DETERMINING THE SUSCEPTIBILITY OF CLOUD ALBEDO TO CHANGES IN DROPLET CONCENTRATION WITH THE ADVANCED VERY HIGH-RESOLUTION RADIOMETER

Combustion processes that produce greenhouse gases also increase cloud condensation nuclei (CCN) concentrations, which in turn increase clou d droplet concentrations and thereby cloud albedo. A calculation of cl oud susceptibility, defined in this work as the increase in albedo res ulting from the addition of one cloud droplet per cubic centimeter (as cloud liquid water content remains constant), is made through the sat ellite remote sensing of cloud droplet radius and optical thickness. T he remote technique uses spectral channels of the Advanced Very High R esolution Radiometer (AVHRR) instrument on board NOAA polar-orbiting s atellites. Radiative transfer calculations of reflectance and effectiv e surface and cloud emissivities are made for applicable sun and satel lite viewing angles, including azimuth, at various radii and optical t hicknesses for each AVHRR channel. Emission in channel 3 (at 3.75 mum) is removed to give the reflected solar component. These calculations are used to infer the radius and optical thickness that best match the satellite measurements. An approximation for the effect of the atmosp here on the signal received by the AVHRR is included in the analysis. Marine stratus clouds, as well as being important modifiers of climate , are cleaner than continental clouds and so likely to be of higher su sceptibility. Analysis of several stratus scenes, including some conta ining ship tracks, supports this expectation. The retrieved range of s usceptibilities for all marine stratus clouds studied varied by about two orders of magnitude. This variation implies that climate studies t hat include possible marine stratus albedo modification from anthropog enic CCN are incomplete without accounting for existing susceptibiliti es.