INTERPRETATION
DISCUSSION
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Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University Fort Collins, Colorado |
December 14 , 2005
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Mesoscale vortices across the Great Lakes during the winter are observed as arctic air is warmed from below by the relatively warm water. Forbes and Merritt (1984) discuss lake-effect vortices over the western Great Lakes, where they are more commonly observed relative to the eastern Great Lakes. Laird (1999) investigated the existence of coexisting mesoscale lake-effect vortices over the western Great Lakes. This discussion will focus on a rare occurence over the eastern Great Lakes that occured on December 13, 2005.
The synoptic scale conditions favorable for wintertime mesoscale lake-effect vortices include 1) large difference between the lake and low-level air temperature, 2) convergence over the lake, 3) unstable environment at low-levels and 4) weak surface flow and horizontal pressure gradients (Forbes and Merritt 1984; Hjemfelt 1990). These conditions existed on December 13, 2005. Great Lakes water temperatures were in the 4 to 10 degrees Celsius range over the eastern Great Lakes. The 12:00 UTC surface analysis shows a 1028 mb high just northeast of the Great Lakes leading to relatively weak surface winds and horizontal pressure gradients. Also, note the convergence over the lakes caused by the land breeze circulation.
GLERL Lake Surface Temperatures |
1200 UTC Surface Analysis |
An inspection of the 12:00 UTC sounding from Alpena, MI shows an unstable environment at low-levels. The difference between the 850 mb and lake surface temperature was approximately 22 degrees Celsius. This is much greater than 13 degrees which is considered the minimum to initiate lake-effect snow (Holroyd, 1971) and corresponds to a dry adiabatic lapse rate.
1200 UTC Alpena, MI Sounding |
The GOES-12 visible satellite loop shows simultaneous mesoscale lake-effect vortices over Lakes Huron, Erie and Ontario. Over Lakes Erie and Ontario, convergence boundaries caused by the land breeze circulation develop vortices. Over Lake Huron multiple convergence boundaries exist.
Visible Loop |
Location of vortices and lake names |
The 10.7 um IR imagery views this event from earlier on (06:15 UTC) through the daytime. The vortex over Lake Huron is readily observed at night. We also observe the convergence boundaries form over the south shoreline of Lakes Erie and Ontario then advect northward before developing vortices. The MODIS visible image from 18:10 UTC provides us with a higher resolution (0.5 km) compared to the 1 km visible GOES imagery shown above. This is a true color image which allows us to see additional details such as sediments and ice cover over the lakes.
10.7 um Infrared Loop |
1810 UTC MODIS Imagery |
References:
Forbes, G.S., and J.M. Merritt, 1984: Mesoscale vortices over the Great Lakes in wintertime. Mon. Wea. Rev., 112, 377-381.
Hjemfelt, M.R., 1990: Numerical study of the influence of environmental conditions on lake-effect snowstorms on Lake Michigan. Mon. Wea. Rev., 118, 138-150.
Holroyd, E.W., III, 1971: Lake effect cloud bands as seen from weather satellites. J. Atmos. Sci., 28, 1165-1170.
Laird, N.F., 1999: Observation of coexisting mesoscale lake-effect vortices over the Western Great Lakes. Mon. Wea. Rev., 127, 1137-1141.