Steven J. Ghan

Droplet Nucleation: Physically-Based Parameterization and Validation

[1]xa0One of the greatest sources of uncertainty in simulations of climate and climate change is the influence of aerosols on the optical properties of clouds. The root of this influence is the droplet nucleation process, which involves the spontaneous growth of aerosol into cloud droplets at cloud edges, during the early stages of cloud formation, and in some cases within the interior of mature clouds. Numerical models of droplet nucleation represent much of the complexity of the process, but at a computational cost that limits their application to simulations of hours or days. Physically-based parameterizations of droplet nucleation are designed to quickly estimate the number nucleated as a function of the primary controlling parameters: the aerosol number size distribution, hygroscopicity and cooling rate. Here we compare and contrast the key assumptions used in developing each of the most popular parameterizations and compare their performances under a variety of conditions. We find that the more complex parameterizations perform well under a wider variety of nucleation conditions, but all parameterizations perform well under the most common conditions. We then discuss the various applications of the parameterizations to cloud-resolving, regional and global models to study aerosol effects on clouds at a wide range of spatial and temporal scales. We compare estimates of anthropogenic aerosol indirect effects using two different parameterizations applied to the same global climate model, and find that the estimates of indirect effects differ by only 10%. We conclude with a summary of the outstanding challenges remaining for further development and application.

Modeling Activities in the Department of Energy’s Atmospheric Sciences Program

The Department of Energys Atmospheric Science Program (ASP) conducts research pertinent to radiative forcing of climate change by atmospheric aerosols. The program consists of approximately 40 highly interactive peer-reviewed research projects that examine aerosol properties and processes and the evolution of aerosols in the atmosphere. Principal components of the program are instrument development, laboratory experiments, field studies, theoretical investigations, and modeling. The objectives of the Program are to 1) improve the understanding of aerosol processes associated with light scattering and absorption properties and interactions with clouds that affect Earths radiative balance and to 2) develop model-based representations of these processes that enable the effects of aerosols on Earths climate system to be properly represented in global-scale numerical climate models. Although only a few of the research projects within ASP are explicitly identified as primarily modeling activities, modeling actually comprises a substantial component of a large fraction of ASP research projects. This document describes the modeling activities within the Program as a whole, the objectives and intended outcomes of these activities, and the linkages among the several modeling components and with global-scale modeling activities conducted under the support of the Department of Energys Climate Sciences Program and other aerosol and climate research programs.

Simulation of Climate Forcing by Aerosols

The largest source of uncertainty in estimates of the radiative forcing governing climate change is in the radiative forcing due to anthropogenic aerosols. Current estimates of the global mean of the aerosol radiative forcing range from –0.3 to –3.0 watts per square meter (Wm-2 ) which is opposite in sign and possibly comparable in magnitude to the +2 Wm-2 forcing due to increasing greenhouse gases. We have developed a global aerosol and climate modeling system that provides arguably the most detailed treatment of aerosols and their impact on the planetary radiation balance of any model, but our estimates of radiative forcing have been hindered by our lack of access to high performance computing resources. We propose to use the MSCF to conduct a series of simulations with and without emissions of a variety of aerosol particles and aerosol precursors. These extensive simulations will enable us to produce much more refined estimates of the impact of anthropogenic emissions on radiative forcing of climate change. To take full advantage of the parallelism available on the MSCF MPP1, we will apply the Global Array Toolkit to dynamically load balance the reactive chemistry component of our model. We will adapt our modifications of the morexa0» serial NCAR Community Climate Model CCM2 to the parallel NCAR CCM3.10. «xa0less