GCM Comparison
GCM type
In a spectral GCM, vertical exchanges are computed in grid point space, whilst horizontal flow is computed in spectral space, i.e., atmospheric fields are held and manipulated in the form of waves. The variation of any quantity around a latitude zone can be represented as a summation of a number of waves. Model resolution is governed by the wave number, with the truncation number representing the number of waves which are resolved around a latitude zone. The 'R' and 'T' refer to the method of truncation, rhomboidal and triangular, respectively, whilst the number indicates the number of waves in each latitude zone, e.g., R15 indicates that there are 15 waves in the latitude zone and rhomboidal truncation has been used.
In a finite grid GCM, modelling of the atmosphere is carried out by dividing it into a series of boxes, usually evenly spaced in latitude and longitude.
Flux corrections
The atmosphere and ocean interact through fluxes of heat, momentum and fresh water. In transient GCMs, the atmosphere and ocean models are generally run independently before being coupled together. This coupling of the atmosphere and ocean component models can highlight discrepancies in the surface fluxes that may lead to a drift away from the observed climate. This climate drift may be reduced by flux adjustment whereby the heat and freshwater fluxes, and possibly the surface stresses, are modified before being imposed on the ocean by the addition of a correction or adjustment. Flux adjustment terms are calculated from the difference between the modelled surface fluxes and those required to keep the model close to current climate. After running the model for a period suitable for the calculation of average flux adjustments, these terms are applied throughout the control and climate change experiments. The main purpose of flux adjustment is to ensure that any perturbation in forcing is applied about a realistic reference climate so that distortion of the major climate feedback processes is minimized. As climate modelling advances the need for flux adjustments will be reduced.
