Grandey, BS; Stier, P; Wagner, TM (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.
Strong positive relationships between cloud fraction (f(c)) and aerosol optical depth (tau) have been reported. Data retrieved from the MODerate resolution Imaging Spectroradiometer (MODIS) instrument show positive f(c)-tau relationships across most of the globe. A global mean f(c) increase of approximately 0.2 between low and high tau 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 tau data, which should be less affected by retrieval artefacts, it is demonstrated that a large part of the observed f(c)-tau signal may be due to cloud contamination of satellite-retrieved tau. For longer MACC forecast time steps of 24 h, which likely contain less cloud contamination, some negative f(c)-tau relationships are found. The global mean f(c) increase between low and high tau conditions is reduced to 0.1, suggesting that cloud contamination may account for approximately one half of the satellite-retrieved increase in f(c). ECHAM5-HAM general circulation model (GCM) simulations further demonstrate that positive f(c)-tau relationships may arise due to covariation with relative humidity. Widespread negative simulated f(c)-tau 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.