Wednesday, 17 January 2007: 5:15 PM
Large eddy simulation of coherent structures in cold air outbreaks
214C (Henry B. Gonzalez Convention Center)
Simulations in the Cloud-Free-Path (CFP) region of a northwesterly flow cold air outbreak (CAO) over Lake Michigan show multiple scales of coherent structures that are "rooted" through various depths of the convective planetary boundary layer. At least five distinct length scales of coherent structures are identified, including a description of the manner in which convective structures self-organize and transfer their energy from the smaller 3-D scales to larger 2-D and 3-D flows. Model results are compared to the Wisconsin lidar observations of convective structures produced in the steam fog patterns in the CFP region for model runs with 10m x 10m x 10m grid resolution and 2km x 2km x 1km domain size, out to ten large eddy turnover times. These results, in general, support the findings of the previous LES model simulations of this CAO, but differences are noted. This study has also extended the analysis and interpretation of the coherent structures and their selective self-organization into larger scale flows (a representation that is different from the traditional view of thermal coalescence in convective PBLs). Three different CS are seen at the lower levels (especially within the superadiabatic surface layer) and are predominantly of 3-D planform. However, a larger 2-D structure emerges from within the surface layer, and dominates through the lower half of the mixed layer. A 3-D structure emerges from within the 2-D pattern and dominates the upper half of the mixed layer.