Cloud droplet size distributions in low-level stratiform clouds

A database of stratus cloud droplet (diameter <50 mu m) size distribution p arameters, derived from in situ data reported in the existing literature, w as created, facilitating intercomparison among datasets and quantifying typ ical values and their variability. From the datasets, which were divided in to marine and continental groups, several parameters are presented, includi ng the total number concentration, effective diameter, mean diameter standa rd deviation of the droplet diameters about the mean diameter, and liquid w ater content, as well as the parameters of modified gamma and lognormal dis tributions. In light of these results, the appropriateness of common assump tions used in remote sensing of cloud droplet size distributions is discuss ed. For example, vertical profiles of mean diameter, effective diameter, an d liquid water content agreed qualitatively with expectations based on the current paradigm of cloud formation. Whereas parcel theory predicts that th e standard deviation about the mean diameter should decrease with height, t he results illustrated that the standard deviation generally increases with height, A feature common to all marine clouds was their approximately cons tant total number concentration profiles: however, the total number concent ration profiles of continental clouds were highly variable. Without cloud c ondensation nuclei spectra, classification of clouds into marine and contin ental groups is based on indirect methods. After reclassification of four s ets of measurements in the database, there was a fairly clear dichotomy bet ween marine and continental clouds, but a great deal of variability within each classification. The relevant applications of this study lie in radiative transfer and clima te issues, rather than in cloud formation and dynamics. Techniques that inv ert remotely sensed measurements into cloud droplet size distributions freq uently rely on a priori assumptions, such as constant number concentration profiles and constant spectral width. The results of this paper provide a b asis for evaluating the sensitivity of these techniques. In particular, the re were large enough differences in observed droplet spectral widths to sig nificantly affect remotely sensed determinations of cloud microphysics.