Great Lakes surface geographical outlines evident on water vapor imagery

February 23rd, 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)

A cold and dry arctic air mass (morning minimum temperatures) was in place over the Great Lakes region on 23 February 2015. This arctic air mass was sufficiently cold and dry throughout the atmospheric column to allow the outlines of portions of the surface geography of the Great Lakes to be seen on GOES-13 (GOES-East) 6.5 µm water vapor channel images (above; click image to play animation).

In addition to the commonly-used 4-km resolution 6.5 µm water vapor channel on the GOES Imager instrument, there are also three 10-km resolution water vapor channels on the GOES Sounder instrument (centered at 6.5 µm, 7.0 µm, and 7.4 µm). A 4-panel comparison of these water vapor channel images (below; click image to play animation) provides the visual indication that each water vapor channel is sensing radiation from different layers at different altitudes — for example, the surface geographical outlines of the Great Lakes are best seen with the Sounder 7.4 µm (bottom left panels) and the Imager 6.5 µm (bottom right panels) water vapor channels.

GOES-13 Sounder 6.5 µm, 7.0 µm, 7.4 µm, and Imager 6.5 µm water vapor channel images (click to play animation)

GOES-13 Sounder 6.5 µm, 7.0 µm, 7.4 µm, and Imager 6.5 µm water vapor channel images (click to play animation)

An inspection of GOES Sounder and Imager water vapor channel weighting function plots (below) helps to diagnose the altitude and depth of the layers being sensed by each of the individual water vapor channels at a variety of locations. For example, the air mass over Green Bay, Wisconsin was cold and very dry (with a Total Precipitable Water value of 0.87 mm or 0.03 inch), which shifted the altitude of the various water vapor channel weighting functions to very low altitudes; this allowed surface radiation from the contrasting land/water boundaries to “bleed up” through what little water vapor was present in the atmosphere, and be sensed by the GOES-13 water vapor detectors. In contrast, the air mass farther to the south over Lincoln, Illinois was a bit more more moist, especially in the middle/upper troposphere (with a Total Precipitable Water value of 4.20 mm or 0.17 inch) — this shifted the altitude of the water vapor channel weighting functions to much higher altitudes (to heights that were closer to those calculated using a temperature/moisture profile based on the US Standard Atmosphere).

GOES-13 Sounder and Imager water vapor channel weighting function plots for Green Bay WI, Lincoln IL, and the US Standard Atmosphere

GOES-13 Sounder and Imager water vapor channel weighting function plots for Green Bay WI, Lincoln IL, and the US Standard Atmosphere

In addition to the temperature and/or moisture profile of the atmospheric column, the other factor which controls the altitude and depth of the layer(s) being detected by a specific water vapor channel is the satellite viewing angle (or “zenith angle”); a larger satellite viewing angle will shift the altitude of the weighting function to higher levels in the atmosphere. Recall that the water vapor channel is essentially an Infrared (IR) channel — it generally senses the mean temperature of a layer of moisture or clouds located within the middle to upper troposphere. In this case, the sharp thermal contrast between the cold land surfaces surrounding the warmer Great Lakes was able to be seen, due to the lack of sufficient water vapor at higher levels of the atmosphere to attenuate or block the surface thermal signature.

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.

On a separate — but equally interesting — topic: successive intrusions of arctic air over the region allowed a rapid growth of ice in the waters of Lake Michigan. A 15-meter resolution Landsat-8 0.59 µm panochromatic visible image viewed using the SSEC RealEarth web map server (below) showed a very detailed picture of ice floes along the western portion of the lake, as well as a patch of land-fast ice in the far southern end of the lake.

Landsat-8 0.59 µm panochromatic visible image (click to enlarge)

Landsat-8 0.59 µm panochromatic visible image (click to enlarge)

The motion of the band of ice floes along the western  edge of Lake Michigan was evident in 1-km resolution GOES-13 0.63 µm visible channel images (below; click image to play animation) — along the east coast of Wisconsin, southwesterly winds gusting to around 20 knots were acting to move the ice floes away from the western shoreline of Lake Michigan.

