Tutorial: GrADS Hovmöller diagram

Hovmöller diagrams are very useful for determining the speed of various atmospheric disturbances, as it contours a variable in both time and space.  Typically, the spatial variable is on the x-axis and the time variable on the y-axis.

It's very easy to plot a simple Hovmöller diagram in GrADS.  To do this, you only need to set your varying dimensions to longitude and time.  The axes will arrange longitude on x, and time on y.  In this tutorial, the example will be a Hovmöller diagram of 500mb meridonal wind from the GFS model.  The block of code below will set up and plot this data.

    'sdfopen http://nomads.ncep.noaa.gov:80/dods/gfs_2p5/gfs_2p520130428/gfs2p5_12z'

    'set parea 1.5 10.0 0.5 8.0'

    'set lat 35'
    'set lon -135 -60'
    'set lev 500'
    'set gxout shaded'
    'set display color white'
    'set t 1 33'
    'clear'
    'set yflip on'
    'd vgrdprs'
    'cbar'

A few things to note about this block of code:

1. Both latitude and Height are fixed.  This allows for longitude and time to vary
2. The command 'set yflip' flips the y-axis around so that time starts at the top, rather than the bottom of the plot (normal Hovmöller display).
3. The first number in the 'set parea' command is 1.5 giving the left side of the plot a large margin in order to contain the labeled dates on the y-axis.  A smaller margin will cut these off.

The above block of code will create this image:

Hovmöller Diagram of meridional wind at 500mb

Notice, that you can easily track the speed and frequency of the atmosphere disturbances as they propagate across the domain by taking the slope of the wind packets.

Download this script