HR: 0800h
AN: A41C-10    [Abstracts]
TI: CCN Contrasts Below and Above California Stratus
AU: * Hudson, J G
EM: hudson@dri.edu
AF: Desert Research Institute, 2215 Raggio Pkwy, Reno, NV 89512-1095, United States
AB: Detailed aircraft measurements of cloud condensation nuclei (CCN) spectra associated with extensive cloud systems off the central California coast are presented. These measurements include the wide supersaturation (S) range (2-0.01%) that is important for these polluted stratus clouds. Measurements below ~0.1% S are not possible with most other CCN instruments. Concentrations up to 200 km from the coast were usually characteristic of continental/anthropogenic air masses. The most notable feature was the consistently higher concentrations above the clouds than below. Extensive comparisons throughout the eleven flights between two CCN spectrometers operated at different but overlapping S ranges displayed the precision and perhaps accuracy of these difficult spectral determinations. Good agreement within the S overlap range centered at 0.1% for a variety of concentrations and spectral shapes tends to verify the accuracy of the measurements over the entire S ranges. There are enough channels of resolution in these instruments to provide differential spectra, which produce more rigorous and precise comparisons than traditional cumulative presentations of CCN concentrations. Differential spectra are also more revealing than cumulative spectra. Only one of the eleven flights exhibited typical maritime concentrations. Average below cloud concentrations over the two hours furthest from the coast for the 8 flights with low polluted stratus was 614±233 at 1% S, 149±60 at 0.1% S and 57±33 at 0.04% S per cubic cm. Immediately above cloud average concentrations were respectively 74%, 55%, and 18% higher. Although cloud droplet concentrations are generally thought to be established at the base and thus related mainly to cloud base CCN, entrainment of air from above clouds also affects cloud microphysics. The fact that this air often contains significantly different CCN concentrations is important to efforts to understand these clouds and how they are disposed to the indirect aerosol effect. Concentration variability among those 8 flights was a factor of two. Variability within each flight excluding distances close to the coast ranged from 15-56% at 1% S. However, CN and probably CCN concentrations sometimes varied by less than 1% over distances of 1-9 km. This concentration consistency is important for understanding cloud microphysics.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Joint Assembly