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In the previous task, we have seen that introducing some uncertainty into the forecast
ens_to_an.mv
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by starting from different initial conditions and enabling the stochastic parameterizations in IFS can result in significant differences in the RMSE (for this particular case and geographical region).
The purpose of this task is to explore the difference in more detail and look in particular at the 'ensemble spread'.
Refer to the storm track plots in the handout in this exercise.
Use the ens_to_an.mv icon and plot the MSLP and wind fields. This will produce plots showing: the mean of all the ensemble forecasts, the spread of the ensemble forecasts, the operational HRES deterministic forecast and the analysis.
Q. How does the mean of the ensemble forecasts compare to the HRES & analysis?
Q. Does the ensemble spread capture the error in the forecast?
Q. What other comments can you make about the ensemble spread?
If time:
- change the 'run=' value to look at the mean and spread for other forecast lead times.
- set the 'members=' option to change the number of members in the spread plots.
e.g. try a "reduced" ensemble by only using the first 5 ensemble members: "members=[1,2,3,4,5]".
Task 3: Spaghetti plots - another way to visualise spread
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Cumulative distribution function (CDF) The figures to the right illustrate the relationship between a normal distribution and its associated cumulative distribution function.The CDF is constructed from the area under the probability density function along the x-axis. The CDF gives the probability that a value on the curve will be found to have a value less than or equal to the corresponding value on the x-axis. For example, in the figure right, the probability for values less than or equal to X=0 is 50%. The Note that the shape of the CDF curve is related to the shape of the normal distribution. The width of the CDF curve is directly related to the value of the standard deviation of the probability distribution function. For our ensemble, the width is then related to the 'ensemble spread'. For a forecast ensemble where all values were the same, the CDF would be a vertical straight line. |
The probability distribution function of the normal distribution |
Cumulative distribution function for a normal Figure from Wikipedia |
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