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IntroductionThe ECMWF operational ensemble forecasts for the western Mediterranean region exhibited high uncertainty while Hurricane Nadine was slowly moving over the eastern N. Atlantic in Sept. 2012. Interaction with an Atlantic cut-off low produced a bifurcation in the ensemble and significant spread, influencing both the track of Hurricane Nadine and the synoptic conditions downstream. The HyMEX (Hydrological cycle in Mediterranean eXperiment) field campaign was also underway and forecast uncertainty was a major issue for planning observations during the first special observations period of the campaign. This interesting case study examines the forecasts in the context of the interaction between Nadine and the Atlantic cut-off low in the context of ensemble forecasting. It will explore the scientific rationale for using ensemble forecasts, why they are necessary and how they can be interpreted, particularly in a "real world" situation of forecasting for an observational field campaign. | Panel |
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| title | This case study is based on the following paper which is recommended reading |
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| Pantillon, F., Chaboureau, J.-P. and Richard, E. (2015), 'Vortex-vortex interaction between Hurricane Nadine and an Atlantic cutoff dropping the predictability over the Mediterranean, http://onlinelibrary.wiley.com/doi/10.1002/qj.2635/abstract |
In this case studyIn the exercises for this interesting case study we will: - Study the development of Hurricane Nadine and the interaction with the Atlantic cut-off low using the ECMWF analyses.
- Study the performance of the ECMWF high resolution (HRES) deterministic forecast of the time.
- Use the operational ensemble forecast to look at the forecast spread and understand the uncertainty downstream of the interaction.
- Compare a reforecast using the May/2016 ECMWF operational ensemble with the 2012 ensemble forecasts.
- Use manual clustering to characterize the behaviour of the ensembles and compare the results with clustering based on principal component analysis (PCA; see Pantillon et al).
- Study the performance of the ECMWF ensemble forecasts trough RMSE curves.
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If the plotting produces thick contour lines and large labels, ensure that the environment variable LC_NUMERIC="C" is set before starting metview. |
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Data is provided for a single 10 day forecast starting from 20th Sept September 2012.
Data is provided at the same resolution as the operational model, in order to give the best representation of the Hurricane and cut-off low iterations. This may mean that some plotting will be slow.
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For this exercise, you will use the Metview icons in the folder 'HRES_forecast' shown above. hres_1x1.mv & hres_2x2.mv : these work in a similar way to the same icons used in the previous task where parameters from a single lead time can be plotted either in a single frame or 4 frames per page.
hres_xs.mv : this plots a vertical cross section and can be used to compare the vertical structure of Hurricane Nadine and the cut-off low.  : for this exercise, this icon can be used to overlay the forecast track of Nadine (and not the track from the analyses as in Exercise 1)
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You can use the hres_1x1.mv and hres_2x2.mv icons in the same way as Exercise 1 when looking at the analyses.
Although the macros will animate the data for the whole forecast, for this task concentrate on the forecast for the first 5 days.
For example, to plot geopotential at 500hPa and MSLP using the hres_1x1.mv macro, right click, select Edit and put:
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- "vo850", "mslp" : vorticity at 850hPa and MSLP - low level signature of Nadine and disturbance associated with the cutoff low.
- "r700", "mslp", : MSLP + relative humidity at 700hPa - with mid-level humidity of the systems.
- "pv320K", "mslp" : 320K potential vorticity (PV) + MSLP - upper level conditions, upper level jet and the cutoff signature in PV, interaction between Nadine and the cut-off low.
- "wind850", "w700" : Winds winds at 850hPa + vertical velocity at 700hPa (+MSLP) : - focus on moist and warm air in the lower levels and associated vertical motion.
- "t2", "mslp" : 2m-temperature and MSLP MSLP - low level signature of Nadine and temperature.
- "mslp", "wind10" : MSLP + 10m winds - interesting for Nadine's tracking and primary circulation.
- "t500","z500" : Geopotential geopotential + temperature at 500hPa - large scale patterns, mid-troposphere position of warm Nadine and the cold Atlantic cutoff.
- "eqpt850", "z850" : Geopotential geopotential + equivalent potential temperature at 850hPa - lower level conditions, detection of fronts.
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| title | Questions to consider |
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Q. What changes are there to the vertical structure of Nadine during the forecast?
Q. What is the fate of the cut-off and Nadine?
Q. Does this kind of Hurricane landfall event over the Iberian peninsula happen often? |
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- "pt", "pv" : Potential potential temperature + potential vorticity vorticity - to characterize the cold core and warm core structures of Hurricane Nadine and the cut-off low.
- "r", "w" : Humidity humidity + vertical motion : motion - another view of the cold core and warm core structures of Hurricane Nadine and the cut-off low.
- "pv", "w" ("r") : Potential potential vorticity + vertical velocity (+ relative humidity) : - a classical cross-section to see if a PV anomaly is accompanied with vertical motion or not.
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Now look at the analyses from 20th Sept September to observe what actually happened.
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If time, use the other icons such as an_2x2.mv and an_xs.mv to look at the cross-section through the analyses and compare to the forecast cross-sections from the previous exercises.
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Plot forecast difference maps to see how and when the forecast differed from the analyses.
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 Image Removed : this plots a single parameter as a difference map between the operational HRES forecast and the ECMWF analysis. Use this to understand the forecast errors.
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Use the hres_to_an_diff.mv icon and plot the differences between the z500, MSLP and other fields to how the forecast differences evolve.
Also try the ctrl_to_an_diff.mv icon which plots the difference but this time using the ensemble control forecast.
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Q. How does the behaviour of Nadine and the cut-off low differ from the HRES deterministic forecast and the ensemble control forecast?
Q. Did the ensemble spread from the previous exercises represent the uncertainty between the analyses and the HRES forecast?
Q. Was HRES a good forecast? |
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3: RMSE "plumes" for the ensemble
This is similar to the previous exercise, except the RMSE curves for all the ensemble members from a particular forecast will be plotted.
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Q. How do the HRES, ensemble control forecast and ensemble mean compare?
Q. How do the ensemble members behave, do they give better or worse forecasts?
Q. Is the spread in the RMSE curves the same in using other pressure levels in the atmosphere? |
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3: Difference stamp maps
Use the stamp_diff.mv plot to look at the differences between the ensemble members and the analysis. It can be easier to understand the difference in the ensembles by using difference stamp maps.
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