model.ctlGrADS descriptor file (0.7 kb)
model.grbGrADS (GRIB) data file (579 kb)
model.gmpGrADS gribmap index file (4 kb)
This data file is described by the data descriptor file
model.ctl. You may want to look at this file before
continuing. The data descriptor file describes the actual data file,
which in the case contains 5 days of global grids that are 72 x 46
elements in size.
Please download these 3 files to a local directory before proceeding.
To start up GrADS, enter:
If the grads executable is not in your current directory, or if it is not in your PATH somewhere, you may need to enter the full pathname, ie:
GrADS will prompt you with a landscape vs. portrait question; just press enter. At this point a graphics output window should open on your console. You may wish to move or resize this window. Keep in mind that you will be entering GrADS commands from the window where you first started GrADS -- this window will need to be made the 'active' window and you will not want to entirely cover that window with the graphics output window.
In the text window (where you started grads from), you should now
see a prompt:
ga-> You will enter GrADS commands at this
prompt and see the results displayed in the graphics output
The first command you will enter is:
You may want to see what is in this file, so enter:
One of the available variable is called
ps, for surface pressure.
We can display this variable by entering:
d is short for
will note that by default, GrADS
will display an X, Y plot at the first time and at the lowest
level in the data set.
Now you will enter commands to alter the
display command (and
implicitly, the access,
output of the data) will do things with respect to the current
dimension environment. You control the dimension environment
clearclears the display
set lon -90sets longitude to 90 degrees West
set lat 40sets latitude to 40 degrees North
set lev 500sets level to 500 mb
set t 1sets time to first time step
d zdisplays the variable 'z'
In the above sequence of commands, we have set all four GrADS dimensions to a single value. When we set a dimension to a single value, we say that dimension is "fixed". Since all the dimensions are fixed, when we display a variable we get a single value, in this case the value at the location 90W, 40N, 500mb, and the 1st time in the data set.
If we now enter:
We have set the X dimension, or longitude, to vary. We have done this by entering two values on the set command. We now have one varying dimension (the other dimensions are still fixed), and when we display a variable we get a line graph, in this case a graph of 500mb Heights at 40N.
We now have two varying dimensions, so by default we get a contour plot. If we have 3 varying dimensions:
we get an animation sequence, in this case through time.
In this case we have set the Y (latitude) and Z (level)
dimensions to vary, so we get a vertical cross section. We have
also displayed two variables, which simply overlay each other.
You may display as many items as you desire overlaid before you
Another example, in this case with X and T varying (Hovmoller plot):
Now that you know how to select the portion of the data set to view, we will move on to the topic of operations on the data. First, set the dimension environment to an Z, Y varying one:
Now lets say that we want to see the temperature in Fahrenheit instead of Kelvin. We can do the conversion by entering:
Any expression may be entered that involves the standard operators of +, -, *, and /, and which involves operands which may be constants, variables, or functions. An example involving functions:
to calculate the magnitude of the wind. A function is provided to do this calculation directly:
Another built in function is the averaging function:
In this case we calculate the 5 day mean. We can also remove the mean from the current field:
d z - ave(z,t=1,t=5)
We can also take means over longitude to remove the zonal mean:
We can also perform time differencing:
This computes the change between the two fields over 1 day. We could have also done this calculation using an offset from the current time:
d z(t+1) - z
The complete specification of a variable name is:
name.file(dim +|-|= value, ...)
If we had two files open, perhaps one with model output, the other with analyses, we could take the difference between the two fields by entering:
display z.2 - z.1
Another built in function calculates horizontal relative vorticity via finite differencing:
Yet another function takes a mass weighted vertical integral:
Here we have calculated precipitable water in mm.
Now we will move on to the topic of controlling the graphics output. So far, we have allowed GrADS to chose a default contour interval. We can override this by:
We can also control the contour color by:
We can select alternate ways of displaying the data:
This is not very smooth; we can apply a cubic smoother by entering:
We can overlay different graphics types:
and we can annotate:
draw title 500mb Heights and Vorticity
We can view wind vectors:
Here we are displaying two expressions, the first for the U component of the vector; the 2nd the V component of the vector. We can also colorize the vectors by specifying a 3rd field:
You may display pseudo vectors by displaying any field you want:
Here the U component is the wind speed; the V component is moisture.
We can also view streamlines (and colorize them):
Or we can display actual grid point values:
We may wish to alter the map background:
To alter the projection:
set lon -140 -40
set lat 15 80
set mpvals -120 -75 25 65
set mproj nps
set gxout contour
set cint 30
In this case, we have told grads to access and operate on data from longitude 140W to 40W, and latitude 15N to 80N. But we have told it to display a polar stereographic plot that contains the region bounded by 120W to 75W and 25N to 65N. The extra plotting area is clipped by the map projection routine.
This concludes the sample session. At this point, you may wish to examine the data set further, or you may want to go through the GrADS documentation and try out the other options described there.