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Description of Gaussian, fixed, fixed offset, regular, curvilinear grids

Gaussian Grids

A Gaussian grid is one where each grid point can be uniquely accessed by one-dimensional latitude and longitude arrays (i.e. the coordinates are orthogonal). The longitudes are equally spaced while the latitudes are unequally spaced according to the Gaussian quadrature.

Gaussian grids do not have points at the poles. Typically, the number of longitudes is twice the number of latitudes (i.e. 128 longitudes and 64 latitudes). Given the number of latitudes the NCL function gaus will generate the Guass distributed latitudes.

More details are located at:
An Introduction to Three-Dimensional Climate Modeling
W.M. Washington and C.L. Parkinson
University Science Books, 1986
ISBN 0-935702-52-0

Fixed Grids

A fixed grid is one where each grid point can be uniquely accessed by one-dimensional, monotonically increasing or decreasing arrays (i.e. the coordinates are orthogonal). In cartesian coordinates, these may refer to the "x" and "y" coordinates, while on the globe these are longitude and latitude arrays. The grid spacing may be different in the latitude (y) and longitude (x) coordinates, but it is constant. The special case where the grid spacing is the same in the latitude/longitude directions is called an "equally spaced" grid. Pole points may or may not be present. Some examples include: 1x1, 2x5, 2.5x2.5 degree grids.

Fixed-Offset Grids

A fixed-offset grid is analogous to the fixed grid, but refers to the special case where the latitude/longitude grids are offset for the traditional Greenwich Meridian or poles.

Regular Grids

Like fixed grids, regular grids can be can be uniquely accessed by one-dimensional, monotonically increasing or decreasing arrays (i.e. the coordinates are orthogonal). However, these grids can have irregular spacing in each direction. One example is the "MOM" grid, which is irregular in latitude: closely spaced points near the poles and equator but more sparsely spaced at mid-latitudes.

Curvilinear Grids

Curvilinear grids refer to grids where grid pints cannot be uniquely accessed by a pair of one-dimensional coordinate arrays. These grids require a pair of two-dimensional arrays to describe grid point locations.