Introduction

In these exercises we will look at a case study of the interaction between a Hurricane and Atlantic cut-off low in the context of ensemble forecasting. During the course of the exercises, we will explore the scientific rationale for using forecast ensembles, why they are necessary and how ensemble forecasts can be visualized. In this interesting case study, the HyMEX field campaign was also underway. We will see how the forecast products were used during this campaign.

This case study is based on the paper (recommended reading):

Pantillon, F., Chaboureau, J.-P. and Richard, E. (2015), 'Vortex-vortex interaction between Hurrican Nadine and an Atlantic cutoff dropping the predictability over the Mediterranean, http://onlinelibrary.wiley.com/doi/10.1002/qj.2635/abstract

In practise many cases are aggregated in order to evaluate 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.

Recap

ECMWF operational forecasts consist of:

HRES : T1279 (16km grid) highest resolution 10 day deterministic forecast
ENS : T639 (34km grid) resolution ensemble forecast (50 members) is run for days 1-10 of the forecast, T319 (70km) is run for days 11-15.

Saving images and printing

To save images during these exercises for discussion later, you can either use:

"Export" button in Metview's display window under the 'File' menu to save to PNG image format. This will also allow animations to be saved into postscript.

or use the following command to take a 'snapshot' of the screen:

ksnapshot

Virtual machine

If using the metview/openifs virtual machine with these exercises the recommended minimum memory is 4Gb. These exercises use a relatively large domain with high resolution data. Some of the plotting options can therefore require significant amounts of memory. If the virtual machine freezes when running metview, please try increasing the memory assigned to the VM.

Outline of the exercises

Study track & development of Hurricane Nadine and the N.Atlantic cut-off low
Study HRES & ENS forecasts of the time
Study HRES & ENS forecasts using the 2016 ECMWF ensemble forecast
.... PCA analysis ..
...etc...

Starting up metview

Before starting metview, you may need to type a command to copy the exercise files to your user account.

metview

Please enter the folder 'openifs_2016' to begin working.

Exercise 1. The ECMWF analysis

Study development and behaviour of Hurricane Nadine
...

Task 1: Hurricane Nadine

Plot and animate MSL + 500hPa maps showing track of Nadine
> 1 : Nadine MSLP and T2m (or better SST) tracking 15-20 september
> 2 : Satellite views on the 20th (provided by Etienne, if possible to put on the VM)
> 3 : Studying of the horizontal maps (analysis + forecasts)
> 4 : Studying and building of the vertical x-sections (analysis + forecasts)
> 5 : Beyond D+5 deterministic scenario : MSLP only

Exercise 2.

Task 1 : forecast error

Task 2 : compare forecast to analysis

Task 3 : visualize ensembles (plumes, ensemble spread, spaghetti, stamp, CDO)

--> These 3 tasks from last year are very interesting. To gain time maybe that we should put a group on each item for task 3 or suppress task 2 ? The CDO adds a "statistical" taint to the workshop do you think we can adapt it to our case ?

Task 4 : PCA and clustering

If not possible in Metview I can make the students plot with NCL figures 5, 6 and 7 from Pantillon.

Figure 5 shows that EOF1 accounts for 3/4 of the variance. This dipole pattern is typical when tropical interact with mid latitudes. No need to spend a lot of time on this.

Figure 6 is much more interesting. It Allows to see that we can choose 2 clusters containing approximately half of the members. The deterministic forecast is close to the two outliers and the control and the analysis belong to cluster 1.

From Figure 7 we see that cluster 1 corresponds to a cutoff moving eastward over Europe and cluster 2 to a weak ridge over western Europe.

It would be great if we could also do the cluster composite of rainfall from Figure 8 : cluster 1 shows impact on precipitation over The Cévènes whereas cluster 2 shows weak precipitation over the Cévènes.

The plot of the cluster member tracks of Nadine and the cutoff from figure 10 is also very interesting to me, I think we should do it. We see more clearly that cluster 1 exhibits a weak interaction between cutoff and low and cutoff over Europe. In cluster 2, there is a strong interaction between the cutoff and Nadine and Nadine makes landfall over the Iberian penisula (in model world, is it realistic ?). I don't know if the tracking is easy to do in Metview as it implies to track the cutoff and the low for each member.

Like you said in a previous mail, there is a possibility of interactivity for figures 7 (MSLP and Z500 composites) 8 (wind and RR composite) and 10 (member track). We have to identify by a number the cluster members and if make the students group the members to create the cluster composites. I think it is a good idea.

Task 5 : Sensitivity experiments to the SST coupling

As I am writing I am beginning to wonder if we should not make 2 groups : one for task 4 and one for task 5. Tasks 1-3 would be for all students. This would allow to keep the CDO task. What do you think ?

Appendix

Datasets available

The following datasets are available on the Virtual Machine for this workshop:

Acknowledgements

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