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What is NetCDF?

Geopoints is Metview's own format for storing irregularly distributed geographical data (typically observations). A geopoints file is an ASCII file containing a header section and a data section consisting of several columns, so it can be regarded as specialised CSV (Comma Separated Values) file.

NetCDF (Network Common Data Form) is a binary format for array-oriented scientific data. The full specification of the format, as used in Metview, can be found on the NetCDF overview page.

The official homepage of NetCDF is hosted by UCAR: http://www.unidata.ucar.edu/software/netcdf/There are some different 'flavours' of the format, which described here in detail.

The NetCDF icon

NetCDF files are represented by this icon in the user interface:

Examining

NetCDF contents

The contents of a Geopoints NetCDF file can be inspected with the Geopoints NetCdf Examiner, which can be started up from the user interface (right-click examine on the icon).

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Visualisation

Geopoints can be directly visualised in Metview on a map view. In the user interface just right-click visualise on the icon to get a plot with the default settings. These plots can be further customised with Symbol Plotting (scalar values) and Wind Plotting (vector values) icons.

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Visualisation with Table Visualiser

Since Geopoints is a CSV data format it can be also visualised with the Table Visualiser icon, which can handle generic CSV data files.

Geopoints and BUFR

The typical way to handle BUFR data in Metview is to filter it into a Geopoints file using the Observation Filter and the Bufr Picker icons. The resulting data is then visualised or further processed with Metview's rich Geopoints API. 

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Operators between GRIB and Geopoints

It is atypical scenario in Metview that we have forecast data in GRIB format and some corresponding observations in Geopoints (probably filtered from BUFR) and we want to compute the difference between them. Using Metview's script interface it can be done in a single line like this (supposing g is a fieldset while gpt is a geopoints object):

gpt_diff = g - gpt

For a full fledged example just click the image below.

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Conversion between Geopoints and GRIB

Direct visualisation of NetCDF is not available in Metview but it is implemented via the NetCDF Visualiser icon. With this icon we can specify the view type, the variable and the dimensions (with slicing) to generate the actual plot. Visualisation is supported both on maps and xy-charts for scalar and vector data, as well.

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NetCDF as storage format for analysis views

Metview uses NetCDF as an internal format to store the data prepared for the so-called analysis views (various sections and profiles). The data preparation icons of these views (including Cross Section Data, Average Data, Hovmoeller Data, Thermo Data and Vertical Profile Data) all store their results in NetCDFA GRIB fields can be directly exported into Geopoints using the Grib To Geopoints icon. The other direction is also possible with the Geopoints To Grib icon, which offers various interpolation techniques to compute the grid point values from the values available at scattered locations.

Script language support

Metview provides full support for Geopoints NetCDF from its Macro and Python interfaces. This includes:

• accessing dimensions, variables and attributes
• performing arithmetic on variables' values

The list of available functions for Geopoints NetCDF can be found on the Geopoints the NetCDF Functions page.