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One of Metview's most powerful features is its data processing ability. Data from various sources can be combined and manipulated using high- or low-level commands.
Fieldset Manipulation
A fieldset is a collection of fields stored physically in GRIB files. Metview has many built-in functions and operators to manipulate fieldsets, from simple arithmetic operators to vertical integrations through the atmosphere. Many of these operations are available from the icon-based user interface, all are available in the Macro language. The results can be stored on disk, or passed to other functions for further manipulation.
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- apply some shading to the data with a Contouring icon, only contouring wind speeds above 20m/s
- note that we use Wind Plotting icons to modify the plotting of vector fields, but this is now a scalar field, so we use the Contouring icon
- overlay the original wind field and change the colour of the arrows to 'charcoal' with a Wind Plotting icon
- shade the land and the sea with a Coastlines icon
Returning a Computation for Further Interactive Use
The result of the above macro does not have to live entirely inside the macro - it can be passed back to the user interface or used as input to other icons. Do this:
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This passes the fieldset speed back to the user interface. Try it by right-clicking on the macro's icon and selecting examine, save or visualise. This icon can also be dropped into an existing Display Window to plot the data there. It could also be used as an input to another icon, for example a Simple Formula icon, for further processing.
Writing the result of a computation to disk
Again, modify the last line of the compute_wind_speed icon so that we now instead write the result to a file:
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A full path could also have been supplied if you do not wish the file to be written to the current directory.
Field Interpolation and Sub-area Extraction
Field Interpolation
Metview's GRIB Filter icon has parameters which enable the interpolation of data onto a new grid, or the extraction of a sub-area of data. This can be useful if you wish to compare two fields which are currently at different resolutions (e.g. from different model runs) - both fields need to be on the same grid before Metview can perform computations between them.
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The result will be the same data interpolated onto a 2.5x2.5 degree grid. Visualise the result and apply the supplied gridpoints icon to confirm the new grid. Visualise the two fields side-by-side with coloured contour shading to also confirm that they look very similar in terms of their data values.
Sub-area Extraction from a Fieldset
The GRIB Filter icon also allows the extraction of a sub-area from a fieldset. Create a new one and, using the Area parameter, extract just the data over a selected region for any of the supplied GRIB files. The area is defined as a list of 4 numbers, forward slash-delimited, representing the North/West/South/East coordinates of the area.
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