Wednesday, 12 February 2003: 3:45 PM
Westerly flow cold air outbreak over Lake Michigan during Lake-ICE
The Lake-Induced Convection Experiment (Lake-ICE) has provided special field data that capture interesting boundary layer structures during a westerly flow cold air outbreak (CAO) on 13 January 1998. Vertical cross-sections prepared from these data, extending from upstream over Wisconsin out across Lake Michigan, show the modifying effects of land-water contrast on boundary layer mixing, entrainment, heating and moisture flux. The cross-sections show lake heating up to 9ºC of warming near the Michigan shoreline, and 0.7 gm/kgm of lake moistening. Both land soundings and aircraft flight legs over the lake, along with lidar observations, have been used to show distinctly different heights in vertical mixing of heat and moisture, as well as off-shore down-welling and subsidence effects in the atmosphere. Cross-section analysis shows evidence of a new 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 and the effectiveness of vertical mixing of moist plumes over the lake.
Characteristic turbulence statistics for select aircraft flight legs have also been calculated and analyzed to show evidence of multiple scales of coherent structures, 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. Where possible, these field observations and boundary layer features will be compared to results from Large Eddy Simulations of CAOs over Lake Michigan.