Modeling Activities in the Department of Energy’S Atmospheric Sciences Program
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Modeling Activities in the Department of Energy’s Atmospheric Sciences Program Jerome D. Fast, Steven J. Ghan, and Stephen E. Schwartz March 2009 Modeling Activities in the Department of Energy’s Atmospheric Sciences Program Jerome D. Fast1, Steven J. Ghan1, and Stephen E. Schwartz2 1Pacific Northwest National Laboratory 2Brookhaven National Laboratory March 2009 Executive Summary The Department of Energy's 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 Earth's radiative balance and to 2) develop model-based representations of these processes that enable the effects of aerosols on Earth's 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 Energy's Climate Sciences Program and other aerosol and climate research programs. 2 Modeling Activities in the Department of Energy’s Atmospheric Science Program Jerome D. Fast1 and Steven J. Ghan1, and Stephen E. Schwartz2 1Pacific Northwest National Laboratory 2Brookhaven National Laboratory March, 2009 The focus of research performed by the Department of Energy’s Atmospheric Sciences Program (ASP) is improving the understanding of 1) atmospheric processes governing the formation, transformation, and removal of atmospheric aerosols (suspended solid or liquid particulate matter) associated with energy-related and other emissions and 2) the properties of these aerosols pertinent to radiative forcing of climate change. The reconfiguration of ASP since 2004 was brought about in recognition of the substantial contribution of anthropogenic aerosols to radiative forcing of climate change over the industrial period and of the large uncertainty in present estimates of this forcing. As described in some detail by Ghan and Schwartz [2007], the approach employed by ASP to develop the required understanding of atmospheric processes that govern aerosol evolution and aerosol radiative forcing needed by the climate modeling community consists of following broad categories of research activities: Instrumentation development that provides improved methodologies of measuring the amounts and properties of atmospheric aerosols as well as other quantities that affect aerosols for both field measurements and laboratory experiments. Laboratory experiments that determine the dependence of controlling variables and concentration of trace gases and particles or relate optical and cloud nucleating properties of aerosols to size-dependent composition under controlled conditions. Field measurements that quantify properties of particulate matter in the atmosphere, identify important transformation processes, and permit the evaluation aerosol processes represented in models. Theory that provides a framework for interpreting laboratory and field experiment results and their extension beyond the conditions measured. Modeling which develops and evaluates numerical treatments of aerosol processes on a variety of spatial scales, from zero-dimensional (i.e. box model) to local, regional, and global three-dimensional representations. Research in ASP is conducted by a highly interactive portfolio of peer-reviewed research projects conducted by investigators at DOE National Laboratories, other Federal Laboratories, universities, and the private sector. ASP is located organizationally within DOE’s Climate and Environmental Science Division and interacts with other programs in that division, including the Atmospheric Radiation Measurement (ARM) Program and the Climate Change Prediction Program (CCPP). More broadly, ASP interacts with other research activities on atmospheric composition and radiative forcing carried out under the auspices of the U.S. Climate Change Science Program (CCSP) and similar international research endeavors. This document describes ongoing modeling activities in ASP, shows the connections of these activities to each other and to other activities in the broader climate change research community, and identifies products and deliverables of ASP research. Gaps in the current portfolio of ASP modeling activities are identified and means of filling these gaps are also addressed. 3 1. Current Approach 1.1 Overview of Modeling in ASP A fundamental premise of the approach employed by ASP is that a quantitative description of important atmospheric processes that affect aerosols must be accurately represented in climate models. Many of these processes, such as chemical reactions, condensation, and coagulation are inherently local. But these local processes are coupled with larger-scale meteorological processes, such as transport and mixing, cloud- aerosol interactions, and wet and dry deposition that affect the concentration, composition, and size distribution of atmospheric particles. The interactions of local and larger-scale processes are often non- linear and complex. Representing aerosol processes in climate models necessitates development and evaluation of models suitable for application at a variety of scales: local (zero-dimensional) property and process models, regional-scale models, and global-scale models. The numerical representations of aerosol processes are often highly parameterized because of computational constraints, especially so for global-scale climate models. The overall multi-scale approach of modeling and the relationship between ASP laboratory, field, and modeling activities is outlined in Fig. 1. Figure 1. Relationship of ASP research components associated with different modeling approaches. The product of a laboratory experiment is a new or improved theoretical representation of a specific property or process, generally in zero dimensions. In some instances such representations can be immediately incorporated into global models, but usually they first need to be incorporated into more complex zero-dimensional models that take into account multiple aerosol properties and processes (arrow 1, Fig. 1). In this way, the behavior of a new or improved theoretical representation can be assessed when it interacts with representations of other particle processes and the performance can be quantified using measurements made during field studies (arrow 2, Fig. 1). In contrast with zero-dimensional aerosol property and process models that assume uniform conditions within a volume element (i.e. ‘box’) of the atmosphere, the domains of three-dimensional models encompass substantial gradients in aerosol concentrations, aerosol transformation rates, and meteorological conditions that influence these concentrations and rates. Three-dimensional models are required to simulate the full life cycle of the aerosol, coupling particulate transformation processes that occur in individual grid cells with a description of the transport and mixing between grid cells, emissions of primary particles and precursor gases, atmospheric radiation, cloud processing, and removal of particles and precursor gases by wet and dry deposition. Grid cells employed by large-scale models, however, do not generally resolve gradients in aerosol properties that are commonly observed. Consequently, many atmospheric processes pertinent to aerosols must be dealt with by employing parameterizations. But these parameterizations also introduce uncertainties into models. 4 In general, there are three types of three-dimensional atmospheric models: large-eddy simulation, regional, and global. Large-eddy simulation models employ grid spacings of 10 – 100 m so that local atmospheric processes can be resolved, such as the interaction of aerosols and cloud microphysics and the chemical transformation of aerosols within boundary layer eddies. Regional models, which employ horizontal grid spacings of 1 - 50 km resolve much of the spatial and temporal variability in concentrations and properties of aerosols and aerosol precursors commonly observed in field studies. These models are also able to include quite explicit representations of aerosol processes because of the limited domains and time periods for which such models are commonly applied. Therefore, it is desirable to test modules obtained from zero- dimensional aerosol property and process models within regional models (arrow 3, Fig. 1) to evaluate their performance by comparison with field study measurements (arrow 4, Fig. 1). Global models employ relatively coarse horizontal grid spacings (e.g. 50 – 100 km) and simplified treatments of aerosol processes to limit the computational expense associated with simulating atmospheric conditions over long time scales (decades to centuries). After