GOES-15 10.7 µm IR images (click image to play animation)

McIDAS images of 4-km resolution GOES-15 10.7 µm IR data (above; click image to play animation) showed the transition of Tropical Depression #13 into Tropical Storm Lee in the northern Gulf of Mexico on 02 September 2011. Deep convection persisted within the eastern semi-circle of the system, with large areas exhibiting cloud top IR brightness temperatures of -80º C and colder (violet color enhancement) prior to Lee reaching tropical storm intensity.

A comparison of AWIPS images of 1-km resolution POES AVHRR 0.63 µm visible channel and 10.8 µm IR channel data (below) revealed a few vigorous overshooting tops embedded within the convective region, with one feature located southeast of the circulation center having an IR brightness temperature value of -93º C (darker violet color enhancement).

POES AVHRR 0.63 µm visible channel and 10.8 µm IR channel images

About 2 hours later, a similar comparison of 1-km resolution MODIS 0.65 µm visible channel and 11.0 µm IR channel images (below) showed overshooting tops with IR brightness temperatures as cold as -89º C.

MODIS 0.65 µm visible channel and 11.0 µm IR channel images

===== 03 SEPTEMBER UPDATE =====

GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (above; click image to play animation; also available as a QuickTime movie) revealed a curious “dual-vortex” structure to Tropical Storm Lee early in the day on 03 September 2011, with convective bursts developing in the vicinity of each of the two vortices. Later in the day, it appears that these dual vortices merged into a single vortex, as the center of Lee approachd the coast of Louisiana.

A comparison of 1-km resolution MODIS 6.7 µm and 4-km resolution GOES-13 6.5 µm water vapor images (below) showed that a significant tongue of dry continental air was being drawn into the western portion of the circulation of Lee.

MODIS 6.7 µm and GOES-13 6.5 µm water vapor channel images

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GOES-13 Visible (0.63 µm) image and auto-detected Overshooting Tops (click image to toggle)

Overshooting tops above a thunderstorm suggest an updraft powerful enough to penetrate through the tropopause into the stratosphere. In mid-latitudes, overshoots are well-correlated with both heavy precipitation and severe weather. In the tropics, overshooting tops are linked to intensification in tropical systems. In the image toggle above of an area of disturbed weather that may become a named tropical system in the Gulf of Mexico over the weekend, visible imagery and infrared data are shown with the automatically-detected overshooting tops (red being a more intense OT, that is, a bigger temperature difference between the tower top and the surrounding anvil cirrus through which the OT penetrates). Overshooting tops over the Continental United States are shown here (computed using data from GOES-13); Data from MSG are used to compute OTs over the tropical Atlantic, as shown here.

GOES-13 IR/WV (10.7 µm - 6.5 µm) used to detect Overshooting Tops

Difference fields between infrared data at 10.7 µm and the so-called water vapor channel (6.5 µm) can also be used to infer overshooting tops because Hot Towers push water vapor into the stratosphere and this alters the difference between the two channels. The image above, taken from the CIMSS Tropical Weather website, shows maxima in the difference field in the region where strong convection is occurring. (Click here for the toggle between the visible image and a similar WV/IR difference image). The automated detection algorithm seems to miss some towers; the wv/ir difference field identifies too many.

As the system in the Gulf of Mexico intensifies, the signal from the IR/WV difference and the number of autmomated detections should both increase. Intensification for now is inhibited by high shear over the region. The CIMSS Tropical Weather website offers information on this developing Gulf system.

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GOES-13 10.7 µm IR images

Tropical Storm 12L intensified on the evening of 31 August to become Hurricane Katia, the second Atlantic basin hurricane of the 2011 season. GOES-13 10.7 µm IR images from the CIMSS Tropical Cyclones site (above) showed the development of deep organized convection both east and west of the storm center.

A DMSP SSMIS 85 GHz microwave image (below) revealed a closed eyewall had formed (although the southern portion of the eyewall was thin and poorly-defined).

DMSP SSMIS 85 GHz microwave image

Katia existed in an environment of relatively low 850-200 hPa deep layer wind shear (below), which was one factor that favored further intensification.

GOES-13 IR image + Deep layer wind shear product

An AWIPS image of EUMETSAT MetOp ASCAT scatterometer surface winds (below) indicated that the radius of tropical storm force winds had increased since earlier in the day as Katia intensified.

ASCAT scatterometer surface winds

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MODIS true color Red/Green/Blue (RGB) images

A sequence of daily 250-meter resolution MODIS true color Red/Green/Blue (RGB) images from the SSEC MODIS Direct Broadcast site (above) showed the development and evolution of the smoke plume emanating from a swamp fire that was burning in the Bayou Sauvage National Wildlife Refuge near New Orleans during the 26 August – 31 August 2011 time period. The change in daily wind directions resulted in very different smoke dispersion patterns on each day. Smoke from this fire caused air quality alerts to be issued for the New Orleans and Baton Rouge areas.

AWIPS images (below) of the 1-km resolution MODIS 0.65 µm visible channel data at 19:16 UTC (2:16 pm local time on 30 August) showed the curving smoke plume; about 9.5 hours later, the fire “hot spot” (black to red to yellow color enhanced pixels) was seen on a 1-km resolution MODIS 3.7 µm shortwave IR image at 04:42 UTC (11:42 pm local time on 30 August).

MODIS 0.65 µm visible image + MODIS 3.7 µm shortwave IR image

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