GOES-11 visible images

Well, not really — but a very interesting cloud plume formed yesterday and streamed off the 20,318-ft (6194-m) summit of Denali (“Mt. McKinley”) in southern Alaska on 10 June 2009, which almost had the  appearance of a volcanic eruption plume. GOES-11 visible images (above) showed this thin cloud plume spreading out as it curved to the southeast and then to the south, eventually moving over Anchorage and then the Kenai Peninsula.

A 1-km resolution NOAA-18 AVHRR 10.8 µm IR image (below) indicated that IR brightness temperatures were quite warm for such a high-altitude cirrus plume, barely reaching the -15 to -20º C range. Due to the thin nature of this cloud plume, a significant amount of radiation from the warmer ground surfaces below was bleeding upward through the thin cloud layer and reaching the satellite detectors.

NOAA-18 AVHRR 10.8 µm IR image

A false-color NOAA-18 Red/Green/Blue (RGB) image (below) showed the “transparent” nature of the cloud plume, with snow cover features on the ground clearly recognizable beneath the cloud.

NOAA-18 AVHRR false color RGB image

Tracking the location of the leading edge of the thin cloud plume feature was difficult using single-channel satellite imagery, which underscores the importance of using multi-spectral satellite products such as the 10.8 – 12.0 µm IR difference  (below) to correctly analyze the cloud location. Areas where the IR difference product reached +7 to +10 K (cyan colors) corresponded to the thicker portions of the cloud plume.

NOAA-18 IR difference product (10.8 - 12.0 µm, channel 04 - 05)

The cloud plume was curving to the southeast and then to the south due to the presence of a ridge of high pressure aloft over southern Alaska, as seen on an AWIPS image of the GOES-11 IR channel with an overlay of the GFS model 500 hPa winds (below).

GOES-11 10.7 µm IR image + GSF 500 hPa winds

A tip of the hat to Emily Niebuhr, UW-Madison / AOS graduate student, who is currently up in Alaska and brought this to our attention!

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GOES-12 6.5 µm water vapor images

AWIPS images of the GOES-12 6.5 µm “water vapor channel” (above) showed the development of widespread severe convection across parts of Kansas on 09 June 2009. Of particular interest was the appearance of an arc of significantly warmer/drier air (signified by the darker blue colors) along the upwind (western) edge of the thunderstorm cloud as the convection continued to intensify. This water vapor image signature likely indicates areas of well-defined compensating subsidence along with the possible “detrainment” of dry stratospheric air around the cloud edge. Note that the upstream cirrus cloud features also appeared to erode and disappear as they arrived at this region of subsidence along the western cloud edge.

There was one pilot report of moderate turbulence at an altitude of 38,000 feet around 16:00 UTC (below), which was near the water vapor signature of compensating subsidence.

GOES-12 water vapor image + pilot reports of turbulence

A comparison of the 1-km resolution MODIS 6.7 µm water vapor channel and the 4-km resolution GOES-12  6.5 µm water vapor channel data (below) shows that MODIS water vapor brightness temperature values were as warm as -26.5º C (yellow color enhancement) just to the west of the thunderstorm edge — the warmest GOES-12 water vapor brightness temperature value was -29º C.

MODIS 6.7 µm water vapor + GOES-12 6.5 µm water vapor images

A similar comparison of MODIS and GOES-12 water vapor images about 10 hours later (below) displayed a very pronounced signature of a gravity wave train along the western edge of the ongoing thunderstorm complex. There were no pilot reports of turbulence in that particular region at that time.

MODIS 6.7 µm water vapor and GOES-12 6.5 µm water vapor images

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MODIS true color and false color images

It’s a sure sign that Summer can’t be far off when Hudson Bay in Canada finally begins to lose its ice coverage — and a comparison of MODIS true color (channels 01/04/03) and false color (channels 01/07/07) Red/Green/Blue (RGB) images from the SSEC MODIS Direct Broadcast site (above) showed that large portions of the bay were indeed ice-free on 09 June 2009. Much of Hudson Bay appeared bright white on the true color image, but the false color image revealed that the bright white appearance was due not only to thick ice, but also a thin layer of supercooled water droplet cloud that had  formed as warmer air moved over the cold ice surface. The ice in the bay (along with deeper areas of snow cover on the ground, and thick high clouds composed of ice crystals) appeared as darker red features on the false color image.

A comparison of the 17:11 UTC Terra and the 18:56 UTC Aqua MODIS false color images (below) indicated that the thin supercooled water droplet cloud layer above the ice was moving during that short time period, as evident by the changes in the location of the cloud edges.

MODIS false color images

While this type of RGB imagery is currently not available on today’s 8-bit AWIPS system, changes to the AWIPS II architecture will allow  24-bit RGB images to be created and displayed; a simulated AWIPS RGB false color image is shown below.

Simulated AWIPS MODIS false color RGB image

You may have noticed the appearance of several very thin linear features on the true and false color images above; these were contrail segments from aircraft flying over the region, which were more easily seen on an AWIPS image of the MODIS near-IR “cirrus detection” channel (below). In addition to ice crystal clouds, the “cirrus detection” imagery is also useful for detecting the presence of airborne particles that are good scatterers of light (such as smoke, dust, volcanic ash) — and the large patches of brighter gray shades seen over Hudson Bay could be areas of smoke aloft from recent fire activity in Alaska and northern Canada.

