Cloud Mask: Results, Frequency, Bit Mapping, and Validation UW Cloud Mask Working Group

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

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Cloudy casesas determined by AIRS cloud mask

Clear casesas determined by AIRS cloud mask

Sample AIRS/MODIS Cloud Mask Histogram

MODIS Cloud Mask

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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.