The 1.37 um near-IR channel 4 is one of four near-IR channels on the GOES-16 ABI instruments. At 1.37 um, this channel is unique in that it is in a strong water vapor absorption region (Fig. 1). This means that surface and low-level features will appear dark or indistinguishable due to absorption of solar energy by water vapor before said energy can reach the low-levels/surface. The exception is in a really dry atmosphere, when reflectance off of low clouds and even the surface can be detected. Since most of the atmospheric moisture is concentrated in the lowest part of the troposphere, solar reflectance off of high clouds will still be detected at this wavelength. In fact, reflectance off of high clouds (bright) contrasts nicely with the dark background of little to no reflectance. That, along with a spatial resolution of 2 km, allow features such as thin cirrus clouds and contrails to be easily detected, leading to the nickname, “cirrus” band. Other features such as blowing/lofted dust and thick smoke can stand out in band 4 as well, though not to the same degree as high clouds.
Below is an example of the cirrus band over the southwest US during the day on March 16, 2017. Numerous cirrus clouds and contrails can be seen floating west to east over California. To the south, the scene is very dark due to water vapor absorption and lack of upper level clouds. To the north in N Nevada in a relatively dry atmosphere, mountain wave clouds can be seen.
For comparison, the 0.64 um visible channel is shown over the same area. Notice that the contrails are comparatively difficult to discern against the bright background of the land surface.
And finally, direct comparison of the scene between the two channels at 2102 UTC.
-Bill Line, NWS
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