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

Surface Cold Front over the High Plains of Texas

April 3rd, 2018 |

Hourly GOES-16 ABI Low-Level Water Vapor Infrared (7.34 µm) Imagery, and hourly observations, 0700-1600 UTC on 3 April 2018 (Click to enlarge)

A cold front moving southward along the western Great Plains showed a distinct signature in GOES-16 Water Vapor Imagery.  The hourly animation above, with surface observations, shows the front in the Low-Level Water Vapor passing over stations where winds shift from westerly and southwesterly to strong northerly.  The feature is far more trackable in GOES-16 ABI Imagery with a 5-minute cadence as is typical over CONUS, as shown below for both low-level water vapor infrared imagery (Band 10, 7.34 µm) and upper-level water vapor infrared imagery (Band 8, 6.19 µm). The infrared imagery allowed a precise determination of when the cold front would reach a location. (In fact, because a GOES-16 Mesoscale Sector was placed over west Texas, the time of arrival could be observed down to the minute, as shown in this animation of the clean window (10.3 µm) infrared imagery from GOES-16).

GOES-16 ABI Low-Level Water Vapor Infrared Imagery (7.34 µm), 0832-1637 UTC on 3 April 2018 (Click to animate)

GOES-16 ABI Upper-Level Water Vapor Infrared Imagery (6.19 µm), 0832-1637 UTC on 3 April 2018 (Click to animate)

Visible Imagery after sunrise (below) shows that some surface cloudiness was associated with this feature — but other parts were clear.

GOES-16 ABI “Red” Visible Imagery (0.64 µm), 1252-1637 UTC on 3 April 2018 (Click to animate)

It is not common for surface features to appear in the Upper-Level Water Vapor Imagery, even when the surface is near 900 mb, as over the High Plains of west Texas. Weighting Functions show from which layers in the atmosphere energy detected by the satellite originates. The Weighting function from Amarillo TX at 1200 UTC on 3 April is shown below.  The low-level water vapor weighting function — shown in magenta — shows contributions from the surface, but the upper-level water vapor weighting function — shown in green, shows contributions ending about 200 mb above the surface, at around 700 mb.  A conclusion might be that the depth of the cold air quickly increases to around 200 mb behind the front.  Thus is can appear in the Upper-Level water vapor imagery.   The cold front passes Amarillo (here is a meteorogram) shortly before 1200 UTC (and before the Radiosonde was launched).  The radiosonde from Dodge City Kansas, however, at 1200 UTC, shows a cold layer about 200 mb thick.  (Here is the Amarillo Sounding for the same time;  it’s shown in the Weighting Function plot below as well).

Clear-Sky Weighting Functions from Amarillo TX, 1200 UTC on 3 April 2018 (Click to enlarge)

Interpretation of water vapor imagery is simplified if you use information from weighting functions to understand the three-dimensional aspect of the water vapor imagery.