Further development of a hybrid-isentropic GCM

The UK Universities' Global Atmospheric Modelling Programme hybrid-isentrop ic general-circulation model (HIGCM) uses a flexible sigma-theta-p vertical coordinate, where p is pressure, theta is potential temperature, and sigma = p/p(*) where p(*) is surface pressure. Three major improvements to the H IGCM are presented. The first improvement is a modification to the vertical -difference scheme so that spurious vertical motions in the isentropic doma in are minimized. The second improvement is a modification to the implement ation of the radiation scheme so that it is now able to damp, and does not itself create, noisy temperature profiles; this allows the model to be run without ad hoc extra vertical diffusion and so allows a cleaner comparison with sigma-theta simulations. The third improvement is to extend the isentr opic domain up to the top of the model thus allowing sigma-theta or sigma-t heta-p simulations to be performed. Idealized baroclinic instability life-cycle experiments are used to investi gate the impact of the new vertical scheme on the dynamical core of the HIG CM. The reduction in spurious vertical velocities is found to be substantia l whilst the impact on the global conservation properties and overall evolu tion is found to be very small. These simulations also show that the common ly used del(2n) form of scale-selective dissipation can seriously compromis e global energy conservation when model-layer thicknesses have significant horizontal gradients. The impact of the isentropic coordinate on the climate of the full GCM is i nvestigated by performing perpetual January simulations using sigma-theta, sigma-theta-p and sigma-p vertical coordinates. The most robust response to the isentropic coordinate is a warming of the southern hemisphere high-lat itude lower stratosphere. In the northern hemisphere the largest changes in zonal mean temperature are in the polar stratosphere. The possible mechani sms by which the isentropic coordinate may yield these changes are describe d and investigated. The results strongly suggest that many of the potential benefits of the isentropic coordinate are realized, to some extent at leas t, with the HIGCM.