Introduction of a hybrid vertical co-ordinate to an atmospheric general circulation model

Abstract

Both the R21 and T63 versions of the CSIR0-9 (MARK II) Atmospheric General Circulation Model (AGCM) utilise a sigma vertical co-ordinate (a=p/Ps) which was introduced to compensate for the influence of uneven surface topography on atmospheric circulation patterns. In addition, climate model simulations may also be performed with pressure as a vertical co-ordinate. Trough the depth of the atmosphere, over high surface topography, the formulation of the sigma vertical co-ordinate system allows for numerical analyses to be carried out at higher geometric levels than equivalent pressure co-ordinate analyses, where the effect of topography is ignored. If compared with pressure co-ordinates, sigma coordinate simulations therefore usually provide better results at lower altitudes (near the Earth's surface), but often fail to perform well in the upper atmosphere where the influence of surface topography is in reality much less profound. The hybrid vertical co-ordinate system (or 11-system) addresses this problem by converging to a sigma vertical co-ordinate system (or a-system) near the Earth's surface, while gradually changing to a pressure vertical co-ordinate system (or psystem) in the upper atmosphere. The CSIR0-9 (MARK II) AGCM utilises the flux formulation of the atmospheric equations. In this study a 11-system is introduced to the non-linear dynamics of the model. A 5-year seasonal cycle 11-system control run has been performed. Climate parameters from this simulation are compared with output results from a different 5-year a-system control run. Both model simulations were started with exactly equivalent initial conditions. Actual climate fields extracted from ECMWF analyses are finally used to compare and validate model output from both the and 11-systems. The study reveals noticeable improvements by 11-system simulations at stratospheric pressure-levels over high surface topography