On initialization for primitive equation models formulated for an alternating grid

In a primitive equation model which uses an alternating grid for the integration, it is necessary to specify initial data for two sets of grid points. It is shown that improved results are obtained if the balance equation is solved for both sets, rather than for only one, and the other one being deduced by interpolation, although the improvement is modest. The requirement of zero initial divergence, imposed by the balance equation, causes gravity waves to appear. The behaviour of these waves is studied, and they are found to be of some importance for atmospheric processes governed by the vertical velocity.