What is the octahedral grid ?

The octahedral grid has been inspired by the Collignon Projection of the sphere onto an octahedron.

It is a form of reduced Gaussian grid with the same number of latitude lines located at the same latitude values as those of a original Gaussian grid but with the number of longitude points at each latitude circle computed according to the formula:

\begin{eqnarray*}
\mbox{N}_{lat}(lat_N) & = & 20 \\
\mbox{N}_{lat}(lat_i) & = & \mbox{N}_{lat}(lat_{i+1}) + 4,  \mbox{ for } i=\mbox{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 original reduced Gaussian grid where there are 'jumps' between blocks of latitudes with a constant number of longitude points (a restriction imposed by the Fast Fourier Transform routines being used up to IFS cycle 41r1).

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

Note in particular that the octahedral grid has 4N + 16 longitude points at the latitude circle closest to the equator whereas the original 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 original reduced Gaussian grid.For example, the N1280 O1280 octahedral grid has about 20% fewer grid points than the equivalent N1280 linear original reduced grid. Generally, an octahedral grid of resolution N has 4xNx(N+8) grid points.

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

Comparison of the resolution variation with latitude for the reduced Gaussian grids

Variation of the resolution (characteristic length scale) with latitude for the reduced Gaussian grids.  The full red and orange curves show the resolution for the original reduced Gaussian grids at N1024 and N1280 (T_C1023 and T_C1279, respectively).  Note that the resolution remains more or less constant at 10km and 8km, respectively, as the latitude varies.  The corresponding curves for the N1024 O1024 (T_CO1023) and N1280 O1280 (T_CO1279) octahedral grids are shown by the dashed and full blue lines, respectively.  The resolution for the N1280 O1280 octahedral grid varies from about 8 km at the equator, increasing to about 10 km at 70^oN and 70^oS before decreasing again towards the poles. The resolution of the N640 original reduced Gaussian grid used for HRES at IFS cycle 41r1 is at about 16 km. Also shown are the regular Gaussian grids at N1024 F1024 (black dashed line) and N1280 F1280 (black full line).

Comparison of the zonal variation in resolution between original reduced and octahedral grids

Comparison of the zonal variation in resolution for the N1280 original reduced Gaussian grid (left) with the corresponding O1280 octahedral grid (right) on the surface of the sphere