Atmos. Chem. Phys. Discuss., 12, 30805-30823, 2012
www.atmos-chem-phys-discuss.net/12/30805/2012/
doi:10.5194/acpd-12-30805-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Investigating relationships between aerosol optical depth and cloud fraction using satellite, aerosol reanalysis and general circulation model data
B. S. Grandey1,*, P. Stier1, and T. M. Wagner1
1Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, UK
*now at: Singapore-MIT Alliance for Research and Technology, Singapore

Abstract. Strong positive relationships between cloud fraction (fc) and aerosol optical depth (τ) have been reported. Data retrieved from the MODerate resolution Imaging Spectroradiometer (MODIS) instrument show positive fcτ relationships across most of the globe. However, these relationships are not necessarily due to cloud–aerosol interactions. Using state of the art Monitoring Atmospheric Composition and Climate (MACC) reanalysis-forecast τ data, which should be less affected by retrieval artifacts, it is demonstrated that a large part of the observedfcτ signal may be due to cloud contamination of satellite-retrieved τ. For longer MACC forecast time-steps of 24 h, which likely contain less residual cloud contamination, some negative fcτ relationships are found. ECHAM5-HAM general circulation model (GCM) simulations further demonstrate that positive fcτ relationships may arise due to covariation with relative humidity. Widespread negative simulated fcτ relationships in the tropics are shown to arise due to scavenging of aerosol by precipitation. Wet scavenging events are likely poorly sampled in satellite-retrieved data. Quantifying the role of wet scavenging, and assessing GCM representations of this important process, remains a challenge for future observational studies of aerosol–cloud–precipitation interactions.

Citation: Grandey, B. S., Stier, P., and Wagner, T. M.: Investigating relationships between aerosol optical depth and cloud fraction using satellite, aerosol reanalysis and general circulation model data, Atmos. Chem. Phys. Discuss., 12, 30805-30823, doi:10.5194/acpd-12-30805-2012, 2012.
 
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