Abstract
Abstract. Seven and a half years (June 2006 to November 2013) of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol and cloud layer products are compared with collocated Ozone Monitoring Instrument (OMI) aerosol index (AI) data and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloud products in order to investigate variability in estimates of biannual and monthly above-cloud aerosol (ACA) events globally. The active- (CALIOP) and passive-based (OMI-MODIS) techniques have their advantages and caveats for ACA detection, and thus both are used to derive a thorough and robust comparison of daytime cloudy-sky ACA distribution and climatology. For the first time, baseline above-cloud aerosol optical depth (ACAOD) and AI thresholds are derived and examined (AI = 1.0, ACAOD = 0.015) for each sensor. Both OMI-MODIS and CALIOP-based daytime spatial distributions of ACA events show similar patterns during both study periods (December–May) and (June–November). Divergence exists in some regions, however, such as Southeast Asia during June through November, where daytime cloudy-sky ACA frequencies of up to 10 % are found from CALIOP yet are non-existent from the OMI-based method. Conversely, annual cloudy-sky ACA frequencies of 20–30 % are reported over northern Africa from the OMI-based method yet are largely undetected by the CALIOP-based method. Using a collocated OMI-MODIS-CALIOP data set, our study suggests that the cloudy-sky ACA frequency differences between the OMI-MODIS- and CALIOP-based methods are mostly due to differences in cloud detection capability between MODIS and CALIOP as well as QA flags used. An increasing interannual variability of ∼ 0.3–0.4 % per year (since 2009) in global monthly cloudy-sky ACA daytime frequency of occurrence is found using the OMI-MODIS-based method. Yet, CALIOP-based global daytime ACA frequencies exhibit a near-zero interannual variability. Further analysis suggests that the OMI-derived interannual variability in cloudy-sky ACA frequency may be affected by OMI row anomalies in later years. A few regions are found to have increasing slopes in interannual variability in cloudy-sky ACA frequency, including the Middle East and India. Regions with slightly negative slopes of the interannual variability in cloudy-sky ACA frequencies are found over South America and China, while remaining regions in the study show nearly zero change in ACA frequencies over time. The interannual variability in ACA frequency is not, however, statistically significant on both global and regional scales, given the relatively limited sample sizes. A longer data record of ACA events is needed in order to establish significant trends of ACA frequency regionally and globally.
Highlights
The above-cloud aerosol (ACA) phenomenon, wherein significant active-based backscatter and passive-based scattered solar radiances are induced by particles above what are predominately lower-tropospheric clouds, has gained an increased amount of attention from the scientific community (e.g., Haywood et al, 2004; Wilcox et al, 2009; Coddington et al, 2010; Devasthale and Thomas, 2011; Wilcox, 2012; Kacenelenbogen et al, 2014)
Divergence exists in some regions, such as Southeast Asia during June through November, where daytime cloudy-sky ACA frequencies of up to 10 % are found from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) yet are non-existent from the Ozone Monitoring Instrument (OMI)-based method
Using a collocated OMI-Moderate Resolution Imaging Spectroradiometer (MODIS)-CALIOP data set, our study suggests that the cloudy-sky ACA frequency differences between the OMI-MODIS- and CALIOP-based methods are mostly due to differences in cloud detection capability between MODIS and CALIOP as well as QA flags used
Summary
The above-cloud aerosol (ACA) phenomenon, wherein significant active-based backscatter and passive-based scattered solar radiances are induced by particles above what are predominately lower-tropospheric clouds, has gained an increased amount of attention from the scientific community (e.g., Haywood et al, 2004; Wilcox et al, 2009; Coddington et al, 2010; Devasthale and Thomas, 2011; Wilcox, 2012; Kacenelenbogen et al, 2014). Global oceans are covered with clouds nearly 70 % of the time (e.g., Rossow and Schiffer, 1999), with almost non-existent corresponding ground-based verification data of ACA phenomena This exacerbates the impact of ACA effects globally, limiting characterization of any quantitative impact and frequency of occurrence almost exclusively to satellite-based measurements. The OMI aerosol index (AI), computed using the difference between observed and calculated ultraviolet (UV) radiances (Torres et al, 2007), has been used to locate UV-absorbing aerosols suspended over bright cloud decks (e.g., Yu et al, 2012; Torres et al, 2012) This technique, originally used on the Total Ozone Mapping Spectrometer (TOMS), can only be used to detect UV-absorbing aerosols, such as biomass burning smoke and desert dust aerosols, and is sensitive to underneath cloud properties (e.g., Yu et al, 2012; Alfaro-Contreras et al, 2014). We highlight regions susceptible to ACA occurrence, establishing a baseline for future ACA-induced biases in satellite cloud property retrievals overall
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