Understanding Ensembles


Ensemble charts are very important when it comes to forecasting in the medium term because they help us gauge how accurate a particular model run is going to be. For this guide we’re going to be using the GFS Ensemble Suite to get a basic idea of how we can use them to improve medium term forecasting.

Below is the chart I will be referring to in this guide, so it might be easier to open it in a new tab to make switching between the two easier.

 

For now I’d like you to ignore the lines at the bottom of the chart, and focus on the lines around half way up. There three lines we need to pay particular interest to. The first one being the bold red line which runs straight across the graph, this shows us what the average temperature for this time of year is, the second line we need to pay close attention to is the white line. This line gives us a mean of all the other lines, ensemble members, on the graph. The third and final line I’d like to mention is the solid green line, this line shows us the GFS Operational model which is what we would be viewing in Wetterzentrale as the pressure patterns chart.

All the other lines in the mix are perturbation members, they are run along with the GFS Operational but have slightly different starting data and are run at a much lower resolution. The closer all these lines are together the more accuracy we can have on them being correct, for example if we look at the left hand side of the chart we can see these lines are very close together, almost forming a single line in fact. This tells us all perturbation members are forecasting exactly the same thing, this gives us high confidence in the forecast.

Looking to the right of the chart, we can see these lines spread out and diverge significantly, this gives us very little confidence on the outcome during this period and therefore an accurate prediction cannot be made. The chart above shows us the temperatures at 850hPa with the temperatures being on the Y Axis just to the left of the graph. The X Axis at the bottom shows the date of forecast.

We can see by looking at the chart above that we’re going to see some above average temperatures between the period 19-25th October, the white mean line going above the red average line indicates this, the perturbation members are also relatively close together, giving confidence in this outcome.

The white line then moves below the red average line indicating temperatures cooling to below average from around the 26th October onwards, however perturbation members diverge at this point so there’s little confidence on this outcome.

The lines at the bottom of the chart indicate precipitation. The more “spikes” there are in this part of the chart and the higher they go the higher the chance of some wet and unsettled weather. You can see that even here, the further out into the forecast period we move, the more these lines diverge and confidence drops away.

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