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Seven and a half years (June 2006 to November 2013) of Cloud-Aerosol Lidarwith Orthogonal Polarization (CALIOP) aerosol and cloud layer products arecompared with collocated Ozone Monitoring Instrument (OMI) aerosol index (AI)data and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloudproducts in order to investigate variability in estimates of biannual and monthlyabove-cloud aerosol (ACA) events globally. The active- (CALIOP) andpassive-based (OMI-MODIS) techniques have their advantages and caveats forACA detection, and thus both are used to derive a thorough and robustcomparison of daytime cloudy-sky ACA distribution and climatology. For thefirst time, baseline above-cloud aerosol optical depth (ACAOD) and AIthresholds are derived and examined (AI  =  1.0, ACAOD  =  0.015) foreach sensor. Both OMI-MODIS and CALIOP-based daytime spatial distributions ofACA events show similar patterns during both study periods (December–May)and (June–November). Divergence exists in some regions, however, such asSoutheast Asia during June through November, where daytime cloudy-sky ACAfrequencies of up to 10 % are found from CALIOP yet are non-existent fromthe OMI-based method. Conversely, annual cloudy-sky ACA frequencies of20–30 % are reported over northern Africa from the OMI-based method yetare largely undetected by the CALIOP-based method. Using a collocatedOMI-MODIS-CALIOP data set, our study suggests that the cloudy-sky ACAfrequency differences between the OMI-MODIS- and CALIOP-based methods aremostly due to differences in cloud detection capability between MODIS andCALIOP as well as QA flags used. An increasing interannual variability of ∼  0.3–0.4 % per year (since 2009) in global monthly cloudy-sky ACAdaytime frequency of occurrence is found using the OMI-MODIS-based method.Yet, CALIOP-based global daytime ACA frequencies exhibit a near-zerointerannual variability. Further analysis suggests that the OMI-derivedinterannual variability in cloudy-sky ACA frequency may be affected by OMIrow anomalies in later years. A few regions are found to have increasingslopes in interannual variability in cloudy-sky ACA frequency, including theMiddle East and India. Regions with slightly negative slopes of theinterannual variability in cloudy-sky ACA frequencies are found over SouthAmerica and China, while remaining regions in the study show nearly zerochange in ACA frequencies over time. The interannual variability in ACAfrequency is not, however, statistically significant on both global and regionalscales, given the relatively limited sample sizes. A longer data recordof ACA events is needed in order to establish significant trends of ACAfrequency regionally and globally.</p

Investigating the frequency and interannual variability in global above-cloud aerosol characteristics with CALIOP and OMI