Effect of atmospheric gases, surface albedo and cloud overlap on the absorbed solar radiation

Recent studies have provided new evidencethat models may systematically underestimate cloud solar absorption compared toobservations. This study extends previous work on this "absorptionanomaly' by using observational data together with solar radiative transferparameterisations to calculate f_s (the ratio of surface andtop of the atmosphere net cloud forcings) and its latitudinal variation for arange of cloud types. Principally, it is found that (a) the zonal mean behaviourof f_s varies substantially with cloud type, with the highestvalues obtained for low clouds; (b) gaseous absorption and scattering canradically alter the pattern of the variation of f_s withlatitude, but gaseous effects cannot in general raise f_s tothe level of around 1.5 as recently determined; (c) the importance of thegaseous contribution to the atmospheric ASR is such that whilst f_srises with surface albedo, the net cloud contribution to the atmospheric ASRfalls; (d) the assumed form of the degree of cloud overlap in the model cansubstantially affect the cloud contribution to the atmospheric ASR whilstleaving the parameter f_s largely unaffected; (e) even largeuncertainties in the observed optical depths alone cannot account fordiscrepancies apparent between modelled and newly observed cloud solarabsorption. It is concluded that the main source of the anomaly may derive fromthe considerable uncertainties regarding impure droplet microphysics ratherthan, or together with, uncertainties in macroscopic quantities. Further,variable surface albedos and gaseous effects may limit the use ofcontemporaneous satellite and ground-based measurements to infer the cloud solarabsorption from the parameter f_s