Michael C. MacCracken

The Use of General Circulation Models to Predict Regional Climatic Change

Abstract Equilibrium simulations using the best-available general circulation models to estimate the sensitivity of the climate to a doubling of the atmospheric carbon dioxide concentration are in broad general agreement that the global annual average surface air temperature would increase 2.5 to 4.5 K. However, at finer spatial scales, the range of changes in temperature and precipitation predicted by different computer models is much broader. Many shortcomings are also apparent in the model simulations of the present climate, indicating that further model improvements are needed to achieve reliable regional and seasonal projections of the future climatic conditions.

UNDERSTANDING CLIMATIC IMPACTS, VULNERABILITIES, AND ADAPTATION IN THE UNITED STATES: BUILDING A CAPACITY FOR ASSESSMENT

Based on the experience of the U.S. National Assessment, we propose a program of research and analysis to advance capability for assessment of climate impacts, vulnerabilities, and adaptation options. We identify specific priorities for scientific research on the responses of ecological and socioeconomic systems to climate and other stresses; for improvement in the climatic inputs to impact assessments; and for further development of assessment methods to improve their practical utility to decision-makers. Finally, we propose a new institutional model for assessment, based principally on regional efforts that integrate observations, research, data, applications, and assessment on climate and linked environmental-change issues. The proposed program will require effective collaboration between scientists, resource managers, and other stakeholders, all of whose expertise is needed to define and prioritize key regional issues, characterize relevant uncertainties, and assess potential responses. While both scientifically and organizationally challenging, such an integrated program holds the best promise of advancing our capacity to manage resources and the economy adaptively under a changing climate.

Parametric study of the effects of arctic soot on solar radiation

A numerical radiative transfer model has been used to simulate the possible effects of carbonaceous aerosol on the Arctic climate. Because of uncertainties in aerosol properties in the Arctic, the sensitivity of modeled components of the solar radiation budget to ranges in aerosol parameters was determined. It was found that differences in assumed aerosol absorption, concentration and vertical distribution were more important than differences in aerosol size distribution. Aerosol absorption was parameterized through the index of refraction assuming an aerosol composed of soot and sulfates. Modeling the radiative effects of the springtime phenomena of Arctic soot leads to an increase in average heating rates of 0.01–0.06 K day−1 in the lowest 1–5 km of the atmosphere under cloud free conditions. The higher value is similar to estimated heating rates at northern latitudes that would result from doubling atmospheric CO2 concentrations. The hemispheric average temperature change due to the Arctic soot is, however, estimated to be almost two orders of magnitude less than for doubling of CO2 concentrations.

Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves

This scientific assessment examines changes in three climate extremes—extratropical storms, winds, and waves—with an emphasis on U.S. coastal regions during the cold season. There is moderate evidence of an increase in both extratropical storm frequency and intensity during the cold season in the Northern Hemisphere since 1950, with suggestive evidence of geographic shifts resulting in slight upward trends in offshore/coastal regions. There is also suggestive evidence of an increase in extreme winds (at least annually) over parts of the ocean since the early to mid-1980s, but the evidence over the U.S. land surface is inconclusive. Finally, there is moderate evidence of an increase in extreme waves in winter along the Pacific coast since the 1950s, but along other U.S. shorelines any tendencies are of modest magnitude compared with historical variability. The data for extratropical cyclones are considered to be of relatively high quality for trend detection, whereas the data for extreme winds and waves ar...

CLIMATE CHANGE SCENARIOS FOR THE U.S. NATIONAL ASSESSMENT

In support of the U.S. National Assessment of the Potential Consequences of Climate Variability and Change, climate scenarios were prepared to serve as the basis for evaluating the vulnerability of environmental and societal systems to changes projected for the twenty-first century. Since publication of the results of the assessment at the end of 2000, the National Research Councils report Climate Change Science: An Analysis of Some Key Questions, and the U.S. governments U.S. Climate Action Report—2002 have both relied on the assessments findings. Because of the importance of these findings, it is important to directly address questions regarding the representativeness and usefulness of the model-based projections on which the findings were based. In particular, criticisms have focused on whether the climate models that were relied upon adequately represented twentieth-century conditions and whether their projections of conditions for the twenty-first century were outliers. Reexamination of the approa...

