As previously described, the data used for this comparison is daytime data from TERRA MODIS between April 1 and April 8 2003. The MODIS cloud mask data are mapped to the same projection as the CLAVR-x data. Both zonal and global comparisons are included for this time period. Cloud amount is calculated by dividing the number of cloudy pixels (both confidently cloudy and probably cloudy) in a grid cell by the total number of pixels in the grid cell.
Table 9 shows the MODIS vs. CLAVR-x statistical scores. These scores indicate that the agreement between MODIS and CLAVR-x is slightly poorer than for ISCCP and CLAVR-x. However, this may be due to the difference in the amount of time included in the comparison (7 days vs. 1 month.) Weekly averages will tend to show more structure and
variability in global cloud amount and distribution than monthly averages. Despite this fact, the scores in Table 9 indicate moderate agreement between MODIS and CLAVR-x, with 20% or better agreement for nearly 65% of the pixels. When polar regions are excluded, this figure increases to almost 78% of the pixels. Disagreement greater than 60% occurs only slightly more than 1% of the time, and in the polar regions only. Globally, MODIS observes slightly more cloud than CLAVR-x.
Figure 10 shows the comparison of the CLAVR-x and MODIS total cloud amounts for April 1-8, 2003. The upper panels are the mean day-time cloud amounts for MODIS (right) and CLAVR-x (left). The image in the lower left shows the difference between MODIS and CLAVR-x while the zonal averages are given in the lower right. Because the period of
comparison is 7 days rather than one month as in the other comparisons, the global fields of cloud amount in Figure 10 show more structure than the monthly averages shown in previous comparisons. The CLAVR-x and MODIS fields show many similarities. However, the
difference plot in Figure 10 (lower left) reveals regions where the differences exceed 20%. For example, during this period, MODIS produced more cloud over Antarctica, the Arctic, and high latitude land surfaces in the Northern Hemisphere that are likely snow covered. These
differences between MODIS and CLAVR-x at high latitudes are also evident in the zonal averages (lower right). In addition, zonally averaged MODIS cloud amount exceeds that of CLAVR-x for almost all zones. Analysis of the spatial differences in the MODIS – CLAVR-x difference outside of the areas discussed above indicates that differences are distributed fairly uniformly, and are not concentrated in any single area. This uniform distribution of the MODIS – CLAVR-x differences indicates that cloud amount differences are not due differences in the detection of any one cloud type such as tropical cirrus. While MODIS has spectral channels missing from AVHRR that improve cirrus detection, the uniform distribution of the differences coupled with the non-uniform distribution of cirrus indicates that sensitivity to cirrus does not appear to be dominant factor in the MODIS – CLAVR-x differences. Overall, for this period, the global MODIS cloud amount is 8% higher than that from CLAVR-x. Much of this
difference is due to the differences at high latitudes. Because the zonal averages of CLAVR-x are in rough agreement with APP for the July and January months studies, it is unclear if this
difference at high latitudes is a weakness of CLAVR-x revealed by the improved spectral information from MODIS.
5. Conclusions
The results of this study support that the CLAVR-x cloud mask performs
consistently with other cloud mask products such as ISCCP, MODIS, and UW-HIRS. The fact that CLAVR-x includes multiple cloud mask classifications (clear, partly clear, partly cloudy and cloudy) as opposed to a simple binary (clear or cloudy) classification in its calculation of total cloud amount accounts for much of the difference between CLAVR-x and these other products.
However, the zonal mean trends in cloud amount exhibited by CLAVR-x are consistent with these other products. By selecting cases that cover a variety of seasons, this study has shown that CLAVR-x daytime cloud amount is reliable in both summer and winter cases. In addition, this study has shown that CLAVR-x has improved upon CLAVR-1 in two important respects.
First, cloud amounts from CLAVR-x may be used reliably from satellites with either morning or afternoon equator crossing times. Second, CLAVR-x has added a more rigorous algorithm for the detection of snow and ice. This has improved upon the CLAVR-1 cloud detection in the polar regions, as documented by the high degree of agreement between CLAVR-x and APP cloud amount. These improvements have been made while maintaining good agreement between CLAVR-x and CLAVR-1 in areas where CLAVR-1 has historically performed well, namely in the mid-latitudes for afternoon orbiting satellites. In light of these improvements, and the potential of the AVHRR data record being extended for an additional 14 years, CLAVR-x may prove to be a very useful tool for future studies of global clouds and their climatology.
Preliminary investigation of nighttime cloud amount by examination of the diurnal average cloud amount is encouraging, however, further study is needed to verify CLAVR-x nighttime cloud
amount reliability. In addition, future work should include regional studies that analyze
CLAVR-x cloud properties at different levels for a variety of cloud systems. This type of future study will help to identify the specific conditions under which the largest differences between CLAVR-x and other products exist. Future studies should also include comparisons of other satellite retrieved properties such as surface temperature, clear-sky albedo, and cloud top properties.
Acknowledgments
Funding for this research was provided by the NOAA Polar Program
(NA07EC0676). The authors would also like to thank Richard Frey for providing assistance with MODIS cloud mask processing, and Donald Wylie for providing data from UW-HIRS.
The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or U.S.
Government position, policy, or decision.
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