HR: 0800h
AN: A51E-0838    [Abstracts]
TI: Study of the Aerosol Indirect Effect by Large-Eddy Simulation of Marine Stratocumulus
AU: * Lu, M
EM: julialu@caltech.edu
AF: California Institute of Technology, California Institute of Technology 210-41 1200 E. California Blvd. , Pasadena, CA 91125
AU: Seinfeld, J H
EM: seinfeld@caltech.edu
AF: California Institute of Technology, California Institute of Technology 210-41 1200 E. California Blvd. , Pasadena, CA 91125
AB: A total of 73 3D LES simulations of marine stratocumulus clouds covering both nighttime and daytime conditions were performed to explore the response of cloud optical depth ($\tau$) to various aerosol number concentration ($N_a$ = 50--2500 cm$^{-3}$) and the co-varying meteorological conditions (large-scale subsidence rate and SST). The idealized FIRE and the ASTEX Lagrangian 1 sounding profiles were used to represent the lightly and heavily drizzling cases, respectively. The first and second aerosol indirect effects are identified. Through statistical analysis, $\tau$ is found be to both positively correlated with $N_a$ and cloud liquid water path (LWP) with a higher correlation associated with LWP, which is predominantly regulated by large-scale subsidence and SST. Clouds with high LWP occur under low SST or weak large-scale subsidence. $\tau$ as a function of $N_a$ and SST (or large-scale subsidence rate) is also derived statistically, which can aid in isolating the cloud optical depth change due to the variation in CCN from the LWP variability associated with different meteorological conditions. Introduction of a small amount of giant sea salt aerosol into the simulation lowers the number of cloud droplets activated, results in larger cloud droplets, and initiates precipitation for non-drizzling polluted clouds. However, giant sea salt aerosol is found to have a negligible effect on $\tau$ for lightly precipitating cases, while resulting in a relative reduction of $\tau$ of 2%-66% (increasing with $N_a \geq$ 1000 cm$^{-3}$) for heavily precipitating cases, suggesting the impact of giant sea salt is only important for moist and potentially convective clouds. Finally, a regression analysis of the simulations shows that the second indirect effect is more evident in clean than polluted cases. The second indirect effect is found to enhance (reduce) the overall aerosol indirect effect for heavily (lightly) drizzling clouds, that is $\tau$ is larger (smaller) for the same relative change in $N_a$ than considering the Twomey (first indirect) effect alone. The aerosol indirect effect is lessened in daytime conditions and is dominated by the Twomey effect. This study suggests that concurrent observation of cloud LWP and $N_a$ is needed for assessing the aerosol indirect effect.
DE: 3354 Precipitation (1854)
DE: 1610 Atmosphere (0315, 0325)
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
DE: 0350 Pressure, density, and temperature
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting