Case description

In this exercise we will apply the basic Metview techniques to produce the plot shown above: a map showing part of North America with precipitation and mean sea level pressure fields plotted during the time of Hurricane Sandy. We will also attach a marker to show a specific location on the map (New York City) and customise the legend and title.

Check that the supplied 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!

Steps

Setting the View

With a new Geographical View icon, set up a cylindrical projection with its area defined by its lower-left corner [20^oN, 110^oW] and its upper-right corner [70^oN, 30^oW].

Set up a new Coastlines icon with the following:

• the land coloured in cream
• the coastlines in grey
• the grid as a grey dashed line

Plotting the Mean Sea Level Pressure field

Plot the GRIB file msl.grib into this view using a new Contouring icon. Plot black isolines with an interval of 5 hPa between them. Since this will be plotted on top of another field, it would also be a good idea to increase the thickness of the isolines. Activate the legend in the Contouring icon and set the legend text for this icon to "MSLP".

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

Generate 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. Do it using the Generate Macro button and we will gain a little extra functionality for free. The generated macro will be called MacroFramework1; Metview rescans its folders every few seconds, so it might not appear immediately. Right-click and choose visualise to obtain our normal on-screen plot, or choose execute to generate a PDF file of the plot. Look at the generated Macro code to see how this is done! Different output formats will be studied in more depth in Working with graphical output.

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

Extra Work

Try the following if you have time.

Add another location marker

Washington DC is quite close, at coordinates 38.5N, 77W. Create another Input Visualiser icon (or make a copy of the New York one) and set these co-ordinates. Create another Symbol Plotting icon to plot this marker in green and ensure it has a correct name in its legend entry. Drop the two icons into your existing plot.

Using the cursor data to examine the grid point values

Activate the Cursor Data tool and see what it shows you when you have multiple data layers in your plot. Note that both sets of data are scaled from their units in the GRIB files.

Experiment with different contouring schemes

The precipitation could of course be shaded differently - try some different colour schemes.

By setting Contour Min Level to 5, you can choose to quickly show only those areas with 5mm or more precipitation (look at a global map to see all of these areas).

Metview has a built-in facility for using the same contouring styles for certain fields as ecCharts does. Create a new Contouring icon and set Contour Automatic Setting to Ecchart and Grib Scaling of Derived Fields to On. With this setting, various parameters from the GRIB data are used to choose a contouring style (if it exists in the ecCharts style library). In fact, this same icon can be used for both fields!

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.

Construct the second line of the title by extracting the date and time from the MSLP field and converting into an appropriate string - do this in the Macro code.

Hints:

• this procedure will be covered in Customising Your Plot Title
• if you have a fieldset variable called msl_grib, the following line will extract the date at which the field is valid:
  • msl_date = valid_date(msl_grib)
• 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  (ensure that we read the msl data and extract its date before defining the 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 and projections

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

Try a different Geographical View icon and set the projection to, for example, Mollweide. Drop this into the plot, then update the macro to use this new view (to do this, edit the macro, then drop your view icon into the editor towards the bottom of the code, where the view is defined; a little editing will be required to use your new view instead of the original one).