Cloud Detection
Here I present results of the cloud detection method used to
identify clear-sky HIRS observations. The cloud detection
method utilizes the technique used by ISCCP in that it
identifies cloudy observations by spatial and temporal constrast
testing. The following figure gives histograms of the HIRS
channel 8 surface channel observations for N12 observations for
January 2, 1992. The spatial contrast test compares HIRS 8
observations from adjacent observations and identifies cloudy
observations for those with relative cold values. Temporal
contrast test compares observations from the same location from
the day previous and following the current observation and
assigns the observation as clear, cloud, mixed cloud or
undecided. Combining the two tests through a logic table gives
the final cloud/no cloud result. Only observations assigned as
clear are saved as clear-sky.
Cloud detection short-term test
A long-term cloud detection test is needed to remove regions
with persistent cloud cover. Long-term tests require gather
statistics on HIRS 8 over 15 and 30 days. Further comparison of
the clear-sky observations from the short-term test yield
remove the persistent cloud cover regions. The following figure
gives the results for N12 on January 30, 1992.
Cloud detection long-term test
Validation of the cloud detection algorithm with ISCCP AVHRR
data indicate good agreement between the two methods. Using
ISCCP DX pixel-grid data, we made direct comparisons with N12
observations from January 8, 1992 with the Pathfinder clear-sky
data. The first figure gives the all-sky HIRS 8 observations
over Western Africa for 3-9Z. High clouds can be seen by the
grey shading. Since N12 is a morning satellite, the desert
regions appear relatively cool. The second figure gives the
clear-sky results from the Pathfinder data. White areas
indicate clear skies. They correspond reasonably well with the
warmest HIRS 8 observations; however, large clear-sky regions
are still identified over the relatively cool desert regions.
The third figure gives the clear-sky regions as is identified
from ISCPP AVHRR data. The clear-sky regions are larger than
identified by Pathfinder. The convervative cloud test used in
Pathfinder results in 23% of the cloudy areas in Pathfinder to
be identified as clear with the ISCCP data. However, only 2% of
region for the Pathfinder data identified as clear are found to
be cloudy with ISCCP, thus indicating very good agreement
between the methods.
N12 HIRS 8 all-sky observations
N12 HIRS 8 clear-sky observations
N12 ISCCP AVHRR clear-sky observations
Last modified: Wed May 2 11:51:20 MDT 2001