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Cloud Mask:
Results,
Frequency,
Bit Mapping,
and Validation
UW Cloud Mask Working Group
Overview
• Cloud mask
• Changes for Version 4
• Aqua and Terra comparisons
• Evaluation Activities
Our approach to the MODIS Cloud Mask, is for each pixel to provide a confidence flag
that indicates how certain we are that the pixel is clear.
RestrictionsRestrictions Real time execution Computer storage (4.8 g bytes per day) Comprehension
Thresholds range from 0 to 1
Some tests see cloud, some don’t
Cloud Mask Changes for v4.0• Band 2 (0.86 µm) ocean thresholds in sun-glint areas are now dynamically calculated based on the value of sun-glint angles. Reduces false cloudy designations and improves clear sky vs. low cloud discrimination.
• Coastal NDVI threshold for clear-sky restoral test changed from 0.3 to 0.4. Reduces false clear determinations in heavily vegetated river basins.
• Added new clear-sky restoral tests for non-snow covered land surfaces; 3.7-3.9 µm and 3.9-11 µm brightness temperature differences and 1.2/0.55 µm ratio tests. Reduces false cloud and uncertain determinations in arid and semi-arid regions.
• Set new 1.38 µm thresholds for thin cirrus test. There are separate thresholds for land, water, and snow/ice surfaces. These assume corrected 1.38 µm data.
• A 13-11 µm clear-sky restoral test was added for polar night conditions. Improves detection of clear skies in regions characterized by deep polar inversions.
• A new 11-3.9 µm cloud test for polar night conditions was added with dynamically calculated thresholds based on 11 µm brightness temperatures. Improves detection of low clouds.
Improved Performance of the
MODIS Cloud Mask.
Semi-arid Regions
MODIS Direct Broadcast Scene 17:17 UTC 28 May 2002
ClearProbably ClearUndecidedCloud
Old New
Evaluation
• Scene visual analysis
• Comparison with ground based
• Comparison with aircraft
• Geographic distribution
• Comparison with other satellite instruments
Aqua MODIS from February 3, 2003
MODIS Band 27 Hudson Bay
Very Dry
Aqua MODIS from February 3, 2003
MODIS Band 26 Hudson Bay
Aqua Cloud MaskBand 26 Cloud Test without Correction
Aqua Cloud MaskFinal Result without Correction
Green conf. clearCyan prob. clearRed uncertainWhite cloudy
Aqua MODIS from February 3, 2003
MODIS Band 7 Hudson Bay
Aqua Cloud MaskSurface Snow/Ice Test
Gray = snow/ice
Aqua MODIS from February 3, 2003
MODIS Band 2 Sahara Desert
Aqua Cloud MaskBand 26 Cloud Test without Correction
Aqua Cloud MaskFinal Result without Correction
Green conf. clearCyan prob. clearRed uncertainWhite cloudy
Radar/lidarMODISCloudMODISUncertainMODISProbably ClearMODISClearClear1968565175Low Cloud8204389Middle Cloud44313060High Cloud14163241591010871
Evaluation of MODIS Cloud Mask
Comparison of cloud heights from the Micropulse Lidar/ Millimeter Cloud Radar (MPL/MMCR) atthe DOE ARM SGP CART site to MODIS cloud mask results. The MODIS cloud mask algorithmand MPL/MMCR agreed on the existence of clear or probably clear 86% of the time (86+ 65/175) and 92% of the time that a cloud was present. An uncertain result occurred in less than 3% of the total comparisons.
