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 fc-τ relationships across most of the globe. A global mean f c increase of approximately 0.2 between low and high τ conditions is found for both ocean and land. 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 artefacts, it is demonstrated that a large part of the observed fc-τ signal may be due to cloud contamination of satellite-retrieved τ. For longer MACC forecast time steps of 24 h, which likely contain less cloud contamination, some negative f c-τ relationships are found. The global mean fc increase between low and high τ conditions is reduced to 0.1, suggesting that cloud contamination may account for approximately one half of the satellite retrieved increase in fc. ECHAM5-HAM general circulation model (GCM) simulations further demonstrate that positive fc-τ relationships may arise due to covariation with relative humidity. Widespread negative simulated fc-τ relationships in the tropics are shown to arise due to scavenging of aerosol by convective precipitation. Wet scavenging events are likely poorly sampled in satellite-retrieved data, because the properties of aerosol below clouds cannot be retrieved. 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. © Author(s) 2013.
CITATION STYLE
Grandey, B. S., Stier, P., & Wagner, T. M. (2013). Investigating relationships between aerosol optical depth and cloud fraction using satellite, aerosol reanalysis and general circulation model data. Atmospheric Chemistry and Physics, 13(6), 3177–3184. https://doi.org/10.5194/acp-13-3177-2013
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