ODB Seminar 2107 examples

Preparations

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.

Then run the following command from the command line:

cp -Rf ~cgx/tutorial/odb_seminar_2017 ~/metview/

This will copy all the examples into your metview home folder. Soon you will see a folder called 'odb_seminar_2017' appear on your main Metview desktop. Double-click to enter it. You should see the following contents:

ODB Exercise

This exercise shows how to retrieve ODB data from MARS, examine its structure then plot and process it in different ways. Enter folder 'main' to start the exercise.

Retrieving the ODB data from MARS

The 'ret_temp' MARS Retrieval icon is already prepared for you to fetch Land TEMP ODB data from MARS fro a given date. Edit the icon (right-click & edit) and see what parameters are set. The most important ones are as follows:

Close the icon editor and 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. The data is now cached locally. To see what was retrieved, right-click examine the icon. This brings up Metview's ODB Examiner tool. Here we can see the metadata (Columns tab) and the actual data values themselves as well (Data tab). Close the ODB Examiner.

To save the ODB data from the cache to disk, right-click Save result on the Mars Retrieval icon and save as 'temp.odb'. A few seconds later an ODB Database icon with the given name will appear in your folder.

Using the ODB Visualiser

We will visualise the 500 hPa temperature values from this ODB using the 'vis_temp' ODB Visualiser icon.  The query we need to perform is as follows:

select 
  lat@hdr, 
  lon@hd, 
  obsvalue@body 
where 
  varno = 2 and vertco_reference_1=50000

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 by clicking the Save button at the bottom-right of the icon editor (or click Ok to save and close the editor).

Right-click visualise the icon to generate the plot. Then drag the the provided Symbol Plotting, Coastlines, Legend and Text Plotting icons into the plot for further customisation. Having done so you should see something like this.

Inspecting the Data Values in the Plot

Data values at the cursor position can be inspected with the Cursor Data Tool, which can be activated by pressing on the gun-sight icon in the toolbar of the Display Window. The Cursor Data Tool displays the co-ordinates of the current cursor position and the information for the nearest data point to this position.

You may find hard to use the Cursor Data Tool in data dense regions. To overcome it launch the Magnifier by pressing on the magnifier icon in the toolbar and navigate it to your area of interest in the plot.

Enable the sidebar of the Display Window with the button and select the Data tab then the Metadata panel inside it (and select the 'vis_temp' layer at the top if it is not yet selected). You will see some statistics about the data you plotted and a histogram as well.

Now switch to the Values panel. This features a list showing all the plotted data.  In the bottom-left corner click on the button to activate the Data probe. The probe is synchronised with the list. Try to drag it around in the plot, or change its position through the list. The Magnifier might help you position the Data probe more accurately.

Writing a Macro

We write a Macro to produce the same temperature map as we plotted with icons.

Create a new Macro icon (in an empty area of the desktop right-click and select Create new macro) and edit it. First, drag your 'vis_temp' ODB Visualiser icon into the Macro editor just below the line containing the #Metview macro text.  Next, drag your 'symbol' Symbol Plotting icon into the  editor after the text the editor already holds. Next, add the following line to the macro:

plot(vis_temp,symbol)

Now click on the play button in the Macro editor to run the Macro. You should see a nice plot popping up. 

A more advanced version of this macro is provided you in 'plot.mv'. It uses all the icons we used to customise the original plot, allows selecting the pressure level to plot and automatically adjusts the symbol plotting to to current value range.

Overlaying with GRIB data

The 'fc.grib' GRIB icon contains a 12 h global forecast valid for the date and time of our TEMP ODB data. Double-click the icon to inspect its fields with the GRIB Examiner.

Re-visualise the 500 hPa temperature ODB data with the 'vis_temp' and other icons as you did before. To overlay the 500 hPa temperature forecast we need to filter the matching filed from GRIB file. We will use the  't500_fc' GRIB Filter icon that is already set up for this task. Just drag it into the plot, then drag the 't_cont' Contouring icon as well to customise the contour lines.

Forecast-observation difference

The 'diff.mv' Macro computes the difference between the forecasts stored the 'fc.grib' GRIB file and the observations stored in the 'temp.odb' ODB. This is achieved by using the following steps:

1. the ODB query is performed and the resulting data is converted into Geopoints (this is Metview's own format to stored scattered geospatial data)
2. the matching GRIB filed read and interpolated to the observation points
3. the difference is computed between forecast and observation

Edit it 'diff.mv' Macro and visualise it using the play button. You should see now a difference plot. Try a different level/parameter at the top of the macro. 

Wind plotting

The 'wind.mv' Macro plots wind on a given pressure level from the 'temp.odb' ODB. It is not a trivial task because the u and v wind components cannot be retrieved from our ODB in a single query. The macro overcomes this difficulty by using the following steps:

1. two ODB queries are performed: one for the u and one for the v wind component
2. the resulting data is converted into Geopoints
3. the macro plots the wind data as Geopoints

Edit it 'wind.mv' and visualise it using the play button. Try to set a different level at the top of the macro. 

Tephigram plotting

Macro 'tephi.mv' demonstrates how to extract and plot TEMP ODB data into a tephigram (it is a type of thermodynamic diagram to show atmospheric  profiles). Edit the macro and visualise it. Try to change the station specified at the top of the macro code.

Other Examples

There are some other examples presented i

Enter folder 'radiance_map'. The ODB database "ASMUA.odb" stores AMSU-A brightness temperature observations. Right-click examine to start up the ODB examiner and study the Columns and Data tabs in it. 

Example 2: Wind Plotting and Overlay - Scatterometer

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