2001 Yearly Mean Cloud Fraction from Terra MODIS
Boreal Spring Mean Cloud Fraction from Terra MODIS (Mar. - May 2001)
Boreal Summer Mean Cloud Fraction from Terra MODIS (Jun. - Aug. 2001)
Boreal Autumn Mean Cloud Fraction from Terra MODIS (Sep. - Nov. 2001)
Boreal Winter Mean Cloud Fraction from Terra MODIS (Dec. 2001 - Feb. 2002)
Aqua/Terra Cloud Mask Comparison
Terra MODIS Daytime Cloud Frequency from MOD35January 2003
Aqua MODIS Daytime Cloud Frequency from MOD35January 2003
Terra MODIS Average Daytime Band 1 Clear-sky Reflectance August 24, 2002
Aqua MODIS Average Daytime Band 1 Clear-sky Reflectance August 24, 2002
Terra MODIS Maximum Daytime Band 1 Clear-sky Reflectance August 24, 2002
Aqua MODIS Maximum Daytime Band 1 Clear-sky Reflectance August 24, 2002
Terra MODIS Average Daytime Band 31 Clear-sky Tbb (K)August 24, 2002
Aqua MODIS Average Daytime Band 31 Clear-sky Tbb (K)August 24, 2002
Terra MODIS Average Daytime Band 27 Clear-sky Tbb (K)August 24, 2002
Aqua MODIS Average Daytime Band 27 Clear-sky Tbb (K)August 24, 2002
Terra MODIS Frequency of Confident Clear (%)
August 24, 2002
Aqua MODIS Frequency of Confident Clear (%)
August 24, 2002
Terra MODIS Frequency of Probably Clear (%)
August 24, 2002
Aqua MODIS Frequency of Probably Clear (%)
August 24, 2002
Terra MODIS Frequency of Uncertain (%)
August 24, 2002
Aqua MODIS Frequency of Uncertain (%)
August 24, 2002
Terra MODIS Frequency of Confident Clear (%)
August 24, 2002
Terra MODIS Frequency of Cloud (%)
August 24, 2002
Aqua MODIS Frequency of Cloud (%)
August 24, 2002
3-Hourly Cloud Changes Measured by Terra and Aqua
Shown at left are zonal values of daytime land Terra and Aqua total high cloud frequency (top), and high, opaque cloud frequency (bottom) from August 24, 2002. The latter are mostly cold convective towers. With a local observing time of about 1:30 pm, roughly three hours later than Terra, we expect the Aqua measurements to indicate more convective activity and hence more thick, high clouds and more high clouds in general. This is clearly seen in the tropics and northern hemisphere where solar heating is greatest. For reference, the same data is plotted for ocean surfaces (right) where we would not expect to see changes in high clouds due to solar heating. Differences between Terra and Aqua are small as we expect, especially for high, opaque clouds
July 2002 AVHRR CLAVR/MODIS Cloud Mask Comparison
Global Cloud Detection Comparison
July 2002 cloud frequency from Terra MODIS Collection 3 cloud mask and NOAA-16 CLAVR.
Global Clear-sky Composites
July 2002 mean clear-sky 0.66 µm reflectances from MODIS Terra Collection 3 (top) and NOAA-16 CLAVR.
6 September 2002 AIRS/MODIS Cloud Mask Study
Granule 127
Not Cloudy Uncertain Probably Confident Determined Clear Clear
MODIS cloud mask
cMODIS fractionscloudy uncertain
probably clear confident clear
AIRS cloud flag. All confident clear
cle
ar
no
t cl
ea
r
Cloudy casesas determined by AIRS cloud mask
Clear casesas determined by AIRS cloud mask
Sample AIRS/MODIS Cloud Mask Histogram
MODIS Cloud Mask
Fra
ctio
n of
occ
uran
cecl
ear
case
sF
ract
ion
of o
ccur
ance
clou
dy c
ases
Range bins (%) of MODIS pixels within AIRS FOVsfor each MODIS cloud mask class
E.g. when the AIRS cloud mask said it was clear, ~72 percent of the time 90-100 percent of the MODIS pixels within those AIRS FOVs were determined to be confident clear (green) by the MODIS cloud mask, ~12 percent of the time 90-100 percent of the MODIS pixels within those AIRS FOVs were determined to be probably clear (cyan) by the MODIS cloud mask, ~0 percent of the time 90-100 percent of the MODIS pixels within those AIRS FOVs were determined to be uncertain (red) by the MODIS cloud mask, and ~3 percent of the time 90-100 percent of the MODIS pixels within those AIRS FOVs were determined to be cloudy (white) by the MODIS cloud mask.
