added fields to inhance mapping and filtering possibilities for forec…

…ast data

README.md

@@ -30,6 +30,10 @@ The fields in the table below can be used in these parts of STAC documents:
 | forecast:reference_datetime  | string | **REQUIRED.** The *reference* datetime: i.e. predictions for times after this point occur in the future. Predictions prior to this time represent 'hindcasts', predicting states that have already occurred. This must be in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). |
 | forecast:horizon             | string | The time between the reference datetime and the forecast datetime. Formatted as [ISO 8601 duration](https://en.wikipedia.org/wiki/ISO_8601#Durations), e.g. `PT6H` for a 6-hour forecast. |
 | forecast:duration            | string | If the forecast is not only for a specific instance in time but instead is for a certain period, you can specify the length here. Formatted as [ISO 8601 duration](https://en.wikipedia.org/wiki/ISO_8601#Durations), e.g. `PT3H` for a 3-hour accumulation. If not given, assumes that the forecast is for an instance in time as if this was set to `PT0S` (0 seconds). |
+| forecast:param               | string | Name of the model parameter that corresponds to the data, e.g. `T` (temperature), `P` (pressure), `U`/`V`/`W` (windspeed in three directions). |
+| forecast:mode                | string | Denotes whether the data corresponds to the control run or perturbed runs. Allowed values are `ctrl` or `perturb`. |
+| forecast:member              | integer | Specifies the member (sample number) of perturbed runs, e.g. `1`. |
+| forecast:level               | integer | Index of the vertical level of the height coordinate system used in the forecast model, e.g. `4`. |
 
 ### Additional Fields from other extensions