Surface features seen in GOES Water Vapor imagery

January 19th, 2019 |
GOES-17 Low-level Water Vapor (7.3 µm) images, plus topography [click to play animation | MP4]

GOES-17 Low-level Water Vapor (7.3 µm) images, plus topography [click to play animation | MP4]

* GOES-17 images shown here are preliminary and non-operational *

A comparison of GOES-17 Low-level Water Vapor (7.3 µm) images with topography (above) revealed that radiation being emitted by the higher elevations of the Brooks Range in northern Alaska was able to be sensed by the 7.3 µm detectors — in spite of the very large satellite viewing angle (or zenith angle) of around 75 degrees.

The GOES-17 ABI Water Vapor band weighting functions calculated using 12 UTC rawinsonde data from Fairbanks, Alaska (below) showed that the presence of cold, dry air within the middle to upper troposphere had shifted the peak pressure of the 7.3 µm weighting function downward to 753.63 hPa (corresponding to an altitude of 7053 feet) — which was at or below the elevation of much of the higher terrain of the Brooks Range. There was very little absorption of upwelling surface radiation by the small amount of water vapor that was present within the middle/upper troposphere, allowing the cold thermal signature of the higher terrain to be observed on the Water Vapor imagery.

GOES-17 Water Vapor weighting functions calculated using 12 UTC rawinsonde data from Fairbanks [click to enlarge]

GOES-17 Water Vapor weighting functions calculated using rawinsonde data from Fairbanks, Alaska [click to enlarge]

On the following day (19 January), a very cold/dry arctic air mass was moving southward across the Upper Midwest and Great Lakes — the coldest temperature in the US that morning (including Alaska) was -42ºF at Kabetogama, Minnesota — and the outline of Lake Superior was very apparent in GOES-16 (GOES-East) Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor imagery; in fact, a portion of the northwestern shoreline was even faintly visible in Upper-level (6.2 µm) Water Vapor images (below).

GOES-16 Low-level (7.3 µm), and Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm), and Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

Plots of the GOES-16 Water Vapor band weighting functions calculated using 00 UTC rawinsonde data from International Falls, Minnesota (below) showed some radiation contribution coming from near or just above the surface. As a result, a signature of the strong surface thermal contrast — between a relatively warm Lake Superior (water surface temperatures in the 30s F) and the adjacent cold land surface temperatures (generally -10 to -20ºF) — was able to reach the satellite with minimal absorption by water vapor aloft.

GOES-16 Water Vapor band weighting functions, calculated using rawinsonde data from International Falls, Minnesota [click to enlarge

GOES-16 Water Vapor weighting functions, calculated using rawinsonde data from International Falls, Minnesota [click to enlarge]

===== 21 January Update =====

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images, with rawinsonde sites plotted in cyan [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images, with rawinsonde sites plotted in cyan [click to play animation | MP4]

In the wake of a large winter storm, arctic air spread across the eastern US on 21 January (minimum temperatures,– and the outline of the coasts of Maryland, Virginia and North Carolina could clearly be seen on GOES-16 Low-level (7.3 µm) Water Vapor images (above). In addition, the coast of the Albemarle-Pamlico Sound  and the Outer Banks of central North Carolina could even be seen for a short time on Mid-level (6.9 µm) Water Vapor imagery (for example, at 1502 UTC).

This cold/dry air mass set new daily records for lowest rawinsonde-measured Total Precipitable Water at Greensboro in central North Carolina (0.04 inch), Roanoke/Blacksburg in western Virginia (0.02 inch) and Wallops Island on the Eastern Shore of Virginia (0.05 inch). GOES-16 Total Precipitable Water product showed values in the 0.01 to 0.09 inch range in the vicinity of Roanoke and Greensboro. In plots of the GOES-16 water vapor weighting functions for those 3 rawinsonde sites (below), note the very strong contributions of radiation directly from or just above the surface for the 7.3 µm and 6.9 µm spectral bands.

