HR: 1340h
AN: A23C-0822    [Abstracts]
TI: The impact of humidity above stratiform clouds on indirect aerosol climate forcing
AU: * Ackerman, A S
EM: andrew.ackerman@nasa.gov
AF: NASA Ames Research Center, MS 245/4, Moffett Field, CA 94035 United States
AU: Kirkpatrick, M P
EM: michael.kirkpatrick@utas.edu.au
AF: University of Tasmania, Private Bag 65, Hobart, TAS 7001 Australia
AU: Stevens, D E
EM: dstevens@llnl.gov
AF: Lawrence Livermore National Laboratory, L-103, Livemore, CA 94551 United States
AU: Toon, O B
EM: btoon@lasp.colorado.edu
AF: University of Colorado, Campus Box 392, Boulder, CO 80309 United States
AB: Increases in concentrations of cloud condensation nuclei are consistently observed to enhance cloud droplet concentrations. For clouds with a fixed volume of condensed water, droplets become smaller as their numbers increase, thereby increasing cloud optical depth and albedo. The resulting increase in cloud reflectivity is expected to offset some of the global warming resulting from accumulation of greenhouse gases, but the magnitude of this indirect aerosol effect is highly uncertain. Smaller droplets are less efficient at producing precipitation, and decreased precipitation is expected to result in increased cloud water and cloud cover, further increasing cloud reflectivity. Yet polluted marine boundary-layer clouds are not generally observed to hold more water. Here we use model simulations of stratocumulus clouds to show that suppression of precipitation from increased droplet concentrations leads to increased cloud water only when sufficient precipitation reaches the surface, a condition favored when the overlying air is moist. Otherwise, suppression of precipitation from increased droplet concentrations enhances entrainment of overlying dry air, thereby reducing cloud water and diminishing the indirect aerosol effect.
DE: 3354 Precipitation (1854)
DE: 3307 Boundary layer processes
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0345 Pollution--urban and regional (0305)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting