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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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convert -delay 75 -rotate "90<" in.ps out.gif


Exercise 1. Hurricane

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Nadine and the cut-off low

ECMWF analyses to the 20th September 2012

In this exercise, the development of Hurricane Nadine and the cut-off

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flow up to the 20th September 2012 is studied.

Begin by entering the folder labelled 'Analysis':

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For these tasks, use the metview the row labelled ''
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bgColorborderColorwhitelightgrey
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titleMetview icons
icons in
Analysis
folder

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.

Task 1: Mean-sea-level pressure and track

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

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

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Plot track of Nadine. Drag Drag and drop the mv_track.mv icon onto the map.

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This will add the track of Hurricane Nadine. Although the full track of the tropical storm is shown from the 10-09-2012 to 04-10-2016, the ECMWF analyses (for the purpose of this study) only show 15-09-2012 to 25-09-2012.

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

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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.

Task 2: MSLP and 500hPa geopotential height

This task creates Figure 2. from Pantillon et al.

Right-click the mouse button on the 'an_1x1.mv' icon and select the 'Edit' menu item.

An edit window appears that shows showing the Metview macro code used to generate the plot. During these exercises you can change the top lines of these macros to alter the choice of parameters and plot types.

Code Block
languagebash
titleList of fields available
#Available# Available parameters:
# mslp,t2,wind10,speed10,sst
# t,z,pt,eqpt [850,700,500,200]
# wind,speed,r[925,850,700,500,200]
# w700, vo850, pv320K

The surface fields (single level) are: MSLP mslp (mean-sea-level-pressure), t2 (2-metre temperature (t2), wind10 (10-metre wind arrows (wind10), speed10 (wind-speed at 10m (: sqrt(u^2+v^2): speed10), sst (sea-surface temperature (sst).

The upper level fields are: t (temperature (t), z (geopotential (z), pt (potential temperature (pt), eqpt (equivalent potential temperature (eqpt), wind arrows (wind arrows), speed (wind-speed (speedas contours), r (relative humidity (r).

These fields have a list of available pressure levels in square brackets.

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Some extra fields are also provided: w700 (vertical velocity at 700hPa (w700), vo850 (relative vorticity at 850hPa (vo850) and pv320K (potential vorticity at 320K).

Wind fields are normally plotted as coloured arrows. To plot them as wind barbs add the suffix '.flag'. e.g. "wind10.flag" will plot 10m wind as barbs.

 

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titlePlot mslp and z500

With the edit window open, find the line that defines 'plot1':

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languagebash
#Define plot list (min 1- max 4)
plot1=["mslp"]                      # use square brackets when overlaying multiple fields per plot

Change this line to:

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plot1=["z500.s","mslp"]

Thesuffix '.s' means plot the 500hPa geopotential as a shaded plot instead of using contours (this style is not available for all fields).

Click the play button and then animate the map that appears.


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titlePlot PV at 320K

Change the value of "plot1" again to animate the PV at 320K (similar to Figure 13 in Pantillon et al).

Code Block
plot1=["pv320K"]

You might add the mslp or z500 fields to this plot.

You can add multiple lines e.g.

Code Block
plot1=["z500.s","mslp"]



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titleQuestions

Compare the animation of the z500 and mslp fields with Figure 1. from Pantillon et al.

Q. When does the cut-off low form (see z500)?
Q. From the PV at 320K (and z500), what is different about the upper level structure of Nadine and the cut-off low?

Task 3: Changing geographical area

Right-click on 'an_1x1.mv' icon and select  'Edit'.

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