Strong arctic cold front: grass fires, blowing dust, and a lee-side frontal gravity wave

March 17th, 2015 |
GOES-13 3.9 µm shortwave IR channel images (click to play animation)

GOES-13 3.9 µm shortwave IR channel images (click to play animation)

After a day of record high temperatures in parts of Nebraska — the 91º F at North Platte set a new record high for the month of March, and was also the earliest temperature of 90º F or above on record at that site — a strong arctic cold front plunged southward across the state late in the day on 16 March 2015. With strong winds (gusting to 40-50 knots at some locations) in the wake of the frontal passage and dry vegetation fuels in place, GOES-13 3.9 µm shortwave IR images (above; click image to play animation) showed the “hot spot” signatures (black to yellow to red pixels) associated with a number of large grass fires that began to burn across the state.

The strong northwesterly winds behind the cold front also lofted dry soil into the boundary layer, creating blowing dust whose hazy signature was evident on GOES-13 0.63 visible channel images (below; click image to play animation). Visibility was reduced to 7 miles at some locations.

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

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

After sunset and into the pre-dawn hours on 17 March, a lee-side frontal gravity wave signature could be seen on GOES-13 6.5 µm water vapor channel images (below; click image to play animation). This warmer/drier (darker blue color enhancement) arc on the water vapor imagery followed the position of the surface cold front, which meant that the upward-propagating frontal gravity wave reached altitudes where the water vapor channel was sensing radiation.

GOES-13 6.5 µm water vapor channel images (click to play animation)

GOES-13 6.5 µm water vapor channel images (click to play animation)

As the frontal gravity wave was approaching the Kansas/Oklahoma border region around 05 UTC, a pilot reported light to moderate turbulence at altitude of 6000 feet (below).

GOES-13 6.5 µm water vapor channel image with pilot report of turbulence

GOES-13 6.5 µm water vapor channel image with pilot report of turbulence

A 4-panel comparison of the three Sounder water vapor channels (6.5 µm, 7.0 µm, and 7.4 µm) and the standard Imager 6.5 µm water vapor channel (below; click image to play animation) showed that the southward propagation of the frontal gravity wave signature was most evident on the Sounder 7.0 µm and Imager 6.5 µm images, although there was also a more subtle indication on the Sounder 7.4 µm images. The new generation of geostationary satellite Imager instruments (for example, the AHI on Himawari-8 and the ABI on GOES-R) feature 3 water vapor channels which are similar to those on the current GOES Sounder, but at much higher spatial and temporal resolutions

GOES-13 Sounder 6.5 µm (upper left), 7.0 µm (upper right), 7.4 µm (lower left), and Imager 6.5 µm (lower right) - click to play animation

GOES-13 Sounder 6.5 µm (upper left), 7.0 µm (upper right), 7.4 µm (lower left), and Imager 6.5 µm (lower right) – click to play animation

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GOES-13 Sounder and Imager water vapor channel weighting functions for North Platte, Nebraska

GOES-13 Sounder and Imager water vapor channel weighting functions for North Platte, Nebraska

The depth and altitude of the layer from which a particular water vapor channel is detecting radiation is shown by plotting its weighting function — for example, at North Platte, Nebraska (above), the Imager 6.5 µm plot (black) and the 7.0 µm plot (green) exhibited lower-altitude secondary peaks around the 500 hPa level — while farther to the south at Dodge City, Kansas (below) these 2 water vapor channel plots had their peaks located slightly higher in the atmosphere. Even though the bulk of the radiation was being detected from higher altitudes (due to the presence of moisture and cirrus clouds aloft over much of the southern Plains region), the sharp signal of the lower-altitude cold frontal gravity wave was strong enough to be seen in the deep layer average moisture brightness temperature depicted in the water vapor images.

GOES-13 Sounder and Imager water vapor channel weighting functions

GOES-13 Sounder and Imager water vapor channel weighting functions

Strong cold front and a lee-side frontal gravity wave

January 17th, 2012 |
GOES-13 10.7 µm IR channel images (click image to play animation)

GOES-13 10.7 µm IR channel images (click image to play animation)

A strong cold front moved southward across the south-central US on 17 January 2012, dropping temperatures as much as 20 degrees F in 1-2 hours with wind gusts of 30-40 knots. The cold air behind the front (lighter gray enhancement) was clearly evident on AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data (above; click image to play animation).

