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These exercises demonstrate some basic functionality of Metview, showing how to retrieve data from MARS, examine the data's structure, compute the differences between different data sets and visualise them.

First start Metview; at ECMWF, the command to use is metview for this tutorial (see Metview at ECMWF for details of Metview versions). You should see the main Metview desktop which looks something like Figure 1.

In Metview, all operations can be performed via icons. All icons are available via the Create New Icon context menu of the Metview desktop.

You will create some icons yourself, but some are supplied for you - please download the following file and save it in your $HOME/metview directory:

You should see it appear on your main Metview desktop, from where you can right-click on it, then choose execute to extract the files. You should now (after a few seconds) see a solutions metview_intro folder which contains the solutions and also some additional icons required by these exercisesicons we will work with. You should work in the main Metview desktopthis folder, not the embedded solutions folder.

This exercise introduces ODB data and some ways that Metview can use it. To save time, we will mostly use pre-prepared icons. Enter the ODB folder to do these exercises.

Retrieving the ODB data

The 'ret_temp' MARS Retrieval icon is already prepared for you to fetch Land TEMP ODB data from MARS from 3 days ago. Edit the icon to see which parameters are set. The most important ones are these:

Close the icon editor and perform the data retrieval by choosing execute from the icon's context menu. Right-click examine the icon to bring up Metview's ODB Examiner tool. Here you can see the metadata (Columns tab) and the actual data values themselves (Data tab). Close the ODB Examiner.

Save a local copy of the ODB data to the current folder by right-clicking Save result on the ret_temp icon; save as 'temp.odb'. A few seconds later an ODB Database icon (Figure 12) with the given name will appear at the bottom of your folder. We will work with this to avoid repeating the retrieval.

Using the ODB Visualiser

We will select and visualise the 500 hPa temperature values from our ODB using the 'vis_temp' ODB Visualiser icon.

Now edit the vis_temp icon.

First, drop your ODB Database icon into the ODB Data field.

Next, specify the where statement of the query in the ODB Where parameter as:

varno = 2 and vertco_reference_1=50000

Save these settings, then right-click visualise the 'vis_temp' icon to generate the plot. Then drag the the provided Symbol Plotting, Coastlines, Legend and Text Plotting icons into the plot for further customisation. Metview's plot window has many tools for inspecting data values, described in detail in the standalone tutorial "Using ODB with Metview". Do not close the plot window yet.

Overlaying with GRIB data

The 'fc.grib' GRIB icon contains 12 h global forecasts of temperature and wind at different vertical levels, valid for the date and time of our TEMP ODB data.

To overlay the 500 hPa temperature forecast we need to filter the matching field from the GRIB file. The 't500_fc' GRIB Filter icon is already already set up to perform this task. Just drag 't500_fc' into the plot, then drag the 't_cont' Contouring icon into the plot as well to customise the contour lines (Figure 13).

Further ODB work

If you have time, inspect and run the supplied macros:

• 'diff.mv' - computes and plots the difference between the ODB observation data and the GRIB model forecast
• 'plot_wind.mv' - extracts U and V wind components from the ODB data, converts to geopoints format and plots the result
• 'plot_tephi.mv' - computes and plots a tephigram for a given station ID

The results can be seen in Figure 14.