A hough spectral model for three-dimensional studies of the middle atmosphere

A three-dimensional framework is developed for studying the middle atmosphe re in relation to upper-tropospheric structure. The numerical model is form ulated from the primitive equations in isentropic coordinates, which direct ly characterize diabatic processes forcing the Brewer-Dobson circulation. I t is anchored in observed tropospheric variability, so integrations provide middle atmospheric behavior that tracks observed variations in the upper t roposphere. The numerical framework is versatile and computationally efficient. It achi eves enhanced performance by incorporating eigenfunctions of the primitive equations to represent structure spectrally in all three coordinates. Scale -selective dissipation can then be applied entirely at sixth order, which l eaves ail but the shortest vertical scales undamped. This feature allows ve rtical diffusion to be made small enough to represent stratospheric transpo rt as advective (rather than diffusive) for most of the scales carried in t he integration. Transport across the model's lower boundary, which is posit ioned near the tropopause, is calculated prognostically from diabatic proce sses in the middle atmosphere, in concert with tropospheric influences impo sed at the bottom. Integrations in which different tropospheric influences are represented can then be used to provide an understanding of how transpo rt and chemical composition depend on processes in the middle atmosphere an d in the troposphere. Integrations forced by observed tropospheric behavior are validated against climatological structure, as well as tracer behavior deduced from satellit e measurements. The isentropic formulation, together with sixth-order verti cal dissipation, enable potential vorticity to be conserved quite accuratel y. The results throw light on the three-dimensional structure of the Brewer -Dobson circulation and how it follows from diabatic processes operating in the middle atmosphere and tropospheric processes operating below.