Opposite Effects of Absorbing Aerosols on the Retrievals of Cloud Optical Depth from Space-borne and Ground-based Measurements

Absorbing aerosols above or within cloud layers have drawn much attention in recent years, due to substantially enhanced absorption of solar radiation that may affect reflection at the top of the atmosphere (TOA). The retrieval of cloud properties is usually conducted without any regard to aerosols. This study illustrates that retrievals of cloud optical depth (τc) from space-borne and ground-based sensors are both affected by such aerosols and lead to opposite biases. A ground-based retrieval algorithm is developed for the simultaneous retrieval of τc and cloud droplet effective radius using spectral irradiance measurements from a multi-filter rotating spectroradiometer and liquid water path (LWP) data from a microwave radiometer deployed in China. The algorithm is applied to data acquired from 17 May 2008 to 12 May 2009 at a heavily polluted site in the heart of the Yangtze Delta Region in China. The ground-based retrieval of cloud droplet effective radius increases with increasing LWP. MODIS retrievals tend to overestimate (underestimate) LWP when cloud LWP is less (greater) than about 200 g/m2. Model tests show strong sensitivities to the retrieval of τc from ground and space-borne sensors under varying absorption, loading and vertical distribution conditions. For absorbing aerosol mixed with cloud, τc tends to be under-estimated from space, but over-estimated from the ground, leading to very poor agreement between ground-based and Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals. Their differences increase with increasing τc. This finding suggests that in a turbid atmosphere with absorbing aerosols, the aerosol effect should be considered, or it would mislead any validation using satellite and ground-based retrievals.