Tuesday, May 22, 2012

4491.txt

cc: pmcox@meto.gov.uk, p.j.valdes@reading.ac.uk, jfbmitchell@meto.gov.uk, sfbtettatXYZxyzo.gov.uk
date: Fri, 28 Aug 1998 17:29:45 +0100
from: Peter Cox <pmcoxatXYZxyzo.gov.uk>
subject: Re: writing Thematic proposal
to: Keith Briffa <k.briffaatXYZxyz.ac.uk>

Keith,

I attach a draft of the model description/development section of the
NERC Thematic proposal which I produced by editing Simon's contribution
and adding some bits on the carbon cycle. Let me know if you need
anything else.

Cheers

Peter
Believable predictions of climate change require models which both
reproduce the current climate and its variability, and also respond
realistically to previous climatic forcings (e.g. well documented changes
in CO2 and solar insolation). The model development programme at the Hadley
Centre mirrors these goals, in aiming to improve the representation of the
processes already included in GCM's, and also to incorporate new processes
and related feedbacks which may be relevant to future (and indeed past)
climatic changes.

On both counts, the third generation Hadley Centre coupled climate model,
HadCM3, must be considered a significant advance over its predecessor.
HadCM3 (Gordon et al,?) was successfully developed to remove the need for
ocean flux-corrections and multi-century integrations have already been
completed which show minimal climate drift. A suite of improvements were
made to both the ocean and atmosphere components of the GCM. The ocean
model resolution was increased to 1.25 degrees by 1.25 degrees (from
3.25 by 2.5 degrees) and this along with new parametrisations of mixing
processes (due to Gent and McWilliams,? and Visbeck,?) has resulted in
a much improved simulation of ocean currents. The atmospheric component of
HadCM3 includes a state-of-the-art radiation code (Edwards and Slingo,?),
which allows explicit representation of the major greenhouse gases and
aerosols and a parametrisation of momentum transport by convection (Kershaw
and Gregory, ?). A new land surface scheme, called ``MOSES'', accounts for
the effects of soil water phase changes and the impact of CO2 concentrations
on transpiration (Cox et al, 1998). HadCM3 represents the first step in the
development of an Earth System Model in that it includes a sulphur-cycle
sub-model which converts prescribed emissions into aerosol concentrations,
which can influence the Earth's radiation budget both directly and indirectly
through their influence on the properties of clouds.

Other work at the Hadley Centre is also extending the remit of the GCM
by including formerly prescribed elements as interactive components of the
model. A variant of HadCM3 is being used as the basis for the first coupled
climate-carbon cycle model. "HadCM3LC" will incorporate models of the ocean
and land carbon cycle which have been developed at the Hadley Centre in
collaboration with partners from elsewhere. "HadOCC" represents chemical
uptake of carbon dioxide by the ocean and includes a simple 4-box model
of ocean biology which was developed at the Southampton Oceanography Centre.
An interactive vegetation model, "TRIFFID", resolves the terrestrial biosphere
into five plant functional types which can grow and spread based on the
competition between the types and the carbon balance of each. Land surface
parameters (such as albedo and roughness) are updated based on the changing
vegetation distribution and structural properties, so that physical climate
-vegetation feedbacks can be included as well as those associated with CO2.

The model developments outlined above will require additional and new methods
of validation. Slow processes such as forest migration and carbon uptake
cannot be adequately validated by detailed observation of the current
climate system and its variability. Here, palaeoclimate data is likely to be
especially useful, as it offers the possibility of testing the ability of
Earth System models to reproduce past equilibrium states of climate,
vegetation and CO2. An effective partnership with the Hadley Centre in this
area would provide valuable guidance on future model developments and
improvements. With this in mind we would aim to complement the experiments
which the Hadley Centre plans to carry out before the end of 1999:

1) A 1000-year simulation with constant forcing.

2) An ensemble of 4 simulations in which the model will be forced with
anthropogenic forcings (Greenhouse gases, sulphates and ozone) for the
period 1850--2000.

3) An ensemble of 4 simulations in which the model will be forced with
changes in solar irradiance (Lean et al) and Volcanic aerosol changes
(Sato et al) for the period 1850--2000.

4) An ensembles of 4 simulations in which the model will be forced
with changes in well mixed Greenhouse gases for the period 1850--2000.

5) One of the anthropogenic and one of the well mixed greenhouse gas
simulations will be continued to 2100.

6) Control (zero CO2 emissions) and anomaly (anthropogenic emissions)
coupled climate-carbon cycle simulations for the period 1850--2100.

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