Droplet Activation in Cloud Updrafts

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Computer models of clouds and climate rest on assumptions that relate aerosol particles to the cloud droplets that form on them. Observations have revealed uncertainties in these relationships. For example, the concentration of cloud droplets computed from the measured sizes and chemical composition of aerosol particles usually overestimates the observed concentration. The aerosols that are capable of serving as centers for drop formation are the ones with an affinity for water (hygroscopic). Of these, the particles important for natural clouds in the atmosphere are those on which droplets form at relatively low values of ambient supersaturation. The cloud-nucleating ability of an aerosol population is measured by instruments called cloud condensation nucleus counters (CCN counters), which record the number of cloud droplets formed per unit volume of air as a function of the applied supersaturation. This project is an investigation of droplet formation in different types of clouds based on airborne observations with a CCN counter and other equipment. The work will be conducted as part of two field projects that are independently supported by NSF: DYCOMS-II, a study of marine stratocumulus clouds off the coast of California in July 2001; and a Wyoming-based study of continental cumulus clouds in the summer of 2002. The main objective of DYCOMS-II (Dynamics and Chemistry of Marine Stratocumulus) is to obtain high-resolution aircraft data (using the NCAR C-130) on temperature, humidity, vertical air velocity, and cloud properties, that can be used for validating large-eddy simulations of mixing processes in the marine boundary layer. This project contributes to DYCOMS by providing CCN measurements and the study of aerosol-droplet relationships. Likewise, it is an important component of the continental cloud study, based on observations with the Wyoming King Air aircraft. In both of these field projects, the airborne Wyoming Cloud Radar (a mm-wavelength Doppler radar) will be employed for measurements of vertical air motion near cloud base, a parameter that is needed to test the theoretical relationship between the activity spectra of CCN and the drops they produce. This information has not been available in previous aerosol studies, and may help to resolve some of the reported discrepancies between aerosol properties and droplet counts. The ultimate goal of the research is to improve the parameterization of clouds in large-scale models of atmospheric circulation and climate.