Abstract
In constructing a numerical model of the atmosphere, we must choose an appropriate vertical coordinate. Among the various possibilities, isentropic vertical coordinates such as the θ-coordinate seem to have the greatest potential, in spite of the technical difficulties in treating the intersections of coordinate surfaces with the lower boundary. The purpose of this paper is to describe the θ-coordinate model we have developed and to demonstrate its potential through simulating the nonlinear evolution of a baroclinic wave.
In the model we have developed, vertical discretization maintains important integral constraints, such as conservation of the angular momentum and total energy. In treating the intersections of coordinate surfaces with the lower boundary, we have followed the massless-layer approach in which the intersecting coordinate surfaces are extended along the boundary by introducing massless layers. Although this approach formally eliminates the intersection problem, it raises other computational problems. Horizontal discretization of the continuity and momentum equations in the model has been carefully designed to overcome these problems.
Selected results from a 10-day integration with the 25-layer, β-plane version of the model are presented. It seems that the model can simulate the nonlinear evolution of a baroclinic wave and associated dynamical processes without major computational difficulties.
