forecasts. This forecast process depends on scientific understanding of complex physical and chemical processes in the atmosphere and detailed simulation of momentum, heat, energy, and molecular exchange between the land, ocean, and atmosphere. The radiant energy from the Sun is converted into thermal energy and then into the kinetic energy of winds and storms, all involving a highly nonlinear exchange of energy among small- and large-scale phenomena and a continuing exchange of water and chemical constituents between the atmosphere, oceans, and land surface. Because of these continuous exchanges, it is difficult to talk about atmospheric science without also considering ocean, land, and ice.

Skill in atmospheric prediction builds on scientific understanding converted into the mathematical expressions that become a numerical model of Earth and its atmosphere, oceans, and land surface, including detailed treatments of land and ocean biogeochemistry as well as the radiative impacts of atmospheric aerosols and gas-phase chemistry. The ocean is the planet’s reservoir of thermal energy and water, the land surface alters energy fluxes, and plants in both the ocean and on land maintain the oxygen balance. The atmosphere is the high-speed transport system that links them together and drives toward a thermodynamic equilibrium that is never attained. The central task facing atmospheric scientists is to unite sufficiently powerful science and sufficiently powerful computers to create a numerical counterpart of Earth and its atmosphere with the similitude required to manage weather, climate, and environmental risk with confidence in the years ahead.

This chapter identifies the major frontier challenges that the atmospheric sciences are attacking in order to realize that central task. In order to identify those challenges, the committee relied on several recent reports, including the following:

• European Centre for Medium-Range Weather Forecasts, 2006, ECMWF Strategy 2006-2015. Available at http://www.ecmwf.int/about/programmatic/2006/index.html.

• Intergovernmental Panel on Climate Change (IPCC), 2007, Climate Change 2007: The Physical Science Basis, Working Group I report, Cambridge University Press.

• National Research Council (NRC), 1998, The Atmospheric Sciences Entering the Twenty-first Century, Washington, D.C: National Academy Press.

• NRC, 2000, From Research to Operations in Weather Satellites and Numerical Weather Prediction: Crossing the Valley of Death, Washington, D.C.: National Academy Press.

• NRC, 2001, Effectiveness of U.S. Climate Modeling, Washington, D.C.: National Academy Press.

• NRC, 2002, Abrupt Climate Change: Inevitable Surprises. Washington, D.C.: National Academy Press.

• National Science Foundation (NSF), 2000, NSF Geosciences Beyond 2000: Understanding and Predicting Earth’s Environment and Habitability. Available at http://www.nsf.gov/geo/adgeo/geo2000.jsp.

• University Corporation for Atmospheric Research (UCAR). 2005. Establishing a Petascale Collaboratory for the Geosciences: Scientific Frontiers and Establishing a Petascale Collaboratory for the Geosciences: Technical and Budgetary Prospectus. Technical Working Group and Ad Hoc Committee for a Petascale Collaboratory for the Geosciences. Available at http://www.geo-prose.com/projects/petascale_tech.html.

Members of the committee also consulted the peer-reviewed literature in the atmospheric sciences, especially the references listed at the end of this chapter. The committee had extensive discussion with invited guests in a daylong workshop, details of which are included in Appendix B. Further discussions were undertaken with the directors and senior executives of the National Centers for Environmental