17. Grid Rotation

This chapter was contributed by Michael Eby ( eby@uvic.ca). A grid rotation is one of the simplest types of grid transformations that can be applied to a spherically gridded model. This transform is particularly useful for studies of high latitude oceans where the convergence of lines of longitude may limit time steps or where the ocean contains a pole (as in the Arctic). The idea is to define a new grid in which the area of interest is far from the grid poles. In limited domain models, the pole can be rotated outside of the domain. For global models, one possibility is rotate the North Pole to ($40^\circ$ W,$78^\circ$ N) which puts the North pole in Greenland and keeps the South pole in Antarctica. Other uses include rotating the grid to match the angle of a coastline or to provide more flexibility in specifying lateral boundary conditions.

Option rot_grid modifies the Coriolis term to handle a rotated model grid which is specified using three Euler angles for solid body rotation. The Euler angles are computed by defining the geographic latitude and longitude of the rotated north pole and a point on the prime meridian as described below.

To make this option easier to use, several rotation routines are provided within module rotation.F. The driver may be used to help define the rotation, write a file of geographic latitudes and test the rotation with idealized data. Other routines demonstrate how to interpolate scalar and vector data from a geographically gridded data set, to a rotated model grid. To run the driver, use the script run_rotation.