Observations from Combined MODIS and CALIPSO ATrain Instruments

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Observations from Combined MODIS and CALIPSO
A-Train Instruments Steve Ackerman and Robert
Holz University of Wisconsin-Madison Cooperative
Institute of Meteorological Satellite Studies
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CALIOP and MODIS make very different measurements
with different sampling characteristics. To
correctly compare, collocation must be done
carefully!
MODIS 250 m resolution image with the CALIOP
sampling represented by the red line. The finer
resolution of CALIOP makes careful collocation
important in an analysis of combined data streams.
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Careful Collocation
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Global Comparison
 
Comparison of MODIS cloud detection with
collocated observations from CALIPSO for the
entire month of August 2006. Over 5 million
observations went into the analysis.
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AUGUST 2006
The fractional agreement between the MODIS and
CALIPSOCALIOP cloud mask for clear FOV. The
fraction agreement calculated at 5-degree
resolution. A grid cell with perfect MODIS
agreement will have a fractional of 1 (red) while
regions of poor agreement are colored blue.
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The number of MODIS FOV for each grid cell used
to generate the fraction agreement.
AUGUST 2006
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AUGUST 2006
The fractional agreement between the MODIS and
CALIOP cloud mask for cloudy FOV. The fraction
agreement calculated at 5-degree resolution.
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AUGUST 2006
The number of MODIS FOV for each grid cell used
to generate the fraction agreement.
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February 2007
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February 2007
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February 2007
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February 2007
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CALIPSO/MODIS Comparison
black CALIPSO,red MODIS CO2
Comparison is helping to understand performance
of MODIS CO2 Slicing derived cloud altitudes.
Black points are MODIS cloud heights.
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Derived cloud top altitude comparison
August 2006
As expected, for thin clouds, the MODIS (IR
passive approach) is sensitive to a layer below
the physical cloud top.
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August 2006
The mean cloud top height differences for each
5-degree grid box the mean cloud height
difference (MODIS CALIOP) is calculated. A
negative difference (blue) results when mean
MODIS cloud height is below the CALIOP.
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February 2007
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Stratus problem
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MODIS Marine Stratus Cloud Height Over-Estimation
found and fixed
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High cloud differences expected
Holz et al 2006
A schematic of the lidar integrated cloud optical
depth at the level of the passive IR cloud top
retrieval.
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High cloud differences
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Summary

• Over 80 cloud detection agreement, with a strong
  regional dependence.
• Collection-5 MODIS marine stratus have cloud tops
  altitude too high.
• Bias in high cloud altitude is as expected.

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Backup
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Summary

• The average fractional agreement for MODIS IFOVs
  with CALIOP is 82 and 90 for clear and cloudy
  scenes respectively.
• For both day and night the MODIS cloud mask is
  in closer agreement with CALIOP for cloudy scenes
  than for clear.
• MODIS CO2 cloud height biases of cirrus are lower
  than CALIOP altitudes by 2.5 km, consistent
  with expectations for comparison with lidar.
• MODIS CO2 cloud height biases of cloud lower than
  5 km are lower than CALIOP altitudes by 0.2 km.
• Combined Retrievals for visible and infrared
  optical properties

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MODIS Aqua Collection 5 Research-LevelMultilayer
Flag vs. CALIPSO
CALIPSO single and multilayer layer (gt3 km)
co-locations, August 2006, 60 latitude
single
positive multilayer tests
no retrieval
gt single layer conservative, (though optically
thin cirrus detection by CALIPSO not expected to
be seen by MODIS)
gt relatively few false positives (13)
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The impact of nadir only sampling
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The impact of nadir only sampling