Warren Washington and Gerald Meehl, National Center for Atmospheric Research
Bert Semtner, Naval Postgraduate School
John Weatherly, U.S. Army Cold Regions Research and Engineering Laboratory

This figure depicts a short simulation from the Parallel Climate Model (PCM). For the atmosphere, the figure shows vectors depicting the winds in the lowest model layer, and shows the sea level pressure as lines of constant pressure. The surface temperatures are shown in color, and the sea ice is shown in grayscale. (Illustration: Gary Strand, NCAR)

Research Objectives
The main purpose of this research is to use the Parallel Climate Model (PCM) and the Community Climate System Model version 2 (CCSM2) for studies of anthropogenically forced climate change simulations with higher resolution and more detailed model components. Because it is difficult to separate anthropogencic climate change from natural climate variability, it is necessary to carry out ensembles of simulations in order to find the statistically significant climate change signal.

Computational Approach
(1) With collaborators, we have developed an ocean component that uses the finite difference Parallel Ocean Program (POP) with a displaced North Pole. This model was modified from the original average resolution of 2/3° latitude and longitude to allow increased latitudinal resolution near the equator of approximately 1/2°. (2) The sea ice model component is entirely new. The thermodynamic part of the model uses the physics from C. Bitz's University of Washington ice model. It allows for five or more ice thickness categories and elaborate surface treatment of snow and sea ice melt physics. The elastic-viscous-plastic physics uses the E. Hunke and J. Dukowicz approach to the solution of the ice dynamics. (3) The atmospheric component is the massively parallel version of the NCAR Community Climate Model version 3 (CCM3). This model includes solar and infrared radiation, boundary physics, and precipitation physics. (4) The coupler's design allows the component models to execute concurrently as separate executables, or sequentially within a single executable, with the information exchange achieved by message passing (MPI). Since the component grids are different, there is an interpolation scheme for passing information between the atmosphere component grid and the ocean/sea ice grid, which has been designed to run efficiently on distributed memory architectures. Chris Ding and collaborators at NERSC have improved the performance of the coupler on parallel computers.

Accomplishments
During this past year, we completed many climate change simulations. Some of the simulations are of "business as usual" scenarios, in which there are no constraints on the use of greenhouse gases and aerosols. We have also performed various stabilization simulations. In both cases we completed five simulations that have different initial conditions. Using an ensemble is essential for evaluating the natural variability and to separate the climate change signal from the climatic noise. The societal interest is increasingly on regional changes.

Significance
The DOE Climate Change Prediction Program is focused on developing, testing and applying climate simulation and prediction models that stay at the leading edge of scientific knowledge and computational technology. The intent is to increase dramatically both the accuracy and throughput of computer model-based predictions of future climate system response to the increased concentrations of greenhouse gases. The PCM and CSM simulations have been highlighted in many parts of the latest report from the Intergovernmental Panel on Climate Change (IPCC).

Publications
J. W. Weatherly and Y. Zhang, "The response of the polar regions to increased CO₂ in a global climate model with elastic-viscous-plastic sea ice," J. Climate 14, 268 (2001).

A. Dai, T. M. L. Wigley, G. A. Meehl, and W. M. Washington, "Effects of stabilizing atmospheric CO₂ on global climate in the next two centuries," Geophysical Research Letters (in press).

A. Dai, G. A. Meehl, W. M. Washington, T. M. L Wigley, and J. M. Arblaster, "Ensemble simulation of 21st century climate changes: Business as usual vs. CO₂ stabilization," Bulletin of the American Meteorological Society (in press).

http://www.cgd.ucar.edu/pcm