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 prior to IFS cycle 41r2).
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 octahedral grid has about 20% fewer grid points than the equivalent N1280 linear 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 resolution with latitude for the reduced Gaussian grids. The full red and yellow curves show the resolution for the original reduced Gaussian grids at N1024 and N1280 (TC1023 and TC1279, 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 (TCO1023) and N1280 (TCO1279) octahedral grids are shown by the dashed and full blue lines, respectively. The resolution for the N1280 octahedral grid varies from about 8 km at the equator, increasing to about 10 km at 70oN and 70oS 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 (black dashed line) and N1280 (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 octahedral grid (right) on the surface of the sphere
Gaussian grid descriptions
Descriptions of the Gaussian grids introduced for the horizontal resolution upgrade at IFS cy41r2 and used for HRES and ENS are available:
- HRES: N1280 Gaussian grid
- ENS Leg A (days 1 to 10): N640 Gaussian grid
- ENS Leg B (days 10 to 15 and monthly extension to day 46): N320 Gaussian grid
The descriptions provide the latitude values and the number of longitude points at each latitude for both the original and octahedral reduced Gaussian grids.
The number of points at each latitude is encoded as the PL array in the Grid description section of the GRIB header of a grid point field. This is accessible with grib_api as the pl key.
As of grib_api 1.14.0, a new computed key isOctahedral is introduced which allows users to query the grid type. For an octahedral grid, isOctahedral=1; otherwise, isOctahedral=0.
Background
Spectral representation of the IFS
The IFS uses a spectral transform method to solve numerically the equations governing the spatial and temporal evolution of the atmosphere. The idea is to fit a discrete representation of a field on a grid by a continuous function. This is achieved by expressing the function as a truncated series of spherical harmonics:
\[ \begin{equation*} A(\lambda,\mu,\eta,t) = \sum_{l=0}^{\mbox{T}} \sum_{|m|\leq l}^{\mbox{T}} \psi_{lm}(\eta,t) Y_{lm}(\lambda,\mu) \end{equation*} \]where μ = sinθ with λ the longitude and θ the latitude of the grid point, T is the spectral truncation number and Y lm are the spherical harmonic functions.
The spectral coefficients ψlm are computed from the discrete values known at each point of a Gaussian grid on the sphere by
- a Fast Fourier Transform in the zonal direction followed by
- a slow/fast Legendre transform in the meridional direction.
At each time step in the IFS:
- derivatives, semi-implicit correction and horizontal diffusion are computed in spectral space;
- explicit dynamics, semi-Lagrangian advection and physical parametrizations are computed in grid point space.
The representation in grid point space is on the Gaussian grid. The grid point resolution is determined by the spectral truncation number, T.
Gaussian grids
Naming convention
The Gaussian grids are defined by the quadrature points used to facilitate the accurate numerical computation of the integrals involved in the Fourier and Legendre transforms. The grids are labelled by N where N is the number of latitude lines between the pole and the equator. For example, for the N640 Gaussian grid, there are 640 lines of latitude between the pole and the equator giving 1280 latitude lines in total.
The grid points in latitude, θk , are given by the zeros of the Legendre polynomial or order N: P N 0 (μk=sinθk) = 0. A consequence of this is that a Gaussian grid has:
- latitude lines which are not equally spaced;
- no latitude points at the poles;
- no line of latitude at the equator;
- latitude lines which are symmetric about the equator.
Regular (or full) Gaussian grid
A regular Gaussian grid has the following characteristics:
- there are 4N longitude points along each latitude circle;
- each latitude circle has a grid point at 0o longitude;
- the longitudinal resolution in degrees is 90o/N;
- the points get closer together (i.e. more crowded) as the latitude increases towards the poles;
- the total number of grid points is 8N2.
Reduced (or quasi-regular) Gaussian grid
A reduced Gaussian grid:
- has the same number of latitude lines (4N) as the corresponding regular Gaussian grid;
- has a grid point at 0o longitude on each latitude circle;
- has a decreasing number of longitude points towards the poles;
- has a quasi-regular grid spacing in distance at each latitude;
provides a uniform CFL (Courant–Friedrichs–Lewy) condition.
Up to and including IFS cycle 41r1, ECMWF has used a original reduced Gaussian grid. This has 4N longitude points at the latitude nearest to the equator, with the number of longitude points reducing in blocks as the latitudes approach the poles.
With the horizontal resolution increase at IFS cycle 41r2, ECMWF introduces a slightly different form of the reduced Gaussian grid which is referred to as the octahedral reduced Gaussian grid or, more simply, the octahedral grid.
