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VIIRS

AVHRR Cloud Overlap Detection

VIIRS Cloud Overlap Detection

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Cloud Overlap Detection with VIIRS


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The VIIRS cloud overlap algorithm consists of two sets of spectral tests.  If either is passed, then cloud over is said to be present in a given pixel.  The first set of tests is the same used in the  AVHRR algorithm.  The second set of tests is described below.

The following additional spectral properties are exploited in the VIIRS algorithm.  In the 1.65 um region of the spectrum, ice particles absorb radiation much more strongly than water particles.  Thus, the radiation reflected back to the satellite at 1.65 um will be greater when an optically thick water cloud is present compared to an optically thick ice cloud.  Further, in the 1.38 um region, water vapor is a strong absorber of radiation, so the radiation detected by a satellite at this wavelength will mainly be from the upper troposphere, unless the atmosphere is very dry.  Due to this fact, the 1.38 um band is very effective at detecting cirrus clouds.  If both the 1.65 um reflectance and the 1.38 um reflectance are greater than some specified thresholds, there is a good possibility that both a high cloud and a lower water cloud are present in a given satellite field-of-view.

Simulations were performed in order to better understand the relationship between the 1.65 um reflectance and the 1.38 reflectance.  Thresholds of 1.65 um reflectance were created as a function of 1.38 um reflectance for a variety of viewing and illumination angles.  Threshold values are determined by plugging a 1.38 um reflectance value (in fractional form) into a fourth degree polynomial whose coefficients are given a water surface and a grass surface  (i.e. general vegetated surface).  The 1.38 um threshold was set to 0.025 (2.5%) for water/ice/snow surfaces and 0.027 (2.7%) for all other surfaces.  If the actual 1.65 um reflectance is greater than the threshold value and the 1.38 um reflectance is greater than its threshold and the 11 um brightness temperature is less than 280 K and the 1.65 um/0.65 um reflectance ratio is less than 1.0 and the split window brightness temperature test described here is passed when the 1.38 um reflectance is less than 0.08 (8%) (otherwise the split window brightness temperature test is not applied) then this cloud overlap test is passed.  This test should NOT be used over known desert surfaces.



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