A cold frontal boundary was moving southeastward through the Great Lakes region on 02 August 2007; Terra MODIS true color imagery (above) showed a very hazy air mass along and ahead of the advancing frontal boundary, with a very clean air mass behind the front. The dew point temperature at Madison, Wisconsin dropped from 70º F to 42º F in the 24-hour period following the frontal passage. Also evident on the MODIS image is a thin light-colored streak (oriented southwest-to-northeast) across northeastern Wisconsin, which was the damage path from a long-track tornado that occurred nearly 2 months earlier on 07 June 2007.

The IDEA MODIS aerosol optical depth (AOD) product several days earlier (below) revealed a high AOD signal (orange to red enhancement) over the northern Rocky Mountains region, due to thick smoke from numerous wildfires that were burning in Idaho and western Montana. Much of this smoke was subsequently transported eastward along the cold frontal boundary (MODIS AOD image Java animation).

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Our recent fascination with the MODIS Sea Surface Temperature (SST) product continues, with AWIPS imagery of the MODIS SST that revealed an intricate pattern of cold water eddies across the southern portion of Hudson Bay in Canada on 30 July 2007 (above; upper left panel). The 2 MODIS image sets are about 90 minutes apart, and the image animation indicates that these cold water eddies were moving rather rapidly westward during that short time interval — this water feature motion was in the opposite direction of the boundary layer winds, which were light westerly to northwesterly around the northern periphery of a surface anticyclone that was centered over northern Ontario.

There were patches of low-level cloudiness over the water in the eastern and western portions of the satellite scene — these clouds showed up as darker (warmer) features on the MODIS 3.7µm IR images (above; lower right panel) due to the solar radiation reflected off the tops of these water droplet clouds. If you look closely, you can also see several small white “specks” in the water near the middle of the MODIS visible images (above; lower left panel) which were also moving westward — these were small ice floes that were floating in the still-cold waters of Hudson Bay (the coldest SST values seen in that area on this day were around 37º F or +3º C, dark blue enhancement). The ice floes did not exhibit a darker signal on the MODIS 3.7µm IR image, since the component of solar radiation reflected off ice surfaces is minimal.

These small ice floes were more clearly depicted in false-color composite images using the 250-meter resolution visible channels 1 and 2 from the Terra abd Aqua MODIS instruments (above). Hudson Bay retained significant ice cover well into the month of June 2007 (MODIS true color image | MODIS false color image: ice features have a red enhancement), and a good deal of ice was still present as recently as early July 2007 (MODIS true color image | MODIS false color image: ice features have a red enhancement).

A comparison of GOES-13 and GOES-12 visible channel images (Java animation, below) better showed the motion of these ice floes during the 6-hour period from 14:02-20:15 UTC. Note the improved image navigation and registration (INR) evident with the GOES-13 satellite: the coastline and island features remain fairly steady from image to image, in contrast with the GOES-12 images which exhibit a notable amount of “wobble” in the animation. Using McIDAS, we tracked the speed of 2 different ice floe features on the GOES-13 imagery: one of the fastest-moving ice floes was seen to have a speed of about 1.8 kilometers per hour (1 knot), while most of the other ice floes seemed to be moving more slowly at a speed of around 0.5 kilometers per hour (0.3 knots). Using the AWIPS “Distance Speed” tool, the speed of displacement of the ice floes and cold water eddies between the 2 MODIS images was found to be about 3 kilometers per hour (2 knots).

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The low-lying areas of the Wisconsin River and other tributaries that drain into the upper Mississippi River are favored areas for nocturnal valley fog formation, as shown by the MODIS “fog/stratus product” and topography image comparison (above).

AWIPS images of the MODIS and GOES-12 fog/stratus product from 29 July 2007 (below) demonstrated important differences in the detection of these narrow fingers of river valley fog (yellow to orange enhancement) that were forming during the nighttime hours over parts of southwestern Wisconsin, southeastern Minnesota, and northeastern Iowa. The 1-km resolution MODIS fog/stratus product at 07:50 UTC (below, left) was able to give a more precise indication of the areas where fog was beginning to form, while the corresponding 4-km resolution GOES-12 fog/stratus product at 08:01 UTC (below, right) could only provide a vague signal that fog was starting to develop over portions of the region.

The fog continued to increase in depth and areal coverage later that night, with surface visibility eventually dropping to 0.15 miles at Wisconsin sites Lone Rock KLNR and Boscobel KOVS (0.01 inch of precipitation was also recorded at each location as mist developed) — and as the fog thickened, the GOES-12 fog/stratus product did begin to exhibit a better signal of the fog structure (especially over the lower Wisconsin River valley: Java animation).

A comparison of the AWIPS GOES-12 fog/stratus product and GOES-12 visible channel image at 13:15 UTC (below) shows that much of the river valley fog was still present after sunrise; however, note that after sunrise the fog features on the GOES-12 fog/stratus product began to change in appearance from yellow or orange enhanced features (11-3.9µm brightness temperature difference values around -7º to -8º C) to darker gray enhanced features (11-3.9µm brightness temperature difference values of around -4º to -5º C) — this is due to the fact that the 3.9µm channel is very sensitive to solar radiation reflected off the tops of water droplet cloud features (such as fog), which increases the brightness temperature value sensed by the 3.9µm detectors (and therefore decreases the 11-3.9µm brightness temperature difference that constitutes the fog/stratus product). Because of the solar reflection issue, the fog/stratus product is not valid during daytime hours.

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An mesoscale phenomenon that sometimes emerges out of Mesoscale Convective Systems (MCS) is the Mesoscale Convective Vortex (MCV). Intense latent heating within the rain core of an MCS can help spin up a vortex that will occasionally live on even as the MCS that spawned it withers away. The spin-up can be visualized as a potential vorticity response to latent heating in mid levels that increases static stability and therefore increases the potential vorticity, inducing spin. The spin development can also be viewed in terms of changes in height via the Quasi-geostrophic height tendency equation: latent heat above causes height falls below and the development of cyclonic spin.

Atmospheres that support the development of MCVs have things in common. Abundant moisture and low stability are important. It’s also common to have low values of vertical wind shear; that is, the wind profile is fairly uniform. The degree of uniformity together with the amount of moisture and instability help determine if the MCV will be sustained. The key to persistence is ongoing warming through latent heat release at mid levels.

On 24 July, a large MCS over the northern Plains spawned an MCV that moved eastward and southward into Minnesota. The loop is above. Focus on the large cloud mass over the Dakotas that moves towards central Minnesota. The cyclonic spin of the MCV is subtle but present.

This MCV traversed a region of plentiful moisture, as shown by surface dewpoint plots at 15z and at 21z. Dewpoint plots at 850 hPa also show plenty of moisture over the upper midwest, and plots of 700-400 hPa wind speed shearshow a region of small shear over the upper midwest. You can also check out Chanhassen’s upper air sounding at 12z on the 24th and 00z on the 25th.

What do the conventional data show? Abundant moisture in a region of small vertical shear. That is precisely the kind of atmosphere that supports MCVs.

Added: A visible loop that more clearly shows the cyclonic spin of the MCV is available here. (Caution: this is a 12 megabyte animated gif)

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