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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AWIPS image of GOES-11 6.7 µm water vapor + GOES satellite winds + PIREPs

There was a pilot report (or PIREP) received over Annette Island, Alaska (station identifier ANN) on 05 April 2009 which noted “stronger winds than forecast” at an altitude of 29,000 feet. The winds reported by the aircraft were from a direction of 190º at a speed of 76 knots — and nearby GOES-derived atmospheric motion vectors (above) generally had speeds of around 50 knots or less.

However, note the strong dry-to-moist (dark blue to white) gradient seen on the 8-km resolution GOES-11 water vapor imagery above — this is a common signature that often occurs along the axis of a strong jet streak. This strong water vapor gradient is more well-defined when viewed on a 1-km resolution MODIS water vapor image from 21:40 UTC (below).

MODIS 6.7 µm water vapor image

An animation of 4-km resolution GOES-13 6.5 µm water vapor imagery (below) also shows this strong “jet streak gradient” signature just to the west of Annette Island (ANN). Even though the viewing angle from the GOES-13 satellite (positioned over the Equator at 105º West longitude) was quite large, the 4 km spatial resolution of the data still allowed the gradient to show up quite well.

GOES-13 6.5 µm water vapor images

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GOES-11 visible, 3.9 µm IR, 10.7 µm IR, and IR "split window difference" images

The Mt. Redoubt volcano in Alaska experienced its 19th explosive event (in a series that began on 23 March) on 04 April 2009. GOES-11 visible, 3.9 µm shortwave IR (IR2), 10.7 µm IR window (IR4), and 10.7-11.0 µm “split window difference” images (above) showed that the southeastward advection of the volcanic plume became increasingly difficult to follow a few hours after the eruption.

However, the volcanic plume likely contained a good deal of water vapor, which made it easier to track on GOES-11 6.7 µm “water vapor channel” imagery (below) as it moved toward and eventually south of 50º N latitude  after about 21:00 UTC.

GOES-11 6.7 µm water vapor images

Images of the MODIS 1.3 µm “cirrus detection channel” at 20:45 and 22:35 UTC (below) exhibited a signal of the leading edge of the volcanic plume as it approached and moved south of 50º N latitude (between 144º W and 142º W longitude). This MODIS near-IR channel is sensitive to particles that are efficient scatterers of light (such as smoke, haze, dust, ash), so these types of airborne particles to show up as slightly brighter features on grayscale-enhanced MODIS “cirrus detection channel” imagery.

Terra and Aqua MODIS near-IR "Cirrus detection channel" images

The extent of the long-range transport of the Redoubt SO2 plume was even more obvious on the AIRS Ozone Monitoring Instrument (OMI) SO2 24-hour composite image for 04 April (below).

AIRS OMI SO2 24-hour composite image

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