Variability of optical depth and effective radius in marine stratocumulus clouds
M. Szczodrak, P. H. Austin and P. B. Krummel
Journal of the Atmospheric Sciences , 2001, 58, pp. 2912-2926

Abstract

Radiance measurements made by the Advanced Very High Resolution Radiometer (AVHRR) on board five NOAA polar orbiting satellites were used to retrieve optical depth ($\tau$) and effective radius ($r_{eff}$) for 31 marine boundary-layer clouds over the eastern Pacific Ocean and the Southern Ocean near Tasmania. Both the liquid water path and a scaled measure of the droplet number concentration (Nsat) can be inferred from the satellite measurements. The measurements show marked variability in Nsat within some scenes, indicative of little or no mixing between clean and polluted marine air on spatial scales of 256 km. In the majority of scenes, however, the effective radius increases as the 1/5 power of optical depth. This is consistent with approximately uniform values for Nsat. in these scenes. In-situ aircraft measurements were made simultaneously with six AVHRR overpasses as part of the Southern Ocean Cloud Experiment (SOCEX II). The clouds sampled by these flights were significantly thicker than the typically 200 m thick eastern Pacific stratocumulus, with large vertical and horizontal variability. On five of the six flights, aircraft measurements of the cloud top effective radius were well-matched by the satellite retrievals.