In these exercises we will look at a case study using a forecast ensemble.The case study will look at severe wind-storms that hit Europe in late 2013 (see separate ECMWF article by Hewson et al, ECMWF Newsletter 139).
You will start by studying the evolution of the ECMWF HIRES forecast and ensemble forecast for this event. Then you will run your own OpenIFS forecast for a single ensemble member at lower resolutions and work in groups to study the OpenIFS ensemble forecasts.
Caveat: In practise many cases are aggregated in order to evalute the forecast behaviour of the ensemble. However, it is always useful to complement such assessments with case studies of extreme events, like the one in this exercise, to get a more complete picture of IFS performance and identify weaker aspects that need further exploration. |
ECMWF operational forecasts consist of:
metview |
Dates 24th - 29th October 2013.
Key questions:
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Suggested fields to plot: MSL, Z200, 10m wind and visualize the storm track.
Use the metview macros to plot different days and compare to analysis and plot forecast differences.
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Again using the ECMWF operational forecast, look now at the 50 ensemble forecasts. These are at a lower resolution (T639) than the HIRES (T1279).
Key questions:
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Suggested plots:
Note that as only 6hrly wind gust data is available from the operational forecasts, we have supplemented the 3hrly fields using forecast data.
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TO DO: RMSE & CDF (concepts need explanation)
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See separate handout.
In this exercise, OpenIFS will be run on the ECMWF Cray XC30 to create a forecast for the storm at T319 resolution using only the stochastic schemes in the model. All forecasts are started from the same initial conditions based on the analysis.
Aim is to understand the impact of these different methods on the ensemble
Experiments available:
These are at T319 with start dates: 24/25/26/27 Oct 00Z for 5 days with 3hrly output.
Plots:
Look at ensemble mean and spread for all 3 cases.
Look at different ensemble sizes?
Ensemble perturbations are applied in positive and negative pairs. For each perturbation computed, the initial fields are CNTL +/- PERT. (need a diagram here)
If time:
For more information on the stochastic physics scheme in (Open)IFS, see the article:
Shutts et al, 2011, ECMWF Newsletter 129.
Many people have contributed to this exercise and their contributions are acknowledged, in particular from ECMWF: Glenn Carver, Linus Magnusson, Martin Leutbecher, Sandor Kertesz, Iain Russell, Erland Kallen. From University of Oxford: Aneesh Subramanian, Peter Dueben, Peter Watson, Helen Christensen.
These will disappear in the final handout.
Need to think how to group some of these activities so that people on a row are working together.
Retrieve data from MARS for all apart from the OpenIFS experiment the participants will run themselves. Note that operational ensemble runs at T319 are also available if we want to use them to compare 40r1 with OpenIFS? (see Linux for MARS script) wind gust data is not simply max windspeed over 6hr period, it includes convective component (as well as conversion from windspeed to gust speed). need to fake windgust data! |
In exercise 1, the aim is to study the error between the forecast and the analysis. Key points: The forecast developed the storm too early, even for the forecast on the 27th. Storm was too far west. plot Z200 to see position of the jetstream. For the ensemble, plotting the mean will remove some features - might be useful for class to see this. Note how spread is small for northerly low centre where uncertainty is much less. Make sure they use 4 frame layout and plot 25th, 26th, 27th + analysis. Also, 25th, 26th & their difference |
Question. How best to organise the experiments? Each user has an account or use one account with multiple directories? Linus suggested running a script that reorders the data to have 1 file with all ensemble members for each field of interest. Do this after all members have run? |
Suggests doing 2 more runs: EDA only and SV only. The perturbations from these have different characteristics and as we don't yet know what the results look like it might be useful to have these runs. (I may not have all the details right - check with Linus who apparently did a PhD thesis on this) Relate the perturbations to the baroclinic zones (ie. large scale flow). While the cyclone is developing the baroclinicity will be high and the EDA/SV perturbations will be more significant than when the storm is more developed with fronts when it will be more stable. Should see that SV are quite artificial, some pertubations will grow rapidly and then die out. By contrast the perturbations in the EDA will be larger in the initial conditions. Maybe we should plot the Eady index? Erland says very important to plot these and see what we get in order to refine the questions and direction we want participants to take. |
Linus explained that with the OpenIFS runs will have differing amounts of uncertainty, so the spread should noticeably change for points near the track in the analysis. This is particularly because of (a) timing error between the analysis & fc, (b) the ensemble tracks being more to the north of the analysis track. So Amsterdam for instance should see much less spread as the uncertainty in the ensemble is reduced. Parameters that do not have a Gaussian like distribution in the ensemble can be problematic. |