Coherent structures and transitional patterns in convective boundary layers

Field observations during Lake-ICE in January 1998 have shown further evidence of unique convective structures within PBLs during Cold-Air Outbreaks (CAOs) over Lake Michigan. These are suggestive of the ability of either buoyancy or mechanical shear to dominate (over the other) to form regular patterns of 2-d (rolls) or 3-d (cells), on at least different scales, from hundreds of meters to a few kilometers. Large Eddy Simulation (LES) modeling is also used to show the modulating effect of snow precipitation on a given initial convective pattern (or geometry). Evidence is also presented to show a similar effect over land, where the onset of snow showers in a pre-existing pattern of convective rolls, causes a transition to a regular pattern of open mesoscale cellular convection.