I have always enjoyed nature and in particular, I love watching the sky, there is so much to see and I always want to know "why" things happen.
By: Tom Niziol , 6:11 PM GMT on February 01, 2013
Recently there were some very interesting features spinning away on the Great Lakes as seen by the GEOS-East satellite. Although somewhat masked by higher thin, cirrus clouds, you can still make out the circulations or "spins" going on across three of the bodies of water which make up the Eastern Great Lakes. In Figure 1, over Georgian Bay (northernmost) you can see a rather large meso-vortex spinning away in the cloud field as it moves south along the Bay. On Lake Erie and Ontario there are smaller scale vortices which are barely visible under the higher cirrus clouds.
Figure 1: GOES-East visible satellite loop of mesoscale vortices across the eastern Great Lakes from 1301 UTC through 1931 UTC January 26, 2013. You can plainly see the spin in the clouds over Georgian Bay to the north and smaller spins which are visible at times through the high cirrus clouds on eastern Lake Erie as well as western Lake Ontario. (images courtesy College of DuPage NEXLAB)
Mesoscale vortices occur on the Great Lakes when prevailing winds are very weak and the air-lake temperature difference is significant. This allows for the development of local land breezes that move from the cold shores toward the middle of the warmer lakes. Where the lakes are more "bowl-shaped" there is better potential to develop the vortices, but they can spin up anywhere on the lakes if the mesoscale (or small scale) wind field sets up properly. On the morning of January 26th, 2013 we had very weak flow over the region as you can see by the surface map in Figure 2. High pressure extended to the west of the Great Lakes. There was a "thermally-induced" trough across the Great Lakes on an axis from Lake Ontario back through Georgian Bay which was a result of the warmth being emitted from the aggregate of all of the lakes. As a result you see a weakness in the pressure field that extends across Lakes Ontario, Erie and Georgian Bay and I am certain that if you did a very detailed surface analysis you could find a low center over each of those lakes in the vicinity of the mesoscale vortices.
Figure 2: Mean Sea Level Pressure and temperature at 1500 UTC January 26, 2013. Notice the broad, strong High pressure to the west of the Great Lakes. A "thermal trough" extends on a northwest-southeast axis across the eastern Great Lakes. As a result, we end up with weak pressure fields over the areas where the mesoscale vortices spun up. (image courtesy College of DuPage NEXLAB)
The overall surface pressure pattern was conducive to the development of the circulations on the lakes. The WSR-88D radar in Buffalo, NY was close enough to sample the Lake Ontario vortex. Figure 3(top) shows the reflectivity associated with the vortex. Figure 3 (bottom) shows the base velocity. If you look closely you can see the telltale signs of rotation in the radial velocity, i.e. a wind component moving away from the radar right next to one moving toward the radar. The arrows show the signal and in fact, there is a second rotation that I encircled within this band just to the right of the first.
Figure 3: WSR-88D radar images at 1415 UTC January 26, 2013. (Top) 0.5 degree reflectivity showing pattern of the circulation on the west end of Lake Ontario. (Bottom) 0.5 degree base velocity showing rotational couplets over the west end of the lake. The arrows show the base velocity along the radar radial either toward (green) or away (purple) from the radar. This is a classic signature that shows rotation at those points. (images courtesy College of DuPage NEXLAB)
Mesoscale vortices are not that uncommon on the Great Lakes. In fact, I wrote about my first experience with one of these features way back in 1984 in the magazine "Weatherwise". One of my Weather Channel colleagues, Dr. Greg Forbes, co-authored a manuscript on this topic as well. In that research he found 14 occurrences of mesoscale vortices on the Great Lakes in just a 4-year period. The vortices generally do not produce much in the way of snowfall since they occur in light wind situations often associated with arctic High pressure. In theses situations there is usually a strong low-level subsidence inversion present that limits convective cloud growth. However, in some cases they can be part of mesoscale lake bands that produce a lot of snow. One such case occurred back on January 7-9, 2011 on Lake Michigan when a mesoscale vortex slid southward down the long axis of the lake along an existing narrow band of snow that had been pummeling the South Bend area (Figure 4a). That band, not necessarily the mesovortex, produced a storm total over 3 feet of snow near South Bend IN. Just 3 years earlier I found a nearly carbon copy of the 2011 event. This one was from February 20, 2008 but did not produce anywhere near the snowfall the 2011 event did. So, if you want to see one of these features, keep your eye on satellite imagery, look for days when an arctic High has built down across (or near) the Great Lakes, you want a weak wind field and significant air-lake temperature differences. If you are lucky you will get to see a different "spin" on lake-effect snow, enjoy !!
Figure 4: Other examples of mesoscale vortices. (a) January 8, 2011 (courtesy NASA EOSDIS) and (b) February 20, 2008 (courtesy College of DuPage NEXLAB).
As always, if you have any questions or comments you can send them to me at email@example.com
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