Atmospheric stationary wave modelling

Stationary waves are vitally important in the general circulation of the atmosphere. These planetary scale waves are generally attributed to large scale orography and diabatic heating. The purpose of the present study is to investigate stationary waves as a linear response to the large scale orography and steady-state diabatic heating by developing a linear steady-state spectral model with the primitive equations in the global domain, in which Rayleigh friction and Newtonian cooling as well as biharmonic horizontal diffusion are included. We use pertubation theory to linearize the nonlinear system of primitive equations with respect to the zonally symmetric component of the dependent variables. The structure of the dependent variables is described by truncated series of spherical harmonics in the horizontal and orthogonal functions in the vertical. The model solution is obtained by way of linear superimposition of zonal waves calculated in the spectral domain for each zonal wave. The three dimensional wave activity flux, which was derived by Plumb (1985) for linear quasi-geDstrophic stationary waves on a zonal mean flow, as well as the EP flux are used as a diagnostic method for studying the three dimensional structure of wave propagation. The Plumb flux is a conservable measure of the wave activity flux which reduces to EP flux in the zonal mean, and is non-divergent for steady, conservative, linear waves. The results of this thesis may present new insights into forcing mechanisms of 3 the stationary waves. The numerical experiments for the response to idealised orography in northern middle latitudes and actual global orography show that the northern middle latitude orographic forcing, in particular the Tibetan Plateau, plays the most important role in the maintenance of the orographically forced stationary waves in the middle and upper troposphere as well as lower stratosphere, while forcing by the orographic effect of the Antarctic plateau is also very important, but restricted only to the lower troposphere of the southern high latitudes. It is also shown that middle latitude orographic forcing can play an important role in the maintenance of the planetary scale motion in the subtropics and tropics. The stationary waves induced by the climatological thermal forcing have a comparable amplitude with those forced by orographic forcing except in the lower troposphere of the Antarctic area. It might be considered that between these two categories of forcing, the orographic forcing was more important in the lower troposphere, particularly in high latitudes of the southern hemisphere, but having a lesser role in the upper troposphere and lower stratosphere in the northern hemisphere, for the maintenance of the planetary stationary waves in northern winter. This was also supported by the longitude-pressure cross-sections of geopotential height, from which it might be concluded that the thermal and orographic forcing were comparably important in the middle latitudes of the northern hemisphere, while the circulation in the high latitudes of the southern hemisphere is primarily orographic. However, due to the simplicity of the model, the results presented in the thesis must be treated with caution.