Several versions of a simplified or "intermediate" model of the global atmospheric circulation have been developed at the University of Reading from the primitive-equations baroclinic model of Hoskins and Simmons. The main IGCM versions are
IGCM1 : Portable version of the original adiabatic baroclinic model, with an option for Newtonian forcing and Rayleigh drag to obtain a stable climate.
IGCM2 : Includes advected trace species, simplified moist parameterizations and a cheap "radiation scheme" (constant tropospheric cooling!).
IGCM3 : Intermediate climate model which includes more sophisticated radiation and real-Earth surface conditions.
Further developments external to Reading, based on IGCM3, are
The adiabatic version, IGCM1, is freely available. Access to IGCM2 and IGCM3 is restricted to members of the Department of Meteorology at the University of Reading and collaborating researchers.
Link to OLD IGCM departmental information (Department of Meteorology only).
Hoskins, B.J. and Simmons, A.J. (1975): A multi-layer spectral model and the semi-implicit method. Q. J. R. Meteorol. Soc., 101, 637-655.
(Abstract and PDF version)
In the beginning was the Hoskins and Simmons (1975) primitive-equations baroclinic global model using the spectral transform method......
IGCM1 - Adiabatic / Newtonian forcing
The adiabatic model was developed throughout the 1970s and 1980s, with changes to the vertical scheme, time-splitting of the linear diabatic processes and optimisation for Cray vector computers.
Original documentation (PDF, 16Mb).
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A flexible scheme for linearised forcing (Newtonian heating and Rayleigh friction) was included in a separate model version by Ian James and his group.
SGCM documentation (PDF, 27Mb)
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The two versions were merged and made portable by converting to (almost) ANSI Fortran-77 ...... creating IGCM1.
New Legendre transforms: notes (3.8Mb)
BGFLUX2 transforms: notes (4.8Mb)
Source code listing of IGCM1.
Link to Loïs Steenman-Clark's modelling support information, including source code for IGCM1.
Test job for IGCM1 - the LC1 baroclinic lifecycle:
(formatted initial data)
IGCM1 version including orographic growth and regional heating for monsoon studies:
of its contents (password required).
IGCM2 - Simple moist parameterizations
In addition to the adiabatic dynamics and optional Newtonian forcing of IGCM1, IGCM2 includes
- an arbitrary (non-zero) number of passively advected trace species
- an optional simple parameterization of moist physical processes:
- surface fluxes of momentum and sensible and latent heat
- dry and moist convective adjustment schemes
- stratiform precipitation
- the (in)famous and incredibly cheap "Reading Radiation Scheme" (prescribed tropospheric cooling!).
- Version 2.0 is the original version by Piers Forster, now superceded by 2.1.
- Version 2.1 includes optional mass correction and extensive code changes.
- Version 2.2 includes optional TVD vertical advection of the trace species.
The moist parameterizations were developed in the mid 1980s for the Cray-specific adiabatic model. Apart from convection, which is a simplified version of the original (1986) Betts-Miller convective adjustment scheme, the parameterizations were based on earlier schemes from the late 1970s, developed at Reading.
Because the moist parameterizations are extremely simplified, IGCM2 is available to collaborators only, to prevent inappropriate use of the model. Contact us to use the model and access the files linked below.
IGCM 2.0 :
Source code -
IGCM 2.1 :
IGCM 2.2 :
Link to tar file containing nupdate program library and sample "Dynamical Core" model and diagnostic jobdecks for IGCM 2.2 (223kb gzipped, 778kb unzipped).
Listing of tar file contents.
Embryonic documentation of moist processes in IGCM2/3.
IGCM3x - Intermediate-complexity climate model
IGCM3 was developed in the mid 1990s, mainly by Piers Forster, Robin Glover and Suzanne Rosier, for climate sensitivity studies.
More information is available at the separate IGCM3x
Diagnostics - the Flux Program
Several versions of a comprehensive diagnostics package exist for the IGCM.