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:
Type
Ob
Repres
Bufr
Date
-3
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:
Data
Drop your obs icon here
Output
Geographical Points
Parameter
012004
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:
Type
Fc
Levtype
Sfc
Param
2t
Date
-5
Step
48
Grid
1.0/1.0
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.
