11-MU-M EMISSIVITIES AND DROPLET RADII FOR MARINE STRATOCUMULUS

The results of a new multispectral infrared retrieval scheme for obtai ning fractional cloud cover and 11-mum emissivity are compared with th ose of the spatial coherence method which obtains fractional cloud cov er assuming that the clouds are opaque at infrared wavelengths. Both m ethods are applied to 4-km NOAA advanced very high resolution radiomet er global area coverage data for 250-km-scale regions containing singl e-layered marine stratocumulus off the coast of South America. The ave rage 11-mum emissivity for low-level clouds is found to be between 0.7 0 and 0.85. The low emissivity is evidently due to the thinning of clo uds at their edges. Semitransparent cloud edges evidently make up a su bstantial portion of the area covered by such clouds. This result indi cates that cloud cover obtained using the spatial coherence method is underestimated by 0.1 to 0.2, as has been claimed in a previous study. The fractional cloud cover for the ensemble of 250-km-scale regions s tudied here increased slightly from 0.60 for daytime observations to 0 .63 for nighttime observations. The 11-mum emissivity also increased s lightly, but about half of the increase was related to the increase in cloud cover and a decrease in the relative area covered by cloud edge material. Presumably, the other half was due to an increase in cloud liquid water. Cloud height showed no significant change. The average e ffective droplet radius increased from 9.3 mum for daytime observation s to 10.2 mum at night.