Seminar 8 Sept 2003

Jan Kleissl, doctoral candidate, Center for Environmental and Applied Fluid Mechanics, Dept. of Geography and Environmental Engineering, Johns-Hopkins University

Turbulent transport in the atmospheric boundary layer: Lidar measurements and Large Eddy Simulation

Turbulent transport in the atmospheric boundary layer (ABL) plays a crucial role in emission and deposition of aerosols. Lidar (Light Detection and Ranging) and LES (Large Eddy Simulation) research is presented to elucidate special and general features of turbulent transport in the ABL. A lidar was operated daily during summer 2002 and measured vertical profiles of aerosol backscatter over Baltimore. During a Canadian forest fire smoke episode in Baltimore on July 7, 2002 downward mixing with severe effects on surface PM2.5 concentration could be observed in great detail.

Large Eddy Simulation (LES) has become an important simulation tool for turbulent flows in the environment. In 3d LES of the atmospheric boundary layer the domain size is on the order of kilometers, while the horizontal grid spacing is on the order of tens of meters. LES separates the scales of motion in resolved and subgrid-scales (SGS). Improving our knowledge about SGS physics and the development of SGS models are essential for accurate predictions of integrated effects of turbulence features, such as transport of momentum and scalars in the atmosphere. An experimental study with forteen 3D sonic anemometers (HATS - Horizontal Array Turbulence Study) is used to determine SGS model parameters, such as the Smagorinsky coefficient, under various flow conditions. Results indicate that the Smagorinsky coefficient is reduced near the ground and in stable atmospheric conditions. Furthermore, the ability of new generation dynamic SGS models to capture the behavior of the coefficient is tested with the field data.