Regions Where the MODIS Cloud Mask is Known to Have Difficulty in 
             the Detection of Clear versus Cloud
                   15 April 2002
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  The MODIS Cloud Mask (MOD35) takes advantage of the unique 
  spectral signatures of land, water, cloud and atmospheric components 
  found in many of the MODIS 36 bandwidths.  Spectral threshold tests 
  applied to scenes based upon surface composition and solar illumination
  are combined to determine a confidence that a single pixel is 
  unobstructed by cloud or aerosol. Given the complexity of the 
  earth/atmosphere system, there are some regions where the cloud mask 
  does not perform optimally.  Research continues on improving the 
  mask in these regions. The regions include:

	Semi-arid Regions - An area with spectral characteristics in between 
                            those of a desert and vegetated surfaces.
                            Example:  Central Plains of the US
                                      in early spring.
        
	Extreme Sunglint Regions - areas within the strongest specular
                            reflection region over water.
 
	High Deserts - Cold dry, highly reflective areas can be falsely
                       identified as cloud.  
                       Example: Takli Makan Desert in Western China.
 

	Nighttime Polar Regions - Very difficult regions.  Clear regions
                      can be falsely identified as cloud; however,
                      little validation is available.
                      
	Nighttime in General - Very little research has been done to 
                      evaluate the performance of the cloud mask at 
                      night.  Research and improvements have focused
                      on responding to our users' needs, almost all of
                      which are land/ocean daytime concerns.

  
        Cloud Shadows - A cloud shadow bit is included within the 
                        cloud mask product; however, the shadow algorithm
                        itself requires more research and should not be
                        used.

  If you need better results in one of the regions mentioned above,
  the THRESHOLDS.dat file located in the cloudmask/cmd directory contains
  the individual spectral test thresholds that are used in combination to
  determine the confidence of clear sky. The thresholds can be tuned for use
  over a specific region.  For more information on the algorithm itself,
  please see the CM_JGRPAPER.pdf document or the Cloud Mask Algorithm
  Theoretical Basis Document at:
   http://modis-atmos.gsfc.nasa.gov/_docs/atbd_mod06.pdf

   Kathleen Strabala  kathy.strabala@ssec.wisc.edu