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So a Product Definition Template Number of 8 means "deterministic and interval-based". If you chose 11 then your product represents would represent an ensemble member for which you get .
In the latter case the GRIB message will have the key "perturbationNumber" but not in the former case because it is not model an ensemble.
Now there are other kinds of product groups which follow this pattern. Let's look at the atmospheric chemical constituents:
| Chemical | Deterministic | Ensemble |
|---|---|---|
| Instantaneous | 40 | 41 |
| Interval-based | 42 | 43 |
For meaning of codes, see http://apps.ecmwf.int/codes/grib/format/grib2/templates/4/
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