Views in Metview

A fundamental concept in Metview is the View. A View specifies the following definitions in a Metview plot:

• type of visualisation (e.g. geographical map, cross section, vertical profile, tephigram)
• parameters specific to that plot type (e.g. geographical area, cross section line, min/max axes values)
• plot position within the page (several plots can share a page)
• how to overlay different data in the same plot (explored in Handling Time in Metview)
• plot decoration (e.g. draw a frame around the plot)

If you do not provide a View, then Metview will use a sensible default View. For instance, when you visualise a GRIB icon, a default Geographical View containing a global map in a Cylindrical projection is used.

The following customised Geographical View shows some of these concepts including plot position and coloured frames.

The Geographical View   

This is the default View for plotting geographic-based data. This View was discussed previously in Customising Your Plot.

The Cartesian View    

This view is for plots that are not on a map, and will be covered in another session: Graph Plotting in Metview.

The Annotation View    

This will be covered in another session: Layout in Metview.

The Cross Section View    

The Cross Section View icon is a plotting specification for cross section plots along a given transect line.

Create a new Cross Section View icon, Visualise it and drop the t_fc24.grib icon into the resulting Display Window. A default cross section along the Equator is generated.

To customise the transect line (coordinates along which the cross-section is calculated), Edit the Cross Section View icon and either click on the Geography Tool button to bring up an editor (or type the coordinates by hand).

Save/OK the changes and re-visualise the data with this new cross section.

Note that you can still drag any valid Contouring icons you may have into the Display Window when visualising a cross section. For instance, apply the given shade icon. You may want to customise it and try different configurations.

Inspect the GRIB icon (right-click on it and choose examine) to see the type of input data this View requires - it should be gridded data (rather than spherical harmonics) and it should contain fields at multiple vertical levels. This view accepts data stored in either pressure levels or model levels (optionally supply a Logarithm of Surface Pressure field to transform model levels to pressure levels).

The Vertical Profile View    

The Vertical Profile View icon is a computation/plotting specification for vertical profiles.

Create a new Vertical Profile View icon, Visualise it and drop the t_fc24.grib icon into the Display Window. The result shows a vertical profile at a point (or averaged over an area). Experiment with this icon in a similar way to how you did with the Cross Section View icon.

The Graph Plotting icon is the visual definition used for the plotting of graphs (e.g. lines, curves and bar charts).   

To customise the line displayed in this plot, create a new instance of Graph Plotting and rename it to vdline. Edit it, setting the following parameters:

Save/OK the changes and drag it into the Display Window.

The Average View    

The Average View icon is a plotting specification for average (zonal or meridional) cross-section plots over an area.

Create a new Average View icon, Visualise it and drop the t_fc24.grib icon into the Display Window. A default meridional average over the global area is generated. Notice the horizontal axis - it only contains E/W co-ordinates, because the data values have been averaged along N/S meridional lines; for each point of longitude, there is one computed value per 2D field. With multiple fields in the vertical direction we can produce this plot.

Experiment with this icon in a similar way to how you did with the Cross Section View icon. You can use a Contouring icon, e.g. the shade icon, to style the contours of the plotting.

The Hovmøller View   

The Hovmoeller View icon is a computation/plotting specification for Hovmøller diagrams along a specified arbitrary transect line or a rectangular area. The diagram displays a two-dimensional graph with latitude or height as one axis, and time as the other.

Create a new Hovmoeller View icon, Visualise it and drop the t_ts.grib icon into the Display Window. A default diagram derived from a transect line along the Equator is generated. 

Three types of Hovmøller diagrams can be produced:

1.  Area Hovm     - diagram derived from an input rectangular area
2.  Line Hovm      - diagram derived from an input transect line
3.  Vertical Hovm - diagram derived from an input rectangular area and a set of levels.

For now, only consider the Area Hovm type and try a different transect line. As previously, you can use a Contouring icon to style the contours of the plotting.

This view requires data at different time steps. Examine the GRIB icon to see the fields used for this example.

The Thermo View   

The Thermo View icon is a plotting specification for Thermodynamic diagram plots from a suitable GRIB or BUFR data source. In such a diagram, temperature, humidity (represented by the dew point) and wind values are displayed with respect to pressure. Note that only the Tephigram diagram is currently available, although there exist other types of thermodynamic diagrams, such as Skew-T, Emagram and Stuve.

Create a new Thermo View icon, Visualise it and drop the tquv_pl.grib icon into the Display Window. A default diagram related to a geographical location [0,0] is generated.

