Of warm lakes and roost rings

September 26th, 2007 |

NOAA-15 AVHRR 10.8µm IR image

A NOAA-15 1-km resolution Advanced Very High Resolution Radiometer (AVHRR) 10.8µm IR image centered over Madison, Wisconsin at 11:15 UTC or 6:15 AM local time on 26 September 2007 (above) shows that several of the smaller inland lakes across southern Wisconsin were still relatively warm (IR brightness temperatures as warm as +17ºC, red enhancement) — the surrounding land areas exhibited significantly cooler IR brightness temperatures around +8ºC (yellow enhancement), in agreement with most of the 11 UTC surface temperatures at METAR sites. The cyan-colored features in the northwestern and northcentral portions of the image were patches of stratus cloud, which exhibited cloud top IR brightness temperatures around +3ºC.

The corresponding GOES-12 10.7µm IR image (below) showed similar warm signatures for a few of the larger inland lakes (but only as warm as +10 to +12ºC); however, many of the smaller lakes could not be resolved by the 4-km resolution GOES-12 IR data. Also note the patch of cold cirrus cloud over far southeastern Wisconsin (dark blue to violet enhancement): a “transverse banding” structure was evident with this cirrus feature on the AVHRR IR image (a signature of potential high-altitude turbulence), which was not as apparent on the GOES-12 IR image.

GOES-12 10.7µm IR  image

About 25 minutes after the time of the IR images above, an interesting orinthological meteorological radar signature of bird “roost rings” was observed. Large numbers of birds dispersed from their nocturnal roost sites during the early morning hours to begin feeding, and as the flock crossed the radar beam, ring-like signatures were seen on the Milwaukee/Sullivan radar composite reflectivity product (below; QuickTime animation) as the birds spread out across the area. Similar radar signatures are also occasionally seen with bat swarms.

At least two of the roost rings appear to have originated from the general vicinity of some of the larger (warmer) lakes noted on the MODIS and GOES IR imagery above: Beaver Dam Lake (located northeast of Madison, KMSN), and Lake Koshkonong (located southeast of Madison). As close as one of the larger roost rings came to Dane County Regional Airport in Madison, one also has to wonder if the high density of birds may have potentially been an aviation hazard for a brief period of time?

KMKX radar base reflectivity (Animated GIF)

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