The color-enhanced Channel 4 GOES-12 IR image above shows a crescent-shaped region of cooler cloud tops over the upper midwestern part of the United States, stretching from extreme northeastern Kansas northeastward to eastern Lake Superior. (The blue enhancement shows cloud-top brightness temperatures around 250 K whereas the isolated pixels of yellow enhancement over central Wisconsin correspond to temperatures around 220 K). Such clouds are called baroclinic leafs and they are associated with temperature gradients and jets in the troposphere and herald the beginning stages of cyclogenesis. If cyclogenesis were to proceed, the leaf would become more S-shaped before developing into a comma cloud. (This link is a large satellite loop showing the evolution from baroclinic leaf in the central Pacific to occluded cyclone off the west coast of Canada). Note the sharp western edge of the leaf. To the east of that edge, moist air has risen from the lower troposphere, cooling and saturating (and producing precipitation) as it rises. Air may be rising west of the edge as well, but origins of the air to the west are much higher in the atmosphere, where moisture is limited; limited moisture and restricted upward motion prevents airmass saturation.

The steady rain that fell over southern Wisconsin underneath this leaf was accompanied by surface temperatures in the low- to mid-50s, nearly 20 degrees below normal. The steady upward motion allowed rain to persist for more than 8 hours, as is typical in the development of extratropical cyclones whose signature in the infrared imagery begins as the leaf shown in this image.

Baroclinic leafs develop in regions of enhanced temperature gradients. Thus, they are an uncommon feature over the United States in summer when temperatures over the United States show little north-south contrast. Expect to see more leafs over the United States in the next 7 months.

Update: The linked images from 10 September 2007 at 1225 UTC and 1730 UTC show the region of baroclinicity (the enhanced horizontal temperature gradient) associated with the leaf. Note the big changes in temperature along the 315K isentropic surface plotted from NAM model output. Temperature is equivalent to pressure on an isentropic surface; the isentropic surface will be very strongly sloped in the region under the leaf, and motion perpendicular to the front will be strongly upward.

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The poleward surge of elevated Total Precipitable Water (TPW) values (50-65 mm, orange to red enhancement) associated with Tropical Storm Gabrielle could be clearly seen on an animation of MIMIC TPW covering the 48-hour period from 09 to 10 September 2007 (above). Note that toward the end of the animation on 10 September, a plume of dry continental air (TPW values of 30-40 mm, blue to cyan enhancement) was entrained into the circulation of downgraded Tropical Depression Gabrielle as the cyclone moved away from the US East Coast.

As Gabrielle began to move northeastward across the Atlantic Ocean, it traveled along the north wall of the Gulf Stream — the warm waters in that region likely helped to prolong Gabrielle’s duration at tropical depression intensity. An 8-day composite of MODIS Sea Surface Temperature (SST) data (below) revealed that water temperatures along the axis of the Gulf Stream were still as warm as 83º F (28ºC) during early September.

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The IDEA MODIS Aerosol Optical Depth (AOD) product (above) indicated that the “dry air plume” moving eastward off the US coast also contained elevated levels of particle pollution, which was supported by the large number of surface AirNOW sites (below) from Georgia to North Carolina that reported “Moderate” concentrations of particle pollution (51-100 µg per cubic meter, yellow circles).

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Tropical Storm Gabrielle made landfall over North Carolina early in the day on 09 September 2007. GOES-12 Rapid Scan Operation (RSO) visible channel imagery at 5-10 minute intervals (above) shows the low-level circulation of Gabrielle emerging from beneath a quasi-stationary cluster of convection and then moving slowly north/northeastward across eastern North Carolina during the afternoon hours. GOES-12 10.7µm IR imagery (below) depicts cold cloud top brightness temperatures of -60º to -79º C (red to black to white enhancement) associated with the convective cluster — these cold cloud tops did manage to move slightly inland across the Morehead City NC region (located near the center of the IR images) after about 15 UTC (11 am local time), and the resulting rainfall amounts along the coast were as high as 8.60 inches at Harlowe.

An AWIPS image of the 1-km resolution MODIS 11.0µm IR channel (below) shows better detail in the cloud top temperature structure compared to the 4-km resolution GOES-12 IR imagery above. An overlay of cloud to ground lightning strikes indicates that lightning activity was fairly minimal with this cluster of thunderstorms.

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Gabrielle was declared a Subtropical Storm by the National Hurricane Center late in the day on 07 September 2007, and reached Tropical Storm status in the western Atlantic Ocean on 08 September. While some convection was associated with Gabrielle, GOES-12 IR imagery from the CIMSS Tropical Cyclones site (above) indicated that the convection was located well to the northwest of the circulation center; GOES-12 visible imagery (below) showed that the circulation center of Gabrielle remained exposed during much of the day.

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