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Comment: Proposed resolution increases

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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

Proposed resolution changes

Atmospheric model

 
 HRESENS
Leg ALeg B / C
 
Current
Upgrade
Current
Upgrade
Current
Upgrade
Spectral
TL1299
TCO1279
TL639
TCO639
TL319
TCO319
Gaussian grid
N640
N1280
N320
N320
N160
N320
Horizontal grid resolution
~16 km
~9 km
~30 km
~xx km
~60 km
~xx km
Dissemination (LL)
0.125
TBC
0.25
TBC
0.5
TBC

Model Level
Vertical resolution

137
137
91
91
91
91
  • There will be no change in the number and definition of the pressure levels.
  • The horizontal resolution of the ENS re-forecasts will be increased to match that of ENS.
  • The horizontal resolution of the two ENS constant-resolution forecasts provided for calibration and validation purposes will be increased:
 
 ENS calibration / validation
High resolutionLow resolution
 
Current
Upgrade
Current
Upgrade
Spectral
TL639
TCO639
TL319
TCO319
Gaussian grid
N320
N640
N160
N320
Horizontal grid resolution
~50 km
~xx km
~60 km
~xx km

Model Level
Vertical resolution

91
91
91
91

Wave model

 HRESENS
Leg ALeg B / C
 
Current
Upgrade
Current
Upgrade
CurrentUpgrade
Lat/Long
0.25
0.125
0.5
0.25
0.5
0.25
Horizontal grid resolution~28km~14km ~55km~28km~55km~28km
Dissemination (LL)
0.25
TBC
0.5
TBC
0.5
TBC
Frequencies
36
36
30
30
25
25
Directions
36
36
24
24
12
12
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.

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