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

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

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

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.

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

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 1. A present-climate simulation at T170 (512 x 256 cells) Forced with climatological observed sea-surface temperatures 12 simulated years completed 2. 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

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

Observations on Some Computational Issues

We Have Run the High Resolution Model on A Variety of Machines LabMachineVendorModel# nodes CPUs/ node CPU type Notes LLNLWhiteIBMRS/6000 SP (NH-2) Power Mhz World’s # 1 LLNLFrostIBMRS/6000 SP 6816 (NH-2) Power Mhz NERSCgseaborgIBM4 NERSCseaborgIBMRS/6000 SP 20816Power Mhz World’s # 2 LLNLTC2000Compaq1184EV Mhz LLNLPCR???128?2Pentium 4 Linux cluster

Computational Issues… speedup curve on NERSC gseaborg machine

A Preliminary Look at Selected Results: Simulations of Present Climate

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

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

Variables and primary validation data sets

Effects of Increasing Resolution From T42 -> T170

Model Improvement: AMIP 1 vs. AMIP 2 CLTcloud fraction Pprecipitation PSLsea level pressure PRWprecipitable water SHsensible heat T hPa temperature τuτu zonal wind stress τvτv meridional wind stress U hPa zonal wind V hPa meridional wind Z hPa geopotential height

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

Effects of Increasing Resolution From T170->T239 old

Effects of Tuning on Spatial Patterns of Results at T170 old

Tuning Reduces Biases

Precipitation Over USA

DJF Precipitation over USA gets Better as Resolution Increases

JJA Precipitation Over USA

As Resolution Increases, Convective Precipitation Decreases…

… and Large-Scale Precipitation Increases

Arctic Climate and Sea Ice

Many Climate Models Simulate Arctic SLP Distribution Poorly…

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

Effects of Increased Greenhouse Gases

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

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

T42

T170

T239

What Have We Learned? It is possible to run short global climate simulations at 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.

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 CO 2 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;

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.

We will make model output available for analysis If interested, contact me: