Droplet Concentration and Spectral Broadening in Southeast Pacific Stratocumulus Clouds

Several airborne field experiments have been conducted to verify model descriptions of cloud droplet activation. Measurements of cloud condensation nuclei and updrafts are inputs to parcel models that predict droplet concentrations and droplet size distributions (spectra). Experiments conducted within cumulus clouds have yielded the most robust agreement between model and observations. Investigations of stratocumulus clouds are more varied, in part because of the difficulty of gauging the effects of entrainment and drizzle on droplet concentrations and spectra. Airborne lidar is used here to supplement the approach used in prior studies of droplet activation in stratocumulus clouds.

A model verification study was conducted using data acquired during the southern hemispheric Vamos Ocean Cloud Aerosol Land Study Regional Experiment. Consistency between observed and modeled droplet concentrations was achieved, but only after accounting for the effects of entrainment and drizzle on concentrations produced by droplet activation. In addition, predicted spectral dispersions were 74 % of the measured dispersions following correction for instrument broadening. This result is consistent with the conjecture that differential activation (at cloud base) and internal mixing - the processes simulated – are important drivers of true spectral broadening.