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The analysis data was valid for 3 days ago; this new icon retrieves a 48-hour forecast data generated 5 days ago, so it is also valid for 3 days ago. You don't need to separately execute and visualise the icon - if you visualise it, the data will automatically be retrieved first. The plot title will verify that this data is valid for the same date and time as the analysis data. It also contains the same set of vertical levels.

1. Retrieve temperature analysis
   1. examine the retrieved data
   2. plot it and use a nice contour shading icon
2. Retrieve temperature forecast
3. Compute the difference
4. Plot the difference and use pos & neg contour icons
5. Generate a macro from the plot

Observation - analysis difference

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1. examine the retrieved data
2. plot obs data using standard synop station plotting

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1. plot using advanced symbol plotting

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Compute the forecast-analysis difference

Create a new Simple Formula icon by taking a copy from the Macros icon drawer. Rename the copy to fc_an_diff. Edit the icon, ensure that the first tab is selected (F+G) and that the operator is minus ( - ). Drop your temperature_forecast icon into the Parameter 1 box, and drop temperature_analysis into the Parameter 2 box. Save the icon and visualise it. The difference will be computed and the result plotted. Note that all 6 fields in each data icon are used in the computation - the result is a set of 6 fields.

Automating the whole procedure

Ensure that the difference fieldset is visualised with the contouring applied. To generate a Metview Macro script from this plot, click the Generate Macro button (also available from the File menu). A new Macro script will be generated - have a look at it to confirm that it contains code to retrieve all the data, compute the difference and plot the result. Run the macro to obtain the plot. By default, the macro is written so that it will produce an interactive plot window; it will generate a PostScript file if it is run with the execute command, or if it is run from the command line:

metview4 -b  <macro-name>

Metview Macro is a rich, powerful scripting language designed for the high-level manipulation and plotting of meteorological data. For example, see List of Operators and Functions.

Exercise 2: forecast - observation difference

This exercise builds on exercise 1, but uses observation data in BUFR format instead of analysis fields.

Retrieving the observation data

Create a new Mars Retrieval icon and rename it to obs. Edit it and set the following parameters:

Retrieve the data and examine it. Metview's BUFR Examiner displays the contents of the BUFR data. Each message contains many readings. If you visualise the data, you will see a standard display of synoptic observations.

Extracting the 2 metre temperature

Create a new Observation Filter icon from the Filters icon drawer, and rename the copy to filter_obs_t2m. With this icon we will extract just the 2m temperature into Metview's custom ASCII format for scattered geographical data - geopoints. Set these parameters:

If you examine this icon now, you will see the result: a table of geo-located temperature values. When you visualise the data, you will see that the actual values are plotted as text on the screen; we can do better than this. From the solutions folder, drop the coloured_symbols icon into the Display Window. The dark_land_sea_shading icon may also help make the points easier to see.

Retrieving the forecast data

Create a new Mars Retrieval icon, rename it to t2m_forecast, and set these parameters:

The retrieved data is the 48-hour forecast made 5 days ago for 2-metre temperature.

Computing the forecast-observation difference

This is just the same as before, using a Simple Formula icon. Drop t2m_forecast into the Parameter 1 box, and filter_obs_t2m into the Parameter 2 box. Visualise the result - you will see that the result of a field minus a scattered geopoints data set is another geopoints data set. For each geopoint location, the interpolated value from the field was extracted before performing the computation.

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1. Overlay with the original temperature field?
2. Generate a macro from the plot