A very cold arctic air mass had been building over central Alaska and northwestern Canada during the latter half of November 2006 (surface air temperatures colder than -40 F/-40 C have been reported daily over that region since 21 November). A NOAA-15 AVHRR 10.8µm “IR window channel” image centered over the southern Yukon Territory (above) on 27 November (surface analysis) revealed that the coldest air (-40 to -50 C, darker blue enhancement) was settling into the lower elevations of the river valleys. Narrow lakes along and south of the Yukon Territory / British Columbia border exhibited significantly warmer IR brightness temperatures (-10 to -20 C, orange to yellow enhancement), due to heat radiating upward through the snow and ice covered lake surfaces.

A similar IR image centered a bit farther east over the Northwest Territories (below) showed warmer brightness temperatures over the higher terrain of the Mackenzie and Selwyn Mountains (-20 to -30 C, yellow to cyan enhancement) — those higher terrain features rose above the level of the strong temperature inversion which was trapping the coldest air near the surface at lower elevations. This IR image also revealed a comparatively warm signature (0 to -20 C, red to yellow enhancement) from the snow and ice covered surface of Great Bear Lake in the northern portion of the image. The 1-km resolution of the NOAA-15 AVHRR instrument showed the small-scale structure of these temperature features much better than the “4-km” resolution of GOES-11 (which had degraded to an effective resolution of about 12 km, due to the ~65 degree satellite viewing angle) — this is quite apparent looking at a NOAA-15 / GOES-11 IR image fader (Java applet).

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An extensive area of mountain waves was apparent on the “water vapor channel” images from GOES-11 and GOES-13 on 22 November 2006 (above). Animation of these images (QuickTime | Java) shows that the mountain waves were present for several hours across a good deal of the Northwest US, becoming well-defined over Wyoming after about 10:00 UTC. Due to the relatively dry air mass that was present over that region, the GOES-11 water vapor channel weighing function was peaking at a fairly low altitude (around 600 hPa, or about 3 km above ground level). The improvement in spatial resolution of the water vapor channel (from 8km on GOES-11 to 4 km on GOES-13) allows such mountain wave features to be detected with better clarity; in addition, the 6.5µm water vapor channel on GOES-13 is spectrally wider than the 6.7µm water vapor channel on GOES-11, which accounts for some of the improved mountain wave detection capability. This improved 4 km resolution water vapor channel is also available on GOES-12.
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Wind speed isotachs at the 500 hPa level (below) indicated that strong winds associated with a passing jet stream axis were responsible for generating these mountain waves (also note the corresponding gradient in GOES Sounder total column ozone, poleward of the axis of strongest winds). This mountain wave signature on water vapor channel imagery is an indicator of turbulence potential; while there were no pilot reports of turbulence during that 05-14 UTC time period, the Graphical Turbulence Guidance product did indicate a Moderate to Severe potential for turbulence across the region (over Wyoming in particular).

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Some interesting photos of “hole punch clouds” were captured on 15 November 2006 — the photos (which appeared on the 16 November Spaceweather.com site) were were taken at Stevens Point in central Wisconsin. A QuickTime animation of GOES-12 visible channel and 3.9µm shortwave IR images (above) revealed a series of aircraft dissipation trails (or “distrails”) drifting northeastward between Madison and Stevens Point during the day; particles in the aircraft exhaust were acting as ice nuclei, causing any supercooled cloud droplets to glaciate and also helping other existing cloud ice crystals to increase in size — these larger ice crystals then descended under the influence of gravity, creating precipitation-induced “holes” and “streaks” in the cloud layers aloft. A 500-meter resolution Aqua MODIS true color image centered on Madison shows better details of the structure of 2 of the northwest-to-southeast oriented “distrails” that were located north of Madison at the time of the satellite overpass.

Other MODIS images and products that were available on AWIPS included the 1000-meter resolution 3.7µm shortwave IR channel (below); the brighter (colder) curved cloud signature in this image suggests that one of the aircraft had recently made a loop in the area between Madison (KMSN) and Wisconsin Dells (KDLL). It is likely that military jets from Volk Field Air National Guard Base (KVOK) were performing training exercises north of Madison, with the jet exhaust helping to initiate some of the interesting cloud patterns that were visible both on the ground and via satellite.

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It’s straightforward to differentiate between high clouds and the ground in infrared satellite imagery because of large temperature differences. As clouds get closer and closer to the surface, their temperature gets closer and closer to the surface temperature, and brightness temperature differences can become insignifcant. At night, when visible data are sadly lacking, what’s a forecaster to do? For example, look at the infrared image below. Where would you say the low clouds are over Texas? How certain are you?

There are times when low clouds can be identified in standard 11-micron imagery, for example by masking surface terrain features that are usually visible — such as river valleys. So instead of seeing the feature in the image, a smooth surface is apparent. If you are unfamiliar with a region for which you are forecasting, however, or if the region is flat. . .
Satellites include other bands that can be useful at night. One is the 3.9-micron band on GOES, or the 3.7-micron band on MODIS. This band is important for low cloud detection because emissivity at that wavelength is smaller for low clouds and fog. Hence, less radiation is emitted and the cloud will appear to be colder. Consider the 3.9 micron image below from the same time as the 11-micron image above:

Consider the region in east-central Texas now. It shows somewhat colder in the 3.9-micron image than its surroudings — the difference in brightness temperature between the two images is about 4 K. A second region of apparent low cloudiness is the region in the Los Angeles basin, and offshore of Los Angeles near the Channel Islands. These differences become all the more distinct in a difference between the two channel temperatures is plotted:

The dark regions of this map are where low clouds / fog are likely present. They show up because of colder brightness temperatures in the 3.9 micron band — thus the difference (11-3.9) is positive. This difference image will also highlight high cirrus clouds. Scattering by the ice crystals means more radiation will be detected by the satellite at 3.9 microns (compared to 11), and the cloud will thus appear warmer. The spectral difference over cirrus clouds will be negative. This is especially true during the day.
Similar behavior occurs in MODIS imagery:

The color-enhancement here distinctly highlights the low clouds (yellow and orange, where 3.7-micron brightness temperatures are cooler) and the high clouds (black, where 3.7-micron temperatures are warmer).

The emissivity differences that allow low-cloud detection continue during the day; however, the differences are overwhelmed by the solar signal. In other words, much more radiation at 3.9 microns (compared to 11 microns) is reflected and scattered towards the satellite during the day. So the character of the difference between the two channels changes sign. During the day, the 3.9 micron brightness temperatures becomes warmer than the 11 micron brightness temperatures.

Cirrus clouds detection improves during the day, as the scattering from incoming solar radiation enhances the signal at 3.7 and 3.9 microns relative to 11 microns; the amount of solar radiation at 3.7-3.9 microns is larger than the amount at 11 microns, and ice crystals scatter 3.9-micron radiation more effectively.

Note that in the 1415 UTC image above, the low clouds over east Texas shortly after sunrise now show as white — the temperature difference between the two bands is negative. Off the coast of California, where it’s still night-time, the temperature difference is still positive. In the 1531 UTC image below, the sun has risen over Southern California as well, and the character of the difference there flips as well.

The visible image below, from shortly after sunset, confirms the presence of clouds over east-central Texas.

More information on low cloud detection at night can be seen here.

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