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VIIRS

AVHRR Cloud Overlap Detection

VIIRS Cloud Overlap Detection

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Cloud Overlap Detection with AVHRR


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The AVHRR algorithm utilizes a visible reflectance (~0.65 um) and brightness temperatures from the infrared window region (~11 um and ~12 um) of the spectrum.  The premise of the AVHRR algorithm is that for a single layer cloud, the 0.65 um reflectance and the 11 - 12 um brightness temperature difference (split window brightness temperature difference) should behave as predicted by plane parallel radiative transfer simulations.  In general, as a single layer cloud becomes optically thick, its reflectance increases and its split window brightness temperature difference dcreases.  In the case of a semi-transparent cirrus cloud overlying a lower water cloud, the vertical separation has little effecton its reflectance but a large effect on the split window brightness temperature difference.  Given a sufficient temperature difference between the cirrus cloud and the lower water cloud, the difference in transmission through the cirrus cloud at 11 um and 12 um will generally result in a split window brightness temperature difference that is much larger than that predicted by plane parallel theory for a single-layer cloud with a similar reflectance.  The detection of overlap in the AVHRR algorithm is fundamentally a detection of this deviation from plane parallel behavior.  

Simulations were performed in order to better understand the relationship between the 0.65 um reflectance and the split window brightness temperature difference.  Split window brightness temperature difference thresholds were created as a function of 0.65 um reflectance for a variety of viewing and solar zenith angles.  Threshold values are determined by plugging a 0.65 um reflectance value (in fractional form) into a fourth degree polynomial whose coefficients are given here.  If the actual split window brightness temperature difference is greater than the threshold value and the 0.65 um reflectance is greater than 0.30 (30%) and the 11 um brightness temperature is less than 270 K then the cloud overlap test is passed.  This algorithm should NOT be used over known snow/ice surfaces or deserts.



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