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

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 cloudysky 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.

A study of 15-year aerosol optical thickness and direct shortwave aerosol radiative effect trends using MODIS, MISR, CALIOP and CERES

“…Table 6 includes estimates for global oceans and for selected regions as defined in Table 3 from all four studies. It is interesting to note that positive trends in AOT (both from passive and active methods), SWARE and above-cloud AOT are found over the Bay of Bengal and the Arabian Sea (Alfaro-Contreras et al, 2016). Negative trends are found, across all four studies, over the Mediterranean Sea and the eastern coast of the US.…”

“…Finally, trend analyses from this study are intercompared with results from several selected studies (e.g., Zhang and Reid, 2010;Alfaro-Contreras et al, 2016;Toth et al, 2016). Consistency in increasing/decreasing AOT trends is found among the studies, using passiveand active-based instruments, over cloud-free and cloudy skies, as well as using narrowband and broadband observations over regions such as the Bay of Bengal, the Arabian Sea, the eastern US coast and the Mediterranean Sea.…”

“…It should be noted that only overocean data are used for the studies utilizing passive-based instruments (Zhang and Reid, 2010; current study). The estimated trends from the active-based studies (Alfaro-Contreras et al, 2016;Toth et al, 2016) included both land and ocean CALIOP data. Also, different data sampling, data screening and filtering methods are applied for different studies.…”

“…Similarly, the MODIS and CERES instruments on board the Aqua satellite have also been in operation for 14 years (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016). Taking advantage of these longer-term datasets from the Aqua and Terra satellites, several studies have already examined temporal variations in aerosol optical thickness (AOT) both on regional and global scales (e.g., Zhang and Reid, 2010;Hsu et al, 2012;Li et al, 2014;Alfaro-Contreras, 2016;Toth et al, 2016). For example, using 10 years (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) of Collection 5 (C5) Terra and Aqua MODIS Dark Target (DT) AOT data, Zhang and Reid, (2010) found a negligible AOT trend over global oceans but documented three regions with statistically significant increases in aerosol loadings, including the Indian Bay of Bengal, the Arabian Sea and the eastern coast of China.…”

Mapping the Twilight Zone—What We Are Missing between Clouds and Aerosols

“…By comparing these months, the seasonal differences in aerosol and cloud dynamics are considered, which are, induced by the shift of the inner-tropical convergence zone (ITCZ), seasonal differences in biomass burning, or dust aerosols [2,[21][22][23]. Figure 4 shows the global distribution of the occurrence frequency of all classes, as the ratio of amount of the pixels of each class and all pixels, for February and August respectively.…”