Introduction to coupled climate modelling

Andy Pitman

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.

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

Climate models work …

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]

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

Climate models work

Isotherms show temperature, colours show error

Climate models work

Rainfall [annual]

Which is observed and which is modelled ?

Climate models work

• Adequately at regional scales [say averages over >9 grid squares]

• For prognostic and diagnostic quantities

• but be very very careful:

Most climate models work

Or at least, some models work

40% error

30-50% error

Most climate models work

Or at least, some models work

25% error40% error

30-50% error

Climate models work

Climate models are not equal …

observedobserved

Climate models work

Though perhaps don’t look too close

MAM precip over eastern Pacific

obs

Bad, does not matter

Bad &matters Good and matters

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

• 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 …

Climate models work

CSIRO Mk3L

• Personal view – do not use this model for this:

Personal view – don’t use it for this

What should we use it for?

Lat Lon levels Total

4 x 7 40 48 5 9600

3.3 x 5.6 54 64 10 3584

2.8 x 2.8 64 128 15 122880

1.25 x 1.25 144 288 30 1244160

You can run 35 of the 3.3 x 5.6 model for one run of the 2.8 x 2.8

What should we use it for?

• For multi-realization experiments

What should we use it for?

• For multi-realization experiments – why ?

What should we use it for

Models do not agree … but physics or chaos ?

… sensitivity of adding new processes [nitrogen]

What should we use it for?

• Understanding abrupt change

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

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

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

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