IFS is a model of two resolutions: the spectral resolution determining the number of retained waves and the corresponding grid onto which these waves can be transformed. A greater spectral resolution can improve the forecast quality whilst reduced Gaussian grid resolutions can improve the efficiency of the model. However, aliassing of the quadratic (and cubic) terms in the model equations can result from using linear grids, which needs to be compensated for in the model formulation.
Supported spectral and grid resolutions
The table below summarizes the supported between spectral resolution, gridpoint resolution and the recommended timestep for optimum performance.
Truncation
Grid
Spacing at Equator
Recommended timestep (min)
Application
Tl1279
N640
16km / 0.1406o
10
Operational forecast resolution (40r1)
Tl1023
N512
20km / 0.1758o
10
-
Tl799
N400
25km / 0.225o
12
-
Tl639
N320
31km / 0.28125o
15
ERA-5 high resolution
Tl511
N256
39km / 0.352o
15
-
Tl399
N200
50km / 0.45o
20
Operational ensemble resolution
Tl319
N160
63km / 0.5625o
20
ERA-5 ensemble
Tl255
N128
78km / 0.703o
45
ERA-Interim
Tq213
N160
63km / 0.5625o
20
-
Tl159
N80
125km / 1.125o
60
ERA-40
Tq106
N80
125km / 1.125o
60
-
Tl95
N48
209km / 1.875o
60
-
Tq63
N48
209km / 1.875o
60
-
Tq42
N32
310km / 2.813o
30
Development/testing only
Tq21
N16
626km / 5.625o
30
Development/testing only
T21 and T42 use the Eulerian dynamical core and a regular Gaussian grid.
T63 and above use the reduced Gaussian grid.
Tl denotes a linear grid, Tq denotes a quadratic grid.
Note that a linear grid allows a higher spectral truncation for the same number of gridpoints as a quadratic grid. Linear and quadratic in this sense means enough gridpoints are available to compute the linear and quadratic terms in the equations respectively.