Coherent structures in cold air outbreaks over Lake Michigan during lake-ice
Abstract
The Lake-Induced Convection Experiment has provided special field data during a westerly flow cold air outbreak on 13 January 1998, which has afforded the opportunity to examine an evolving convective boundary layer in unprecedented detail. Vertical cross-sections prepared from these data, extending from upstream over Wisconsin across Lake Michigan, show the modifying effects of land-water contrast on boundary layer mixing, entrainment, heating and moisture flux. Through this analysis, an unusually interesting case of lake-effect air mass modification was discovered. The data show distinctly different heights in vertical mixing of heat and moisture, as well as off-shore downwelling and subsidence effects in the atmosphere. Analysis shows evidence of a new observational feature, the moisture internal boundary layer (MIBL) that accords well with the often recognized thermal internal boundary layer (TIBL). The “interfacial” layer over the lake is also found to be unusually thick and moist, due in part to the upstream conditions over Wisconsin, as well as the effectiveness of vertical mixing of moist plumes over the lake (also seen in the turbulence statistics). This suggests that the atmosphere can be more effective in the vertical mixing of moisture than heat. Four scales of coherent structures (CSs) with differing spatial and temporal dimensions have been identified. The CSs grow in a building block fashion with buoyancy as the dominating physical mechanism for organizing the convection (even in the presence of shear). Characteristic turbulence statistics show evidence of these multiple scales of CSs, ranging from the smallest (microscale) in the cloud free path region near the Wisconsin shore, to the largest (mesoscale) in the snow-filled boundary layer near the Michigan shore. A Large Eddy Simulation (LES) model has been deployed to study the effects of buoyancy and shear on the convective structures in lake-effect boundary layers. The model simulations have been divided into a two-part study: (1) the general relationship of surface heat flux versus wind shear to study the convective geometry, and (2) specific simulations of convection analogous to what is seen in the CFP region for the 13 January 1998 event. Model simulations will also address the existence of multiple scales of CSs.
Degree
Ph.D.
Advisors
Agee, Purdue University.
Subject Area
Atmosphere
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