HR: 15:10h
AN: A33G-07    [Abstracts]
TI: Aerosol Scavenging by Cirrus Clouds: Evidence from Polarization Lidar Measurements
AU: * Sassen, K
EM: ksassen@gi.alaska.edu
AF: University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775, United States
AU: Zhu, J
EM: jzhu@gi.alaska.edu
AF: University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775, United States
AB: Under some conditions polarization lidar observations of ice clouds suggest the depletion of aerosol particles in the vicinity of the cloud margins and just below cloud base. This is particularly evident for cirrus that form in connection with transported desert dust layers, because these dust particles can be relatively large and hence strongly backscattering and depolarizing. Indications of aerosol scavenging primarily through the ice nucleation and gravitational capture processes, as well as phoretic forces where ice crystals are evaporating, can then produce a noticeable decrease in lidar backscattering and depolarization in clear (but recently cloudy) air. (In contrast, aerosol loss due to Brownian diffusion may not be as noticeable because these generally smaller particles are relatively weak backscatterers.) Presumably, previously cloudy air with its depleted interstitial aerosol is disclosed by variable fallstreak production and evaporation effects. The action of clouds is of great importance to the redistribution and removal (through precipitation) of aerosols suspended in the atmosphere. Polarization lidar examples will be given of aerosol-depleted halos surrounding cirrus cloud elements and backscatter troughs just below the base of undulating cirrus layers.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0322 Constituent sources and sinks
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting