Cloud Overlap
Detection with AVHRR
<----- note this is a
new webpage under development, not all sub pages are complete ----->
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.