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Comment: Comment from Glenn
Section

 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 the shortest wavelength is represented by 2 grid points → 4N

Mathinline
\simeq
 2(TL + 1)

quadratic grid: each the shortest wavelength is represented by 3 grid points → 4N 

Mathinline
\simeq
3(TQ + 1)

cubic grid: each the shortest wavelength is represented by 4 grid points → 4N 

Mathinline
\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 IFS cycle 41r1.  For the planned resolution upgrade, the cubic representation is used with the notation TC used to indicate the spectral truncation.

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