Most users of water vapor satellite imagery interpret the patterns they see as variations in moisture within the middle to upper troposphere — and for the most part, this is often a good first-order assumption. However, one must keep in mind that the water vapor channel is essentially an InfraRed channel, which is sensing the average temperature of a layer of moisture — and the altitude and depth of the layer of moisture being detected can change significantly, based upon such factors as the temperature and/or moisture profile of the atmospheric column, and the viewing angle of the satellite.

During an unusually cold arctic outbreak over the north-central US during the 06 January – 07 January 2014 period, the outline of various portions of the Great Lakes (in particular, Lake Superior, Lake Michigan, and Lake Erie) could actually be seen on GOES-13 6.5 µm water vapor channel imagery (above; click image to play animation). So, how is it possible to see surface features on water vapor channel satellite imagery?

In helping to understand the vertical location and vertical extent of features seen on water vapor imagery, plots of the water vapor “weighting function” (or “contribution function”) can be generated by taking into account the temperature and moisture profile of that location, along with the satellite viewing angle (or “zenith angle”). For this example, plots of GOES-13 Imager 6.5 µm water vapor weighting functions for Green Bay, Wisconsin (below) showed how the altitude and depth of the moisture layer being sensed by the water vapor channel decreased from 12 UTC on 05 January to 12 UTC on 06 January as the core of the cold arctic air moved over the western Great Lakes region. After that time, both the altitude and depth of the moisture layer being detected (as seen on the water vapor channel weighting function plots) began to increase to approximately their pervious values as somewhat warmer and more moist air began to replace the arctic air mass.

Getting back to seeing the outlines of portions of northern Lake Superior, western Lake Michigan, and western Lake Erie: what was being seen on the water vapor imagery was not necessarily the actual surface per se, but the signal of the strong temperature gradient between the cold snow-covered land surfaces and the still-unfrozen waters — and the signal of this strong surface temperature gradient was “bleeding upward” through what little moisture was present in the atmospheric column, and reaching the GOES-13 Imager water vapor detectors.

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A strong winter storm affected much of the central and eastern US during the 02 January – 03 January 2014 period. A map of SSEC RealEarth 24-hour snowfall totals (above) shows how widespread the resulting snowfall was, with amounts as high as 24 inches in Massachusetts abd 22 inches in New York (WPC storm summary).

As the storm system departed over the Atlantic Ocean on 03 January, an AWIPS image comparison of the 17:53 UTC (12:53 PM Eastern time) Suomi NPP VIIRS 0.64 µm visible channel data and the corresponding false-color “snow vs cloud discrimination” Red/Green/Blue (RGB) product (below) showed the areal coverage of snow on the ground (varying shades of red on the RGB image). Some patches of supercooled water droplet clouds (varying shades of white on the RGB image) could be seen streaming off of Lake Erie and Lake Ontario; in fact, a closer look revealed mesoscale bands of “lake-effect snow” downwind of the Finger Lakes in western New York, and also downwind of Lake Champlain along the New York/Vermont border.

Cold air moving southward in the wake of the storm crossed the western Gulf of Mexico, moved through the Chivela mountain pass in southern Mexico, and eventually emerged over the Pacific Ocean in the Gulf of Tehuantepec — this type of mountain gap wind flow is known as a Tehuano wind event or a “Tehuantepecer”. An image of Metop ASCAT surface scatterometer winds at 02:36 UTC (below) showed that a large area of northerly gale force winds (red wind barbs) was present over the Gulf of Tehuantepec, with maximum remotely-sensed wind speeds of 41 knots. The tropical surface analysis (cyan) displayed the fractured cold frontal boundary that had advanced into southern Mexico; behind the cold front along the Gulf of Mexico coast at Veracruz (station identifier MMVR), the surface visibility at the time was reduced to 6 miles due to blowing sand (time series of MMVR surface reports). Surface reports at Ixtepec (station identifier MMIT) along the Gulf of Tehuantepec were sparse, but did show northerly winds gusting to 37 knots at 17 UTC (time series of MMIT surface reports).

Daytime images of GOES-13 0.63 µm visible channel data on 03 January (below; click image to play animation) showed the hazy plume of blowing dust and sand moving southwestward, with the boundaries of the strong Tehauno winds marked by long, narrow rope clouds.

A signature of the dry air (darker blue color enhancement) associated with the Tehuano winds could be seen on the MIMIC Total Precipitable Water product (below).

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A train derailment occurred about one mile west of Casselton, North Dakota at 20:10 UTC (2:10 PM local time) on 30 December 2013 — ten cars of a westbound train transporting grain initially derailed, which then caused an eastbound train transporting crude oil to also derail. A large fire and multiple explosions erupted from the engine and 18 cars of the derailed eastbound train, which were carrying crude oil from the Bakken oil shale field region in northwestern North Dakota. McIDAS images of GOES-13 0.63 µm visible channel and 3.9 µm shortwave IR data (above; click image to play animation; also available as a QuickTime movie) showed the dark smoke plume beginning with the 20:15 UTC visible image, which then quickly fanned out to the south-southeast by 21:45 UTC. On the corresponding shortwave IR images, a darker gray fire “hot spot” accompanied the initial visible image signature of the smoke plume at 20:15 UTC, which later became very hot (dark black) on the 21:15 UTC image.

