Overview
Time is a very important dimension in meteorology, and there are many things to keep in mind. Here we will explore how Metview handles the fourth dimension.
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Overlaying Time Steps
Inspect the supplied GRIB files: z500_fc.grib contains geopotential forecasts made in one run, but for six different forecast steps; z500_an.grib contains analysis fields for two times. Visualise the supplied Geographical View icon and drop the forecast GRIB icon along with its corresponding Contouring icon into the Display Window, and then drop the analysis GRIB icon along with its corresponding Contouring icon too. Go through the frames of animation. The fields have been overlaid, but if you look at the times and dates in the title, you will see that they do not match. Metview has simply plotted the first field of each data file together, then the second, and so on. We can make it more intelligent.
Edit the Geographical View icon and set this:
Map Overlay Control | By Date |
Save the icon, visualise it and drop the data with their visdefs in again. Go through the animation steps and look at the Frames tab in the Display Window to see what has happened. Now the fields will be overlaid only if their valid date and time match.
Metview's overlay rules
To summarise, Metview's overlay rules can be described like this:
Rule 1
In practice, this means that a GRIB file with 5 fields is visualised as 5 separate plots which you can scroll through.
If you wish to overlay data, then you must provide separate data icons for each 'layer'. Then we are subject to Rule 2:
Rule 2