GCM = general circulation model.

What are the objectives of the GCMs?

General characteristics of GCMs:

• All the models work by defining elements and laws for how a change in one element will effect its neighbor. Initial conditions are defined and then element interactions are computed according to the laws.
• Typically 10 vertical levels. Total number of grid elements modeling is10exp4 to 10exp6.
• Remember that there will be multiple parameters and multiple calculations for each grid element.
• Natural the finer the elements the better resolution your model will have, but the number of calculations and the computer power and speed you need increase by a power law function.

Three major grid type components: atmospheric, oceanic, land.

For an atmospheric element - what will be important properties describing its state?

• temperature
• density
• moisture and heat content
• composition
• thermal properties

For an oceanic element - what will be its important properties describing its state?

• wind stress at surface (output of atmospheric component).
• salinity.
• temperature - this in turn is a function of oceanic circulation patterns (another module).
• thermal properties.
• sea ice models - effects thermal exhange, albedo, water density, perhaps a separate element type.
• turbidity.

For a land element - what will be its important properties describing its state?

• albedo (with a vegetative component or snow/ice component that changes seasonally)
• mean elevation.
• soil moisture.
• thermal properties.
• surface roughness.

What will be the laws for how neighbors effect each other?

• Newton's laws of motion: movement in response to pressure gradients both in the atmosphere and the hydrosphere..
• heat exchange/absorbtion components, e.g. between adjacent atmospheric components and between land and water surfaces and atmospheric component (clouds and moisture very important).
• e.g laws for formation of sea-ice important.

What is input?

• Take into account orbital/seasonal factors for solar influx.
• Average heat exchange characteristics of surface elements.
• Changes in atmospheric composition.
• Changes in vegetative cover.

What will be output of computer?

• temperatures.
• wind patterns.
• precipitation?
• changes in snow cover, ice cover.
• many possibilities.

Challenges:

• Cloud parameterization: mismatch of scale, clouds produce important fluxes of mass, momentum, heat and moisture, and yet they occur at scale much smaller than grid elements of computer model.
• Linking oceanic circulation with atmospheric models.

How can you test how good the computer model is (the discussion question)?

Some thoughts on chaotic systems and projections.

• Lorenz equations - initial conditions sensitivity
• Strange attractors - and general predictability.
• Might not be able to predict specific changes in specific areas.

How accurate are the computer models?

How accurate are they in comparison to any competing methodology?

Readings:

• Anon., 1996, Volcanoes and Global Climate Change; NASA Facts, Goddard Space Flight Center, Greenbelt, Maryland, 4p.
• Larson, R. L., 1991, Geological consequences of superplumes; Geology 19, 963-966.
• Hansen, Goddard Space institute, look at effect on climate of Mt. Pinatubo eruption.