The proposed 1.38um ABI Band has been designed to detect high cirrus clouds.  The current proposal is for the 1.38um band to range from 1.365um to 1.395um.  The purpose of this page is to show how the spectral width of the band will affect cirrus cloud detection.  By using AVIRIS data it was possible to determine a bandwidth which can be considered a maximum allowed width and consequently a bandwidth that is too large.  Near 1.38um the atmosphere strongly absorbs scattered solar radiation from the surface and lower troposphere, this is due to the water vapor in the atmosphere.  Since most of the water vapor in the atmosphere is contained in the lower troposphere, radition scattered at the surface and in the lower troposphere will most likely be absorbed by the atmosphere before it can reach a satellite sensor.  As the bandwidth of the channel increases, the channel begins to exit the atmospheric water vapor absorption feature.  As the bandwidth increases even more, the spectral window is now more transparent to scattered radiation from the surface and lower troposphere.  Surface features and water clouds will now be detected by the satellite and hence the images will be polluted with low cloud and surface features.  A java script loop of the proposed ABI Band, maximum allowable width, and unacceptable width has been made available.  This loop allows an easy look at how spectral width affects cirrus/surface detection.  Click here for the loop.

Above is a difference image of the proposed 1.38um ABI Band (1.365-1.395um) v. 1.38um band with a bandwidth of 1.339-1.424um.  ((1.339-1.424um image) - Proposed 1.38um ABI band.).  Note the radiance differences are multiplied by 1000.  Below are difference image histograms of different 1.38um bandwidths compared to the proposed 1.38um ABI Band.


Difference image histogram (1.355-1.407um) - Proposed 1.38um ABI band.


Difference image histogram (1.348-1.415um) - Proposed 1.38um ABI band.


Difference image histogram (1.339-1.424um) - Proposed 1.38um ABI band.


Difference image histogram (1.346 - 1.395um) - Proposed 1.38um ABI band.


Difference images histogram (1.337 - 1.395um) - Proposed 1.38um ABI band.


Bandwidth Image: Proposed 1.38um ABI Band (red) is centered about the atmospheric water vapor absorption feature.  Radiation detected within this bandwidth will primarily occur from the mid troposphere and above.  Note the term "Maximum Allowable" refers to the bandwidth 1.348-1.415um and the term "Unacceptable Width" refers to the bandwidth 1.339-1.424um as seen in the histograms above.  The maximum allowable width (yellow) has an expanded bandwidth for increased signal strength, but begins to encompass areas outside of the atmospheric water vapor absorption feature.  This expanded bandwidth causes slight detection of scattered radiation from points below the middle troposphere.  Any bandwidth that is beyond the maximum allowable width are deemed unacceptable (green).  A bandwidth as wide or wider than the unacceptable band will receive too much scattered radiation from the surface and lower troposphere.  Images would include low clouds and surface features which is not the goal of the 1.38um ABI band.  Note: On the above image the light blue band is the MODIS 1.38um spectrum.  Return to top.

Currently proposed 1.38um (1.365um - 1.395um).  Click on image to enlarge.  Notice limited radiation detected from water cloud due to channel being located within atmospheric water vapor absorption feature.  To view images similar to the above image select the following links.  Again, note the term "Maximum Allowable" refers to the bandwidth 1.348-1.415um and the term "Unacceptable Width" refers to the bandwidth 1.339-1.424um as seen in the histograms above.  
Maximum Acceptable
Unacceptable Width
MODIS 1.38um