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VAPOR stands for Visualization and Analysis Platform for Ocean, Atmosphere, and Solar Researchers. It is a software system providing an interactive 3D visualization environment. The home of the software is https://www.vapor.ucar.edu.
VAPOR input files
VAPOR input data is described by .vdf (VAPOR Data Format) files. These are XML files containing the name and dimension of all the variables and the path of the actual data files storing the data values. VAPOR stores its data values in .vdc (VAPOR Data Collection) files. These are NetCDF files containing wavelet compressed 3D data. There is a separate file for each variable and timestep organized into a folder hierarchy.
There are a set of VAPOR command line tools that can convert NetCDF input data into this format but there is no such tool available for GRIB. This tutorial shows you how to use Metview's VAPOR Prepare icon to convert GRIB data into the VAPOR format.
VAPOR grids
VAPOR input data must be defined on a 3D grid, which has to be regular horizontally (on a map projection).
It is crucially important to understand the vertical coordinate types of the input data VAPOR can use. Here we discuss only the two types that the Metview VAPOR interface supports
- For layered grids VAPOR expects a parameter specifying the elevation of each 3D level in the input data. This is typically the case for pressure or model level (η levels) data with height or geopotential available (or it can be computed).
- For regular grids the 3D levels are supposed to be equidistant (in the user coordinate space). This type can be used when the data is available on equidistant hight levels.
The situation when When pressure or model level data is present without height information the situation is somewhat special. The grid in this case is not layered but can be regarded as regular in its own coordinate space (pressure or model levels) letting the z axis simply represent pressure or model levels in the 3D scene rendered in VAPOR.
VAPOR uses a right-handed coordinate system which means that :
- the horizontal grid has to start at the SW corner
- the vertical coordinates have to increase along the z axis (upwards)
Supported GRIBs
Only GRIB fields on a regular lat-lon grid are supported at the moment. However, please note that GRIBs can be internally interpolated to a regular lat-lon grid by using the VAPOR_AREA_SELECTION parameter. The parameters to be converted are supposed to have the same validity date and time and the same vertical levels. They also have to be valid on the same grid.
Pressure level data with elevation
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With these settings a VDF file called 'tut_pl.vdf' will be created in the directory you specified. All the other VAPOR data files will be placed into a subdirectory called 'tut_pl_data'.
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This tutorial works only with a small amount of data. However, real life examples can easily result in huge VAPOR files (gigabytes). Therefore you should always carefully select the output path to store the results of the GRIB to VAPOR conversion. |
Now save your VAPOR Prepare icon then right click Execute to run the conversion. The icon will first turn orange then green when the conversion finishes.
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Giving detailed instructions about VAPOR visualisation goes beyond the scope of this tutorial. Here you will learn only the basics about how to visualise 3D data with VAPOR. For an in depth introduction please study the VAPOR tutorials at: |
Stating up VAPOR
Right click Visualise your VAPOR Prepare icon to start up VAPOR. You will see this window popping up:
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If you rotate the cube in the display window (left mouse button) you will see it is flat. We need to scale the vertical axis to get a better view of the whole 3D volume. Go to the Edit → Edit Visualiser Features menu and set the Z Scene Stretch Factor to 200:
Now the full 3D volume is visible:
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Direct volume rendering (DVR)
Having set up the view we you can now visualise our data. E.g. try Click on the DVR (Direct Volume Rendering) tab, select Variable to relative humidity (r), tick Instance 1. Then set Refinement Level to 2 and change the opacity in the Transfer Function editor to like this (drag the control points of the white curve and use the vertical slide on the right of the histogram):
Having done so you should get this scene:
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Please note that this scene was generated by using only low resolution data. The see more details change the Refinement level first to 1 then to 2.
You should see more details appear in the scene:
Further rendering types
There are other types of renderers which we just list here and present a small gallery made with the data used for this tutorial:
- wind barb plotting: see the Barbs tab
- 2D field plotting: see the 2D tab
- cross sections: see the Probe tab
- flow visualisation (streamlines): see the Flow tab
- iso surfaces: see the Iso tab
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For further details please study the VAPOR tutorials at: |
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