Michael Sigmond (University of Victoria) Paul J. Kushner (University of Toronto)

1. Michael Sigmond (University of Victoria) Paul J. Kushner (University of Toronto) John F. Scinocca (University of Victoria, CCCma) The Effect of Removing a Well-Resolved Stratosphere on the Simulation of the Tropospheric Climate, and Climate Change

2. Michael Sigmond (University of Victoria) Paul J. Kushner (University of Toronto) John F. Scinocca (University of Victoria, CCCma) The Effect of Removing a Well-Resolved Stratosphere on the Simulation of the Tropospheric Climate, and Climate Change

3. Motivation: Sigmond et al, 2007, (JGR, in press): Investigated robustness of the simulated response to climate change We forced 2 AGCM with a generic SST perturbation, varied horizontal resolution, and a single tuning parameter, and compared the responses Here: investigate robustness of response to climatechange to changing model top height Or: compare the global warming responses in ‘high-top’ with a ‘low-top’ model Do we need a well-resolved stratosphere to realistically model the future tropospheric climate? (Shindell et al 1998, Fyfe et al. 1999, Gillet et al. 2002)

4. Method: • Use different versions of the Canadian AGCM (T63 resolution) • forcing: 1) double atmospheric CO2 concentration 2) Forcing with ‘best-guess’ SST increase in 2xCO2 world (repeating annual cycle) Ensemble average SST response of 17 AR4 models in A1B scenario (2090-2100 minus 2000-1990) • equilibrium runs • All plots DJF

5. How to compare high-top with low-top models? 1) Take ‘best-tuned’ low-top model and compare it to ‘best-tuned’ high-top model HIGH: - CMAM: state-of-the-art stratosphere resolving GCM - 71 levels with top at 0.001 hPa - Used in several studies with interactive chemistry for stratospheric O3 predictions - Here: dynamical part (no coupling to chemistry) LOW: - GCM3: standard Canadian ‘tropospheric’ model - 31 levels with top at 1 hPa - Used for climate prediction, e.g. in IPCC AR4 report Problem: Model versions have different settings (vertical resolution, tuning, timestep) and physics  differences can be caused by more than just the model lid height

6. How to compare high-top with low-top models? (2) 2) Take low-top model and add layers Problems: - we need to add physics (radiation, non-orographic gravity wave drag) - we need to decrease time step

7. How to compare high-top with low-top models? (2) 2) Take low-top model and add layers Problems: - we need to add physics (radiation, non-orographic gravity wave drag) - we need to decrease time step 3) Take the high-top model and remove layers above a certain height LOWERED: - lowered version of ‘HIGH’: 41 levels with top at 10 hPa, with physics and dynamics as similar to standard CMAM - Not trivial to construct (radiation, sponge layer)

8. LOWERED (only removing levels above 10 hPa) (5y, control) HIGH LOWERED LOWERED-HIGH U T

9. HIGH  LOWERED (- Removing all layers above 10 hPa) - Removing (non-zonal) sponge layer - Remove non-LTE LW radiation module Was not ‘tuned’ for model with 10 hPa top, not needed below 10 hPa - conserve angular momentum in column Instead of letting momentum of gravity waves escape to space, deposit in uppermost layer (see Shaw et al. poster)

10. HIGH vs LOWERED (5y, control) HIGH LOWERED LOWERED-HIGH U T

11. RESPONSE to Climate change(40 year equilibrium runs)

12. ∆ SLP = SLP2xCO2 - SLPcontrol HIGHLOW (0.001 hPa top) (1 hPa top) AO+ Model lid height? LOWEREDLOW-G (10 hPa top) No! Just lowering model lid height does NOT change pattern of response (amplitude ~50%) AO+ AO+

13. ∆u = u2xCO2 - ucontrol HIGH LOW (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top)LOW-G -HIGH and LOWERED responses similar, but LOW response is different -Anomalous LOW response must be caused by difference in physics/model settings in LOW compared to LOWERED/HIGH

14. Which model setting in LOW compared to LOWERED causes the response to be so different? Make setting in LOW equal to that in LOWERED/HIGH and check if responses become more similar

15. ∆ SLP HIGHLOW (Gphil=0.65) (Gphil=1.0) AO+ LOWEREDLOW-G (Gphil=0.65) (Gphil=0.65) AO+ AO+

16. ∆ U HIGHLOW (Gphil=0.65) (Gphil=1.0) LOWERED (Gphil=0.65)LOW-G (Gphil=0.65) Response is more dependent on Gphil than on model lid height!!

17. Ucontrol gphil=0.65 gphil=1.0 HIGH LOWLOW-G gphil=0.65 Close to observations Too weak (waveguide to narrow) Closer to observations

18. Conclusions • Assessing the benefit of including a well-resolved stratosphere on the simulation of climate (change) is not straightforward • Response in standard ‘low-top’ model (no AO response) is different from that in standard ‘high-top’ model (AO+) (for this model) • When only lowering model lid height, the responses do not change very much • By making the orographic gravity wave settings in the standard low-top model consistent with that in the standard high-top model, we can get a very similar response as in the high-top model • The strength of orographic gravity waves appears crucial for response to climate change, more so than the model lid height (in this model) (pretty scary, isn’t it?) Michael.sigmond@ec.gc.ca

19. LOW vs LOW-G LOWLOW-G LOW-G minus LOW gphil=1.0 gphil=0.65 gphil=0.65 CONTROL 2xCO2 Climate Closer to observations

20. ∆T HIGHLOW LOWEREDLOW-G

21. ∆u (40 years) HIGH LOW (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top)LOW-G

22. ∆u (40 years) HIGH LOW (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top)LOW-G

23. ∆u (year 1-20) HIGH LOW (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top)LOW-G

24. ∆u (year 21-40) HIGH LOW (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top)LOW-G

25. ‘Construction’ of LOWEREDstep 1: removing all layers above 10 hPa HIGH LOWERED LOWERED-HIGH U T

26. ‘Construction’ of LOWEREDstep 2: removing (non-zonal) sponge layer HIGH LOWERED LOWERED-HIGH U T

27. ‘Construction’ of LOWEREDstep 3: Remove non-LTE LW radiation module HIGH LOWERED LOWERED-HIGH U T

28. ‘Construction’ of LOWEREDstep 4: conserve angular momentum in column HIGH LOWERED LOWERED-HIGH U T