Regional and Mesoscale Meteorology Branch

CIRA/WMO VLab New Satellite Information

Transmittance for various features in the VIS and Near IR and RGB examples using VIS, Near-IR and IR

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View of multiple graphs comparing the channels on various satellites across the globe

Spectra loops with channel width representation

The graphs are separated into “Visible and Near Infrared” and “Infrared” because of the different means of interpreting the radiance measurements: albedo and brightness temperature. Here, transmittance is used in the “Visible and Near Infrared” because it highlights various absorption regions in the atmosphere that affect measurements. Brightness temperature is used in the Infrared because that is the most familiar to the forecaster.

Visible to Near Infrared

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All graphs: This is a graph of atmospheric transmittances from a radiative transfer model showing a mid-latitude standard atmosphere with no clouds. The region extends from the ultravoilet into the near infrared and encompasses the visible portion of the spectrum. For reference: If the atmosphere were clear (no clouds and no absorbing gases and aerosols), and we were looking down from an orbiting satellite, we would be able to see the surface features. The transmittance would be represented by a straight line across the top at the value of one. But as we can see, this line is very variable from 0.3 to 2.5 micrometers. The width of the channels represents the full width at half maximum. For some applications it is important to remember that the response of the instrument is not zero outside this range, but it is less than 50%.

Last frame in loop(T99_transmission_absorption). There are gases and aerosols that absorb radiation and if we are in regions between 1.3 and 1.4 and 1.8 and 1.9 for example, water vapor absorption (and other minor constituents that absorb) prevents us from seeing surface features. We can also see other narrower regions where there is absorption by other gases.

Infrared

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All graphs: Orientation – On the y-axis is Brightness temperature going from warm at the bottom to cold at the top. The data that make this graph come from the imaging spectrometer AIRS instrument (Atmospheric InfraRed Sounder), on the polar orbiting satellite EOS Aqua. It measures a couple thousand (2378) narrowly spaced wavelength bands in the shortwave to longwave region (approximately 1band every 5 nm). These two examples are of opaque targets for 1) a surface which is clear ground in Nebraska during the day and represented by the open black circles, and 2) a deep convective cloud at night represented by the open purple diamonds. Each of these series of points represents the spectral information for one pixel (~13km x13km at nadir) in an AIRS scene. Example pixels from day and night are shown.

Last frame (BT99 spectra absorption). This graph shows dominant broad absorption regions. Note, there are many other absorbing constituents and regions.

Note: the loops work well on iOS devices but there is a little bug. Read
here for more information.