Preliminary Results of Global Climate Simulations With a High-Resolution Atmospheric Model

1. Preliminary Results of Global Climate Simulations With a High-Resolution Atmospheric Model P. B. Duffy, B. Govindasamy, J. Milovich, K. Taylor, S. Thompson, M. Wehner Lawrence Livermore National Laboratory With help from J. J. Hack, NCAR

2. Contents • Pushing the limits of climate model resolution: • Why do it? • What have we done? • What have we learned?

3. Why Try to Increase Model Resolution? • To allow global climate models to give meaningful results on regional scales. • This will allow assessments of societal impacts of climate change • To improve simulations of terrestrial Carbon cycle • Results sensitive to precip. and surface temperature • Higher resolution • Includes fine-scale detail • Improves results on large scales

4. What Are Our Goals? • Assess computational issues associated with running the model at high resolution; • Evaluate the realism of the simulated climate at high resolution vs. coarse resolution; • Examine resolution dependence of atmospheric response to increased greenhouse gases.

5. Contents • Pushing the limits of climate model resolution: • Why do it? • What have we done? • What have we learned?

6. Six High-Resolution Simulations are Complete or in Progress • All simulations use the CCM3 atmospheric model forced by prescribed sea-surface temperatures (SSTs) • Analagous T42 simulations also have been performed • A present-climate simulation at T170 (512 x 256 cells) • Forced with climatological observed sea-surface temperatures • 12 simulated years completed • An increased GHG simulation at T170 • Forced with predicted SSTs for 2100 based on SST CHANGES from a coarse-resolution coupled model (CSM) simulation • 12 simulated years completed • Both of above use “untuned” (i.e. tuned for T42) model • 3. A present climate simulation at T170 with “tuned” model • 6 simulated years completed

7. Simulations Complete or in Progress… • 4. An AMIP simulation at T239 (720 x 360 cells) • Forced with observed sea-surface temperatures for 1979-1987 • 6 years completed with untuned model 5. A present-climate simulation at T239 • Uses model tuned for T170 • ~1 simulated year completed so far • 6. A present-climate simulation at T340 (!) (1024x512 grid cells) • This is short (1 simulated month)

8. Observations on Some Computational Issues

9. We Have Run the High Resolution Model on A Variety of Machines

10. Computational Issues… • speedup curve on NERSC gseaborg machine

11. A Preliminary Look at Selected Results:Simulations of Present Climate

12. Example“Taylor Diagram” Result of ideal model would be plotted here

13. Two Points About Our Taylor Diagrams 1. Taylor diagrams do not show errors in means (I.e. biases) 2. Comparison was performed on T42 grid Thus, we are assessing how finer resolution affects large-scale results

14. Variables and primary validation data sets

15. Effects of Increasing Resolution From T42 -> T170

16. Model Improvement: AMIP 1 vs. AMIP 2

17. Increasing Resolution vs. Actual Thinking AMIP 1 -> AMIP 2 T42 -> T170

18. Effects of Increasing Resolution From T170->T239 old

19. Effects of Tuning on Spatial Patterns of Results at T170 old

20. Tuning Reduces Biases

21. Precipitation Over USA

22. DJF Precipitation over USA gets Better as Resolution Increases

23. JJA Precipitation Over USA

24. As Resolution Increases, Convective Precipitation Decreases…

25. … and Large-Scale Precipitation Increases

26. Arctic Climate and Sea Ice

27. Many Climate Models Simulate Arctic SLP Distribution Poorly…

28. JJA sea level pressure distribution is more realistic at T170 than at T42

29. Effects of Increased Greenhouse Gases

30. We Calculate SST change from Coarse-Resolution Coupled Model (CSM) Simulation Sea Surface Temperature 1990s 2090s Simulated Time

31. Regional Details of Predicted Climate Changes Can be Very Different at T170

32. T42

33. T170

34. T239

35. What Have We Learned? • It is possible to run short global climate simulations at 50 - 75 km resolution. • CCM3 is reliable but not efficient at these resolutions; • Eulerian spectral dynamics seems to run inefficiently at high resolution. • Physics parameterizations seem robust to an increase in resolution • Most likely, retuning, but not reformulation, is needed. • In most aspects, large-scale results are more realistic at T170 than at T42; T239 is even better. I.e. using high resolution improves the large-scale results. • Partial re-tuning of cloud and hydrological parameters removes biases but has little effect on spatial patterns. • Climate changes due to increased greenhouse gases at T170 vs. T42 are very similar globally, but quite different regionally.

36. What’s Next? With More Funding, We Could • Work with the NCAR Environmental and Societal Impacts Group (ESIG) to design simulations of maximum benefit to the impacts community • Use appropriate CO2 scenarios • Save needed climate statistics • Distribute output to the community; • “Downscale” to fine resolution results of climate forecasts to be performed by NOAA; • Drive a 20 km nested model with output from our global T170 model; • Perform Initial Tendency Error Analysis (ITEA) at high resolution; • Compare results of high-resolution global model to those of a nested model at same resolution;

37. What’s Next?… • Run the new NCAR model (CAM1) at high resolution • Investigate computational behavior (scaling, etc.) • Evaluate simulated climate; • Perform a short coupled-model simulation with the atmosphere at high resolution; • Can high-resolution atmosphere improve simulation of Arctic sea ice? • Force sea ice model with winds, etc. from high-resolution atmospheric model; • Does higher resolution improve simulation of terrestrial C-cycle? • Go to even higher resolution • Etc.

38. We will make model output available for analysis If interested, contact me: pduffy@llnl.gov