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Case description

In this exercise we will use Metview to produce the plots shown above:

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Preparations

XXX Download data

Verify that the data are as expected.

We will prepare the plot interactively using icons. Then, at the end, we will put it all together into a macro. Remember to give your icons useful names!

Creating the map plot

Setting the map View

With a new Geographical View icon, set up a cylindrical projection with its area defined as

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• the land coloured in grey
• the sea coloured as #dcf0ff

Plotting the Mean Sea Level Pressure field

Plot the GRIB file sandy_msl.grib into this view using a new Contouring icon. Plot black isolines with an interval of 5 hPa between them. Animate through the fields to see how the forecast evolving.

Plotting the storm track

The storm track data is stored in the CSV file called sandy_track.txt. If you open this file you will see that it contains the date, time and geographical coordinates of the track points.

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Now drag your Table Visualiser icon the plot to overlay the mean sea level forecast with the track.

Customising the storm track

The storm track in its current form does not look great so you need to customise it with a Graph Plotting icon by setting the

• the track line to black and thick
• the track points to be white filled circles (their marker index is 15) with red outline.

Plotting date/time labels onto the track

To finalise the track plot you need to add the date/time labels to the track points. This can be done with a Symbol Plotting icon by specifying the list of labels you want to plot into the map. Since it would require too much editing you will learn a better (programmatic) way of doing it by using Metview Macro.

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By returning the visual definition your Macro behaves as if it were a real Symbol Plotting icon. So save the Macro and drag it into the plot to see the labels appearing along the track.

Creating the graph plot

Setting the View

With a new Cartesian View icon, set up a view to cater for the graph showing the mean sea level pressure values in hPa by setting

• the x-axis type to date
• the y-axis label to hPa
• the y-axis minimum value to 940 and its maximum is 1000

Computing the minimum pressure along the track

Since this task is fairly complex you will use a Macro for it. The idea goes like this:

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trDate = trDate & [date(val_date(i)) + hour(val_time(i))]

With this Having finished the body of the loop has been finished. The the last step is to define an Input Visualiser and return it. The code you need to add is like this:

vis = input_visualiser
 (
    input_x_type        : "date",
    input_date_x_values : trDate,
    input_y_values      : trVal
)   

return [vis] 

By returning the visual definition you built a Macro behaves as if it were a real Input VisualiserSymbol Plottingicon. Drop it.

Customising the storm track

The storm track in its current form does not look great so you need to customise it Customise the graph with a Graph Plotting icon by setting the

• the track line to black and thickthicker
• the track points to white be blue filled circles (their marker index is 15) with red outline.

Plotting the Precipitation Field

The GRIB file precip.grib contains a pre-processed field of precipitation accumulated over 3 hours before and after the time of the MSLP field (a total of 6 hours of precipitation).

Plot the precipitation data using new Contouring icon. Do this in isolation from the MSLP field until you are happy with the result.

• use the following list of levels for contouring: 0.5, 2, 4, 10, 25, 50, 100, 250  (but remember that in the Contouring icon editor to use a forward slash to separate the items)
• use the following list of colours:  cyan, greenish_blue, blue, bluish_purple, magenta, orange, red, charcoal (use the Contour Shade Colour List helper tool)
• deactivate the contour highlight
• activate the legend

Overlaying Both Fields

Visualise your Geographical View icon and drop the following icons into the Display Window:

• your Coastlines icon
• the msl.grib icon with your MSLP Contouring icon
• the precip.grib icon with your precipitation Contouring icon

Improving the Legend

Create a new Legend icon and change Legend Display Type to Disjoint. Play with the font size and colour and set the legend title to say something about the precipitation field. Drop it into the Display Window to see the results.

Adding the Position of New York City

One easy way to add a place-mark is to use the Input Visualiser icon and combine it with a Symbol Plotting icon.

Edit a new Input Visualiser icon and set the following:

Create a new Symbol Plotting icon to plot this as a red marker (the filled circle is marker index 28) with some text for its legend entry.

Adding a Custom Title

Using a Text Plotting icon, add a custom title as shown in the plot.

Generating a Macro to Reproduce the Plot

• a reasonable size.

Putting it all together

With a new Display Window icon design an A4 portrait layout with two views: your Geographical View icon should go top and your Cartesian View icon into the bottom. Now visualise your icon and populate the view with the dataGenerate a macro which will reproduce your plot with a single click. This can be done either by clicking the Generate Macro button from the Display Window or by editing a new Macro icon, dropping your data and visdef icons into it and adding a plot() command.

Extra Work

Try the following if you have time.

Ensuring the title has the correct date and time

There are various ways we can ensure that the title has the date and time according to the actual data. The default title in fact contains the date and time, but in this exercise we want more control over it.

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• if you have a fieldset variable called msl_grib, the following line will extract the date at which the field is valid:
  • msl_date = grib_get_long(msl_grib, 'validityDate')
• we can do something similar for the validity time
• these are extracted as integer numbers, but can be combined into a proper date variable in Macro:

  • full_date = date(msl_date) + hour(msl_time)

• use the string() function to construct a date string similar to the one used in the current title
  • see String Functions for details of how to use it
• insert this into the mtext() function instead of the current title
• it is now more robust - if you use data from a different date or time, the title will still be correct
• note that this method will not work directly if you want to generate an animation from different time steps of data

Experiment with different backgrounds

Modify the Coastlines icon, for example:

• plot the US state boundaries
• try different land or sea shading colours
• change the frequency of the grid lines

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