P1.26 A study of the effect of the Great Lakes on deep convective systems

Monday, 17 August 2009
Arches/Deer Valley (Sheraton Salt Lake City Hotel)
Thomas E. Workoff, Systems Research Group, Inc. and NOAA/NCEP/WPC, College Park, MD; and D. A. R. Kristovich

The effect cooler lake surfaces have on convective systems and their ability to produce severe weather is poorly known. When spring and summer convective systems move over the Great Lakes, they commonly dissipate or weaken as their surface-based source of buoyant energy is depleted. However, numerous examples during which convective systems strengthen or maintain their intensity over the lakes have been observed. This study examines such an event, which occurred on 26 July 2005. On this date, a line of convective storms developed over eastern Michigan and maintained intensity as it crossed the northern coastline of Lake Erie. Instead of dissipating, the squall line continued to produce severe surface winds and new convective storms were initiated over the lake. This study seeks to understand the impact the relatively cool Marine Boundary Layer (MBL) over Lake Erie had on this severe squall line, as well as gain a better understanding of how various types of convective systems respond to Lake Erie MBLs.

Previous research on squall line propagation and maintenance has shown interactions between the storm's evaporatively cooled cold pool and that of lower-level ambient wind shear plays an integral part in controlling the squall line lifecycle. This interaction is governed by the balance (or imbalance) of the horizontal vorticity generated by the cold pool (negative vorticity) and ambient low-level wind shear. Surface observations from 26 July 2005 show the presence of a cooler, mesoscale airmass located over and north of Lake Erie due to the cooler water surface (~5oC) cooler than the surrounding land). Integration of the 2D horizontal vorticity equation shows that the presence of the cooler MBL introduces buoyancy gradients into the ambient environment, altering the ambient vorticity production and potentially affecting the cold pool/wind shear balance. These calculation indicate that since cold pool strength (C) is relative to the air it is interacting with, the cold pool/wind shear vorticity balance is susceptible to rapid changes of temperature in the ambient environment.

Investigation of the case of 26 July 2005 shows that the presence of the MBL creates an area of increased positive environmental vorticity to the immediate west of the Lake Erie, followed by negative vorticity creation (reducing the positive environmental vorticity) over the lake itself. The expected result of the vorticity alteration associated with Lake Erie is for potential squall line strengthening as the storm approaches the western shore of the lake, followed by the cold pool dominating, accelerating forward, and the storm becoming tilted while over the lake. This cold pool domination and acceleration is further aided by the frictional decrease associated with moving from land to water. This pattern of strengthening followed by cold pool acceleration and updraft tilting is observed in WSR-88D data from Cleveland, OH (KCLE), which shows bow echo formation and the rapid tilting of the squall line's updraft as the convection moves over the lake.

A climatology of convective interaction with Lake Erie was conducted to document convective reaction to the lake surface with varying seasons and time of day. Results of this study will also be presented.

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