Quantifying above-cloud aerosol using spaceborne lidar for improved understanding of cloudy-sky direct climate forcing

Estimates of global mean direct climate forcing by absorbing aerosols located above boundary layer clouds are large, uncertain, and almost entirely unconstrained by observations. Spaceborne lidar offers a new opportunity for global constraints. Here we examine techniques for using liquid water clouds as lidar targets, allowing aerosol optical depth and A ˚ ngström exponent to be deduced directly from aerosol effects on light transmission. Two such techniques are examined using data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The first is a previously reported method based on measurements of cloud depolarization ratio (DR) at 532-nm wavelength. The second is a new method using measurements of cloud color ratio (CR), which is the ratio of the signal from the cloud at 1064 nm to that at 532 nm. Optical depth retrievals from these two methods compare favorably over the eastern tropical Atlantic Ocean during August 2006, when biomass burning aerosols are frequently advected over marine stratiform clouds. The CR technique is mainly sensitive to fine-mode aerosols and essentially insensitive to clouds and coarse-mode dust. Because anthropogenic aerosol is predominantly found in the fine mode, the CR technique can be used to help identify situations where anthropogenic cloudy-sky direct radiative forcing is occurring. We demonstrate this capability using 6 months data over the eastern tropical Atlantic Ocean.