GENIE:Igcm

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

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/

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 and 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 contains a pdf discussing 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  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.

In IGCM3 there are two sets of divisions in the cloud scheme. The first group of cloud types is the set { convective,large-scale } ; the other set is { high,medium,low }. Both sets are used in the radiation subroutine (radn.f). The second set is diagnosed in lscrn.

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.