Characterization of AVHRR global cloud detection sensitivity based on CALIPSO-CALIOP cloud optical thickness information: demonstration of results based on the CM SAF CLARA-A2 climate data record
The sensitivity in detecting thin clouds of the cloud screening method being used in the CM SAF cloud, albedo and surface radiation data set from AVHRR data (CLARA-A2) cloud climate data record (CDR) has been evaluated using cloud information from the Cloud-Aerosol Lidar with Orthogonal Polariza...
Main Authors: | , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-02-01
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Series: | Atmospheric Measurement Techniques |
Online Access: | https://www.atmos-meas-tech.net/11/633/2018/amt-11-633-2018.pdf |
Summary: | The sensitivity in
detecting thin clouds of the cloud screening method being used in the CM
SAF cloud, albedo and surface radiation data set from AVHRR data
(CLARA-A2) cloud climate data record (CDR) has been evaluated using cloud
information from the Cloud-Aerosol Lidar with Orthogonal Polarization
(CALIOP) onboard the CALIPSO satellite. The sensitivity, including its global
variation, has been studied based on collocations of Advanced Very High
Resolution Radiometer (AVHRR) and CALIOP measurements over a 10-year period
(2006–2015). The cloud detection sensitivity has been defined as the minimum
cloud optical thickness for which 50 % of clouds could be detected, with
the global average sensitivity estimated to be 0.225. After using this value
to reduce the CALIOP cloud mask (i.e. clouds with optical thickness below
this threshold were interpreted as cloud-free cases), cloudiness results were
found to be basically unbiased over most of the globe except over the polar
regions where a considerable underestimation of cloudiness could be seen
during the polar winter. The overall probability of detecting clouds in the
polar winter could be as low as 50 % over the highest and coldest parts of
Greenland and Antarctica, showing that a large fraction of optically thick
clouds also remains undetected here. The study included an in-depth analysis
of the probability of detecting a cloud as a function of the vertically
integrated cloud optical thickness as well as of the cloud's geographical
position. Best results were achieved over oceanic surfaces at mid- to high
latitudes where at least 50 % of all clouds with an optical thickness down
to a value of 0.075 were detected. Corresponding cloud detection
sensitivities over land surfaces outside of the polar regions were generally
larger than 0.2 with maximum values of approximately 0.5 over the Sahara and
the Arabian Peninsula. For polar land surfaces the values were close to 1 or
higher with maximum values of 4.5 for the parts with the highest altitudes
over Greenland and Antarctica. It is suggested to quantify the detection
performance of other CDRs in terms of a sensitivity threshold of cloud
optical thickness, which can be estimated using active lidar observations.
Validation results are proposed to be used in Cloud Feedback Model
Intercomparison Project (CFMIP) Observation Simulation Package (COSP)
simulators for cloud detection characterization of various cloud CDRs from
passive imagery. |
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ISSN: | 1867-1381 1867-8548 |