When Water Vapor Channels are Window Channels
GOES-16 Low-Level Water Vapor Imagery (7.3 µm), 1322 UTC on 2 January 2017 (Click to enlarge)
The very cold and dry airmass over the eastern half of the United States during early January 2018 is mostly devoid of water vapor, a gas that, when present, absorbs certain wavelengths of radiation that is emitted from the surface (or low clouds). That absorbed energy is then re-emitted from higher (colder) levels. Typically, surface features over the eastern United States are therefore not apparent. When water vapor amounts in the atmosphere are small, however, surface information can escape directly to space, much in the same way as occurs with (for example) the Clean Window channel (10.3 µm) on GOES-16 (water vapor does not absorb energy with a wavelength of 10.3 µm). The low-level water vapor (7.3 µm) image above, from near sunrise on 2 January 2018, shows many surface features over North and South Carolina, Kentucky, Tennessee and southern Illinois. The features are mostly lakes and rivers that are markedly warmer than adjacent land. (In fact, Kentucky Lake and Lake Barkely in southwest Kentucky are also visible in the 6.9 µm imagery!)
Weighting Functions from 1200 UTC on 2 January for Davenport IA (left), Lincoln IL (center) and Greensboro NC (right) for 6.2 µm (Green), 6.95 µm (blue) and 7.3 µm (magenta), that is, the upper-, mid- and lower-level water vapor channels, respectively, on ABI. Peak pressures for the individual weighting functions are noted, as are Total Precipitable Water values at the station (Click to enlarge)
GOES-16 Weighting Functions (Click here ) describe the location in the atmosphere from which the GOES-16 Channel is detecting energy. The upper-level (6.2 µm) and mid-level (6.95 µm) weighting functions show information originating from above the surface. Much surface information is available at Greensboro, with smaller proportional amounts at Davenport and Lincoln.
The “Cirrus” Channel on GOES-16’s ABI (Band 4, 1.38 µm) also occupies a spot in the electromagnetic spectrum where water vapor absorption is strong. Thus, reflected solar radiation from the surface is rarely viewed at this wavelength. The toggle below, between the ‘Veggie’ Channel (0.86 µm) and the Cirrus Channel (1.38 µm) shows that some surface features — for example, lakes in North Carolina — are present in the Cirrus Channel.
ABI Band 3 (0.86 µm) and ABI Band 4 (1.38 µm) (That is, Veggie and Cirrus channels) at 1502 UTC on 2 January 2018 (Click to enlarge)
Whenever the atmosphere is exceptionally dry, and skies are clear, check the water vapor channels on ABI to see if surface features can be viewed. A few examples of sensing surface features using water vapor imagery from the previous generation of GOES can be seen here.