HR: 1340h
AN: A53A-0143    [Abstracts]
TI: Aerosol indirect effects in small trade cumulus clouds
AU: * Xue, H
EM: huiwen.xue@noaa.gov
AF: NOAA Environmental Technology Laboratory, 325 Broadway, Boulder, CO 80305 United States
AU: Feingold, G
EM: graham.feingold@noaa.gov
AF: NOAA Environmental Technology Laboratory, 325 Broadway, Boulder, CO 80305 United States
AB: Previous studies have suggested that increases in aerosol concentrations lead to higher cloud reflectivity by increasing the number of cloud droplets (first aerosol indirect effect), as well as by reducing precipitation, and increasing liquid water path and fractional cloudiness (second aerosol indirect effect). Here we investigate the effects of aerosols on small warm trade cumulus clouds using a large eddy simulation model with coupled bin-microphysics. Results show that, as expected, pollution causes a reduction in precipitation and an increase in cloud-averaged optical depth, but that the response of cloud fraction and cloud liquid water path is not as simple as suggested by the second indirect effect. For the small cumulus case studied, it is found that droplet evaporation associated with the entrainment plays an important role in determining the cloud properties. Cloud droplets become smaller and evaporate more readily in the polluted case, leading to stronger evaporative cooling and consequently stronger turbulence and entrainment. This in turn enhances droplet evaporation, constituting a positive feedback. Therefore, the polluted clouds are shown to have smaller cloud fraction, smaller cloud sizes and depth. It is suggested that cloud fraction may only increase with increasing aerosol loading for larger clouds that are less susceptible to entrainment and evaporation.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0321 Cloud/radiation interaction
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005