Lee-side cold frontal gravity wave

February 5th, 2018 |

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, middle) and Upper-level (6.2 µm, right) Water Vapor images, with hourly surface wind barbs plotted in cyan [click to play animation]

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, middle) and Upper-level (6.2 µm, right) Water Vapor images, with hourly surface wind barbs plotted in cyan [click to play animation]

As a cold front moved rapidly southward across the Great Plains (surface analyses) on 05 February 2018, the signature of a deep-tropospheric lee-side cold frontal gravity wave (reference) could be seen on GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above; also available as an MP4 animation). In addition, the initial gravity wave was soon followed by a secondary lee-side gravity wave, which could be seen moving southward over the northern Texas Panhandle by the end of the animation.

Plots of the weighting function (or “contribution function”) for each of the three GOES-16 Water Vapor bands (below) are calculated using 05 February/12 UTC rawinsonde data from Dodge City, Kansas — which was south of the cold front at that time. The peak pressure level for all three weighting function plots was in the 442-497 hPa range, giving some indication of the depth of these vertically-propagating gravity waves.

Weighting function plots for each of the three GOES-16 Water Vapor bands, calculated using 05 February/12 UTC rawinsonde data from Dodge City, Kansas [click to enlarge]

Weighting function plots for each of the three GOES-16 Water Vapor bands, calculated using 05 February/12 UTC rawinsonde data from Dodge City, Kansas [click to enlarge]

GOES-16 Water Vapor weighting functions using 06 February/00 UTC rawinsonde data from Amarillo, Texas — where the surface cold front had passed about 3 hours earlier — are shown below. Note that in the drier post-frontal air mass, the peak pressures for the 3 water vapor bands had increased, descending to the 477 to 684 hPa pressure levels. This comparison helps to underscore the dependence of water vapor weighting function height on the temperature and/or moisture profile of the atmosphere.

Weighting function plots for each of the three GOES-16 Water Vapor bands, calculated using 06 February/00 UTC rawinsonde data from Amarillo, Texas [click to enlarge]

Weighting function plots for each of the three GOES-16 Water Vapor bands, calculated using 06 February/00 UTC rawinsonde data from Amarillo, Texas [click to enlarge]

Lee-side cold frontal gravity wave

November 28th, 2017 |

GOES-16 Lower-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Lower-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

* GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing *

As a strong cold front (surface analyses) moved southward from Colorado and Nebraska across New Mexico, Texas and Oklahoma on 28 November 2017, the subtle curved arc signature of a lee-side cold frontal gravity wave could be seen on GOES-16 Lower-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above).

Closer views of imagery from each of the 3 water vapor bands are shown below.

GOES-16 Upper-level (6.2 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Upper-level (6.2 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Mid-level (6.9 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Mid-level (6.9 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Lower-level (7.3 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

GOES-16 Lower-level (7.3 µm) images, with hourly surface wind barbs plotted in yellow [click to play MP4 animation]

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