further understanding of nighttime pollutant transport and transformation. Public health and safety can benefit from improved quantitative prediction of transport of chemical or biological hazardous pollution. The agricultural community will benefit from increased knowledge of physical processes controlling field-scale temperature, including those affecting crop health and those leading to patches of frost and subsequent crop loss, a major challenge in ensuring global food security. Using unique observations taken during the NSF funded Stable Atmospheric Variability ANd Transport (SAVANT) campaign, this work will lead to an improved understanding of impacts of shallow complex terrain on mechanical and thermodynamic properties of the stable boundary layer and to fill the knowledge gap in scale interactions between environmental and local flows, specifically with respect to flow patterns that occur in shallow topography. SAVANT collected concurrent in-situ and remote sensing observations during two intensive months in the fall of 2018. The field setup was designed to investigate causes and effects of cold air drainage (down gully) flow in a shallow gully. Tracer plume releases tracked with multiple lidar (Light Detection and Ranging) systems offer the unique opportunity to examine interacting flows at multiple spatial scales. Two critical factors for stable-layer turbulent mixing in complex terrain, wind shear and the stable stratification, will be used to stratify observations to describe the most energetic turbulence eddies using the tower data within and above the main gully, augmented by spatial shear observations from lidars. Vertical scaling factors for turbulent eddies and the influences of drainage flows will be investigated. Factors leading to non-uniform drainage flows (i.e., pulsing, meandering, and converging flows) will be detailed. The unique combination of tower and 3-D lidar observations will allow for improved parameterizations of energy and mass exchanges, which will improve predictive model capabilities. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.