ECMWF Atmospheric Level Types

Introduction

The ECMWF Integrated Forecasting System (IFS) divides the atmosphere into layers defined as model levels. However, the ECMWF also produces output on other level types. ECMWF atmospheric data held at the BADC is archived on four level types:

Model levels (Operational, ERA-15 and ERA-40 data).
Pressure levels (held for Operational, ERA-15 and ERA-40 data).
Potential Temperature levels (Operational and ERA-40 data).
Potential Vorticity levels (Operational and ERA-40 data).

This page gives a description of each atmospheric level type.

Model levels

Atmospheric models generally divide the atmosphere into a number of distinct layers that are defined in terms of pressure as opposed to altitude. This is largely because the equations of motion, which form the basis for all NWP models, have their simplest form in pressure coordinates. However, there are limitations to using constant pressure surfaces, because like constant height surfaces, they can be "broken" by surface features such as mountains (i.e. parts of the surface intersect with the ground causing problems in the model).

The use of model levels (also often called σ-levels) instead of standard pressure levels addresses the problem of discontinuities by creating atmospheric levels that follow the contours of the Earth's surface in the lower and mid-troposphere. As the altitude increases (i.e. in the upper stratosphere and mesosphere), the effect of the surface orography lessens until the vertical layers in the upper atmosphere are the same as pressure levels.

The ECMWF web pages contain more documentation on model levels.

The current Operational model divides the atmosphere into 91 layers up to 0.1 hPa (about 64 km). The vertical resolution is highest in the planetary boundary layer and lowest in the stratosphere and lower mesosphere. The Operational dataset includes data archived at 31, 60 and 91 model levels. The ERA-40 datasets includes data archived on 60 model levels only.

The BADC archives model level data for the following ECMWF data sets:

Operational data on a regular 1.125° x 1.125° grid from October 1998 - present (see the parameters page for more details).
Operational data at a T106 Spectral resolution from March 1994 - October 2000 (see the parameters page for more details).
Operational data at a T159 Spectral resolution from September 2002 - present (see the parameters page for more details).
Operational data on a Reduced N80 Gaussian grid from September 2002 - present (see the parameters page for more details).
ERA-15 Re-analysis at a T106 Spectral resolution from January 1979 - February 1994 (see the parameters page for more details).
ERA-15 Re-analysis on a Reduced N80 Gaussian grid January 1979 - February 1994 (see the parameters page for more details).
ERA-40 Re-analysis at a T159 Spectral resolution (see status page for details of what years are available and the parameters page for details of the data).
ERA-40 Re-analysis on a Reduced N80 Gaussian grid (see status page for details of what years are available and the parameters page for details of the data).

Model levels are calculated in relation to the surface pressure using a method detailed in the Dynamics and numerical procedures section of the Integrated Forecasting System (IFS) documentation.

Table 1 provides an example of the pressure of the 91 model levels (as used in the current Operational model) when the surface pressure is 1013.250 hPa.

Table 1. Pressure of model levels when the surface pressure is 1013.250 hPa.

Level Number hPa Level Number hPa Level Number hPa Level Number hPa Level Number hPa

1 0.0100 21 15.8844 41 117.5942 61 417.7338 81 925.2226

2 0.0299 22 18.5179 42 125.7453 62 443.3441 82 940.4416

3 0.0568 23 21.4101 43 134.3981 63 470.1659 83 954.0914

4 0.1015 24 24.5653 44 143.5909 64 497.9584 84 966.1707

5 0.1716 25 27.9860 45 153.3538 65 526.4620 85 976.6735

6 0.2768 26 31.6736 46 163.7180 66 555.3989 86 985.6311

7 0.4285 27 35.6281 47 174.7166 67 584.4855 87 993.3027

8 0.6396 28 39.8481 48 186.3837 68 613.4989 88 999.8373

9 0.9244 29 44.3310 49 198.7556 69 642.2899 89 1005.1222

10 1.2985 30 49.0732 50 211.8697 70 670.7310 90 1009.1459

11 1.7781 31 54.0701 51 225.7656 71 698.7032 91 1012.0494

12 2.3800 32 59.3150 52 240.4844 72 726.0656

13 3.1209 33 64.7978 53 256.0690 73 752.6718

14 4.0176 34 370.5061 54 272.5644 74 778.4036

15 5.0860 35 76.4292 55 290.0175 75 803.1575

16 6.3417 36 82.5725 56 308.4774 76 826.8141

17 7.7988 37 88.9589 57 327.9948 77 849.2512

18 9.4706 38 95.6172 58 348.6233 78 870.3798

19 11.3688 39 102.5813 59 370.4182 79 890.1340

20 13.5037 40 109.8913 60 393.4375 80 908.4403

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Pressure levels

Pressure levels are defined as atmospheric levels at constant pressure. Although model levels are more convenient from a modelling perspective, many scientists prefer to use pressure levels in their analyses.

