A general circulation model solves
Answer: A
The primitive equations are integrated forward in time on a three dimensional grid. Simpler models in the hierarchy do use energy balance equations, but they answer different questions. A grid is what makes it three dimensional.
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Match each type of climate model with what it does.
List I
A. Energy balance model
B. Radiative convective model
C. General circulation model
D. Earth system model
List II
1. three dimensional equations on a global grid
2. one or few equations for the planetary balance
3. a GCM with carbon and other cycles included
4. a single vertical column in detail
Answer: B
Correct answer: A-2, B-4, C-1, D-3
The four form a hierarchy of increasing complexity and decreasing tractability. An energy balance model reduces the planet to one or a few equations. A radiative convective model resolves a single vertical column in detail. A general circulation model solves the three-dimensional fluid equations on a global grid. And an Earth system model adds the carbon and other biogeochemical cycles to a GCM. The simpler members are not obsolete - they are what make a result understandable.
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Assertion (A): Simple climate models remain useful despite the existence of complex ones.
Reason (R): They are fast enough to explore a wide range of parameters and their behaviour is easy to understand.
Answer: C
Both are true and the reason gives both advantages, and a result that appears in both a simple and a complex model is a good deal more trustworthy than one that appears in only one. Speed allows a wide parameter sweep.
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Parametrisation in a climate model refers to
Answer: D
Cloud, convection and turbulence all occur on scales below the grid and must be represented rather than resolved. Parametrisation is the single largest source of difference between models. Nothing below the grid can be resolved.
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Assertion (A): Cloud processes must be parametrised rather than resolved in a global climate model.
Reason (R): Individual clouds are far smaller than any practical grid cell.
Answer: A
Both are true and the reason gives the necessity, and cloud parametrisation remains the largest source of the spread in climate sensitivity between models. Clouds are far smaller than a grid cell. The scheme chosen strongly affects the answer.
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Consider the following statements about parametrisation.
1. It represents processes too small to be resolved by the grid.
2. Different choices give different model behaviour.
3. A model with fine enough resolution needs none at all.
Answer: B
The first two are correct. The third is unattainable in practice, since molecular processes will always lie below any feasible grid however fine it becomes. Some parametrisation is always required. Resolution never removes the need entirely.
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A model has a horizontal grid spacing of 100 kilometres. What is the smallest feature it can represent even approximately?
Answer: C
Correct answer: a few hundred kilometres across
A feature needs several grid points to be represented at all, so the effective resolution is some multiple of the grid spacing - typically four to eight times it. A 100 kilometre grid therefore resolves nothing below a few hundred kilometres, which is why fronts, convective storms and mountain valleys must be parametrised rather than simulated.
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Assertion (A): The effective resolution of a model is coarser than its grid spacing.
Reason (R): Several grid points are needed before a feature is represented with any fidelity.
Answer: D
Both are true and the reason gives the rule of thumb, and quoting the grid spacing as though it were the resolution overstates what the model can actually do. Several grid lengths is the usual figure for the smallest feature carried faithfully.
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Spin-up of a coupled climate model is the process of
Answer: A
The deep ocean takes centuries to equilibrate, so a long integration is needed before an experiment can begin. Starting from an unspun state produces drift that can be mistaken for a climate signal. Centuries are needed for the deep ocean.
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Assertion (A): A long spin-up is needed before a coupled model experiment can begin.
Reason (R): The deep ocean equilibrates over centuries and drift would otherwise contaminate the result.
Answer: B
Both are true and the reason gives the necessity, and residual drift is usually removed by subtracting a parallel control run. Without that subtraction a slow trend in the model itself could be mistaken for a response to the forcing applied.
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