A community chemistry-climate model
ACMSU, together with the University of Leeds are collaborating with the
UK Met. Office on the development of a new community chemistry-climate
model. We describe the motivation behind this model here. The primary
contact for the UKCA model at ACMSU is Dr
The role of atmospheric chemistry in the climate system is now recognised
being of central importance. Modelling of the chemistry/climate system
be advanced in recognition of this important coupling. In the modelling
3rd assessment report of the IPCC, tropospheric chemistry modelling using
was performed off-line from other components of the climate model. This
important limitation; chemistry/climate coupling now needs to be included
'on-line' to maintain a state-of-the-art modelling capability. It is also
recognised that certain aspects of the aerosol behaviour in climate models
to be improved in order properly to capture the effect of aerosols on
forcing, particularly the forcing related to changes in cloud microphysics.
Similarly, links with the vegetation, deposition, etc., should also be
To remain at the leading edge of worldclass science it is essential that
community continues to develop our modelling capability, towards the objective
of a fully coupled climate model; this development is the aim of this
drawing on the strengths of the Met Office and NCAS communities to build
for a variety of uses including contributions to future IPCC assessments,
for government and for detailed process studies.
The MO has relied on a series of different models to carry out different
aspects of atmospheric chemistry modelling: the Lagrangian model STOCHEM
has been used for tropospheric chemistry modelling, with EUMETRAC being
used for interactive stratospheric chemistry modelling and with aerosol
modelling being done within the climate model using the default transport
scheme. In NCAS, models in use include a range of tropospheric and stratospheric
schemes implemented in HadAM3 using a revised transport scheme with the
ASAD chemistry module, as well as chemical transport models for the troposphere
and stratosphere. Sophisticated aerosol box models and offline global
aerosol transport models are also being developed with a view to understanding
the evolution of aerosol physical and chemical properties. There is clearly
a case for combining the best aspects of these models in a framework to
create a flexible model environment based on the UM with the added advantage
of being accessible to more users in the research community than the current
generation of models.
The Hadley Centre model presently contains options for interactively
simulating several varieties of aerosol: sulphate, sea-salt, fossil fuel
black carbon, biomass smoke and mineral dust. The direct and (where relevant)
indirect effects of these aerosols can be included if desired. However,
there are some important scientific issues that the current aerosol code
addresses incompletely or not at all. For example, there is only limited
freedom for the size distribution to evolve (apart from the case of mineral
dust where 6 size bins are used). Also there is no treatment of internally
mixed particles (i.e. particles composed of more than one constituent).
New aerosol observations and detailed model simulations highlight the
importance of aerosol types that are not presently included in the current
model; the most important being nitrate and secondary organic aerosol,
the latter formed from biogenic and anthropogenic source gases. Some of
the limitations of the current model reflect the Hadley Centre's requirement
for a scheme affordable in centennial timescale experiments with the computer
power hitherto available.
Looking forward, likely increases in computer power and the need to improve
the representation of important processes both indicate the need to develop
a more sophisticated aerosol code for the Unified Model. It is important
now to design a new aerosol scheme that will be as self-contained and
"modular" as possible.
The model to be developed will have a range of uses from longterm IPCC
assessments through to detailed process studies. It will become the model
of choice for the UK atmospheric science community. The scientific argument
for the project is that this new model will allow a range of interactions
which are known to be essential and which have previously not been fully
included. There are also strategic arguments. First, the human resources
for the development of atmospheric chemistry and aerosol models of high
complexity are currently small in relation to those available in atmosphere
modelling. It is important to harness the world class expertise available
in the MO-NCAS communities. Second, an increasing amount of data (produced
with important scientific and financial contributions from the UK) is
being collected from aircraft, balloon ascents and, more recently, from
satellites, which needs to be used in modelling in order to improve the
science. This new model will maximize UK use of these data.
The model to be developed must be flexible. It must be capable of use
at high resolution with short timescales, as well for runs for much longer
time periods. While its main use will be in a fully coupled mode, for
a range of timescales and with varying complexity (the objective here),
the model will also be used outside this specific project for a further
set of scientific issues. The model developments here will be an important
step towards a fully coupled Earth System model, incorporating all relevant
aspects of the earth system. Finally, it must be capable of easy use by
non-specialists using UMUI.
The model will be a state-of-the art chemistry/climate model, which
will perform centennial timescale integrations for IPCC assessments. A
primary aim will therefore be to study future states of the atmosphere
under different scenarios for the emissions of major greenhouse gases
(CO2, methane, CFCc, HCFCs, HFCs, N2O, etc.) and projected changes in
aerosol composition. The interaction with changing levels of oxidants
and aerosol will be central to these integrations.
Studies of the oxidation capacity of the troposphere, including the two-way
interaction with atmospheric aerosol, will have high priority.
A range of chemistry-climate interactions will be considered, including
the role of temperature and water vapour changes on atmospheric composition,
the impact of changing convection and lightning NOx emissions under changed
climates, and the importance of natural variability including various
known modes of atmospheric variability.
The model will be suitable for use to explore chemistry-ecosystem interactions,
including natural emissions of VOCs and DMS, and deposition of ozone and
nitrogen species in gaseous and particulate forms. An important new area
for investigation will be biogeochemical cycles involving particulate
deposition to the oceans.
Air quality is a major concern during the coming century, with large
increases likely in surface ozone and particulates at some locations.
Study of the interaction between air quality and global change will be
another high priority research area.
The recovery of stratospheric ozone is known to depend on climate change
(and this problem intimately connects the Kyoto and Montreal Protocols).
We will investigate the interactions between stratospheric dynamics (including
the Brewer Dobson circulation, tropopause structure, stratosphere to troposphere
exchange, etc.) and chemistry, and climate change.
The whole atmosphere chemistry scheme to be implemented will make the
technical distinction between troposphere and stratosphere superfluous
and will be ideal for, for example, stratosphere/troposphere exchange
studies and assessment studies of the UTLS. Aerosols-chemistry-radiation
interaction is a developing field for which the model will be ideally