Examine the GRIB icon to see the type of input data this View requires. Fields Temperature and Specific Humidity are mandatory and they will be used to compute the Dew Point parameter. Fieldsets U and V wind components are optional, but if given they will be used to compute the wind vectors. If the data is given in model levels then a Logarithm of Surface Pressure field must be provided too in order to help the conversion to pressure levels fields.

To customise the curves displayed in this plot, you can apply (or edit it first) icon vdline. The changes will be applied to both lines. The ability to customise each line individually (temperature and dew point) is available in the Thermo Plotting icon. Try it!

The Wind Plotting icon is the visual definition responsible for specifying how wind vector data is displayed. It controls the plotting of features such as wind arrows and wind flags.     

To customise the wind flags displayed in the plot, create a new instance of this icon and rename it to vdwind. Edit it, setting the following parameters:

Save/OK the changes and drag it into the Display Window.

Macro example

Let's create a Macro program to analyse the vertical structure of temperature changes in time. This exercise reads two forecast steps, computes the differences and visualises the result in a Cross Section View.

Create a new Macro icon and rename it to xsdiff. Edit it and do the following:

• drop the t_fc24.grib icon into the Macro Editor. A variable called t_fc24_2e_grib is assigned to the value of the read() command, which reads the GRIB data. Rename the variable to simply be t_fc24.

• drop the t_fc96.grib icon into the Macro Editor. Rename the variable to t_fc96.

• compute the differences: diff = t_fc96 - t_fc24

• drop the two contouring icons, neg and pos, into the editor

• drop the xs_europe icon into the editor
• underneath the generated code, type the following line:

plot(xs_europe,diff,neg,pos)

The says, "In the xs_europe view, plot the data field diff using the visual definitions neg and pos."

Your complete macro should look like this:

t_fc96 = read("/path/to/home/metview/training/day_2/analysis views/t_fc96.grib")
t_fc24 = read("/path/to/home/metview/training/day_2/analysis views/t_fc24.grib")

diff = t_fc96 - t_fc24

pos = mcont(
    legend                         : "on",
    # <code omitted for brevity>
    )

neg = mcont(
    legend                         : "on",
    # <code omitted for brevity>
    )

xs_europe = mxsectview(
    line : [55,-6,43,16]
    )

plot(xs_europe,diff,neg,pos)

Now run the macro to generate the plot. You can also omit xs_europe from the plot() command; in this case, Metview will use the default view for GRIB data, which is a Geographic View, giving a map plot.

Finally, Examine the two input GRIB icons to see how the fields differ in terms of date, time and step.

View / Data Modules

Metview uses a netCDF format internally for the results of some computations (this format will be covered in the session Data Part 2). Most of the Views described in this session (i.e. Cross Section, Vertical Profile, Average, Hovmøller and Thermo) do this, but the resulting data file is not available to the user. Therefore, each of these Views has a corresponding Data Module icon. If the intention is to simply plot the result, then the View icons are more useful. But to store the result data, the corresponding Data Module icon is required.

• create both a Vertical Profile View and a Vertical Profile Data icon.
• Edit both to see the differences.

All the parameters related to the visualisation of the result are only in the View icon, and the Data parameter exists only in the Data Module icon.

Now dealing only with the Vertical Profile Data icon:

• drop the supplied input GRIB icon t_fc24.grib into the Data parameter box.
• set the Point parameter to whatever you like and save the icon.
• Examine this icon to see the resulting netCDF file in the NetCDF Examiner.
• Save Result to save the result into a file for storage.

All of this can also be put into a Macro, where the resulting netCDF variable can be further manipulated before being written to a file (or visualised):

• create a new Macro icon and rename it to save_vp; edit it
• into the Macro Editor, drop the Vertical Profile Data icon that you already set up
• write the result to a file

To write a netCDF variable to a file, the syntax is the same as for any other data type:

write('output_file', data)

Your macro should be 3 lines long (well, 3 commands anyway) - one to read the input GRIB file, one to compute the profile and one to write the result to disk.

Extra Work

Hovmoeller Types

Investigate the different types of Hovmoeller diagrams available. Please note that type Vertical Hovm requires the input GRIB data t_ts_nlevels.grib. Examine this data to see that it contains fields from different vertical levels.

Axis Customisation

All of these views allow the ability to customise the rendering of the axes. While the view itself defines the limits and projection parameters for the plot, the axis lines themselves can be customised, for example in terms of colour and title.

Create a new Axis Plotting icon and rename it to H Axis. Edit it to change the colour of the axis and to add an axis title. Repeat the process to create a vertical axis icon.

Edit a Cross Section View icon and drop your icons into the Horizontal Axis and Vertical Axis parameter boxes. Visualise the view to see the results.