| A planetary wave model which is symmetric about the equator has been developed using the linear balance set of equations. The model is formulated on a sphere and spectrally truncated to 4 zonal and 7 meridional modes. There are 11 levels in the vertical with the highest computational level at 5 mb. The model is linearized about an observed January zonal mean basic state and forced by the Northern Hemisphere orography and a wintertime calculated diabatic heating.;In the vicinity of the Himalayas when the wave-coupled LBC is used, the boundary eddies setup perturbation easterlies that locally offset the imposed zonal mean westerlies. Thus the wave-coupled LBC adjusts the lower boundary wave structure so that the total flow tends to circumvent the Himalayas rather than to flow over. When the diabatic heating field is included, the resulting simulation successfully reproduces most of the tropospheric time mean flow characteristics. The model generates zonal mean eddy heat and momentum fluxes that are in good agreement with those observed throughout the troposphere. However, in contrast, the model with wave-decoupled LBC poorly simulates the Siberian High, Aleutian Low and East-Asian Trough. Consequently it produces the lower tropospheric eddy heat flux maxima located 15 degrees of latitude too far south compared with observations and the magnitude of the eddy statistics is overestimated throughout the troposphere.;It is found that both orographic and thermal forcings strongly interact with each other through the wave-coupled LBC. The results indicates that both forcings are important in the troposphere, while the thermal waves are dominant in the stratosphere.;A wave-coupled LBC (lower boundary condition) is employed and contrasted to the wave-decoupled LBC which has been used in many other forced wave models. Thus the eddies at the lower boundary also flow over the mountains and produce vertical motion. In this way, although the model remains linear, zonal waves are coupled through the LBC and the model equations have to be solved for simultaneously. The wave-coupled LBC has significant impact on the forced planetary waves. |
