Exercise 1: forecast - analysis difference

Starting Metview

This short exercise will demonstrate some basic functionality of Metview, showing how to retrieve data from MARS, examine its structure, compute the differences between fields and visualise them.

First start Metview; at ECMWF, the command to use is metview4_new (see Metview at ECMWF for details of Metview versions). You should see a Metview desktop which looks something like Figure 1.

Figure 1 - the Metview desktop	Figure 2 - the Mars Retrieval icon editor

In Metview, all operations can be performed via icons. Most icons are available in the tabbed drawers at the bottom of the Metview desktop.

Retrieving the analysis data

To perform a MARS retrieval in Metview, click the Data Access tab to open it, and drag the Mars Retrieval icon onto your Metview desktop; this will create a copy of the icon for you to customise. Rename the new icon to temperature_analysis by clicking on its name. Edit your icon (right-click & edit, see Figure 2) and set the following parameters:

To save these settings, click the Apply button at the bottom-left of the icon editor.

Inspecting the analysis data

Perform the data retrieval by choosing execute from the icon's context menu. The icon name should turn orange whilst the retrieval takes place, then green to indicate success (if the name turns red, then the retrieval failed and you should look in the output log, available from the output entry in the context menu). The data is now cached locally. To see what was retrieved, right-click examine the icon. This brings up Metview's GRIB Examiner tool (Figure 3). Here we can see that we retrieved six vertical levels of data; this is as expected if we look at the Levelist parameter in the icon editor.

Now visualise the data, again using the icon's context menu. You will see a map plot with the default contouring style in the Display Window (Figure 4).

To plot the data with shaded colours, go to the Visual Definitions drawer (if this drawer is not visible, you may have to either expand your Metview desktop window to the right, or else click on the dots which appear to the right of the last visible drawer). Drop the Contouring icon onto your Metview desktop, rename it shade and edit it, providing these parameters:

Save the icon settings (Apply) and drop this into the Display Window (re-visualise the data if you have closed the plot window). The result should resemble Figure 5. Metview's Contouring icon provides much flexibility in choosing how to display gridded fields; this tutorial uses only simple colour schemes. The Display Window provides a number of facilities for further inspection of the data, not covered here.

Retrieving the forecast data

Create a copy of your temperature_analysis icon (right-click, duplicate) and rename the copy to temperature_forecast. Edit this icon and set the following parameters:

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

1. Retrieve temperature analysis
2. Retrieve observation data
   1. examine the retrieved data
   2. plot obs data using standard synop station plotting
3. Filter temperature from observations
   1. plot using advanced symbol plotting
4. Compute the difference between obs and analysis
5. Plot the difference using advanced symbol plotting
6. Overlay with the original temperature field?
7. Generate a macro from the plot