MODIS "cirrus detection" image

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MODIS visible images

An interesting large “dark feature” showed up on AWIPS images of MODIS visible channel data over Lake Michigan on 04 June 2009 (above). This dark feature appeared to be shifting slowly southward during the 105-minute period between the time of the Terra MODIS image (16:53 UTC, or 11:38 am local time) and the time of the Aqua MODIS image (18:38 UTC, or 1:38 pm local time).

It is revealing to examine the corresponding MODIS Sea Surface Temperature (SST) images (below), which indicate the presence of a significantly warmer area of SST values located within the region of the “dark feature” that was seen on the visible imagery. MODIS SST values were as warm as 52.4º F on the Terra image, and 57.3º F on the Aqua image (darker green colors) — a good 10-15º F higher than the surrounding cooler waters (39-42º F, darker blue colors). There are only 2 buoys in Lake Michigan — Buoy 45007 in the northern portion of the lake (reporting a water temperature of 39º F), and Buoy 45002 in the southern portion of the lake (reporting water temperatures of 41-43º F) — so the MODIS SST imagery provided potentially useful information in an otherwise data-void area.

MODIS Sea Surface Temperature images

As it turns out, this large dark feature on the visible imagery corresponded to portions of the lake water that were very smooth. Since high pressure was centered over this region — note that the NAM12 model surface winds (below) were only in the 0-5 knot range  — the wind-driven wave heights were minimal, allowing for a nearly flat water surface. The presence of very light winds also allowed the “skin temperature” of the water surface to warm very quickly, as we have seen over Lake Michigan in the past. The warmest SST values were located near the center of the “light winds region”, where the winds apparently had been calm for several hours (or longer).

MODIS visible image with NAM12 surface winds

So, why did this large area of “smooth water” appear darker in the MODIS visible images? The answer lies in the fact that with imagery from polar-orbiting satellites such as Terra and Aqua, there is often a significant amount of sun glint off the rough water surfaces below the satellite overpass — these areas of sun glint make the rougher water surfaces appear brighter in visible imagery. However, in the area of the calm winds, the water surface was very flat; this flat water surface then reflected like a mirror (with all the light being reflected back in one direction — but in this case, that one direction was not directly back toward the satellite!).

This idea is supported by the corresonding MODIS 3.7 µm shortwave IR imagery (below) — this particular channel is very sensitive to reflected solar radiation, which makes the rougher waters (which give off a lot of sun glint) appear warmer (darker) on the shortwave IR imagery. The MODIS 3.7 µm IR brightness temperature values were about 20º C colder (denoted by the lighter gray shades) in the region of the smooth water, since there was very little solar radiation being reflected directly back toward the satellite.

MODIS 3.7 µm shortwave IR images

Very fine structure in the edges of the “smooth water areas” can be seen by examining 250-meter resolution MODIS true color imagery (below). This AWIPS MODIS true color imagery viewer is available as part of the CIMSS “MODIS Imagery in D-2D” project (currently only over Wisconsin at this time).

MODIS 250-meter true color image + MODIS 1-km visible image

One final question remains: why wasn’t this “dark signature”  of the calm Lake Michigan waters seen on the “1-km” resolution GOES-12 visible imagery (below)? Once again, the answer revolves around the issue of sun glint: with a geostationary satellite such as GOES-12 (which is positioned over the Equator), the sun glint pattern is generally restricted to the Equatorial regions. The lack of sun glint over Lake Michigan did not allow the GOES-12 visible imagery to be useful for diagnosing those areas of calm winds over the water.

GOES-12 visible images

A hat-tip to Kathy Strabala and Chris Moeller at CIMSS for bringing this case to our attention, and offering a physical explanation of what was going on!

===== 08 JUNE UPDATE =====

Another good case of this type of “calm winds / smooth water” satellite signature presented itself on 08 June 2009 over the northern Gulf of Mexico. An AWIPS image of the MODIS visible channel (below) shows an elongated dark region that was oriented approximately west to east — but in this case, there was also a large bright spot located within the elongated dark region. Note the orientation of the Terra satellite overpass, as shown in the MODIS Orbit Itinerary Viewer to the left of the MODIS visible image: the satellite overpass was just to the left (west) of the bright spot on the visible image. This was due to the geometry of the sun and the satellite at that time: the sun was still to the east of the satellite position during the late morning hours, which affected the path of the rays of sunlight reflection such that the bright reflection spot seen in the visible image was located to the right (east) of the satellite overpass point.

MODIS visible image + Terra satellite overpass geometry

===== 09 JUNE UPDATE =====

The temporal evolution of the changing satellite image appearance of such an area of calm winds / smooth water is demonstrated on GOES-12 visible and 3.9 µm shortwave IR images (below). This particular region of calm winds and smooth water was located off the Pacific coast of Central America on 09 June 2009. Note how the appearance of the area on visible imagery changes from dark to bright to dark again as the sun’s location passes overhead during the rotation of the Earth — and the brightness temperature on the shortwave IR images gets much hotter (darker black in appearance) as the reflected solar radiation becomes the dominant component in the 3.9 µm IR brightness temperature.

GOES-12 visible and 3.9 µm shortwave IR images

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