Climate model
A simplified mathematical representation of the
climate system based on the physical, chemical, and biological
properties of its components, their interactions and feedbacks between
them.
In detail
The climate system can be represented by computer models comprised of
differential equations based on the basic laws of physics, fluid motion
and chemistry. The degree to which the model can simulate the response
of the climate system hinges to a large degree on the level of
understanding of the physical, geophysical, chemical and biological
processes that govern the climate system. They are of varying
complexity depending on the intended use and capabilities and include:
Simple Climate Models
Due to computational expense, simple climate models, calibrated to yield an
equivalent response in temperature and sea level to more complex
climate models. Their simplification is typically in the form of a
reduction in dimensionality to two or even zero. They can be used to
explore the potential sensitivity of the climate to a particular
process over a range of parameters. Simple climate models are also
used within larger integrated assessment models to analyse the costs of
emission reduction and impacts of climate change.
Earth system Models of Intermediate Complexity (EMICs)
These models are designed to bridge the gap between the three dimensional
comprehensive models and simple models. The main characteristic of
EMIOCs is that they describe most of the processes implicit in
comprehensive models, albeit in a more reduced (i.e. more
parameterised) form. They also explicitly simulate the interactions
among several components of the climate system including biogeochemical
cycles. They are computationaly efficient enough to allow for long-term
climate simulations over several tens of thousands of years or a broad
range of sensitivity experiments over several millennia.
Global Climate Models or General Circulation Models (GCMs)
These
are three dimensional climate models that solve the equations for fluid
motion and energy transfer around the globe and integrate these forward
in time. They solve the equations at intervals in time (typically 30
minutes) at a number of points forming a grid over the globe at
different levels in the atmosphere. Horizontally this results in grid
spacing of around 275–300 km over the UK.
Coupled Atmosphere-Ocean Global Climate Models (AOGCM)
Complex
climate model involving coupling comprehensive three-dimensional
atmospheric general circulation models within ocean general circulation
models, with sea-ice models and with models of land-surface processes.
Information about the state of the atmosphere and the ocean adjacent
to, or at the sea surface is used to compute exchanges of heat,
moisture and momentum between the two components. Computational
limitations mean that the majority of sub-grid scale processes are
parameterised.
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A number of countries have developed GCMs. In the UK the Met Office Hadley Centre have developed their own models (e.g. HadCM3).
Due to computing power it is not possible to model each point in space
and time so each of these global climate models breaks the globe into a
grid, usually at a resolution of 265 by 300 km over the UK.
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There are different types of GCM’s depending on whether they
incorporate dynamics from the atmosphere, the ocean or both. Two types
of GCMs have been used in UKCP09, the AGCM and the AOGCM.
Regional Climate Models (RCMs)
Use
of high resolution global models is computationally very demanding
which poses limits to the increase in resolution obtainable. To provide
information on a regional level, regional climate models (RCMs) have
been developed. They model the climate at a higher resolution for a
finite area (limited area at approximately 25–50 km resolution) and are
driven by the boundary conditions of the GCM. With the higher
resolution, including a higher resolution of the underlying geography,
RCMs are more able to simulate climate processes and feedbacks operating at the regional scale.
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The Met Office has developed a regional climate model (HadRM3) that was used in UKCP09.
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More details about the climate models used in UKCP09 are given in Box 2.1 in the UKCP09 Climate change projections report. A commentary about the strengths and weaknesses of climate models is given in Annex 3.
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