Baroclinic Leaf in the Midwest
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.
