A case study of aerosol-cloud-radiation interaction

LLNL's 2-D cloud resolving model, CRM (Chin, 1994; Chin et al., 1995) along with a cloud drop number (Nd) parameterization (Chuang et al., 2002) and aerosol simulation data from LLNL's global chemistry/aerosol model (Chuang, 2005) is used to examine aerosol impacts on optical and microphysical properties of clouds. The CRM used is non-hydrostatic and fully compressible, containing ice microphysics (Lin et al., 1983, except for the warm cloud processes from Soong and Ogura, 1980), longwave (LW) and shortwave (SW) radiation (Fu and Liou, 1993), and surface energy equation (Zhang and Anthes, 1982). Aerosol characteristics as well as CRM's vertical velocity are applied to derive the cloud drop number concentration, which is then used to compute the subsequent auto-conversion process of rainwater and the optical properties of water clouds.

The model is initialized with a warm and moist bubble for a squall-like precipitation system passing through the central facility of ARM SGP site on June 19, 2004. With the parameterized cloud drop number concentration, three different schemes of auto-conversion process (Berry, 1968; Beheng, 1994; Chen and Cotton, 1987) for rainwater are used to compare with the Kessler (1969) type of parameterization to gauge the sensitivity of simulated precipitation to the model auto-conversion process. Although processes associated with ice cloud remain unchanged in all experiments, it is possible that the properties of ice cloud can be influenced through processes between liquid and ice phases. Additionally, an identical threshold value of cloud water mixing ratio (lcwc = 1 g kg-1) is applied to these four autoconversion parameterizations even though individual scheme may have its own optimal value for different application.

Preliminary results indicate that anthropogenic aerosols exert a significant impact on precipitation structure. The time lag to trigger a new convection by cold pool as well as the duration and precipitation associated with the active convective band vary with the representation of autoconversion and aerosol concentration. For Berry and Chen & Cotton schemes, the patterns of surface precipitation rate are similar with or without anthropogenic aerosols but the duration of precipitation with higher aerosol concentration is somewhat longer. In contrast, considerable differences in the evolution of surface precipitation pattern are noticed for Beheng scheme. Our results also suggest that the enhancement of reflected SW by anthropogenic aerosols can be up to 17 - 20 Wm-2 (averaged over the area of 200 x 1 km2). More discussion will be presented in the conference.

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* This work is supported by the Department of Energy Atmospheric Radiation Program and conducted under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.