accumulations

Note

The accumulations source is currently using the mars source internally. This will be changed in the future.

Accumulations and fluxes variables, such as precipitations, are often forecast fields, which are archived for a given a base date (or reference time) and a forecast time (or step). These fields are valid at the forecast time, and are accumulated over a given period of time, with the relation: \(valid\_date = base\_date + step\).

Because the package build datasets according to the valid date of the fields, it must be able to compute the base date and the forecast time from the valid date. Furthermore, some fields are accumulated since the beginning of the forecast (e.g. ECMWF operational forecast), while others are accumulated since the last time step (e.g. ERA5).

The accumulations has some of that knowledge built-in. If the source dataset is unknown, the package assumes that the fields to use are accumulated since the beginning of the forecast, over a 6h period.

The user can specify the desired accumulation period with the accumulation_period parameter. If its value is a single integer, the source will attempt to accumulate the variables over that period. This does not always mean that the data used is accumulated from the beginning of the forecast, but the most recent data available will be used:

For ECMWF operational forecasts, the data is accumulated from the beginning of the forecast. So if the accumulation period is 6h, for the date 2020-01-01 00:00, the source will use the forecast [1] of 2019-12-31 18:00 and the step 6h.
For ERA5, the data is accumulated since the last time step, and steps are hourly, and only available at 06Z and 18Z. So, a 6h accumulation for the date for the date 2020-01-01 13:00 the source will use the forecast of 2020-01-01 06:00 and the step 1-2h, 2-3h, 3-4h, 4-5h, 5-6h and 6-7h.

If the of accumulation_period value is a pair of integers [step1, step2], the algorithm is different. The source will compute the accumulation between the step1 and step2 previous forecast that validate at the given date at step2. For example, if the accumulation period is [6, 12], and the valid date is 2020-10-10 18:00, the source will use the forecast of 2020-10-10 06:00 and the steps 6h and 12h.

Please note that accumulation_period=6 and accumulation_period=[0, 6] are not equivalent. In the first case, the source can use return an accumulation between step 1h and step 7h if it is the most appropriate data available, while in the second case, the source will always return the accumulation between step 0h and step 6h, if available.

input:
  accumulations:
    accumulation_period: 6
    class: ea
    param: [tp, cp, sf]
    levtype: sfc

or:

input:
  accumulations:
    accumulation_period: [6, 12]
    class: od
    param: [tp, cp, sf]
    levtype: sfc