A comparison of GOES-15 (GOES-West, positioned at 135º West longitude) and GOES-13 (GOES-East, positioned at 75º West longitude) 0.63 µm visible channel images (below; click image to play animation) showed how the dark smoke plume appeared from the very different viewing perspectives of the two geostationary satellites. On both sets of images the eastern portion of the smoke plume appeared to have drifted over Interstate 29 (I-29) south of Fargo (FAR), but due to the low sun angle it is likely that this was actually the shadow from the dark smoke plume. Note that the low cloud features cast similar shadows during the late afternoon hours toward the end of the animation.

The corresponding comparison of GOES-15 vs GOES-13 3.9 µm shortwave IR images (below; click image to play animation) also showed differences in the apparent intensity of the fire hot spot, which were dependent upon satellite viewing angle, viewing time, and the opacity of the dense smoke plume overhead. On the GOES-13 21:15 UTC image (which was actually scanning the fire area at 21:17 UTC), a notable increase in IR brightness temperature was seen, with the hot spot exhibiting a brightness temperature of 322 K (48.9º C or 120º F). This was likely the near the time of one of several explosions (video 1 | video 2). GOES-15 was not scanning the fire area at that particular time, so a fire hot spot of that intensity was not evident in the imagery.

===== 31 December Update =====

The intense oil-fueled fire continued to burn into the following night; an AWIPS image of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) data at 09:07 UTC or 3:07 AM local time on 31 December (below) showed the bright glow of the fire near Casselton, as well as the smoke plume which was still drifting to the southeast. The glow of lights from cities and towns appeared somewhat blurry on the DNB image, due to scattering of the light through a thin veil of cirrus clouds that was drifting over the region (VIIRS 11.45 µm IR channel image). Since the Moon was nearly in the New phase, there was very little moonlight to illuminate the smoke plume — airglow and lights from nearby cities and towns helped to make this feature visible on the DNB image. Note that the navigation of the DNB image was slightly off, with the image being shifted a few miles to the southwest; in addition, this particular DNB image was enhanced to provide a darker contrast, eliminating “noise” from the glow of the regional snow cover (which was generally in the 5-13 inch range) to help highlight the smoke plume. A subtle signature of the fire hot spot (a darker gray pixel) could still seen on the corresponding VIIRS 3.74 µm shortwave IR image.

Due to air quality concerns from the toxic smoke plume, residents immediately downwind of the crash site were urged to evacuate. At Fargo’s Hector International Airport (located about 25 miles to the east of Casselton), the surface visibility dropped to 2 miles with haze at 06:53 UTC (12:53 AM local time) on 31 December, as winds shifted to the southwest (time series of surface reports); it is unknown whether this drop in visibility was due to smoke being transported from the accident site, or simply from local sources (such as the widespread burning of firewood in the city, given that the ambient air temperature at the time was -15º F). A WDAY News tower camera photo (below) showed that the dark smoke plume could be seen from downtown Fargo — the tower camera is looking to the west, and the smoke plume is drifting southward (to the left).

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Two signatures of aircraft traffic sometimes seen in satellite imagery are (1) dissipation trails, or “distrails”, and (2) condensation trails, or “contrails”. On 30 December 2013, examples of both were seen over Virginia and West Virgina. Multiple layers of clouds existed over the region as a cold frontal boundary was moving eastward; ahead of the cold front patchy areas of low-level supercooled water droplet clouds were drifting northeastward across North Carolina and Virginia, and examples of aircraft distrails could be seen in a comparison of Suomi NPP VIIRS 0.64 µm visible channel and false-color Red/Green/Blue (RGB) images at 17:29 UTC (above). When aircraft penetrated the supercooled water droplet cloud deck, particles in their exhaust acted as ice condensation nuclei which then created narrow lines of glaciated (ice) clouds in their wake. One particularly vivid example of a distrail was oriented from southwest to northeast over central Virginia. Ice clouds appeared as varying shades of red in the RGB image, in contrast to supercooled water droplet clouds which showed up as brighter white features.

Farther to the west, a wide band of higher-altitude ice clouds existed as part of an elongated warm conveyor belt that was approaching the East Coast of the US. A comparison of Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm IR channel images at 17:29 UTC (below) revealed the presence of widespread contrails over much of West Virginia into western Virginia. The contrails were nearly as cold as the underlying high-altitude cirrus clouds on the 11.45 µm IR image, making their identification more difficult — however, the contrails were quite evident on the shortwave IR image, since their smaller particles were very efficient reflectors of solar radiation (making them exhibit a warmer, darker gray signature).

Other examples of aircraft distrails can be found in previous blog posts.

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