GENIE:Igcm

GENIE-IGCM is an (I)ntermediate complexity (G)eneral (C)irculation (M)odel.

History
The ICGM was initially developed in the department of Meteorology, University of Reading. In Reading the IGCM went through three versions:


 * IGCM1 : Portable version of the original adiabatic baroclinic model (Hoskins & Simmons, 1975), 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. IGCM3 was developed in the mid 1990s, mainly by Piers Forster, Robin Glover and Suzanne Rosier, for climate sensitivity studies.

IGCM3 is currently being used for research at Reading (sometimes refered to as RUGCM). Mike Blackburn is the current point of contact at Reading. Details of the Reading IGCM can be found at: http://www.met.rdg.ac.uk/~mike/dyn_models/igcm/

Summary of changes since IGCM3
GENIE-IGCM is based on IGCM3, with the following changes (up to July 2006). The changes are either scientific or structural. The structural changes do not affect the results (or only affect them at the last decimal place, due to rounding errors).

Structural:


 * Diabatic/Adiabatic/Boundary-Layer split into separate parts.
 * Surface fluxes split into a separate GENIE module.
 * Separation of the ocean and sea ice into separate GENIE modules.
 * NetCDF input/output.
 * All tunable parameters moved into the namelists.
 * Dimensionalisation of the land-surface and boundary layer schemes.
 * Added tests for water and energy conservation.

Scientific:


 * Addition of the Tiedtke convection scheme (as an option).
 * Input files added to run at T42 if required.
 * 7 vertical levels is now the default.
 * Seaice albedo corrected so that the albedo changes when seaice melts.
 * Small bug in the land surface scheme corrected.
 * Ordering of some of the calculations in the land surface scheme changed.
 * Timestep of ocean to 2 days and seaice to 6 hours (both were every atmosphere timestep previously).
 * Small bug in the vegetation-albedo lookup table corrected.
 * Small bug in the vegetation-type input file corrected.
 * Closure of the hydrological cycle by properly accounting for sublimation.

Model structure
The IGCM is a spectral model. Typically run at T21 there have been attempts to run it at T42 also.

In the code the main division in the dynamics is between diabatic (i.e. wet etc) and adiabatic (i.e. dynamics) processes. The top level structure of GENIE-IGCM is:



Convection
There are two different convection schemes available in GENIE-IGCM: the Betts Miller and the Tiedtke. At their hearts the Betts Miller scheme adjusts the atmospheric towards pre-defined thermodynamic profiles which are based on observations whilst the Tiedtke scheme is a mass flux scheme that calculates fluxes of moisture, heat etc in a more 'process-based' approach. A good reference for explaining the different ways of modelling convection is Emanuel (1994).

The original convection scheme used in GENIE-IGCM is the Betts Miller scheme and was developed by Mike Blackburn. A description of this scheme can be found here: http://www.met.reading.ac.uk/~mike/betts_miller.html

In response to poor precipitation values in a tuning exercise (Annan et al, 2005) the Betts Miller scheme is no longer the only convection scheme in GENIE-IGCM. The second convection scheme (implemented by David Cameron and with a cloud diagnostic added by John Hughes) is the Tiedtke convection scheme (Molteni, 2003). Tiedtke, as described in Molteni (2003) is a simplified mass-flux scheme that is triggered by conditional instability. Molteni (2003) use the convection scheme within the SPEEDY intermediate GCM and present analysis of the resulting climatology. Online documentation for the SPEEDY model can be found at: http://www.ictp.trieste.it/~moltenif/speedy-doc.html. This link points to a pdf that discusses the convection scheme in more detail. Initial results using the GENIE-IGCM-tiedtke (presented at GENIEfy meeting October 11th 2006) look promising that this will dramatically improve the prediction of precipitation.

Having two different convection schemes is also highly interesting since it allows comparison studies in which the only change is a change of the convection scheme.

igcm3_diab |                                |                               dgrmlt |                          ______|__________________                          |          |              |                        convec/convec_tiedtke     lscrn |                 |                        |               ___|_____                        |              |         |    ____________________|_________  convmf    vdifsc |      |      |        |      |  dryadj cbcon  cbadj(1)  cudif  cubm

For the Tiedtke scheme the two subroutines are the convective mass flux scheme and the large-scale condensation scheme that is modelled as a relaxation of humidity towards a reference value. (1) - deep convection subroutine.

In IGCM3 there are two sets of divisions in the cloud scheme. The first group of cloud types is the set { convective,large-scale } ; Within convective rain there is deep and shallow convection. In large-scale rainfall there is { high,medium,low }. Both Betts-Miller and Tiedtke schemes use the same large-scale rainfall/cloud scheme. Large-scale rainfall is diagnosed in lscrn.

Both convection schemes use the same models to represent cloud cover. Deep convection cloud cover is proportional to the logarithm of the precipitation rate, whilst shallow convection cloud is set using a pre-defined constant fraction. Large-scale rainfall cloud is a linear function of relative humidity, above a specified threshold humidity.

Published work completed using the IGCM
Annan et al (2005) present the results of a tuning exercise completed using GENIE-IGCM with fixed SSTs. Whilst the tuning significantly improves the temperature field it fails to improve the precipitation field. This study was the motivation behind implementing the tiedtke convection scheme.