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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An AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) revealed a pocket of warmer SST values (72-75ºF or 22-24ºC, orange enhancement) over the mid-lake waters of southern Lake Michigan during the afternoon (around 19:41 UTC, or 2:41 PM local time) on 23 July 2007.

An animation of daily MODIS SST images (below) indicates that this particular warm water feature was not evident on afternoon MODIS SST imagery over southern Lake Michigan on 20 July or 21 July, but the water temperatures did begin to increase over that general region on 22 July.

A time series plot (below) of the air temperature and water temperature from the southern Lake Michigan Buoy 45007 (data courtesy of the National Data Buoy Center) shows that water temperatures at that location (along the northern fringe of the warm SST feature) warmed at a rate of nearly 1ºF per hour during the afternoon hours on 23 July (with a total increase of 9ºF or 5ºC in the 12-hour period from 12:00-00:00 UTC). It is interesting to note that the Buoy 45007 water temperature (blue; measured at 0.6 meter below the site elevation) warmed more quickly than the Buoy 45007 air temperature (red; measured at 4 meters above the site elevation) during the daytime hours (12 GMT to 00 GMT) on both on 22 July and 23 July. Relatively light winds (4 knots or less) and low wave heights during the morning and afternoon allowed for such a rapid warming of the lake’s “skin temperature” (a similar diurnal change in SST values over the Gulf of Mexico and the western Atlantic Ocean — as much as 3 K (5ºF) in regions of light winds — was previously reported using GOES-8/GOES-9 IR satellite data [Wu, Menzel, and Wade, 1999]). The maximum MODIS SST values of 77ºF (25ºC) seen on 23 July were the warmest observed over southern Lake Michigan during the 8-day period from 17-24 July.

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The GOES-13 satellite (launched in May 2006) has been brought out of on-orbit storage for a few weeks of testing and evaluation. A comparison of 1-kilometer resolution visible channel imagery (centered on Sioux Falls, South Dakota) from GOES-13 (above) and GOES-12 (below) demonstrates the fact that over time there is some in-orbit degradation of the GOES visible detectors. Note that the network of cities, towns and highways can be seen in the GOES-13 visible image above, especially across northwestern Iowa (in particular, Highway 60 which runs southwest to northeast: Google maps) and southwestern Minnesota — these towns and roads show up due to the contrast between the higher albedo concrete of the towns and road surfaces (and the adjacent ditches/medians) and the lower albedo of the surrounding fields of dense, mature corn crops. These features were less apparent in the GOES-12 visible image below (GOES-12 was launched in July 2001, and has been the operational GOES-East satellite since March 2003).

Part of the difference in the appearance of the scene can be explained by examining a plot of the spectral response function of the visible channels on GOES-12 and GOES-13 (below; thanks, MatG!): the sharper cutoff for wavelengths beyond 0.7µm on the GOES-13 visible channel (red line) makes it less sensitive to the signal from the mature corn crops (green line), allowing greater contrast between the thick vegetation of the agricultural fields and the more sparsely vegetated cities, towns, and highway corridors. Also of significance is the fact that Highway 60 in Iowa was in the midst of a major reconstruction project to expand it from a 2-lane highway to a 4-lane divided highway, making for a wider non-vegetated space that more easily shows up on the 1-km resolution GOES-13 visible imagery.

A 250-meter resolution “true color” RGB composite image from the polar-orbiting Terra MODIS instrument (below) shows even better detail, with the roadways, cities, and towns standing out very well against the surrounding background of dark green corn crops. Even the square grid network of minor county roads (spaced at 1 mile intervals) can be seen in the MODIS image.

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Increasing wildfire activity was noted across parts of the Great Basin region of the western US on 20 July 2007 — according to the National Fire Interagency Center, there were 43 large fires burning in the states of Idaho and Nevada on that day (NOAA HMS fire and smoke product). AWIPS images of the GOES and MODIS 3.9µm/3.7µm IR channels (above; top 2 image panels) and the 10.7µm/11.0µm (“IR window”) channels (above; bottom 2 image panels) showed a cluster of active fires straddling the Idaho/Nevada border around 05:30 UTC. The 4-km resolution GOES 3.9µm IR image (upper left panel) showed 3 separate areas of somewhat warm “fire hot spots” (dark grey to black pixels), while the 1-km resolution MODIS 3.7µm IR image (upper right panel) revealed a larger number of more intense “fire hot spots” (black to yellow to red pixels).

Since the areal coverage and intensity of the GOES hot spots was diminishing during the nighttime hours, the GOES-derived Wildfire ABBA product (below) did not indicate any fire activity along the Idaho/Nevada border region around 05:30 UTC; therefore, the 1-km resolution MODIS 3.7µm IR imagery shown above would have been the only satellite-based diagnostic of fire activity at that particular time.

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