Versions Compared

Old Version 14

changes.mady.by.user Paul Dando

Saved on

compared with

New Version 15

changes.mady.by.user Paul Dando

Saved on

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

Table of Contents

Section

Horizontal resolution upgrade

The horizontal resolution upgrade is being developed with a trade-off in mind between resolution and computational costs. A number of options of how to produce the most effective combination of horizontal resolutions between 4D-Var, EDA, HRES and ENS have been tested to establish computing costs and to derive possible efficiency gains.

A viable choice was found to employ the so-called cubic Gaussian grid (TC) instead of the current linear Gaussian grid (TL) where the shortest wave is represented by four rather than two grid points. By keeping the spectral truncation the same, more resolution is added in grid-point space to more accurately represent diabatic forcings and advection, which is then controlled through truncation in spectral space. In the current operational configuration the erroneous build-up of energy at the shortest scales is filtered by a lower-than-nominal resolution of the orography, strong horizontal diffusion and a de-aliasing filter. In the future this filtering will be able to be much reduced. The TC option also substantially improves mass conservation.

In order to reduce the computational cost further, a grid modification has been investigated, the cubic, spectral octahedral grid (TCO). The octahedral grid applies a new rule for computing the number of points per latitude circle. It is based on a new mesh that also allows for future implementations of a hybrid spectral – grid point model. The computational cost is reduced by about 25% compared to the cubic grid as fewer grid point calculations are needed and this new grid will also be implemented in the coming high resolution model cycle.

In summary, the anticipated upgrade will have a horizontal resolution that translates to about 9 km in the outer loop of 4D-Var as well as the high-resolution forecast and to about 16 km for the ensemble up to day 10.

Section

Some 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:

Mathdisplay
\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 Ylm 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.

...

Section

Frequently asked questions

Will the change to the cubic 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 standard regular and reduced Gaussian grids ?

Yes, users will still be able to request data, both in dissemination and MARS, of the standard 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 ?

Check LSMUsers 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 vise versa.

Is the new land-sea mask and orography for the cubic 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 ...

Do I need to upgrade the version of GRIB API I use in order to decode data on the cubic octahedral grid ?

Version 1.12.3 of grib_api can decoded fields on the cubic octahedral grid correctly.  At grib_api 1.14.0, a new computed key is introduced which allows users to query the grid type.

Can GRIBEX decode data on the cubic octahedral grid ?

To be checked. But you should not use GRIBEX.

Anchor
accumulated
accumulated
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 cubic octahedral grid.

Is there any change to the vertical resolution at IFS cycle 42r1 ?

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 42r1 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 42r1 this will be the cubic octahedral grid.

What will happen if I retrieve IFS cycle 42r1 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 cubic octahedral grid ?

No, the horizontal resolution upgrade applies only to ECMWF HRES and ENS operational forecasts, including the monthly extension.