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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
Metview's GRIB Filter icon has parameters which enable the interpolation of data onto a new grid, or the extraction of a sub-area. 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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Now create a new GRIB Filter icon with these parameters:
| Data | Drop the u GRIB Filter icon here |
| Grid | 2.5/2.5 |
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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 file.
Conversion Between Fields and Points
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Metview provides two icons, Geopoints to GRIB and GRIB to Geopoints for the purpose of converting between GRIB (gridded) and geopoints (scattered) formats.
Use a GRIB to Geopoints icon to convert the result of the last exercise 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.
Extra Tasks
Computing
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statistics fields
Your forecast-analysis difference macro works on fieldsets containing 6 fields (6 different vertical levels in the atmosphere), so the result also contains 6 fields. Instead of plotting all 6 fields, compute fields which represent the minimum, maximum and means of these fields. For example, if we have 6 fields in fieldset variable fs, then the following code computes a single field, the values of which are the minimums across all 6 input fields:
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min_field = min(fs) |
Plot the results and cross-reference with the original result of 6 fields to confirm that you understand what has been computed.
Adapt the macros from this tutorial so that they compute relative differences rather than absolute differences between the values. By this we mean compute the differences as percentages of the original analysis values. You will need to create new Contouring and Symbol Plotting icons to plot these nicely.
Overlaying wind speed with wind arrows
Write a macro which takes U and V wind components from a GRIB file and plots the wind speed as shaded contours with wind arrows overlaid on top.