AN IMPLICIT TIME-STEPPING SCHEME FOR CHEMICAL-SPECIES IN A GLOBAL ATMOSPHERIC CIRCULATION MODEL

An implicit time-stepping scheme is proposed for the incorporation of a complex chemistry in a three-dimensional general circulation model (3D GCM). The scheme has been tested in a single computational box and in the UGAMP (United Kingdom Global Atmospheric Modelling Programme) 3D GCM. The implicit scheme, which is unconditionally stable, has been shown to provide a computationally more efficient solution than the previously employed explicit scheme, while maintaining an acceptable level of accuracy. The saving achieved is determined by the length of the time step and tests have been carried out wi both constant and variable time steps for the chemistry. An implicit solution with constant time steps, that takes 15% of the computing time of an explicit solution with shorter steps, has good accuracy for all chemical species except those that change rapidly on short time scales. With variable chemical time steps, an implicit solution, with a 50% saving in computing time over an explicit solution with shorter variable steps, is as accurate as the explicit scheme for the longer-lived species but is more accurate for the fastest changing species.