AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Bimodal CCN and Cloud Microphysics
James Hudson, Stephen Noble, Desert Research Institute
Abstract Number: 350 Working Group: Aerosols, Clouds, and Climate
Abstract Bimodal spectra observed by DRI CCN spectrometers in two aircraft field campaigns showed opposite relationships with droplets. Clouds associated with more bimodal CCN had higher droplet concentrations than clouds associated with more unimodal CCN in MASE stratus clouds whereas clouds associated with more bimodal CCN in ICE-T cumuli had lower droplet concentrations than clouds associated with more unimodal CCN. Furthermore, there was an order of magnitude less drizzle in clouds associated with bimodal CCN than clouds associated with unimodal CCN in MASE stratus and orders of magnitude greater drizzle in clouds associated with bimodal CCN than clouds associated with unimodal CCN in ICE-T cumuli. Therefore, chemical cloud processing that predominates in thinner stratus clouds with smaller droplets enhances first (albedo) and second (cloud lifetime) indirect aerosol effects (IAE) whereas predominant coalescence processing in thicker cumulus clouds with larger droplets reduces both IAE.
Surface aerosol bimodality at the Oklahoma ARM site correlated with greater cloud fractions, lower cloud base altitudes, greater liquid water paths, greater negative vertical wind and precipitation. These results provide further proof that clouds are the source of bimodal aerosol/CCN. Particle hygroscopicity (kappa) measurements obtained by matching differential simultaneous CCN and dry particle size spectra showed consistently greater kappa for larger particles. These lower critical supersaturation particles that resulted from cloud processing indicate chemical processing. Because of around-the-clock year-round coverage surface measurements provide more data more economically than aircraft campaigns. Thus, surface aerosol and remote sensing measurements that have now been validated at ARM sites can be used to further advance understanding of cloud processing. Cloud processed CCN may be so good (low critical supersaturations and high kappas) that they may prevent anthropogenic CCN from making cloud droplets, thus precluding IAE except for the anthropogenic contributions to the trace gases that do the chemical processing.