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

2. Overview Cloud mask Changes for Version 4 Aqua and Terra comparisons Evaluation Activities

3. 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. Restrictions Real time execution Computer storage (4.8 g bytes per day) Comprehension

4. Thresholds range from 0 to 1

5. Some tests see cloud, some don’t

6. 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.

7. Improved Performance of the MODIS Cloud Mask. Semi-arid Regions MODIS Direct Broadcast Scene 17:17 UTC 28 May 2002 Clear Probably Clear Undecided Cloud OldNew

8. Evaluation Scene visual analysis Comparison with ground based Comparison with aircraft Geographic distribution Comparison with other satellite instruments

9. Aqua MODIS from February 3, 2003 MODIS Band 27 Hudson Bay Very Dry

10. Aqua MODIS from February 3, 2003 MODIS Band 26 Hudson Bay

11. Aqua Cloud Mask Band 26 Cloud Test without Correction

12. Aqua Cloud Mask Final Result without Correction Green conf. clear Cyan prob. clear Red uncertain White cloudy

13. Aqua MODIS from February 3, 2003 MODIS Band 7 Hudson Bay

14. Aqua Cloud Mask Surface Snow/Ice Test Gray = snow/ice

15. Aqua MODIS from February 3, 2003 MODIS Band 2 Sahara Desert

16. Aqua Cloud Mask Band 26 Cloud Test without Correction

17. Aqua Cloud Mask Final Result without Correction Green conf. clear Cyan prob. clear Red uncertain White cloudy

18. Evaluation of MODIS Cloud Mask Comparison of cloud heights from the Micropulse Lidar/ Millimeter Cloud Radar (MPL/MMCR) at the DOE ARM SGP CART site to MODIS cloud mask results. The MODIS cloud mask algorithm and 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.

19. 2001 Yearly Mean Cloud Fraction from Terra MODIS

20. Boreal Spring Mean Cloud Fraction from Terra MODIS (Mar. - May 2001)

21. Boreal Summer Mean Cloud Fraction from Terra MODIS (Jun. - Aug. 2001)

22. Boreal Autumn Mean Cloud Fraction from Terra MODIS (Sep. - Nov. 2001)

23. Boreal Winter Mean Cloud Fraction from Terra MODIS (Dec. 2001 - Feb. 2002)

24. Aqua/Terra Cloud Mask Comparison

25. Terra MODIS Daytime Cloud Frequency from MOD35 January 2003

26. Aqua MODIS Daytime Cloud Frequency from MOD35 January 2003

27. Terra MODIS Average Daytime Band 1 Clear-sky Reflectance August 24, 2002

28. Aqua MODIS Average Daytime Band 1 Clear-sky Reflectance August 24, 2002

29. Terra MODIS Maximum Daytime Band 1 Clear-sky Reflectance August 24, 2002

30. Aqua MODIS Maximum Daytime Band 1 Clear-sky Reflectance August 24, 2002

31. Terra MODIS Average Daytime Band 31 Clear-sky T bb (K) August 24, 2002

32. Aqua MODIS Average Daytime Band 31 Clear-sky T bb (K) August 24, 2002

33. Terra MODIS Average Daytime Band 27 Clear-sky T bb (K) August 24, 2002

34. Aqua MODIS Average Daytime Band 27 Clear-sky T bb (K) August 24, 2002

35. Terra MODIS Frequency of Confident Clear (%) August 24, 2002

36. Aqua MODIS Frequency of Confident Clear (%) August 24, 2002

38. Terra MODIS Frequency of Probably Clear (%) August 24, 2002

39. Aqua MODIS Frequency of Probably Clear (%) August 24, 2002

40. Terra MODIS Frequency of Uncertain (%) August 24, 2002

41. Aqua MODIS Frequency of Uncertain (%) August 24, 2002

42. Terra MODIS Frequency of Confident Clear (%) August 24, 2002

43. Terra MODIS Frequency of Cloud (%) August 24, 2002

44. Aqua MODIS Frequency of Cloud (%) August 24, 2002

45. 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

46. July 2002 AVHRR CLAVR/MODIS Cloud Mask Comparison

47. Global Cloud Detection Comparison July 2002 cloud frequency from Terra MODIS Collection 3 cloud mask and NOAA-16 CLAVR.

48. Global Clear-sky Composites July 2002 mean clear-sky 0.66 µm reflectances from MODIS Terra Collection 3 (top) and NOAA-16 CLAVR.

49. 6 September 2002 AIRS/MODIS Cloud Mask Study

50. Granule 127 Not Cloudy Uncertain Probably Confident Determined Clear Clear MODIS cloud mask cMODIS fractions cloudyuncertain probably clearconfident clear AIRS cloud flag. All confident clear clear not clear

51. Cloudy cases as determined by AIRS cloud mask Clear cases as determined by AIRS cloud mask Sample AIRS/MODIS Cloud Mask Histogram MODIS Cloud Mask Fraction of occurance clear cases Fraction of occurance cloudy cases Range bins (%) of MODIS pixels within AIRS FOVs for 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.