Tracer Transport Model
An important tool for improving our understanding of the
biogeochemical cycles of atmospheric trace gases that cause climate
change and for studying the release of radioactive gases at the
regional scale is a high resolution three-dimensional global
atmospheric circulation model which can be used to investigate the
sources, removal processes and atmospheric chemistry of these trace
gases. In the past, even low resolution three-dimensional models have
placed enormous computational demands on supercomputers.
This continuing project has involved the development of a
computationally efficient high-resolution simulation model for
atmospheric transport and chemistry, the Australian National
University Chemical Transport Model (ANU-CTM)
(Taylor, 1989, 1991; Taylor et al., 1991) with
parameterised interactions between the oceans and the biosphere.
Models of the sources and sinks of the trace gases CFC-11, CFC-12
(Taylor, 1991), methyl chloroform (Taylor et al.,
1991), methane (Taylor et al, 1991), CO2
(Taylor, 1989, 1993, 1995; Taylor and
Lloyd,1992), radon (Taylor, 1991), CO (Erickson and Taylor,1992; Taylor et al 1995),
N2O (Taylor, 1992; Bouwman and Taylor, 1996)
have been developed and incorporated into the transport model.
Key Physical Processes represented in the model include
- Lagrangian tracer transport which conserves the mass of
tracer by definition thereby avoiding the need for mass fixing
- Wind fields that are based on the monthly mean and
variances of 12 hourly ECMWF data calculated at the United States National Center for Atmospheric
Research by Kevin Trenberth. The method of calculation is as
described in Trenberth (1992). The
data are available on a T42 grid approximately 2.8 degrees in
longitude and latitude. Dr Jay Larson processed the data into a
format suitable for application with ANU-CTM and transferred this data
to ANU.
- Surface topography defined by the surface pressure data
obtained from ECMWF. The model vertical coordinate is pressure.
- Time varying boundary layer using monthly mean estimates
of the boundary layer height derived from the NCAR Community Climate Model
Version 2 by Dr David Erickson III of Atmospheric Chemistry Division of
NCAR.
- Cloud transport of trace gases is based on a modified Tiedke
scheme developed at the Max Planck Institute for Meteorology by Dr
Martin Heimann and employed in the TM2 tracer transport model
The basic approach of the stochastic Lagrangian model is to divide
the troposphere into air parcels of equal mass. Trajectories for these
air parcels are calculated using observed wind field data obtained
from the European Centre for Medium Range Weather Forecasting. While
the simulated air parcels are being transported around the globe, they
can exchange chemical compounds with the oceans, the biosphere and one
another and take up industrial emissions of trace gases. The model
runs on the Silicon Graphics Power Challenge and VP2200 supercomputers
at the Australian National
University Supercomputing Facility. Model runs with up to 1 000
000 air parcels, giving an effective model resolution below 1 x 1
degree, have easily been achieved. Model runs at higher resolution are
possible and will be required in the future to answer questions about
sources and sinks of greenhouse gases at a regional scale and to
investigate the release of radioactive tracers at the regional scale.
Meteorological data ...
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