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

Mesovortex over Lake Ontario

February 17th, 2015
GOES-13 0.63 µm visible channel images (click to play animation)

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

GOES-13 (GOES-East) 0.63 µm visible channel images (above; click to play animation) revealed the presence of a mesocale vortex (“mesovortex”) propagating eastward across the ice-free waters of western Lake Ontario on on 17 February 2015. At the beginning of the animation, also note that there were numerous “hole punch clouds” seen in the stratus cloud deck that covered the western Lake Ontario region during the early morning hours; these holes were likely caused by aircraft inbound/outbound from the Toronto International Airport — particles in jet engine exhaust act as ice nuclei, causing supercooled water droplets to turn into larger, heavier ice particles which then fall out of the cloud to create holes (sometimes described as “fall streaks” due to their appearance).

A closer view using a sequence of MODIS and VIIRS true-color Red/Green/Blue (RGB) images from the SSEC RealEarth web map server site is shown below. There was a significant amount of ice in the northeastern section of Lake Ontario, as well as a ring of offshore ice around other parts of the lake.

MODIS and VIIRS true-color images

MODIS and VIIRS true-color images

A comparison of the 16:31 UTC Terra MODIS 0.65 µm visible channel and the corresponding Sea Surface Temperature product (below) showed that SST values in the ice-free portions of the mesovortex path were generally in the 30 to 34º F  range.

Terra MODIS 0.65 µm visible channel image and Sea Surface Temperature product

Terra MODIS 0.65 µm visible channel image and Sea Surface Temperature product

Turbulence caused by mountain waves and jet stream wind shear

January 30th, 2015
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)

GOES-13 6.5 µm water vapor channel images (above; click to play animation) showed dry air (brighter yellow to orange color enhancement) moving across the Mid-Atlantic and Southeast regions of the eastern US in the wake of a strong cold frontal passage on the morning of 30 January 2015. There were also numerous pilot reports of turbulence, at both low altitudes (plotted in red) and high altitudes (plotted in cyan).

The most obvious feature seen on the GOES-13 water vapor images was the “rippled” signature of mountain waves, which extended far to the lee (southeast) of the Appalachian Mountains (the topographical obstacle to the strong northwesterly boundary layer flow that was causing the waves to initially form). A comparison of 4-km resolution GOES-13 6.5 µm water vapor and 1-km resolution Aqua MODIS 6.7 µm water vapor images (below) demonstrated the benefit of higher spatial resolution for diagnosing the areal coverage of such small-scale mountain waves. Of special note is the pilot report of “severe to extreme” turbulence at 4000 feet over South Carolina.

MODIS 6.7 µm and GOES-13 6.5 µm water vapor channel images, with pilot reports

MODIS 6.7 µm and GOES-13 6.5 µm water vapor channel images, with pilot reports

A comparison of the MODIS 6.7 µm water vapor channel image with the corresponding MODIS 0.65 µm visible channel image (below) showed that the severe to extreme reports in North and South Carolina were examples of Clear Air Turbulence (CAT), since no clouds were apparent in those areas at the time.

Aqua MODIS 0.65 µm visible channel and 6.7 µm water vapor channel images

Aqua MODIS 0.65 µm visible channel and 6.7 µm water vapor channel images

Regarding the numerous high-altitude pilot reports of moderate to severe turbulence, the NAM80 model depicted a 120-knot jet streak over South Carolina at 12:00 UTC, with another 120-knot jet streak approaching from the middle Mississippi Valley region (below). Note that there was strong wind speed shear to the north of the jet stream axis, which is where the bulk of the pilot reports of turbulence were located. Quite often there is an obvious moist-to dry gradient water vapor signature along or just poleward of a strong jet streak axis — but such a signature was not seen with this particular event.

GOES-13 water vapor image with NAM80 250 hPa wind isotachs and pilot reports

GOES-13 water vapor image with NAM80 250 hPa wind isotachs and pilot reports

In response to some of these pilot reports, at 16 UTC a SIGMET (SIGnificant METeorological advisory) was issued for occasional severe turbulence due to jet stream wind shear (below).

