The effects of aerosol particles on the radiative balance of Earth are usually separated into direct clear-sky and indirect cloud effects. However, a recent study conducted by the University of Washington, NASA Langley, and NASA Ames pointed out the existence of a continuum, rather than distinct separation, in cloud and aerosol lidar observables originally thought to naturally separate into cloud and aerosol modes. The chief observable in the study is the integral of attenuated backscatter over the atmospheric column or part of the column (e.g., boundary layer). The continuum in the integrated backsatter, or so-called lidar albedo, implies an inherent a difficulty in separating cloud from aerosol and highlights the difficulty of cloud-aerosol discrimination in satellite imagery. The continuum is considered to be the consequence of at least two factors operating simultaneously for the lower atmosphere. First, substantial hygroscopic growth of PBL aerosol at the high relative humidity in the top of the PBL causes enhanced aerosol backscatter. Second, broken cloud elements and wisps of evaporating cloud extend the lower limit of the cloud backscatter to small values. The two effects lead to an overlap in the expected probability density functions of vertically integrated backscatter from clouds and aerosols, and bring into question the notion of classification of a column, or pixel in satellite imagery, as either cloudy or cloud-free. The study also found via simulations that the climate forcings derived from the sum of independently computed cloud and aerosol effects and from computation for coexisting cloud and aerosol fields can be quite different. These findings provide strong motives for more extensive studies of the apparent cloud-aerosol continuum and potential biases in estimates of aerosol direct climate forcing and cloud microphysical and optical properties due to errors in cloud-aerosol discrimination algorithms used on satellite observations.
The CALIOP instrument on CALIPSO, with its 100 m laser footprint and 333 m horizontal sample spacing and 30-60 m vertical resolution will provide a global data set to investigate aerosol embedded in cloud fields and characterize biases inherent in cloud/aerosol discrimination/clearing algorithms used on satellite imagery. Even these resolutions may lead to biases, however, especially when measurement noise is considered. Higher resolution/fidelity data from the Airborne HSRL (25 m horizontal and 30 m vertical resolution; more than an order of magnitude greater SNR over CALIOP) will be used to assess, via case studies, potential sampling and noise issues in the CALIOP-based assessment of the continuum. The HSRL is scheduled to be deployed in 20-25 underflights of the CALIPSO satellite during the first year of its operation on orbit. These data should provide illuminating case studies to investigate the components (e.g. wisp cloud vs wet aerosol/dust) that give rise to the continuum and provide insight on errors in cloud-aerosol discrimination algorithms as a function of technique and pixel size.
