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Introduction

The 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, which controls 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 they can be interpreted, particularly in a "real world" situation of forecasting for a observational field campaign.

 

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titleThis case study is based on the following paper which is recommended reading

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 study

In 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 principal component analysis (PCA) with clustering techniques (see Pantillon et al) to characterize the behaviour of the ensembles.


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Table of contents

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Note

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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Differences between the 2018 ENM tutorial and the original 2016 tutorial.

  1. Exercise 1. Modified so only goes up to 00Z on the 20/9/2012, not the full track or analyses.
  2. Exercise 6. "Exploring the role of uncertainty" has been completely removed. There is not enough time to examine the OpenIFS forecasts.




Note
titleCaveat on use of ensembles for case studies

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 individual events, like the one in this exercise, to get a more complete picture of IFS performance and identify weaker aspects that need further exploration.

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Info

Please enter the folder 'openifs_2018' to begin working.

Saving images and

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animations

Info

The macros described in this tutorial will write PostScript, PDF and PNG output files to the 'Figures' directory in the 'openifs_2018' folder.  An animated gif is also produced.

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

"Export" button in Metview'

 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.

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languagebash
titleCommand for screen snapshot
ksnapshot

Convert postscript to animated GIF

If you want to create animations, save the figures as postscript and then use the convert command:

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Begin by entering the folder labelled 'Analysis':

TASK 0

Task 1: Satellite images

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titleMetview icons in Analysis folder

Image Removed

an_1x1.mv : this plots horizontal maps of parameters from the ECMWF analyses overlaid on one plot.

an_2x2.mv : this plots horizontal maps of parameters from the ECMWF analyses four plots to a page (two by two).

an_xs.mv : this plots vertical cross-sections of parameters from the ECMWF analyses.

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Warning

TODO: Etienne to send new animations that do not go beyond 20th.

Original 'satellite' folder should go into the 'Forecast errors' folder?

Open the folder 'satellite'.

This folder contains satellite images (water vapour, infra-red, false colour) for 00Z on 20-09-2012 and animations of the infra-red and water vapour images.

Double click the images to display them.

Task 2: Mean-sea-level pressure and track

Right-click on the 'an_1x1.mv' icon and select the 'Visualise' menu item (see figure)Image Removed

After a pause, this will generate a map showing mean-sea-level pressure (MSLP).

Drag and drop the mv_track.mv icon onto the map to add the track of Hurricane Nadine.

Image Removed

In the plot window, use the play button in the animation controls Image Removed to animate the map and follow the development and track of Hurricane Nadine.

You can use the 'Speed' menu to change the animation speed (each frame is every 6 hours).

Info

If the contour lines appear jagged, in the plot window, select the menu item 'Tools -> Antialias'.

This task will look at the synoptic development of Hurricane Nadine and the cutoff low up to 00Z, 20th September 2012. The forecasts in the next exercises start from this time and date.

Warning

TODO:

1. Restrict analyses to go no further than 20th Sept 00Z. (change steps=...)

2. Modify track.csv and remove all dates after 00Z 20th Sept. Move original file track.csv to last exercise on Forecast_errors where the complete track can be used.

3. Move an_xs.mv to the last exercise on Forecast_errors.


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titleMetview icons in Analysis folder

Image Added

an_1x1.mv : this plots horizontal maps of parameters from the ECMWF analyses overlaid on one plot.

an_2x2.mv : this plots horizontal maps of parameters from the ECMWF analyses four plots to a page (two by two).

Right-click on the 'an_1x1.mv' icon and select the 'Visualise' menu item (see figure)Image Added

After a pause, this will generate a map showing mean-sea-level pressure (MSLP).

Use the an_1x1.mv and/or the an_2x2.mv macros to compare the ECMWF analyses with the satellite images.

Drag and drop the mv_track.mv icon onto the map to add the track of Hurricane Nadine.

Image Added

In the plot window, use the play button in the animation controls Image Added to animate the map and follow the development and track of Hurricane Nadine.

You can use the 'Speed' menu to change the animation speed (each frame is every 6 hours).

Info

If the contour lines appear jagged, in the plot window, select the menu item 'Tools -> Antialias'.


Warning
titleClose unused plot windows!

Please close any unused plot windows

Warning
titleClose unused plot windows!

Please close any unused plot windows if using a virtual machine. This case study uses high resolution data over a relatively large domain. Multiple plot windows can therefore require significant amounts of computer memory which can be a problem for virtual machines with restricted memory.

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Wind parameters can be shown either as arrows or as wind flags ('barbs') by adding '.flag' to the end of variable name e.g. "wind10.flag".

Info

Animating. If only one field on the 2x2 plot animates, make sure the menu item 'Animation -> Animate all scenes' is selected.

Plotting may be slow depending on the computer used. This reads a lot of data files.

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Q. What do you notice about the SST field?

Task 5: Satellite images --> Task 0

Warning

This includes animations beyond 20th?

Open the folder 'satellite' (back in the top level folder 'openifs_2018') (scroll the window if it is not visible).

This folder contains satellite images (water vapour, infra-red, false colour) for 00Z on 20-09-2012 and animations of the infra-red and water vapour images. The animations go beyond the 20-09-2012.

Double click the images to display them.

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Animating. If only one field on the 2x2 plot animates, make sure the menu item 'Animation -> Animate all scenes' is selected.

Plotting may be slow depending on the computer used. This reads a lot of data files.


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Q. What do you notice about the SST field?

Task 6: Cross-sections -- delete, just use HRES

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