SPECIFICATION OF ECMWF HYBRID LEVELS

Some of ECMWF's high resolution upper air datasets use a hybrid system for specifying the levels of the data. The system combines the constant pressure level system with the sigma level system based on surface pressure. The ECMWF system is defined such that closer to the earths surface, the levels more closely resemble a pure sigma level, while the higher up you go, the more the levels are like constant pressure levels. The figure¹ below depicts the vertical distribution of the levels.



The vertical coordinate parameters define the edges of the atmospheric layers in terms of the surface pressure (in Pascals). The parameters can be considered as being true at the mid-point (in pressure) of a layer for each point. The formula to use to determine edges of the layers is :




where Ps is the surface pressure and A and B are defined in the table below.


LEVEL A B
1 0.000000 1.0000000000
2 0.000000 0.9922814815
3 0.000000 0.9729851852
4 72.063577 0.9442132326
5 271.626545 0.9077158297
6 638.148911 0.8649558510
7 1195.889791 0.8171669567
8 1954.805296 0.7654052430
9 2911.569385 0.7105944258
10 4050.717678 0.6535645569
11 5345.914240 0.5950842740
12 6761.341326 0.5358865832
13 8253.212096 0.4766881754
14 9771.406290 0.4182022749
15 11261.228878 0.3611450218
16 12665.291662 0.3062353873
17 13925.517858 0.2541886223
18 14985.269630 0.2057032385
19 15791.598604 0.1614415235
20 16297.619332 0.1220035886
21 16465.005734 0.0878949492
22 16266.610500 0.0594876397
23 15689.207458 0.0369748598
24 14736.356909 0.0203191555
25 13431.393926 0.0091941320
26 11820.539617 0.0029197006
27 9976.135361 0.0003908582
28 8000.000000 0.0000000000
29 6000.000000 0.0000000000
30 4000.000000 0.0000000000
31 2000.000000 0.0000000000
32 0.000000 0.0000000000
1 This figure and formula were taken from :
Ritchie, H., Temperton, C., Simmons, A., Hortal, M., Davies, T., Dent, D. and Hamrud, M. (1995) Implementation of the semi-Lagrangian method in a high resolution version the ECMWF forecast model. Mon. Wea. Rev.,123, 489-514