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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

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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

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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 a given GRIB fileany 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.

Conversion Between Fields and Points

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Use a GRIB to Geopoints icon to convert the the GRIB file temperature_analysis.grib to geopoints format. Examine the result to confirm that it is now geopoints and that we have a list of all the individual points. You can also visualise it, applying the supplied symb_colours icon. Converting to geopoints format is an easy way to export the data in an ASCII format.

Use a Geopoints to GRIB icon to convert the geopoints result of the fc_obs_diff icon to GRIB. We now have gridded data derived from scattered points. This icon contains some parameters to help define the output grid and the interpolation process used in the conversion.

Extra Tasks

Computing statistics fields

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• read both files
• use the grib_get_double(fieldset, string) function twice to extract the grid resolution from the first field in uv850 (uv850[1])
  • use the Grib Examiner to find which GRIB_API keys define these parameters
• use the read() command to set the grid on t2mthe temperature field

Extract field values at a set of locations

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