GOES-11 visible channel imagery (above) showed a nice example of lee wave “lenticular clouds” immediately downwind of the Spring Mountains (whose highest peak is Mt. Charleston at 11,918 ft or 3362 m) just to the northwest of Las Vegas, Nevada (Google maps) on 14 September 2007. The vertical motions associated with lee waves can cause moderate to severe turbulence which is a hazard to aviation, but on this particular day there were no pilot reports of turbulence noted in the immediate vicinity of the lenticular cloud formations (McCarran International Airport is located about 20 miles or 50 km southeast of the Spring Mountain range, and no lenticular clouds were seen on the satellite imagery directly over the Las Vegas metropolitan area or the airport itself).

Such lee waves are generated when strong atmospheric flow encounters a barrier to the flow — in this case, the axis of a strong southwesterly jet stream was located over the region, as indicated by an AWIPS image of RUC80 model 250 hPa wind fields overlaid on GOES-11 water vapor channel imagery (below). The lenticular clouds associated with this lee wave were seen to dissipate later in the day as the strongest jet stream winds propagated northeastward away from the region.

In order for these types of stationary lee wave clouds to form and be maintained, there often needs to be a stable layer located at altitudes above the top of the terrain obstruction. In this case, the 12 UTC rawinsonde data from Desert Rock, Nevada (below) did indeed indicate the presence of a shallow stable layer between about 600 and 650 hPa (with a 50 knot wind speed maximum at the 650 hPa level).

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The slow movement of Tropical Storm Humberto over the warm waters of the Gulf of Mexico (above) allowed the system to intensify to become Hurricane Humberto (a Category 1 storm) during the pre-dawn hours on 13 September 2007. Humberto strengthened from a tropical depression (with 35 mph winds) to a hurricane (with wind gusts to 84 mph) in just 18 hours, which is the fastest rate of intensification near landfall ever observed. Humberto also became the first hurricane to make landfall in the US since Hurricane Rita back in September 2005.

GOES-12 IR imagery from the CIMSS Tropical Cyclones site (above) shows the compact cluster of cold cloud top brightness temperatures moving inland across parts of Texas and Louisiana. A hint of a partial eyewall structure was seen in the MIMIC microwave imagery (below). Hurricane Humberto produced heavy rainfall across parts of the southeastern US, with amounts as high as 14.13 inches in Texas.

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Tropical Depression #9 formed early in the day on 12 September 2007, and quickly intensified in the warm waters of the western Gulf of Mexico to become Tropical Storm Humberto (just off the coast of Texas). AWIPS images of the MODIS IR and visible channels (above; upper 2 panels) shows the early stages of a spiral band that began wrapping around the core of the cyclone during the afternoon hours. DMSP SSM/I imagery (above; lower 2 panels) depicted rainfall rates during the morning that as high as 30 mm per hour, and total precipitable water values of 55-65 mm in the near-storm environment.

GOES-12 IR imagery and derived winds products from the CIMSS Tropical Cyclones site (below) indicated that Humberto developed in an environment that was characterized by very low deep layer wind shear (5-10 knots within the 850-200 hPa layer), which was a factor that aided in the intensification from tropical depression to tropical storm.

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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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