The Livermore Regional Air Quality Model: I. Concept and Development

Abstract By using the Eulerian form of the mass conservation equation integrated vertically from the surface to the base of the inversion, two regional air quality models (LIRAQ-1 and LIRAQ-2) have been developed for use in the San Francisco Bay Area. The models consider the complex topography, changing meteorology and detailed source emission patterns in generating surface and vertical average pollutant concentrations with grid resolutions of 1, 2 or 5 km. The focus of LIRAQ-1 is the treatment of transport and dispersion of relatively nonreactive species, accomplished through use of a sophisticated transport prescription. The LIRAQ-2 model, employing a simpler transport scheme, treats photochemically active pollutants and incorporates a photochemical reaction set involving 19 species.

MAP3S: An investigation of atmospheric, energy related pollutants in the Northeastern United States

Abstract The Multi-State Atmospheric Power Production Pollution Study (MAP3S) is a major new atmospheric research program of the U.S. Energy Research and Development Administration. The goal of the MAP3S program is to develop and demonstrate an improved, verified capability to simulate the present and potential future changes in pollutant concentration, atmospheric behavior and precipitation chemistry as a result of pollutant releases to the atmosphere from large-scale power production processes, primarily coal combustion. A major motivation of this program is to be able to provide those agencies charged with the task of meeting the nations energy needs with the knowledge required to assess alternative strategies for generating power while ensuring ample protection of human health and adequate preservation of the natural environment Since coal is the most abundant domestic fossil energy resource and since electric power production is a major and growing sector of our energy economy, this study focuses on the effects of emissions from coal fired electric power plants, particularly sulfur oxide emissions. The study domain is the high population, energy intensive northeastern quadrant of the United States. Research projects are underway to measure present sulfur oxide concentrations and composition, to assess the potential for long range transport to investigate transformation processes in plumes from point and urban sources, to sample precipitation chemistry and improve understanding of scavenging mechanisms and to develop numerical models that can simulate future air quality on sub-continental scales given patterns of anticipated combustion emissions.

The Livermore Regional Air Quality Model: II. Verification and Sample Application in the San Francisco Bay Area

Abstract Topographic, meteorological, source emission and atmospheric pollutant concentration data have been assembled for use in verifying the LIRAQ-1 and LIRAQ-2 regional air quality models in the San Francisco Bay Area. These data, collected primarily during the high-pollution period of 26–27 July 1973, indicate that the temporal and spatial phasing for concentrations of carbon monoxide, ozone and nitrogen oxides can be adequately represented by the models. Sensitivity studies indicate that initial and horizontal boundary conditions as well as grid size and subgrid-scale effects, while very significant in predicting air quality on the local scale, are less important in dealing with regional concentrations of pollutants than are emissions, meteorological conditions and vertical boundary conditions.

Climate Change and Cloud Feedback: The Possible Radiative Effects of Latitudinal Redistribution

Abstract The sensitivity of outgoing longwave flux to changes in cloud cover (∂F/∂Ac) as defined by Cess (1976) must be evaluated carefully to avoid discrepancies arising from the interchange of averaging conventions. In a recent zonal atmospheric model experiment the global value of ∂F/∂Ac was different in sign than in other calculations. This difference in behavior was traced to a latitudinal redistribution of cloud amount and height that occurred in the doubled CO2 experiment. However, when ∂F/∂Ac was evaluated at individual latitudes and then weighted globally, the value of this parameter was consistent with those found by Cess (1976) and Budyko (1974).

On the treatment of advection in flux formulations for variable grid models, with application to two models of the atmosphere

Abstract Numerical simulations using the flux form of the conservation equations on a nonuniform, staggered grid are shown to require careful consideration with respect to the selection of the advective velocities used to calculate the flux transport terms. Results from both a simplified form of an urban boundary layer model and from a complex global climate model demonstrate the appropriateness of the “flux-weighted” averaging technique which is presented in the paper as an alternative to linear averaging.