Uncertainty Assessment of the Vertically-Resolved Cloud Amount for Joint CloudSat-CALIPSO Radar-Lidar Observations

The joint CloudSat-Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) climatology remains the only dataset that provides a global, vertically-resolved cloud amount statistic. However, data are affected by uncertainty that is the result of a combination of infrequent sampling, and a very narrow, pencil-like swath. This study provides the first global assessment of these uncertainties, which are quantified using bootstrapped confidence intervals. Rather than focusing on a purely theoretical discussion, we investigate empirical data that span a five-year period between 2006 and 2011. We examine the 2B-Geometric Profiling (GEOPROF)-LIDAR cloud product, at typical spatial resolutions found in global grids (1.0 degrees, 2.5 degrees, 5.0 degrees, and 10.0 degrees), four confidence levels (0.85, 0.90, 0.95, and 0.99), and three time scales (annual, seasonal, and monthly). Our results demonstrate that it is impossible to estimate, for every location, a five-year mean cloud amount based on CloudSat-CALIPSO data, assuming an accuracy of 1% or 5%, a high confidence level (>0.95), and a fine spatial resolution (1 degrees-2.5 degrees). In fact, the 1% requirement was only met by similar to 6.5% of atmospheric volumes at 1 degrees and 2.5 degrees, while the more tolerant criterion (5%) was met by 22.5% volumes at 1 degrees, or 48.9% at 2.5 degrees resolution. In order for at least 99% of volumes to meet an accuracy criterion, the criterion itself would have to be lowered to similar to 20% for 1 degrees data, or to similar to 8% for 2.5 degrees data. Our study also showed that the average confidence interval: decreased four times when the spatial resolution increased from 1 degrees to 10 degrees; doubled when the confidence level increased from 0.85 to 0.99; and tripled when the number of data-months increased from one (monthly mean) to twelve (annual mean). The cloud regime arguably had the most impact on the width of the confidence interval (mean cloud amount and its standard deviation). Our findings suggest that existing uncertainties in the CloudSat-CALIPSO five-year climatology are primarily the result of climate-specific factors, rather than the sampling scheme. Results that are presented in the form of statistics or maps, as in this study, can help the scientific community to improve accuracy assessments (which are frequently omitted), when analyzing existing and future CloudSat-CALIPSO cloud climatologies.

Automated summary

The study evaluated the uncertainties in the CloudSat-CALIPSO dataset and found that estimating a five-year mean cloud amount at every location is impossible. The existing uncertainties are primarily the result of climate-specific factors.