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

Detecting Surface Features in Water Vapor Channel Imagery (Part 2)

December 18th, 2006 |

GOES-11/GOES-12/GOES-13 water vapor images

Another example of detection of surface features on “water vapor channel” imagery was apparent on 18 December 2006. In this particular case, the “surface” was the high terrain of the Absaroka Range, Wind River Range, and Big Horn Mountains in Wyoming (all of which reach altitudes in excess of 13,000 feet / 4000 m), making it easier to sense radiation from the ground using the 6.5µm/6.7µm water vapor channel. Since this channel is essentially an InfraRed (IR) channel, the cold temperature signature of the snow-covered mountain features (morning temperatures were as cold as -30 F / -34 C at Old Faithful in Yellowstone Park, where 22 inches / 56 cm of snow were on the ground) was very obvious against the warmer background temperature of the surrounding bare ground at lower elevations. Very little water vapor was present within the atmospheric column, so the water vapor channel weighting function (calculated using the Riverton, Wyoming rawinsonde profile) for both GOES-11 and GOES-12 peaked at an altitude just below 500 hPa (very near the altitude of the aforementioned mountain features).

A Java animation of GOES-11, GOES-12 and GOES-13 water vapor imagery shows that the mountain features become more apparent as a drier pocket of air passed over the region. Due to the higher spatial resolution (4km) of the spectrally-wider 6.5µm water vapor channel on both GOES-12 and GOES-13, the mountain features are resolved with greater clarity compared to the 8km resolution 6.7µm channel on GOES-11. In addition, since the mid-tropospheric winds across that region were fairly light (and generally parallel to the orientation of the terrain), there were no “mountain wave” signatures to the lee of these mountain ranges.

GOES-13: Detecting Surface Features in Water Vapor Channel Imagery

December 8th, 2006 |

GOES-13, GOES-12 water vapor images
During Day 2 (08 December 2006) of the GOES-13 post-launch NOAA science test, a cold and dry air mass was moving eastward over the southern Great Lakes region; “water vapor channel” images from the GOES-13 and GOES-12 imagers (above) displayed what appeared to be a typical pattern of 6.5µm brightness temperature values. However, a Java animation of the GOES-13 and GOES-12 water vapor channel images  (with the map overlay removed) shows the outline of the southern portion of Lake Michigan as the pocket of driest air moved across that area.

Detecting surface-based features (or geographical boundaries) on the 6.5µm GOES imager water vapor channel is somewhat unusual, since the radiation sensed by that channel normally originates from the middle troposphere (generally from within the 500-300 hPa layer, or 5-9 km above the surface in a US Standard Atmosphere). However, on this particular day, the air mass located over the Upper Midwest region was rather cold and dry — the GOES-12 water vapor weighting function calculated using the rawinsonde data from Davenport, Iowa at 12 UTC on 08 December (below) indicates that a significant contribution to the water vapor channel radiance at that location was coming from altitudes as low as the 600-700 hPa layer. The warm waters of Lake Michigan were surrounded by relatively cold land surfaces (GOES-13 10.7µm IR image with surface temperature reports), and a signal from this strong thermal contrast was bleeding up through what little water vapor was present within the atmospheric column, allowing the outline of Lake Michigan to be detected on the GOES-12 and GOES-13 water vapor channel imagery.
Davenport, IA water vapor weighting function

GOES-16 water vapor imagery over far northern Canada

August 1st, 2018 |

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

Animations of GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed features moving eastward across Nunavut in northern Canada on 01 August 2018. These images covered the far northern portion of the GOES-16 Full Disk view in AWIPS, and depicted frontal wave disturbances within the polar jet stream over that region.

Due to the large satellite viewing angle or “zenith angle”, the 2 km water vapor image pixel dimension (at satellite sub-point) increased to around 6.4 km or 4 miles (below).

Magnified view of GOES-16 Mid-level (6.9 µm) Water Vapor image, showing the pixel dimension over Nunavut, Canada [click to enlarge]

Magnified view of a GOES-16 Mid-level (6.9 µm) Water Vapor image, showing the pixel dimension over Nunavut, Canada [click to enlarge]

Another effect of the large satellite view angle was a shift of the Water Vapor weighting functions to higher altitudes — plots of the 7.3 µm, 6.9 µm and 6.2 µm weighting functions calculated using 12 UTC rawinsonde data from Baker Lake, Nunavut are shown below. These plots depict the layers of the atmosphere from which emitted radiation was detected by each of the 3 Water Vapor spectral bands on the ABI instrument.

GOES-16 Water Vapor weighting function plots calculated using 12 UTC rawinsonde data from Baker Lake, Nunavut [click to enlarge]

GOES-16 Water Vapor weighting function plots calculated using 12 UTC rawinsonde data from Baker Lake, Nunavut [click to enlarge]