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

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

Sensing the surface on GOES-13 water vapor imagery

January 7th, 2014 |
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)

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 January07 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.

GOES-13 Imager 6.5 µm water vapor channel weighting function plots for Green Bay, Wisconsin

GOES-13 Imager 6.5 µm water vapor channel weighting function plots for Green Bay, Wisconsin

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.

“Seeing the surface” on water vapor imagery

July 8th, 2010 |
MODIS 6.7 µm water vapor image (with and without map overlay)

MODIS 6.7 µm water vapor image (with and without map overlay)

Under normal atmospheric conditions, the weighting function of most water vapor channels tends to peak at altitudes within the 500-300 hPa pressure range, allowing features within the middle to upper troposphere to be viewed on the water vapor imagery. However, under special conditions — for example, either a very dry or a very cold air mass — the altitude of the water vapor weighting function is shifted downward such that we are able to “see the surface” on water vapor imagery. Such was the case with the MODIS 6.7 µm water vapor image over the Baja California region on 08 July 2010 (above), where the outline of the coast was very obvious on the image.

Even though the water vapor channel was not “seeing the surface” per se, a signal of the strong surface thermal contrast (between the very warm land and the much cooler water) was able to override the weak signal from what little middle-tropospheric water vapor was present. Other cases of strong land/water temperature contrasts have been seen on water vapor imagery, such as with very cold and very dry arctic air masses back in February 2007, December 2006, and January 2004.

However, in this case, the signal of the land/water thermal contrast was not evident on the corresponding GOES-11 6.7 µm / GOES-13 6.5 µm water vapor composite image. Because of the large viewing angle of the geostationary satellites (around 40 degrees for GOES-11 and around 55 degrees for GOES-13 for the Baja California region), the water vapor weighting function was apparently shifted upward to a high enough altitude to preclude detection of the surface land/water thermal signal.

GOES-11 6.7 µm + GOES-13 6.5 µm water vapor composite (with and without map overlay)

GOES-11 6.7 µm + GOES-13 6.5 µm water vapor composite (with and without map overlay)

Surprisingly, not even the GOES-11 sounder 7.4 µm water vapor channel image (below) was able to detect the strong surface thermal signal — the weighting function of this channel often peaks much lower in the troposphere (usually around 850-700 hPa). Again, perhaps the large geostationary satellite viewing angle was a factor. With the MODIS instrument flying directly overhead, there was no corresponding upward shift in the water vapor channel weighting function.

GOES-11 sounder 7.4 µm water vapor image (with and without map overlay)

GOES-11 sounder 7.4 µm water vapor image (with and without map overlay)

Detecting Surface Features in Water Vapor Channel Imagery (Part 3)

February 5th, 2007 |

GOES, MODIS water vapor images

The strong arctic outbreak of early February 2007 brought an unusually cold and dry air mass over the northcentral and northeastern US. Water vapor channel imagery from the GOES imager and sounder on 05 February 2007 (above) showed a surprising result once the map overlay was removed (Java animation) — the outlines of parts of the Great Lakes and the Northeast coasts were clearly evident on the imagery. This is somewhat anomalous, given that the water vapor channel imagery normally depicts features in the middle to upper troposphere.

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GOES water vapor channel weighting functions calculated using rawinsonde data from Upton, New York at 00 UTC on 06 February 2007 (below) reveal that the GOES imager 6.5 µm water vapor channel (black plot) was detecting radiation from an atmospheric layer that peaked at an unusually low altitude (near 700 hPa), while the GOES sounder 7.4 µm water vapor channel (red plot) was detecting a significant amount of radiation from near the surface. This enabled a signal of the strong surface thermal contrast (very cold land surfaces adjacent to relatively warm bodies of water) to “bleed up” through what little water vapor was present in the atmospheric column, allowing us to see coastal outlines across the Great Lakes and Northeast US regions on the water vapor channel imagery.
Upton NY water vapor channel weighting functions