4-km resolution GOES-13 6.5 µm water vapor images

15 April 2009 was the last full day of GOES-13 imagery — the satellite was placed back into on-orbit storage on the morning of 16 April. GOES-13 had been brought out of storage during the Summer of 2008, to act as “GOES Central” (at 105º West longitude) and provide imagery through the Fall eclipse period. Larger on-board batteries allow GOES-13 to make imagery available during eclipse periods (when the satellite is in the Earth’s shadow, and the solar panels cannot provide the power necessary to operate the instruments) –  but other improvements to GOES-13 include better image-to-image navigation, and the 4-km resolution water vapor channel that debuted on GOES-12.

The 4-km resolution GOES-13 6.5 µm water vapor imagery (above) displayed widespread mountain wave signatures across much of the western US on 15 April — and many of the smaller-scale areas of mountain waves were unable to be resolved using the 8-km resolution GOES-11 6.7 µm water vapor imagery (below).

8-km resolution GOES-11 6.7 µm water vapor images

Such mountain wave signatures are often a good indicator of the likelihood of turbulence — and there were indeed a large number of pilot reports (PIREPS) of turbulence (including at least 11 reports of severe turbulence) across much of the western US, as seen on AWIPS images of the GOES-11/GOES-12 water vapor channel data (below).

GOES-11 / GOES-12 water vapor channel imagery

AWIPS images of the 1-km resolution MODIS 6.7 µm water vapor channel data (below) displayed even better mountain wave details at 18:46 and 20:31 UTC.

1-km resolution MODIS 6.7 µm water vapor images

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GOES-12 visible images

Multiple clusters of intense deep convection developed over the northeastern Gulf of Mexico during the daytime hours on 13 April 2009. According to the SPC Storm Reports, there were several reports of tornadoes and golfball-size hail in Florida and Georgia as some of these storms moved inland. GOES-12 visible images (above; QuickTime animation) show a detailed look at the intricate cloud top structure of the storms that were developing over the Gulf of Mexico — and since GOES-12 was in Rapid Scan Operations,  images were available at 5-10 minute intervals.

There were a few pilot reports (or PIREPS) of high-altitude turbulence as aircraft were flying over one of the clusters of convection. In particular, one aircraft reported severe turbulence at an altitude of 40,000 feet at 19:57 UTC — soon after the appearance of  a rapidly-developing overshooting top feature on the visible imagery (just to the west of the aircraft location). A comparison of the 4-km resolution GOES-12 10.7 µm IR image and the corresponding 1-km resolution MODIS 11.0 µm IR image closest to the time of the severe turbulence report (below) displayed an area of very cold cloud top temperatures (as cold as -74º C) associated with this rapidly developing overshooting top. The finer spatial resolution of the MODIS IR data was also able to resolve a packet of concentric gravity waves that was propagating outward from the cold overshooting top feature.

In addition, note the “parallax shift” that is evident on the GOES IR image: features are shifted a bit to the west and the north due to the high viewing angle of the GOES-12 satellite (which is positioned at 75º West  longitude over the Equator). There is minimal parallax error when viewing imagery from a polar-orbiting satellite (such as Terra and Aqua, which carry the MODIS instrument), since the satellite passes directly overhead. These 2 images are very close in time:  MODIS actually passed over this region at 19:00 UTC, and the GOES-12 scan which began at 18:55 UTC was probably sampling the Gulf of Mexico region around 18:58 UTC.

GOES-12 10.7 µm IR and MODIS 11.0 µm IR images

The GOES-12 sounder Cloud Top Height product (below) indicated that the highest cloud tops around the time of the MODIS image were around 44,000 feet — this places the highest parts of the thunderstorm about 4,000 feet higher than the altitude of the PIREP of severe turbulence.

GOES-12 sounder Cloud Top Height product

The Blended Total Precipitable Water (TPW) product (below) showed that a southwest-to-northeast oriented axis of higher TPW (values as high as 46-48 mm, darker red colors) was in place across the northeastern Gulf of Mexico, providing the necessary ingredient of moisture required for the formation of deep organized  convection.

Blended Total Precipitable Water product

An AWIPS image of a new experimental CIMSS product is shown below: a “Nearcasting” product that uses GOES sounder-derived  layer Precipitable Water data to highlight regions were mid-level dry air overlays low-level moisture. Such a moisture profile can lead to the release of convective instability. Note the axis of higher Vertical PW Differences (values greater than 0.5 inch, green to yellow colors) that extended across the Florida Panhandle into southern Georgia (where the SPC had issued Tornado Watch #149).

GOES-12 "Nearcasting" sounder-derived Vertical PW Difference product

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GOES-13 visible images

A large outbreak of wildfires occurred across parts of Texas and Oklahoma on 09 April 2009. GOES-13 visible images (above) showed several very large smoke plumes  drifting eastward across far northern Texas and far southern Oklahoma during the afternoon hours.  These fires burned over 145,000 acres, destroyed about 115 homes, closed roadways, and forced the evacuation of schools — and 3 deaths were reported as a direct result of these fires. In addition, the haziness associated with a large cloud of blowing dust was also evident on the visible images, moving overhead just as the fires began to grow in size and intensity. The thick  smoke and blowing dust were restricting  visibilities and causing air quality problems across that region.

AWIPS images of the 4-km resolution GOES-12 3.9 µm shortwave IR channel (below) depicted a number of cluster of very hot pixels, which spread rapidly in areal coverage. Many of the pixel brightness temperatures exceeded the saturation temperature of the GOES-12 3.9 µm sensor — these hottest pixels showed up as dark black in color, displaying “NO DATA” using the AWIPS cursor sampling function.

