Investigating the frequency and interannual variability in global above-cloud aerosol characteristics with CALIOP and OMI
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) c...
Main Authors: | , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-01-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/47/2016/acp-16-47-2016.pdf |
Summary: | 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.</p> |
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ISSN: | 1680-7316 1680-7324 |