DROPLET SPECTRAL BROADENING IN MARINE STRATUS

Broadening of the cloud droplet (diameter < 50 mu m) spectrum with inc reased droplet size was found to depend on the vertical profiles of cl oud water. Clouds with liquid water profiles resembling adiabatic cond itions displayed constant spectral widths. Other clouds displayed broa der droplet spectra and increasing broadness with mean droplet sizes. Less than adiabatic cloud liquid water profiles may be accounted for b y conversion to drops (diameter > 50 mu m, i.e., drizzle). Broad dropl et spectra were most closely associated with drizzle drops. Both the c oncentration, C, and slope, k, of the cloud condensation nuclei (CCN) spectra were theoretically found to affect droplet spectral width. For individual cloud parcels a higher C and lower k each contributed to b roader droplet spectra. When mixing among cloud parcels with different updrafts was considered, the predictions deviated especially at large r mean droplet diameters. Variations in updraft velocity result in dif ferences in droplet concentrations and mean droplet sizes. The predict ions for this internal mixing process showed greater droplet spectral widths for CCN spectra with higher k, especially at the larger mean dr oplet diameters. Instead of the individual parcel predictions of narro wer droplet spectra at larger mean droplet sizes, internal mixing pred icted increasing droplet spectral width with increasing mean droplet s ize.These predictions are consistent with the observations. First, whe n only cloud parcels with small mean droplet diameters (< 11 mu m) wer e considered, the polluted clouds that formed on CCN with higher C and lower k displayed broader droplet spectra than clean clouds. Cloud pa rcels with large mean droplet diameters (> 12 mu m) and large a,were o bserved only in clean conditions where k was high. Increasing droplet spectral width with mean droplet diameter (especially > 12 mu m) is ty pical of many observations here and elsewhere.