The ECMWF converts model level data to pressure levels with a loss of vertical resolution. For example, when the Operational model included 31 model levels, these were available on 15 pressure levels, in the current Operational model 60 model levels are converted to 21 pressure levels.

The BADC archives the following data sets on pressure levels:

Operational data on a regular 2.5° grid from March 1994 - present (see the parameters page for more details).
Operational data at a T106 Spectral resolution from March 1994 - February 1999 (see the parameters page for more details).
Operational data at a T159 Spectral resolution from September 2002 - present (see the parameters page for more details).
Operational data on a Reduced N80 Gaussian grid from September 2002 - present (see the parameters page for more details).
ERA-15 Re-analysis at a T106 Spectral resolution January 1979 - February 1994 (see the parameters page for more details).
ERA-40 Re-analysis at a T159 Spectral resolution (see status page for details of what years are available and the parameters page for details of the data).
ERA-40 Re-analysis on a Reduced N80 Gaussian grid (see status page for details of what years are available and the parameters page for details of the data).

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Potential Temperature levels

The ECMWF Operational and ERA-40 datasets also provide data on 15 isentropic (constant potential temperature surfaces).Table 2 shows the potential temperature of each surface and the corresponding pressures and heights for a 250K dry isothermal atmosphere.

The BADC archives the following data sets on potential temperature levels:

Operational data at a T159 Spectral resolution from September 2002 - present (see the parameters page for more details).
Operational data on a Reduced N80 Gaussian grid from September 2002 - present (see the parameters page for more details).
ERA-40 Re-analysis at a T159 Spectral resolution (see status page for details of what years are available and the parameters page for details of the data).
ERA-40 Re-analysis on a Reduced N80 Gaussian grid (see status page for details of what years are available and the parameters page for details of the data).

Table 2. Isentropic levels and corresponding pressures and heights.

Potential Temperature (K)

Pressure (hPa)

Height (km)

265

815

1.5

275

716

2.4

285

632

3.4

300

528

4.7

315

445

5.9

330

378

7.1

350

308

8.6

370

254

10.0

395

202

11.7

430

150

13.9

475

106

16.4

530

72

19.2

600

47

22.4

700

27

26.4

850

14

31.3

Potential Temperature (K)	Pressure (hPa)	Height (km)
265	815	1.5
275	716	2.4
285	632	3.4
300	528	4.7
315	445	5.9
330	378	7.1
350	308	8.6
370	254	10.0
395	202	11.7
430	150	13.9
475	106	16.4
530	72	19.2
600	47	22.4
700	27	26.4
850	14	31.3

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Potential Vorticity levels

The ECMWF Operational and ERA-40 datasets also provide data on a "PV=+/-2" surface on which the potential vorticity takes the value 2PVU in the northern hemisphere and -2PVU in the southern hemisphere (1PVU = 10^-6 m² s^-1 K kg^-1), provided such a surface can be found searching downwards from the Model level close to 96hPa. Values at this model level are used where the search is unsuccessful.

The BADC archives the following data sets on potential vorticity levels:

Operational data on a Reduced N80 Gaussian grid from September 2002 - present (see the parameters page for more details).
ERA-40 Re-analysis on a Reduced N80 Gaussian grid (see status page for details of what years are available and the parameters page for details of the data).

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Level Number	hPa	Level Number	hPa	Level Number	hPa	Level Number	hPa	Level Number	hPa
1	0.0100	21	15.8844	41	117.5942	61	417.7338	81	925.2226
2	0.0299	22	18.5179	42	125.7453	62	443.3441	82	940.4416
3	0.0568	23	21.4101	43	134.3981	63	470.1659	83	954.0914
4	0.1015	24	24.5653	44	143.5909	64	497.9584	84	966.1707
5	0.1716	25	27.9860	45	153.3538	65	526.4620	85	976.6735
6	0.2768	26	31.6736	46	163.7180	66	555.3989	86	985.6311
7	0.4285	27	35.6281	47	174.7166	67	584.4855	87	993.3027
8	0.6396	28	39.8481	48	186.3837	68	613.4989	88	999.8373
9	0.9244	29	44.3310	49	198.7556	69	642.2899	89	1005.1222
10	1.2985	30	49.0732	50	211.8697	70	670.7310	90	1009.1459
11	1.7781	31	54.0701	51	225.7656	71	698.7032	91	1012.0494
12	2.3800	32	59.3150	52	240.4844	72	726.0656
13	3.1209	33	64.7978	53	256.0690	73	752.6718
14	4.0176	34	370.5061	54	272.5644	74	778.4036
15	5.0860	35	76.4292	55	290.0175	75	803.1575
16	6.3417	36	82.5725	56	308.4774	76	826.8141
17	7.7988	37	88.9589	57	327.9948	77	849.2512
18	9.4706	38	95.6172	58	348.6233	78	870.3798
19	11.3688	39	102.5813	59	370.4182	79	890.1340
20	13.5037	40	109.8913	60	393.4375	80	908.4403