Recorded presentationJ8.6
The Evolution of a Convective Squall Line as it Crossed the Upwind Coast of Lake Erie
Thomas E. Workoff, University of Illinois, Urbana, IL; and D. A. R. Kristovich
The effect cooler lake surfaces have on convective squall lines 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 presentation will focus on such an event 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 high 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 mature, severe squall line.
Surface observations at the time of squall line initiation show environmental temperatures upwind of Lake Erie around 32oC, with a cooler lake surface of 27oC. Regional surface temperature analyses suggest that a relatively cool, stable internal boundary layer (SIBL) was generated over Lake Erie as a result of the warm ambient air flowing out over the cooler lake surface.
Although the SIBL was negatively buoyant, model output from the Global Forecasting System 40km grid (GFS40) shows that the atmosphere still possessed ≈1600J/Kg of convective available potential energy (CAPE) despite the presence of the SIBL (though this was a notable decrease from the ambient environment which had ≈2200-2600J/Kg). This suggests that even though the SIBL decreased the buoyancy of the atmosphere, the atmosphere still had positive potential energy, possibly explaining why the convection did not deteriorate. It is also possible that the convection became elevated atop the SIBL, freely accessing the buoyant air above it.
Another potential factor influencing the squall line is the profile (buoyancy and wind shear) of the ambient environment. A squall line theory originally proposed by Rotunno, Klemp and Weisman (RKW, J. Atmos. Sci., 1988) concludes that squall line intensity is related to the local vorticity balance between the negative horizontal vorticity produced (due to negative buoyancy) at the leading edge of the storm's evaporatively cooled cold pool and the encountered positive horizontal vorticity produced by the ambient environment. While RKW focuses on the ambient environmental vorticity created by wind shear (they assume negligible ambient buoyancy), the case of a SIBL introduces buoyancy-created ambient vorticity and therefore potentially alters the local vorticity balance.
In order to determine the likely ambient vorticity field encountered by the squall line cold pool, SIBL depth was estimated using methods tested in recent publications. SIBL depth, in turn, can be used in the integration of the 2D (inviscid, Boussineq) vorticity tendency equation to estimate the field of vorticity tendency over the lake. These calculations indicate the presence of a narrow, intense area of positive vorticity tendency on the northern shore of Lake Erie, which would lead to enhanced convergence and potential squall line strengthening. Over the rest of the lake, calculations indicate widespread negative vorticity tendency which would potentially allow the cold pool to dominate near the surface and accelerate forward. Cold pool acceleration is also shown to be enhanced by the decrease of friction associated with the movement of the storm from land to water. This pattern of strengthening followed by cold pool acceleration is consistent with WSR-88D data from Cleveland, OH (KCLE). Also, these radar data indicate midlevel rear-to-front flow (rear-inflow-jet, RIJ) descent corresponding with the outflow acceleration, which could lead to high surface winds over and downwind of Lake Erie.
Joint Session 8, Coastal Mesoscale Circulations-I
Wednesday, 14 January 2009, 8:30 AM-10:00 AM, Room 128B
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