Not every part of the US was experiencing the wrath of winter on 26 January 2007 (see the previous  Blog post); an AWIPS image of the 1-km resolution MODIS 11.0 µm InfraRed (“IR window”) channel (above) revealed an interesting distribution of brightness temperatures that were strongly influenced by the topography of the Black Hills region of western South Dakota. Matthew Bunkers (Science and Operations Officer at the Rapid City SD National Weather Service forecast office) provided a nice high-resolution topography image (below) and offered insight toward the interpretation of the MODIS IR image. The most obvious feature evident on the IR image was the zone of darker red enhancement (brightness temperature values of 0º to +10º C, or 32º to 50º F) seen along the northern and eastern periphery of the Black Hills — this was a result of westerly/southwesterly “chinook winds” to the lee of the highest terrain, where downslope winds were warming the air adiabatically (MODIS IR image with surface METAR reports). Several hours after this 04 UTC MODIS image, the temperature at Ellsworth Air Force Base (KRCA) rose from 39º F at 09 UTC to 50º F at 10 UTC, with northwesterly winds gusting to 25 knots (29 mph) in the wake of a cold frontal passage. Another item of interest to note on the MODIS IR image is the narrow filaments of warmer brightness temperatures (red enhancement) that extended from the foothills of the Black Hills eastward across the plains — this signature indicates that the ridge tops in that area remained well-mixed (due to higher wind speeds) and stayed relatively warm compared to the adjacent river valleys (which decoupled, and experienced stronger radiational cooling). Note that the winds were still light (around 5 knots) at Rapid City Regional Airport (KRAP) and Ellsworth Air Force Base (KRCA) at 04 UTC, prior to the arrival of the stronger post-frontal winds. The daytime winds eventually gusted to 47 knots (54 mph) at the Rapid City airport, which prompted the issuance of an air pollution alert for blowing dust in Rapid City (a MODIS true color image from later that day showed that there was no snow on the ground in the Rapid City area and the eastern foothills — only the higher elevations of the northern and western Black Hills still had some residual snow pack). While the corresponding 4-km resolution GOES-12 IR imagery did show the “chinook warming” signature to the lee of the Black Hills, the smaller scale details such as the warmer ridge tops were not apparent.

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A series of Terra and Aqua MODIS “true color” images (Java animation) shows the transition from a dismal “brown Christmas” in December 2006 (above; December had an average monthly temperature of 30.5º F, or 7.5º F above normal)…to snow cover in mid-January 2007 (3-inch snow depth with overnight minimum temperatures of -3º F at Madison on 16-17 January)…to frozen lakes and 5 inches of snow cover at Madison on 25 January 2007 (below). According to the Wisconsin State Climatology Office site, Lake Mendota (the largest of the four lakes in the immediate Madison vicinity) officially froze completely on 20 January 2007 — well past the median freeze date of 20 December, but 10 days shy of the latest freeze on record (which was 30 January 1932). 20 January 2007 ended up being the second-latest Lake Mendota freeze on record.

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January normally isn’t the time of year for large-scale blowing dust events over the Sahara Desert region of northern Africa, but Meteosat-8 false color imagery (above; Java animation) did show a small plume of blowing dust in northern Chad on 18 January 2007. These images utilize Meteosat channels 1, 2, and 3 as the red, green, and blue (RGB) image components to create a false color representation. In these particular images, the plume of blowing dust (light gray color) could be seen moving southwestward across the center of the viewing area; patches of middle to upper level cloudiness farther to the north were moving eastward across the Sahara desert region. There were no surface reports in the immediate vicinity of the apparent dust plumes, but N´Djamena airport (station identifier FTTJ) located just to the south (in southern Chad) did report haze (H) at 14:00 UTC that day (below).

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An AWIPS image of the MODIS visible channel (above) shows fresh snow cover over much of the Upper Midwest region on the afternoon of 16 January 2007, following a winter storm that moved through the area 1-2 days earlier. This satellite scene is cloud-free for the most part, so the variation in visible “brightness” is due to differences in snow cover and/or density of forest vegetation (areas with a dense tree population will still aprear darker, even if there is a deep snow cover on the ground). Cooperative observer reports of new snow depth that morning (not shown; only first-order climate station data are plotted) were greatest across eastern Nebraska (5 inches), northern Iowa and southern Minnesota (8 inches), and southern Wisconsin (6 inches), while lake-effect snow had contributed to even greater snow depths farther to the north (17 inches in northern Wisconsin, and 13 inches in the Upper Peninsula of Michigan).

The swath of fresh snow cover had an important effect on the nocturnal radiational cooling of the surface, and therefore on the minimum temperatures that were reported that morning. An AWIPS image of the MODIS 11.0 µm InfraRed (IR) channel at 08:02 UTC (above) reveals significant variability in the regional “surface temperatures” that were sensed by the satellite — colder IR brightness temperatures (-30 to -40 C, -22 to -40 F) are shown by the darker blue colors on this enhancement. The coldest morning minimum temperatures that were reported by cooperative observers on 16 January included -37 C/-34 F at Embarrass, Minnesota, -32 C/-25 F at Minong, Wisconsin, -31 C/-23 F at Spencer, Iowa, and -30 C/-22 F at Wayne, Nebraska. Note how the larger lakes in northern Minnesota exhibited slightly warmer IR brightness temperatures (near -20 C/-4 F, cyan enhancement) compared to the surrounding land areas; while these lakes were frozen and snow-covered, the satellite was able to sense warmer radiation coming from the lake water below the ice surface.

An AWIPS image of the MODIS IR image at 19:09 UTC (above) shows a similar variability in the regional afternoon IR brightness temperatures. This IR image was near the time of the daily maximum temperatures — areas having little or no snow cover exhibited warmer IR brightness temperatures (0 to +5 C/32 to 41 F, red enhancement), while those locations having deeper snow cover had colder IR brightness temperatures (near -15 to -20 C/+5 to -4 F, green to cyan enhancement). The actual reported maximum air temperatures showed a similar pattern to the deepest snow cover and the colder IR temperatures; the daytime high was only -17 C/+1 F at Mankato, Minnesota, while temperatures reached a balmy +6 C/+43 F at Dickinson, North Dakota (where there was no snow cover). The warmer, unfrozen waters of Lake Superior and Lake Michigan also stand out with their warmer red enhancement (some colder lake-effect snow bands were still seen over parts of the Great Lakes). Use this handy Java image fader to interactively fade between the MODIS visible and IR images shown above (1280 x 1024 screen resolution required). Speaking of warm, unfrozen waters…due in no small part to the 13th warmest December on record, the larger lakes in the vicinity of Madison, Wisconsin were still not frozen (MODIS true color image).

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