The document describes the CSIRO-MK3 climate system model. The MK3 model consists of the CSIRO spectral atmospheric general circulation model (AGCM) coupled to the GFDL MOM2.2 ocean model. The MK3.0 and MK3.5 versions contributed output to CMIP3. The MK3.0 model was improved over MK2 with enhanced parameterizations and less climate drift. The MK3.5 model included further physical improvements such as ocean eddy transport and improved river routing.

2. CSIRO-MK3 MODEL

3. MK3 Model
 An outline of the Mk3 model structure and resolution is now
given. The model consists of the CSIRO spectral AGCM coupled
to an ocean model (the GFDL MOM2.2 ocean model,
Pacanowski 1996).
 The AGCM has been developed as a unified package containing
an atmospheric model, a land surface model, and a polar ice
model. In the description of the AGCM and OGCM to follow, the
horizontal grids used in the models are based around the
“Gaussian” latitudes used in spectral models.
 These latitudes are not evenly spaced in the north-south
(meridional) direction, although they are approximately so.
However, the longitudinal (east west) spacing is regular.

4. Features of mk3.0 climate system model
 Output from two versions of the CSIRO Mk3 model were
contributed to the CMIP3 these are the Mk3.0 and Mk3.5.
 The Mk3.0 model is based on the Mk2 model as described in
Gordon et al. (1997), O’Farrell (1998) and Hirst et al. (2000),
with major numerical, computational and parameterisation
improvements.
 Importantly, the Mk3.0 model is run without any flux adjustments
and suffers from only a moderate amount of climate drift.

5. Continued…
 Briefly, the horizontal resolution of the Mk3.0 atmospheric model
is spectral T63 (approximately 1.875° latitude×1.875° longitude)
with 18 vertical levels (hybrid sigma-pressure vertical coordinate.
 The atmospheric model includes a comprehensive cloud
microphysical parameterization (Rotstayn et al., 2000) and the
convection parameterization is based on that used in the Hadley
Centre model (Gregory et al., 1990)
 This convection parameterization has been linked to the cloud
microphysics scheme via the detrainment of liquid and frozen
water at the cloud top.

6. Continued…
 Atmospheric moisture advection (vapour, liquid, and frozen) is
carried out by the semi-Lagrangian method (McGregor, 1993
 The Mk3.0 ocean model is based upon the Modular Ocean Model
version 2.2 (MOM2.2) of the Geophysical Fluid Dynamics
Laboratory (GFDL) model (Pacanowski, 1996).
 The oceanic component has a horizontal resolution matching that
of the atmospheric model in the east-west direction and twice that
in the north-south direction

7. Continued….
 This represents an approximate grid spacing of 0.9375°
latitude×1.875° longitude and allows the atmospheric model and
ocean model to have matching land-sea masks
 These are an Atmospheric Model Intercomparison Project
(AMIP), present day control (PDcntrl) and the 1%/year CO2
increase from doubling to quadrupling (1%to4x) experiments

8. Improvements in the new mk3.5 climate
system model
 A number of physical parameterization and numerical improvements
were made to the Mk3.0 model to produce the Mk3.5 version
 These include a scheme to control the strength of the ocean eddy-
induced transport, vertical ocean mixing due to wind generated
turbulent kinetic energy, and an improved runoff and river routing
method
 A similar set of experiments was performed and data subsequently
submitted with the Mk3.5 model as for the Mk3.0 model

9. Continued….
 The Mk3.5 was integrated out a further 100 years and only one
long (~1000 years) control experiment was performed
 It is a noteworthy achievement that the flux uncorrected Mk3.5
control model experiment was able to be integrated out for over
1000 years with a negligible surface temperature drift.

10. The atmospheric MK3.5 model structure
The physical parameterizations used in the atmospheric model were
modified to some extent for the Mk3.5 model version. Some minor
changes were made to help with, for example, improved conservation
properties or more exact representation of certain processes. These will
not be detailed here as the overall effects were minor in a climatological
sense.
Radiation
 The Mk3 model time-step is 15 minutes, and the radiation calculations
are undertaken every 2hours. The surface long-wave radiation is
allowed to vary according to the varying surface temperature on a time
step by time step basis.

11. Continued….
 In the Mk3.5 model version, the computation of absorption by the
atmosphere of the surface long-wave radiation between radiation
steps has been improved.
 The effect has been to remove an unrealistic concentration of the
variation in longwave forcing in the boundary layer, especially for
clear skies.
Cloud microphysics
 The minor changes to the atmospheric and surface physical
parameterizations described above altered the overall energy balance
of the AGCM.
 Some small alterations to the cloud microphysics properties were
made to achieve an annual mean energy balance at top-of
atmosphere.

12. Continued…
Surface drag
 The surface drag in the Mk3.0 model for all surfaces (land, ocean,
ice) was computed assuming a zero surface velocity. In the
current coupled model (Mk3.5), the surface drag calculations now
take into account the fact that the sea-ice and oceans have their
own surface velocities.
 For the AGCM in stand alone (non-coupled) mode, the ocean
surface currents on a monthly basis come from ocean surface
current data generated from the OGCM run in stand alone mode.

13. Continued….
River routing scheme
 At coastal points there can be runoff (river outflow) from the
land. In the Mk3.0 model, run off at inland grid points was
transferred, without time delay, by a downslope method to
appropriate coastal ocean grid points.
 In the Mk3.5 model, this scheme has been enhanced whereby
the runoff is transported from grid point to adjacent grid points
(i.e. river flow) with appropriate computed downslope flow
rates.
 This allows for a considerable time delay in some cases (e.g. the
Amazon) between the time that the runoff occurs and the time
that it arrives at the river mouth.

14. Obtaining the MK3 data
 There are 3 typical ways to obtain the Mk3 model data:
 1) become a registered user of the official WCRP(world climate
research programme’s) CMIP3(coupled model inter comparison
project) multi-model dataset
 2) using Open DAP(data access protocol) or
 3) directly from the CSIRO data server.