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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 in order to retrieve BUFR observation data from 3 days ago:

Retrieve the data and examine it. Metview's BUFR Examiner displays the contents of the BUFR data (Figure 8). Each message contains many measurements. If you visualise the data, you will see a standard display of synoptic observations. Figure 9 shows this, using the shaded_coastlines icon from the solutions folder.

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_markers icon into the Display Window. The shaded_coastlines icon may also help make the points easier to see (Figure 10).

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. From the solutions folder, drop both the diff_symb_hot and the diff_symb_cold icons into the plot.

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