GOES-13 6.5 µm water vapor channel images (click image to play animation)

GOES-13 6.5 µm water vapor channel images (click image to play animation)

As the cold front moved southward, a lee-side cold frontal gravity wave was seen along its leading edge on 4-km resolution GOES-13 6.5 µm water vapor channel images (above; click image to play animation). Note the very complex wave structure that was displayed on a 1-km resolution MODIS 6.7 µm water vapor channel image at 08:34 UTC (below). In addition, the MODIS water vapor image showed great detail in the mountain waves across parts of New Mexico and far southwestern Texas, as strong westerly flow was interacting with the terrain in that region.

MODIS 6.7 µm water vapor channel image + Surface frontal analysis

MODIS 6.7 µm water vapor channel image + Surface frontal analysis

 

Jayton, Texas NOAA Wind Profiler time series

Jayton, Texas NOAA Wind Profiler time series

As the cold front passed the Jayton, Texas NOAA wind profiler site (station identifier JTNT2) after about 12 UTC, the transition to a northeasterly flow of cold air was evident (above). Even though the depth of the cold air was not more than about 1.5 km, the lee-side cold frontal gravity wave was able to be seen on the water vapor imagery due to the fact that the cold, dry air mass shifted the peak of the GOES-13 water vapor weighting function down to within the 700-500 hPa pressure level — much lower than the height of the water vapor weighting function of the US Standard Atmosphere air mass (below).

Amarillo, Texas water vapor weighting function vs US Standard Atmosphere water vapor weighting function

Amarillo, Texas water vapor weighting function vs US Standard Atmosphere water vapor weighting function

 

A lee-side frontal gravity wave along the East Coast?

April 2nd, 2008 |

GOES-12 water vapor images (Animated GIF)

Lee-side frontal gravity waves are occasionally seen to the east of the Rocky Mountains, moving southward across the High Plains of the central US (for example, see  03 April 2007, 07 April 2000, and 12 January 1998). However, a satellite signature of what appeared to be a similar type of feature was observed along the US East Coast on 02 April 2008. AWIPS images of the GOES-12 6.5µm “water vapor channel” (above) showed the positions of a cold frontal boundary at 3 hour intervals on that day, while more frequent GOES-12 water vapor images viewed using McIDAS (below) revealed the subtle “gravity wave” feature as it propagated southward off the coast of Virginia and North Carolina during the day. Several coastal stations in Virginia and North Carolina reported northerly to northeasterly surface winds of 30 mph (13 meters per second) or greater after the passage of the gravity wave, with offshore buoy and ship reports as high as 38 mph (17 meters per second).

GOES-12 water vapor images (Animated GIF)

Lee-side frontal gravity wave over New Mexico and Texas

April 3rd, 2007 |

GOES-12 water vapor image

A lee-side frontal gravity wave was seen on GOES-12 6.5µm “water vapor” imagery (above; Java animation) — this feature was associated with a cold frontal boundary that was moving southward across eastern New Mexico and western Texas on 03 April 2007. These types of vertically-propagating gravity waves are stationary with respect to the flow (they follow the cold front) and are forced by nongeostrophic and nonhydrostatic accelerations in the frontal zone (these waves above frontal surfaces are dynamically equivalent to stationary “trapped lee waves” that appear over mountains). Wave breaking induces narrow zones of subsidence that can be sensed as warm/dry bands on the water vapor imagery. Note that the “wave signature” was not readily apparent on the 8-km resolution GOES-11 water vapor channel image; the gravity wave packet eventually became less obvious on GOES-12 4-km resolution water vapor imagery after about 17 UTC, but was still evident on 1-km resolution MODIS water vapor channel imagery at that time. A comparison of GOES-12 visible and water vapor imagery indicates that no clouds were forming along the cold frontal zone in the New Mexico / Texas border region; however, the vertically-propagating gravity waves along the far western end of the front appeared to generate cirrus cloud patches in the foothills of New Mexico.
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AWIPS images of NOAA wind profiler data (below) show that the depth of the cold air was fairly shallow, with winds shifting to northeasterly within the lowest 1 km of the atmosphere at Tucumcari, New Mexico (TCUN5) and Jayon, Texas (JTNT2) as the front moved southward through those locations. Even though the GOES-12 water vapor channel weighting function (calculated using Amarillo, Texas rawinsonde data) peaked at around 450 hPa (about 6.5 km above the surface), a signal of the vertically-propagating wave breaking above the top of the cold front was still evident on the water vapor channel imagery.
AWIPS wind profiler plot