Introduction

Metview has much functionality for meteorological data types stored in ECMWF's MARS archive, for example GRIB, BUFR and ODB. But not all data comes in these formats. Therefore Metview has facilities to handle various other data types, which we will explore here.

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Visualiser Icons

Some formats, such as GRIB, are easy to visualise in Metview - just right-click, Visualise. This is because they are quite constrained in their contents and have enough standardised meta-data for a program to understand how they should be plotted. Some other formats, such as netCDF and tables of ASCII data are not easily interpreted for automatic plotting (which variables/columns should be selected and what do they represent?). Metview introduces the concept of the Visualiser icon, which we will use in some of the following examples.

NetCDF

NetCDF is a binary file for storing multi-dimensional arrays of data and has enjoys wide academic usage.

Examining netCDF

Right-click on the supplied netcdf.nc icon and choose examine to see its structure. It consists of multi-dimensional variables, each of which has its own set of attributes; the file also has a set of global attributes.

Visualising netCDF

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Create a new NetCDF Visualiser icon. Edit it and drop the netcdf.nc icon into the NetCDF Data field. Set the following parameters:

Save the changes, and visualise this new icon. See how the settings in the visualiser icon correspond to the variable names in the data. Now visualise another field from the same file. Use the supplied shading_20_levels icon on the plot.

Handling netCDF in Macro

In Macro, the read() function reads the given netCDF file into a netcdf variable.

Once read in, Metview can perform various manipulations on the data. These are based on the concept of the current variable. Out of the N variables contained in a netCDF file, one is always set to be the current variable. Functions and operators acting upon the netCDF file will act only upon the current variable.

Have a look at the variable v2d inside the netCDF file - its values are stored in degrees Kelvin. As an exercise, we will change them to Celcius. Type the following code into a new Macro:

nc = read('netcdf.nc')
setcurrent(netcdf, XXX)
nc = nc - 273.15

We can then write this to a new file:

 write('new_netcdf.nc', nc)

The only difference between this and the original file should be in the values of the variable v2d. Visualise the new file to check that its values are in Celsius.