Lake-Aggregate Mesoscale Disturbances. Part III: Description of a Mesoscale Aggregate Vortex

Abstract

The structure of a meso-α-scale vortex that developed over the Great Lakes in late autumn is described. Understanding the structure of this particular vortex is important because 1) it altered local- and regional-scale precipitation, 2) it developed in a complicated fashion, and 3) it represents a class of vortices that develop frequently during cold-air outbreaks over the Great Lakes and whose development is not yet understood. Detailed perturbation analyses of the synoptic-scale and mesoscale conditions over the Great Lakes that led to the vortex development are presented using model output from previous numerical simulations that included all of the Great Lakes (WL) and none of the Great Lakes (NL). The initial thermal perturbation was characterized by an elongated warm plume near the surface that advanced southeastward toward the mid-Atlantic coast as cold air overspread the entire lakes region. The plume then became more circular and deepened as it rotated toward the northeast in response to the changing synoptic-scale flow. The perturbation winds revealed several small meso-?-scale circulations that developed during the first 36 h within the warm plume. By 48 h, a 3-km-deep meso-α-scale vortex developed several hundred kilometers to the northeast of Lake Huron with cyclonic flow in the lower half and anticyclonic flow in the upper half. Its size, location, and evolution indicate that it was likely generated by aggregate-lake as opposed to individual-lake heating and moistening. It is therefore referred to as a mesoscale aggregate vortex (MAV). The MAV that developed represents a class of vortices that can be described as inertially stable, meso-α-scale warm-core vortices approximately 500?1000 km wide and 2?4-km deep that develop from aggregate heating and moistening over the Great Lakes. They are usually identifiable on standard surface weather charts as a weak low over the Great Lakes region with 1?3 closed isobars at 2-mb intervals. The outermost closed isobar typically encloses an area approximately as large as that spanned by the upper Great Lakes (e.g., Lakes Superior, Huron, and Michigan). Because the MAV in this study developed nearly 36 h after the coldest air left the region, it is not clear whether other physical mechanisms besides sensible and latent heating were involved, to what extent they were important, and at what stages they occurred. Additionally, the MAV developed within a large elliptical region of lake-aggregate heated air, which suggests the importance of geostrophic adjustment, and from smaller individual-lake scale circulations, which suggests the importance of vortex merger. Development mechanisms will be discussed in a follow-up study.