Here the grid contains three separate zones with interfaces at 100 and 200 km
and, at each interface, the x and y grid spacings are reduced by factors of 2.
The vertical grid spacing increases continuously from 100 km to the top of the
domain. This arrangemnt yields factors of 4 coarsening in the horizontal
directions between the bottom and top of the domain and results in a factor of
2.3 reduction in the number of mesh points ( 3/(1+1/2^2+1/2^4)) and a factor of
16 (2^4) increase in the time step that would be required for a simulation on a
single zone whith fixed horizontal spacing. The grid spacings in the central
portion of the bottom zone are 1000 x 1000 x 500 m but stretching is used to
increase the horizontal spacings between +/- 350 km and the domain boundaries
(at +/- 500 km).