The ECMWF Model Grids

Introduction

1. Spectral Resolution
   • T159 resolution
   • T106 resolution
2. Gaussian Grids
   • N80 Full Gaussian grid
   • N80 Reduced Gaussian grid
3. Regular Latitude-Longitude Grids
   • 1° x 1° grid
   • 1.125° x 1.125° grid
   • 2.5° x 2.5° grid

Information is also provided on the interpolation methods used to convert spectral data to a regular grid.

Spectral Resolution

Only upper air data is stored in spectral form so the complementary surface data sets are provided on a Gaussian grid. The ECMWF data archived at the BADC is stored in two spectral resolutions, T159 and T106.

T159 resolution

The ERA-40 Re-analysis and Operational Analyses data are stored at a resolution of T159. The surface data for this data is provided on an N80 Reduced Gaussian grid.

T106 resolution

The ERA-15 Re-analysis project and Operational Analyses are both stored at a resolution of T106. The surface data for the ERA-15 data set is provided on an N80 Reduced Gaussian grid whilst the surface data for the Operational data set is on an N80 Full Gaussian grid.

Gaussian Grids

There are two types of N80 Gaussian grid, the full and reduced Gaussian grid. The BADC holds ECMWF data stored on both types of Gaussian grid. The ECMWF web site contains information on the different types of Gaussian grid.

N80 Full Gaussian Grid

The N80 Full Gaussian grid is equivalent to a 1.125° grid along its longitudinal axis. This means that it has 320 grid points encircling the earth at each latitude. The grid is symmetric about the Equator with 80 latitudinal points in each hemisphere (see Table 1). The ECMWF surface Operational data held at the BADC is stored on an N80 Full Gaussian grid.

N80 Reduced Gaussian Grid

The N80 Reduced Gaussian grid is more complicated than the N80 Full Gaussian grid. The reduced grid has the same number of latitudinal grid points but the number of longitudinal grid points varies. The number of longitudinal grid points decreases from the equator to the poles as shown in Table 1, with 320 grid points around the equatorial region but only 18 grid points at 89.1416°N and 89.1416°S. This reduces storage time and computing resources when running the model. The ECMWF ERA-15 Re-analysis project and the ERA-40 Re-analysis project surface data is available in the N80 Reduced Gaussian grid.

Table 1. Grid points on the "full" and "reduced" N80 Gaussian grids.

		"Reduced" N80 Gaussian grid	"Full" N80 Gaussian grid
No. from pole	Latitude (degrees)	No. of points on a Lat. circle	No. of points on a Lat. circle
1	89.1416	18	320
2	88.0294	25	320
3	86.9108	36	320
4	85.7906	40	320
5	84.6699	45	320
6	83.5489	54	320
7	82.4278	60	320
8	81.3066	64	320
9	80.1853	72	320
10	79.0640	72	320
11	77.9426	80	320
12	76.8212	90	320
13	75.6998	96	320
14	74.5784	100	320
15	73.4570	108	320
16	72.3356	120	320
17	71.2141	120	320
18	70.0927	128	320
19	68.9712	135	320
20	67.8498	144	320
21	66.7283	144	320
22	65.6069	150	320
23	64.4854	160	320
24	63.3639	160	320
25	62.2425	180	320
26	61.1210	180	320
27	59.9995	180	320
28	58.8780	192	320
29	57.7566	192	320
30	56.6351	200	320
31	55.5136	200	320
32	54.3921	216	320
33	53.2707	216	320
34	52.1492	216	320
35	51.0277	225	320
36	49.9062	225	320
37	48.7847	240	320
38	47.6632	240	320
39	46.5418	240	320
40	45.4203	256	320
41	44.2988	256	320
42	43.1773	256	320
43	42.0558	256	320
44	40.9343	288	320
45	39.8129	288	320
46	38.6914	288	320
47	37.5699	288	320
48	36.4484	288	320
49	35.3269	288	320
50	34.2054	288	320
51	33.0839	288	320
52	31.9624	288	320
53	30.8410	300	320
54	29.7195	300	320
55	28.5980	300	320
56	27.4765	300	320
57	26.3550	320	320
58	25.2335	320	320
59	24.1120	320	320
60	22.9905	320	320
61	21.8690	320	320
62	20.7476	320	320
63	19.6261	320	320
64	18.5046	320	320
65	17.3831	320	320
66	16.2616	320	320
67	15.1401	320	320
68	14.0186	320	320
69	12.8971	320	320
70	11.7756	320	320
71	10.6542	320	320
72	9.53270	320	320
73	8.41120	320	320
74	7.28970	320	320
75	6.16820	320	320
76	5.04670	320	320
77	3.92520	320	320
78	2.80370	320	320
79	1.68220	320	320
80	0.56070	320	320

Regular Latitude-Longitude Grids

1° x 1° grid

Some of the ERA-40 Re-analysis data is archived on a regular 1° x 1° grid.

1.125° x 1.125° grid

The ECMWF Operational data set is archived on a regular 1.125° x 1.125° grid for upper air model levels.

2.5° x 2.5° grid

There are two ECMWF data sets held on a regular 2.5° x 2.5° grid. These are the ERA-15 Re-analysis and the Operational data sets. Both surface and upper air data (on pressure levels) are archived at this resolution.

Interpolation methods

The ERA-15 Re-analysis and Operational data was interpolated using bi-linear interpolation routines provided by the ECMWF (see the ECMWF web pages for details).

The same software and method of interpolation was used to convert ERA-40 data from the N80 Reduced Gaussian grid to a 1° x 1° regular grid are described here.

The BADC also makes the XCONV/CONVSH software package available from the BADC web pages. XCONV is Graphical User Interface that allows the user to visualise, subset and convert data files. CONVSH is a command line utility that allows access to the same features as XCONV via a shell prompt. Some users will download XCONV/CONVSH to interpolate data themselves.

XCONV/CONVSH also uses a form of bi-linear interpolation to convert from one grid resolution to another. Users of the data should be aware that data values will vary slightly depending on the method of interpolation applied. The bi-linear interpolation used for ERA-40 may produce occasional spurious results such as very slightly negative values in precipitation fields. Users who wish to access the data in its original model form should use the spectral and N80 Gaussian gridded data instead of the interpolated regular gridded data.

To ensure that the two interpolation methods did not produce significantly different output the BADC ran a comparison of data sets interpolated by the ECMWF routines and the XCONV/CONVSH package. Figures 1 and 2 show two data sets interpolated from one original spectral data set of temperature on model level 60 (nearest the Earth's surface) on 1 January 1989. The comparison shows that the interpolation routines produce almost identical results.

Figure 1. Level 60 air temperatures (01/01/1989) interpolated by XCONV/CONVSH package.

Figure 2. Level 60 air temperatures (01/01/1989) interpolated by ECMWF routines.

The difference between the two interpolated data sets was less than 0.0012K (around 0.0005%) throughout the globe. This confirms that the there is minimal difference between the XCONV/CONVSH interpolation routine and the ECMWF's own interpolation routines.