Example regular Gaussian grid
Corresponding original reduced Gaussian grid
Relationship between spectral truncation and grid point resolution - linear, quadratic and cubic grids
The relationship between the spectral resolution, governed by the truncation number T, and the grid resolution depends on the number of points on the grid at the equator, 4N, at which the shortest wavelength field is represented:
linear grid: each wavelength is represented by 2 grid points → 4N \( \simeq \) 2(TL + 1)
quadratic grid: each wavelength is represented by 3 grid points → 4N \( \simeq \) 3(TQ + 1)
cubic grid: each wavelength is represented by 4 grid points → 4N \( \simeq \) 4(TC + 1)
Until the implementation of IFS cycle 18r5 on 1 April 1998, the IFS used a quadratic grid. The introduction of the two-time level semi-Lagrangian numerical scheme at IFS cycle 18r5 made possible the use of a linear Gaussian grid reflected by the TL notation. The linear grid has been used since then, up to and including IFS cycle 41r1. At IFS cycle 41r2, the cubic representation is used with the notation TC used to indicate the spectral truncation.
Increasing the horizontal resolution
The relationship between the spectral truncation, T, the number of grid points used to represent the shortest wave (i.e. linear, quadratic or cubic) and the grid point resolution, N, allows for three possible approaches for increasing the horizontal resolution:
- increase the number of wave numbers in the spectral representation but keep the same Gaussian grid by reducing the number of grid points used to represent the shortest wave (i.e., increase T with TQ →TL keeping N constant);
- increase the number of wave numbers in the spectral representation and the Gaussian grid resolution but keep the number of grid points used to represent the shortest wave constant (i.e. increase T and increase N).
- keep the number of wave numbers in the spectral representation constant and increase the Gaussian grid resolution by increasing the number of grid points used to represent the shortest wave (i.e., TL →TC, and increase N).
Frequently asked questions
Will the change to the octahedral grid affect me if I use regular lat-lon data ?
No, users of regular lat-lon data will be unaffected by the change of model grid. They will, however, benefit from the increase of the model horizontal resolution.
Will ECMWF make data available on the original regular and reduced Gaussian grids ?
Yes, users will still be able to request data, both in dissemination and MARS, of the original regular and reduced Gaussian grids. Note, however, that this will be interpolated from the values on the original model grid.See also I make computations involving flux parameters or accumulated fields (for example, to de-accumulate precipitation) and am advised to work on the model grid: which grid should I use ?
I use point data (e.g., for meteograms, vertical profiles, etc) - what do I need to do ?
Users of point data should not that the coordinates of the nearest grid point will have changed. Users should take particular care for coastal points for which the nearest grid point may have changed from a land point to a sea point or vice versa.
Are the new land-sea mask and orography for the octahedral grid available ?
Yes, the new land-sea masks and orography fields for HRES at TCO1279 (N1280), ENS Leg A at TCO639 (N640) and ENS Leg B TCO319 (N320) can be downloaded from ...
I use the orography in spectral representation - will I be affected ?
Although the spectral resolution for HRES remains constant, the spectral orography has changed. If you have this as a static file then this should be updated with the new version.
Do I need to upgrade the version of GRIB API I use in order to decode data on the octahedral grid ?
Version 1.12.3 of grib_api can decode fields on the octahedral grid correctly. As of grib_api 1.14.0, a new computed key isOctahedral is introduced which allows the grid type to be queried. For the octahedral grid, isOctahedral=1; otherwise, isOctahedral=0.
Can GRIBEX decode data on the octahedral grid ?
GRIBEX is no longer supported by ECMWF.
I make computations involving flux parameters or accumulated fields (for example, to de-accumulate precipitation) and am advised to work on the model grid: which grid should I use ?
For performing computations with accumulated fields, users are advised to request data on the octahedral grid.
Is there any change to the vertical resolution at IFS cycle 41r2 ?
No, only the horizontal resolution is increased. The vertical resolution remains at L137 for HRES and L91 for ENS.
What will happen if I retrieve IFS cycle 41r2 data from MARS using grid=av ?
Users retrieving data from MARS with the keyword, grid=av ("archived value") will retrieve data on the model grid. For data from IFS cycle 41r2 this will be the octahedral grid.
What will happen if I retrieve IFS cycle 41r2 data from MARS using grid=1280 ?
This behaviour is unchanged. By default, users retrieving data from MARS with the keyword, grid=1280 will retrieve data on the regular N=1280 Gaussian grid.
Will ERA-Interim fields also use the cubic octahedral grid ?
No, the horizontal resolution upgrade applies only to ECMWF HRES and ENS operational forecasts, including the monthly extension.
Will the ECMWF System 4 Seasonal Forecasts (SEAS) also use the octahedral grid ?
No, the horizontal resolution upgrade applies only to ECMWF HRES and ENS operational forecasts, including the monthly extension.