A comparison of Terra and Aqua MODIS true color images on 13 February 2008 (above) shows that thick ice had formed along the nearshore waters of southwestern Lake Michigan; cold arctic air over the region a few days earlier (the daily maximum/minimum temperatures at Chicago O’Hare on 10 February were +1ºF/-4ºF) led to an increase in thickness and areal coverage of lake ice along the coasts of Wisconsin, Illinois, and Indiana. The times of the 2 MODIS images above were about 111 minutes apart — Terra at 17:29 UTC (11:29am local time) Aqua at 19:10 UTC (1:10pm local time) — and they reveal that large segments of the lake ice were drifting eastward during that short time interval. Note the bright white appearance of the land surfaces, due to widespread deep snow cover (which ranged from about 5-6 inches in the Chicago metro area to 24 inches at Janesville in southern Wisconsin). Slightly darker portions of the visible image are due to a higher density of trees (especially in urban areas, and also along river valleys) — a very low tree density exists in the majority of this particular region due to agricultural land use in the rural areas.

Surface METAR and buoy data plotted on an AWIPS image combination of the MODIS visible channel + the MODIS Sea Surface Temperature (SST) product (below) indicated that surface winds were west-southwesterly at around 10 knots during that time, no doubt aiding the eastward drift of the lake ice; SST values in the ice-free waters of southern Lake Michigan were quite cold, ranging from 33º to 39º F (dark blue to light blue enhancement).

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The previous all-time record for total winter season snowfall in Madison, Wisconsin was 76.1 inches (set in 1978/1979); however, the winter of 2007/2008 decided that would be an easy record to break…and on the morning of 12 February 2008, the old winter season record was finally eclipsed by 1.8 inches of fluffy snowfall. Not content to stop there, the atmosphere conjured up another round of snowfall later in the day, courtesy of a potential vorticity (PV) anomaly that propagated eastward from eastern Iowa into northern Illinois and southern Wisconsin.

AWIPS images of the GOES-12 6.5µm “water vapor” channel (above) showed a warm/dry signature (darker blue enhancement) associated with the core of the PV anomaly — the “dynamic tropopause” (taken to be the pressure of the 1.5 Potential Vorticity Unit surface) was extruded downward to as low as about the 500 hPa pressure level early in the day. PV anomalies tend to induce upward vertical motions as they approach a given area, and in this case the approaching PV anomaly helped to generate another band of moderate snowfall in southcentral Wisconsin, as seen by the radar reflectivites (below) greater than 20 dBz (green enhancement) that added another 2.0 inches to Madison’s ever-growing winter season snowfall total.

An AWIPS image combination of the GOES-12 water vapor imagery (with a different color enhancement) plus the radar base reflectivity is shown below.

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Comparing AWIPS images of the MODIS visible channel, 11.0µm “IR window” channel, 3.7µm “shortwave IR” channel, and 1.6µm near-IR “cirrus channel” (above) showed how certain satellite channels are very useful in the detection of a broad area of aircraft contrails that existed over parts of Illinois, Indiana, and lower Michigan (likely resulting primarily from air traffic to/from Chicago and Detroit) on the morning of 12 February 2008. The 3.7µm shortwave IR and the 1.6µm cirrus channels offered the best depiction of the actual areal coverage of these contrails; the contrail features exhibited a slightly darker signal on the shortwave IR image (due to the smaller ice particle size of the contrails compared to the surrounding cirrus clouds), while they appeared slightly brighter in the cirrus image (since the smaller particles comprising the contrails were better scatterers than those comprising the surrounding cirrus clouds).

Examining other satellite products such as the MODIS Cloud Phase product, MODIS Cloud Top Temperature product, and the GOES sounder Cloud Top Height product (below) from that same time period confirmed that these aircraft contrails existed in an environment that consisted primarily of ice phase clouds (light red enhancement) which exhibited rather cold MODIS cloud top temperatures (-40º to -60º C, cyan to dark blue enhancement) and fairly high GOES sounder cloud top height values (30,000-39,000 feet above ground level, light blue to white enhancement).

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The Tungurahua Volcano in Ecuador began to have a series of eruptions during the first 2 weeks of February 2008 (Washington VAAC advisories). A comparison of 4 different GOES-10 Imager and Sounder products (above) shows the Imager 10.5µm “IR window”, the Imager 10.5-12.0µm “split window difference”, the Sounder 11.0-12.0µm “split window difference”, and the Sounder 7.4-13.3µm “SO2 detection product”. A volcanic ash plume was evident on both the Imager and Sounder split window difference products, moving southwestward away from the volcano at 16:31-16:45 UTC on 06 February 2008. The lack of a signal on the SO2 detection product could have been due to masking by clouds, or the fact that very little SO2 was present in that particular volcanic ash plume.

A comparison of GOES-10 split window difference and IR window images from 06:15-13:15 UTC on 06 February (above) show the improved volcanic ash detection capability of the 11-12µm technique — ash shows up as red features in the split window difference product.

An animation of the GOES-10 visible channel imagery from 06 February 2008 (above) shows the plume of volcanic ash drifting southwestward.

An animation of GOES-10 IR “split window difference” (10.5µm – 12.0µm, top panel) and IR window (10.5µm, bottom panel) imagery from (above) showed two separate pulses of volcanic ash cloud (gray enhancement) that were drifting southwestward on that day. Two days later, on 08 February (below), a new ash cloud was seen to be drifting almost due west.

Then on 10 February (below), two separate ash clouds could be seen — one drifting eastward, and one drifting westward — as changes in wind direction with height (wind shear) moved the volcanic ash plumes in different directions.

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