The ensemble is hybrid in the sense that we use two different model versions; lagged in
the sense that we use successive forecasts. The reason to construct an ensemble in this
way is that the KNMI Harmonie-AROME archive can be used to construct an ensemble where
the spread of the resulting ensemble can represent a realistic ensemble spread and be used
as a pilot for high resolution ensembles which start to become available.
We use each model version to construct 5 ensemble members with a forecast length of the required 72 hours, even though the model makes only 48 hour forecasts.
Construction of a 5-member ensemble with a length of 72h. The blue lines indicate the original Harmonie-AROME 48h forecasts. The red lines indicate how each of the members is constructed.
From the start of the constructed ensemble (tf=0), we use the forecast
that starts there, and also the 4 forecasts that each started 3 hours earlier, for the
first 36 hours for which they all overlap. From there, we use forecasts from successive
runs to a maximum forecast of 72 hours.
Eventually, for all of the (hourly) steps, the date/time/forecast step values are changed to give 5 members that range from tf=0–72 for the same start date/time.