Regular readers of this blog will be quick to recognize the GOES imagery above as a low cloud detection product that exploits the differences in emissivity properties for water droplets that exist between 3.9 and 10.7 µm, two radiation bands that are detected on the GOES imager. The emissivity differences mean that 10.7 µm brightness temperatures will be warmer than 3.9 µm brightness temperatures, so a difference field will highlight where low clouds exist. This can be done with GOES imagery, above, or with MODIS imagery, below. Note that the existence of the low cloud may or may not suggest fog: only the top of the cloud is detected; whether or not the cloud rests on the ground cannot be determined easily from satellite.

Both the GOES and MODIS imagery show slow expansion to the low cloud field over the 3 hours, as might be expected given slow cooling at night. However, careful inspection reveals a variety of regions that show obstructions to visibility but no indication of fog or low clouds. For example, Montgomery (KMGM), Mobile (KMOB) and Tuscaloosa (KTCL) all show fog observations in the absence of a clear signal of detected low cloud. Similar observations occur over eastern Georgia and the Carolinas.

New low-cloud detection algorithms that incorporate model information (from the RUC, or, in the near future, the Rapid Refresh) can quantitatively describe the evolution of the low cloud and fog field. These algorithms were initially developed (and trained) using GOES data and are distributed to the AWIPS environment here at CIMSS. The loop above shows probabilities of Low IFR visibilites over the deep south during the morning. (Click for the 0400 UTC and 0700 UTC examples). Because the GOES-R product synthesizes model and satellite data together, better fog/low cloud detection occurs in regions where high clouds make low cloud detection difficult. Note how the probability of a visibility obstruction increases during the course of the night, and how places that develop fog are among the first to see a LIFR signal.

The visible imagery from GOES-13, below, shows the characteristic erosion of the low clouds, from outside in, during the course of the subsequent day. As usually happens, cumulus development is suppressed in regions where low clouds persist during the morning.

GOES-13 Visible (0.63 µm) (click image to play animation)

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

The water vapor animation from GOES-East on March 12th shows a structure rotating through the upper-level trough, which structure looks very much like a so-called “Sting Jet”. (In the animation above, the sting jet structure crosses the Missouri/Kansas border south of Kansas City, propagates across northern Missouri and eastern Iowa before moving northward into Wisconsin). (A more obvious Sting Jet event is discussed here; A Monthly Weather Review article on Sting Jets is here).

RUC wind analyses show that the sting jet structure was associated with a wind maximum on the 315 Kelvin isentropic surface. This Loop shows the maximum moving from northeastern Missouri into Central Wisconsin between 1000 and 1400 UTC on March 12th. Stability in the lower troposphere on March 12th (as suggested by this sounding from the Quad Cities in Iowa/Illinois) was strong enough to inhibit vertical mixing of stronger upper-tropospheric air down towards the surface. The circulation around the jet was sufficient, however, to generate showers over the upper Midwest, as shown in this loop.

MODIS 6.5 µm water vapor channel image

MODIS water vapor imagery, above, from 0841 UTC on 12 March shows the sting jet structure in north-central Missouri, and curving back to central Nebraska and central South Dakota.

GOES-13 0.63 µm visible image

(Added 13 March: SPC Storm Reports show a rare March tornado north of I-69 in lower Michigan. The visible imagery above, bracketing the observed time of the tornado (near the yellow box), shows a strong thunderstorm. By this time, the possible sting jet has rotated northward into western Ontario, so its influence on the environment in Michigan would be secondary. The sounding from DTX at 2300 UTC shows a favorable low-level wind profile.)

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POES AVHRR 0.86 µm visible channel images

A sequence of AWIPS images of POES AVHRR 0.86 µm visible channel data (above) showed the areal extent of ice in the Bering Sea during the 09 March – 11 March 2012 period. The far southern edge of the ice could be seen moving a bit further to the south during this time.

The corresponding Sea Ice Analaysis chart issued by the Ice Desk at the Anchorage, Alaska National Weather Service forecast office (below) provided a more detailed analysis of the age and thickness of various portions of this ice in the Bering Sea.

National Weather Service Sea Ice Analysis chart

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We received the following in an email from John Goff, lead forecaster at the Burlington, Vermont National Weather Service office:

“Couldn’t help but notice the apparent large ice eddies up in the Gulf of Saint Lawrence this afternoon (3/6) per the TERRA MODIS noon overpass (250m res). I thought that initially these were eddy/small-scale cloud vortices we sometimes see up there, but upon looping the GOES visible imagery on my AWIPS workstation it’s fairly obvious these are not clouds, but ice structures.”

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

Thanks John for the heads-up on this very interesting example! McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (above; click image to play animation) showed the development and motion of the ice eddies on 06 March 2012. Note how many of the ice floe structures began to move toward the northeast by the end of the day, due to increasing southwesterly surface winds across the region.

A comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data and the corresponding MODIS false-color Red/Green/Blue (RGB) image (below) demonstrated the value of using RGB imagery to aid in the discrimination between snow/ice (which appeared as varying shades of red in the false-color RGB image) and supercooled water droplet clouds (which appeared as the brighter white to cyan features on the RGB image).

MODIS 0.65 µm visible channel image + MODIS false-color Red/Green/Blue (RGB) image

An AWIPS image of the 1-km resolution POES AVHRR Sea Surface Temperature (SST) product at 18:55 UTC (below) indicated that SST values over the open waters were in the 30-31º F range (blue color enhancement), while the ice features exhibited colder values in the 23-25º F range (violet color enhancement).

POES AVHRR Sea Surface Temperature product

Finally, a comparison of 250-meter resolution MODIS true color and false color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (below) showed a closer view of the ice eddies, and again demonstrated the value of using various RGB image combinations to discriminate between snow/ice (cyan in the false-color image) and supercooled water droplet low cloud features (white on the true-color image).

MODIS true-color and false-color Red/Green/Blue (RGB) images

CIMSS participation in GOES-R Proving Ground activities includes making a variety of POES AVHRR and MODIS images and products available for National Weather Service forecast offices to add to their local AWIPS workstations. Currently there are 51 NWS offices receiving MODIS imagery and products from CIMSS.

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