GOES-12 3.9 µm IR images

Consecutive images of the 1-km resolution MODIS 3.7 µm shortwave IR channel (below) indicated how quickly the number of fire pixels grew in size and increased in number between 17:42 and 19:28 UTC. On the MODIS images, the hottest pixels were displayed as bright white, as the color scale “wrapped around” to the cold end of the scale (displaying “-110º C” using the AWIPS cursor sampling function) due to the extremely hot temperatures of these fires.

MODIS 3.7 µm IR images

MODIS true color (left) + false color (right) RGB images

A comparison of 250-meter resolution MODIS true color and false color images from the SSEC MODIS Today site (above) offered a closer look at the smoke plumes and the fire hot spots (which appeared as the red-colored areas on the false color image) at 19:28 UTC. At that time, these smoke plumes were drifting eastward toward the Dallas/Fort Worth metro area, as seen on the MODIS true color image displayed using Google Earth (below).

MODIS true color image (displayed using Google Earth)

Very strong winds were seen over the entire region (gusting as high as 76 mph at Frederick, Oklahoma and 67 mph at McLean, Texas), behind a surface dry line and cold front. These strong winds — which helped the fires to grow so quickly –  were aided by the downward transfer of momentum from the middle troposphere. A 4-panel display of GOES Imager and Sounder water vapor channel images (below) revealed a distinct signature of this rapidly-descending dry air (especially evident on the Sounder 7.0 µm channel imagery, lower left panel, and the Sounder 7.4 µm channel imagery, upper right panel). Since the air was so dry, all 4 of the GOES water vapor channel weighting functions peaked at the 500 hPa pressure level and below. The fact that the dew point temperature at Winston, Texas dropped from +7º F at 12:06 pm to -20º F at 3:06 pm local time (as winds gusted to 48 mph) was an indicator of how remarkably dry this air mass was on that particular day.

GOES imager and sounder water vapor images

The GOES sounder Total Column Ozone product (below) indicated that a strong tropopause anomaly was moving eastward through the region — and NAM40 PV1.5 pressures showed that the dynamic tropopause was being brought downward to altitudes as low as the 500 hPa pressure level.

GOES sounder Total Column Ozone product

A NAM40 model cross section oriented from west to east (below) showed the strong descent of the dynamic tropopause over southern Oklahoma — the potential vorticity fields are the colored image portion of the cross section.

NAM40 model cross section

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GOES-13 visible images

GOES-13 visible images (above) displayed the formation of a “standing wave” cloud feature along the Lake Superior shoreline of northeastern Minnesota on 07 April 2009. In addition to the wave cloud, note that you can also see the southeastward drift of lake ice toward the shoreline of the Upper Peninsula of Michigan, driven by strong northwesterly surface winds that persisted during the day (gusting as high as 29 knots at Hancock MI).

This standing wave cloud feature was formed by a vertically-propagating internal gravity wave that resulted from the interaction of the northwesterly flow with the topography of the shoreline (below) — the terrain quickly drops from an elevation of about 2000 feet above sea level (over northeastern Minnesota) to about 600 feet above sea level  (over Lake Superior) in a very short distance.

Topography + surface reports + cross section orientation

GOES-12 10.7 µm IR images (below) showed that the cloud top temperatures associated with this standing wave cloud feature quickly cooled into -20º to -30º C range (cyan to blue colors), getting as cold as -34º C at 19:31 UTC.

GOES-12 10.7 µm IR images

A 1-km resolution MODIS 11.0 µm IR image (below) displayed a minimum cloud top brightness temperature of -38º C at 19:40 UTC. This  coldest IR temperature corresponded to an altitude of about 24,000 feet, according to rawinsonde data from International Falls, Minnesota (INL).

MODIS 11.0 µm IR image + International Falls MN Skew-T plot

However, there appeared to be a veil of thin high-level cirrus clouds streaming southward off the top of the standing wave cloud band, which were likely at a much higher altitude than 24,000 feet — but the satellite was getting a strong thermal signal from the warmer surfaces and mid-level cloud tops that were located directly below the cirrus clouds, which was making the actual cirrus cloud top brightness temperatures appear significantly warmer on the IR image. A MODIS Red/Green/Blue (RGB) composite image — using the MODIS visible, near-IR “snow/ice channel”, and the “IR window channel” images –  helps to get a better sense of the thin ice crystal cirrus clouds (lighter purple in color) that existed at higher altitudes compared to the thicker supercooled water droplet clouds (brighter white colors) that were right along the Minnesota / Lake Superior shoreline. Snow cover and/or frozen lakes appeared as brighter pink features on the RGB image.

MODIS Red/Green/Blue (RGB) composite image

The higher-altitude cirrus clouds are even more obvious in a comparison of 250-meter resolution MODIS “true color” and “false color” images from the SSEC MODIS Today site (below). On the false color image, snow cover, ice, and ice crystal clouds appear as varying shades of cyan (while supercooled water droplet clouds appear as brighter white features).

MODIS 250-m resolution "true color" and "false color" images

A northwest-to-southeast oriented cross section of NAM12 model fields depicted a deep pocket of positive Omega (upward vertical motion, yellow to orange colors) that corresponded to the cloud band along the Minnesota Lake Superior shoreline (below). Note that this Omega feature was vertically tilted in an “upshear”  direction, and extended upward to around the 400 hPa pressure level.

NAM12 model cross section

Another slice of NAM12 model fields along that same NW-SE cross section line showed a distinct region where there was a strong upward component of the ageostrophic vertical circulation (below), which was likely the initial forcing leading to the formation of the standing wave cloud band seen on satellite imagery.

NAM12 model cross section

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