GOES-13 water vapor image with pilot reports and  boundaries of turbulence SIGMET

GOES-13 water vapor image with pilot reports and boundaries of turbulence SIGMET

4-panel images showing the three GOES-13 Sounder water vapor channels (6.5 µm, 7.0 µm, and 7.4 µm) along with the conventional GOES-13 Imager 6.5 µm water vapor channel (below; click to play animation) showed how each channel helped to identify where the pockets of middle-tropospheric dry air were located.

4-panel images showing the three GOES-13 Sounder and the GOES-13 imager water vapor channels (click to play animation)

4-panel images showing the three GOES-13 Sounder and the GOES-13 imager water vapor channels (click to play animation)

The GOES-13 water vapor channel weighting functions plotted using data from the 12 UTC rawinsonde reports from Roanoke/Blacksburg, Virginia and Greensboro, North Carolina are shown below. Due to the very dry middle to upper troposphere, the water vapor channels were able to sense features farther down into the atmosphere than is usually the case — this is illustrated by the relatively low altitude of the water vapor weighting function peaks.

Roanoke/Blacksburg, Virginia water vapor channel weighting function plots

Roanoke/Blacksburg, Virginia water vapor channel weighting function plots

Greensboro, North Carolina water vapor channel weighting functions

Greensboro, North Carolina water vapor channel weighting functions

Compare the 2 examples above with the altitude peaks of the various GOES-13 Sounder and Imager water vapor channels under “normal” conditions, plotted using the US Standard Atmosphere as the sounding profile (below).

GOES-13 water vapor channel weighting functions, calculated using the US Standard Atmosphere sounding profile

GOES-13 water vapor channel weighting functions, calculated using the US Standard Atmosphere sounding profilek

The trans-Atlantic flow of moisture and strong winds

January 14th, 2015
SSEC RealEarth™ Infrared satellite image featured on NBC Nightly News

SSEC RealEarth™ Infrared satellite image featured on NBC Nightly News

The SSEC RealEarth geostationary satellite infrared (IR) image composite shown above (which was first sent out via Twitter by Stu Ostro of The Weather Channel…thanks Stu!) was featured on the NBC Nightly News on 14 January 2015 (link) because it illustrated a vivid example of the trans-Atlantic flow of moisture from a disturbance off the US East Coast to a rapidly-deepening storm approaching the British Isles (surface analysis maps | water vapor images with surface analyses).

A sequence of hourly geostationary satellite water vapor channel image composites (below; click to play animation) showed that there was a clear trans-Atlantic connection in terms of middle to upper tropospheric moisture/clouds, and a comparison of the 20 UTC water vapor image with the corresponding MIMIC Total Precipitable Water product indicated that there was a lower to middle tropospheric moisture connection as well. This type of long and narrow fetch of TPW is often referred to as an “atmospheric river”.

Geostationary satellite water vapor image composites (click to play animation)

Geostationary satellite water vapor image composites (click to play animation)

Another interesting point brought up during the NBC Nightly News segment was the recent presence of unusually strong trans-Atlantic jet stream winds, which has allowed aircraft flying from New York City to London to set record times in terms of conventional passenger aircraft (such as the 08 January flight of British Airways 114). Note the strong dry-to-moist (darker blue to white to green color enhancement) along the northern edge of the trans-Atlantic water vapor image moisture feed: such a moisture gradient often coincides with the axis of a strong jet stream. AWIPS images of water vapor imagery with overlays of MADIS cloud-tracked and water-vapor-tracked winds (below; click image to play animation) showed many high-altitude wind vectors in the vicinity of the jet stream moisture gradient with speeds in the 150-160 knot range (with 175 knots seen on the previous day).

Water vapor images with MADIS atmospheric motion vectors (click to play animation)

Water vapor images with MADIS atmospheric motion vectors (click to play animation)