Introduction to coupled climate modelling

1. Introduction to coupled climate modelling Andy Pitman

2. Outline • What is it • What can you do • What can’t you do • What opportunities exist with Mk3L ? • Assuming you want papers in top journals like Climate Dynamics and J. Climate.

3. background: • Climate models are how we project the future of the Earth’s climate • They are different from Earth System models – Mk3 is a climate system model, not an Earth System Model • It is absolutely not an EMIC

4. Climate models work …

5. Climate models work … • Most of the time. • All models are not equally good, and no one model is best. • No single measure of model performance presently exists [though we are working on it]

6. Climate models work • stunningly well at large spatial scales [continental to global] • Superbly on long time scales [seasonal to multi-annual averages] [they do not all capture all modes of variability equally well] • For prognostic and diagnostic quantities

7. Climate models work Isotherms show temperature, colours show error

8. Climate models work Rainfall [annual] Which is observed and which is modelled ?

9. Climate models work • Adequately at regional scales [say averages over >9 grid squares] • For prognostic and diagnostic quantities • but be very very careful:

10. 40% error 30-50% error Most climate models work Or at least, some models work

11. Most climate models work 40% error 25% error 30-50% error Or at least, some models work

12. Climate models work observed observed Climate models are not equal …

13. Climate models work MAM precip over eastern Pacific obs Though perhaps don’t look too close

14. Bad, does not matter Bad & matters Good and matters

15. Climate models work • Unreliably at local scales • Increasingly unreliably at monthly – daily scales • Combining these to look at fine resolution and short time scales requires • (a) great care; • (b) close contact with model developers

16. Climate models work • Don’t use the models blindly – work with a “real” climate modeller ! • Evaluate for your specific purpose • climate modellers do not assert that the models are reliable below continental scales [IPCC, 2001, Chapter 8] • they remain are our best tool …

17. CSIRO Mk3L • Personal view – do not use this model for this:

18. Personal view – don’t use it for this

19. What should we use it for? You can run 35 of the 3.3 x 5.6 model for one run of the 2.8 x 2.8

20. What should we use it for? • For multi-realization experiments

21. What should we use it for? • For multi-realization experiments – why ?

22. What should we use it for Models do not agree … but physics or chaos ?

23. … sensitivity of adding new processes [nitrogen]

24. What should we use it for? • Understanding abrupt change

26. Summary • In my view, if you want to explore regionally-specific issues, with a state of the art climate model, build links to groups that run such a model [CSIRO Mk3, CCAM or wait for ACCESS] • This is not what the CSIRO-Mk3L is for

27. Summary • A cheap coupled climate model is not to be used as if it were an expensive high resolution model • You must design experiments in full understanding of the strengths and weaknesses of the “L” in CSIRO Mk3L

28. Summary • Weaknesses: • Poorer regional skill • Poorer signal-noise ratio from local perturbations • Strengths • Capacity to run “n” realizations where “n” might be 10, 20, 50 … • Use it for uncerainty analysis, estimation of probability • Use it to build and test parameterization

29. Summary • CSIRO Mk3L is a low resolution coupled climate model • Its weaknesses can be overcome by careful experimental design • If you are not a climate modeller seek advice on design - from a “real” climate modeller – someone who builds rather than uses them • Then do innovative science like Manabe, Staniforth, Murphy etc and bask in the glory of Nature and Science papers