Atmospheric System Research (ASR)

Cloud-Aerosol-Precipitation Interactions

Atmospheric aerosols exert important “indirect effects” on clouds and climate by serving as cloud condensation nuclei (CCN) and ice nuclei that affect cloud radiative and microphysical properties.  For example, an increase in CCN increases the number concentration of droplets enhances cloud albedo, and suppresses precipitation that alters cloud coverage and lifetime.  However, in the case of moist and strong convective clouds, increasing aerosols may increase precipitation and enhance storm development.  Although aerosol-induced indirect effects on climate are believed to have a significant impact on global climate change, estimating their impact continues to be one of the most uncertain climate forcings.  

BNL tackles the interwoven issues using a blend of theory, observations (in situ and remote sensing), and modeling. We focus on improving the parameterizations of aerosol properties and cloud microphysics that are critical for evaluating indirect aerosol effects, such as parameterizations of aerosol hygroscopicity, CCN spectrum, droplet activation, number concentration, and the autoconversion process. This comprehensive, multi-faceted approach enables addressing important issues over a wide range of scales (from cloud-scale to climate model grid scale).  Newly developed parameterizations and/or physical understanding are evaluated using a suite of modeling tools as part of the FASTER project.

Selected Signature Research Accomplishments

• Found that aerosols affect cloud and precipitation by altering the spectral shape of the size distribution of cloud droplets (Liu and Daum, Nature, 2002).
• Developed a systems theory for droplet size distribution (Liu et al., Atmos. Res., 1995; Q. J. Roy. Met. Soc, 1997; J. Atmos. Sci, 1998, 2002).
• Developed a new rain initiation theory that treats rain initiation as a statistical barrier-crossing process (McGraw and Liu, Phys. Rev. Lett., 2003; Phys. Rev. E., 2004).
• Derived a series of analytical expressions for parameterizing the autoconversion process (Liu and Daum, J. Atmos. Sci., 2004; Liu et al., Geophys. Res. Lett., 2004, 2005, 2006, 2007, J. Atmos. Sci., 2006).
• Characterized the influences of aerosol chemical composition, mixing state, and hygroscopicity of organics on CCN spectrum and simulated aerosol indirect effects (Wang et al., Atmos. Chem. Phys., 2008; Wang et al., Atmos. Chem. Phys., 2010; Liu and Wang, Environ. Res. Lett., 2010).
• Provided the first observational evidence that the first indirect effect enhanced aerosol concentrations significantly enhances cloud downwelling surface longwave fluxes at in the Arctic surface (Lubin and Vogelmann, Nature, 2006), which has been studied relative to the surface impact of the shortwave indirect effect (Lubin and Vogelmann, GRL, 2007; Lubin and Vogelmann, Tellus, 2010).