Investigating the Impact of Optically-thin Cirrus Cloud Contamination on Estimates of Direct Aerosol Radiative Forcing Using Collocated MODIS and CALIOP Observations

Abstract:

Cloud contamination is one of the larger sources of uncertainty in passive aerosol optical thickness (AOT) retrievals, due to inherent limitations in separating cloud from aerosol using passive observations. This paper describes efforts to quantify the fraction of aerosol direct radiative forcing attributable to optically-thin cirrus cloud contamination. The study will focus on the MODIS C6 (Collection 6) AOT retrievals using collocated MODIS-CALIOP observations. CALIOP provides the ability to detect optically-thin cirrus and quantify corresponding cloud optical thickness. Using the combined observations we compute the TOA radiative flux for both the total AOT measured by MODIS and the cirrus corrected AOT derived, by subtracting the CALIOP cirrus optical thickness from the cloud contaminated MODIS FOV. We find that based on CALIOP FOV, the MODIS (C5) AOT retrieval filtered using both the MODIS QA (Quality Assurance) and zero AACF (Aerosol Algorithm Cloud Fraction), has cloud contamination of 22.08% over land and 18.01% for ocean cases. The average filtered AOT and contaminated COT are 0.160 (0.161) and 0.056 (0.026) over land (ocean). The cloud contamination increases if only the MODIS QA is used. We will update the data with MODIS C6 AOT. We also find that the amount of cloud contamination has a strong regional dependence. South East Asia, Central Africa, Amazon Basin and North Pacific Ocean are the most cloud-contaminated regions for MODIS aerosol retrieval. The analysis will be presented for four years of observations (2010- 2013).