as those of a standard Gaussian grid but with the number of longitude points at each latitude circle computed according to the formula:

\begin{

eqnarray*}
N_{lat}(lat_N) & = & 20 \\
N_{lat}(lat_i) & = & N_{lat}(lat_{i+1}) + 4 \mbox{ for } i=N-1,\ldots,1
\end{

eqnarray*}

In other words, there are 20 longitude points at the latitude circle closest to the poles with the number of points increasing continuously by 4 at each latitude towards the equator. This is in contrast to the standard reduced Gaussian grid where there are 'jumps' between blocks of latitudes with a constant number of longitude points (a restriction which was imposed by the Fast Fourier Transform routines being used prior to IFS cycle 41r2).

As a consequence, the zonal resolution of the octahedral grid varies more with latitude than for the standard reduced Gaussian grid. This can be seen in the figures.

Note in particular that the octahedral grid has 4N + 16 longitude points at the latitude circle closest to the equator whereas the standard reduced Gaussian grid has 4N longitude points at the latitude circle closest to the equator.

There are also fewer total grid points in the octahedral grid compared to the standard reduced Gaussian grid.

The octahedral grid has been shown to improve the calculation of local derivatives in grid point space.