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Greensboro, North Carolina [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Greensboro, North Carolina [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Roanoke/Blacksburg, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Roanoke/Blacksburg, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Wallops Island, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Wallops Island, Virginia [click to enlarge]

Water Vapor imagery sensing the surface of Hawai’i

November 21st, 2018 |
GOES-17 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

GOES-17 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

* GOES-17 images shown here are preliminary and non-operational *

GOES-17 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) revealed the diurnal cycle of warming and cooling of the summits of Mauna Kea and Mauna Loa on the Big Island of Hawai’i on 21 November 2018. There was even a subtle warming signature of the higher terrain of Maui on 7.3 µm and 6.9 µm imagery. This example helps to underscore the fact that Water Vapor bands are Infrared bands, which — in the absence of clouds — essentially sense the mean temperature of a layer (or layers) of moisture within the troposphere.

The presence of very dry air within the middle/upper troposphere over Hawai’i on 21 November had the effect of shifting the water vapor weighting functions to lower altitudes, as seen on plots for the 3 ABI Water Vapor bands calculated using 00 UTC rawinsonde data from Hilo PHTO (below). This allowed thermal radiation from the higher terrain to pass upward (un-attenuated) through what little high-altitude moisture was present and reach the 7.3 µm / 6.9 µm / 6.2 µm detectors on GOES-17.

Plots of weighting functions for the 3 ABI Water Vapor bands, calculated from 00 UTC rawinsonde data from Hilo PHTO [click to enlarge]

Plots of weighting functions for the 3 ABI Water Vapor bands, calculated using 00 UTC rawinsonde data from Hilo [click to enlarge]

Compare the altitudes and depths of Hilo water vapor weighting function plots on 19 November vs. 22 November (below). Increased moisture within the middle troposphere on 19 November shifted the water vapor weighting function plots for the 7.3 µm and 6.9 µm bands to higher altitudes (and increased the magnitude of the high-altitude contributions for the 6.9 µm and 6.2 µm bands).

Plots of Hilo Water Vapor weighting functions, 19 November vs 22 November at 00 UTC [click to enlarge]

Plots of Hilo water vapor weighting functions, 19 November vs 22 November at 00 UTC [click to enlarge]

Sensing the surface with water vapor imagery

February 6th, 2018 |

GOES-16 Low-level (7.3 µm) Water Vapor images [click to play animation]

GOES-16 Low-level (7.3 µm) Water Vapor images [click to play animation]

As a cold, dry arctic air mass moved across the western Great Lakes on 06 February 2018, portions of the land-water boundaries of Lake Superior, Lake Michigan and Lake Huron were very distinct on GOES-16 (GOES-East) Low-level (7.3 µm) Water Vapor images (above). The motion of low-altitude lake effect clouds were also apparent in the imagery.

Plots of weighting functions for the three GOES-16 ABI Water Vapor bands (7.3 µm, 6.9 µm and 6.2 µm) are shown below, calculated using rawinsonde data from Green Bay, Wisconsin and Gaylord, Michigan. With cold air and low values of Total Precipitable Water at these 2 sites (1.53 mm / 0.06 in and 1.88 mm / 0.07 in, respectively), the height of their weighting functions was shifted to significantly lower altitudes compared to what would be observed in a standard atmosphere. This enabled the contrasting thermal signature of the land/water boundaries to easily reach the satellite sensors, passing through what little moisture existed within the atmospheric column. While the peak of the violet 7.3 µm weighting function plots descended to the 879 hPa pressure level at both sites (which was approximately 1.2 km above the surface), a significant contribution could be seen originating from the surface itself.

Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Green Bay, Wisconsin [click to enlarge]

Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Green Bay, Wisconsin [click to enlarge]

Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Gaylord, Michigan [click to enlarge]

Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Gaylord, Michigan [click to enlarge]

Note that the peaks of the blue 6.9 µm weighting function plots were also anomalously low, reaching the 802 and 754 hPa pressure levels — however, in contrast to the 7.3 µm plots there was very little contribution from the actual surface, and the presence of secondary peaks at higher altitudes led to some absorption and subsequent re-emission of upwelling radiation by that layer of colder moisture aloft. As a result, only the faint outline of Lake Superior and its lake effect clouds were occasionally seen on Mid-level 6.9 µm Water Vapor imagery (below).

GOES-16 Mid-level (6.9 µm) Water Vapor images [click to play animation]

GOES-16 Mid-level (6.9 µm) Water Vapor images [click to play animation]

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)