1. GrADS TUTORIAL This document presents a brief tutorial for Brian Doty's Grid Analysis and Display System (GrADS). The following sample session will give you a feeling for how to use the basic capabilities of GrADS. This sample session takes about 30 minutes to run through. 1.1 Starting and quitting GrADS To start up GrADS, click on: Start > All Programs > OpenGrADS 2.0 > Grads Prompt A (black) command window (see figure below) appears. After the prompt: Landscape mode? ( n for portrait): Type: yes (or y) At this point a second (black) graphics output window is opened on your monitor. You will be entering GrADS commands from the command window and the graphics output will appear in the graphics window. Note: on some computers it may take a while before anything happens after you issued the y(es) command, just have patience! The command window usually needs some relocation (click and drag the frame). If you really wish, you may also do some resizing of the window, but after this operation it may take quite a while before the prompt will be visible again! In the window you should now see a prompt: ga->. You will be entering GrADS commands at this prompt and see the results appear in the black graphics window. To leave GrADS at any time enter: 1
quit The next command you will enter is one of the following: sdfopen d:/atd/month/month1990.nc (if you are using the January 1990 data) sdfopen d:/atd/month/month2002.nc (if you are using the October 2002 data) sdfopen d:/atd/month/month2007.nc (if you are using the January 2007 data) You may want to see what sort of data is actually stored in this file, so enter: q file q is short for query. The data file contains monthly mean values for 1990, 2002 or 2007 of a limited number of variables. One of the available variables is called air, for air temperature. We can display this variable by entering: d air d is short for display. You will note that the graphics output is displayed in the graphics window. If you want to the display before typing the display command, enter: By default, GrADS will display an X, Y plot at the first time and at the lowest level in the data set. 1.2 Changing GrADS environment settings Now you will enter commands to alter the dimension environment. The display command (and implicitly, the access, operation, and output of the data) will do things with respect to the current dimension environment. You control the dimension environment with the set command: c set lon -90 set lat 40 set lev 500 set t 1 (s the display) (sets longitude to 90 degrees West; positive longitude for deg. East) (sets latitude to 40 degrees North; negative latitude for deg. South) (sets level to 500 mb = 50 kpa) (sets time to first time step) (displays the variable 'hgt i.e. geopotential height) 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: set lon -90 90 (X is now a varying dimension) 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. 2
Now enter: set lat 0 90 We now have two varying dimensions, so by default we get a contour plot. If we have 3 varying dimensions: c set t 1 5 we get an animation sequence, in this case through time. Now enter: set lon -90 set lat -90 90 set lev 1000 100 set t 1 d air d uwnd 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 enter the command. Another example, in this case with X and T varying (Hovmöller plot): c set lon -90 90 set lat 40 set lev 500 set t 1 5 1.3 Operations on the data 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 a Z-Y varying one: set lon -180 0 set lat 0 90 set lev 500 set t 1 Now let s say that we want to see the temperature in Kelvin instead of degrees Celcius. We can do the conversion by entering: display air+273.16 3
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: d sqrt(uwnd*uwnd+vwnd*vwnd) to calculate the magnitude of the wind. A function is provided to do this calculation directly: d mag(uwnd,vwnd) Another built-in function is the averaging function: d ave(hgt,t=1,t=5) In this case we calculate the mean over 5 months. We can also remove the mean from the current field: - ave(hgt,t=1,t=5) We can also take means over longitude to remove the zonal mean: -ave(hgt,x=1,x=144) We can also perform time differencing: (t=2)-hgt(t=1) This computes the change between the two fields over 1 month. We could have also done this calculation using an offset from the current time: (t+1) - hgt 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 hgt.2 - hgt.1 Another built-in function calculates horizontal relative vorticity via finite differencing: d hcurl(uwnd,vwnd) 1.4 Controlling GrADS graphics Now we will move on to the topic of controlling the graphics output. So far, we have allowed GrADS to choose a default contour interval. We can override this by: 4
set cint 30 We can also control the contour color by: set ccolor 3 We can select alternate ways of displaying the data: set gxout shaded d hcurl(uwnd,vwnd) This will not display interval values. To obtain a color scale use: run cbar.gs We can apply a cubic smoother by entering: set csmooth on d hcurl(uwnd,vwnd) We can overlay different graphics types: set gxout contour set ccolor 0 set cint 30 and we can annotate: draw title 500mb Heights and Vorticity We can view wind vectors: set gxout vector d uwnd;vwnd 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: or maybe: d uwnd;vwnd;air d uwnd;vwnd;hcurl(uwnd,vwnd) You may display pseudo vectors by displaying any field you want: 5
d mag(uwnd,vwnd) ; air Here the U component is the wind speed; the V component is temperature. We can also view streamlines (and colorize them): set gxout stream d uwnd;vwnd;hcurl(uwnd,vwnd) Or we can display actual grid point values: set gxout grid d uwnd We may wish to alter the map background resolution: set lon -110-70 set lat 30 45 set mpdset hires set digsize 0.2 set dignum 1 d uwnd 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. 1.5 Producing hardcopy output At this point we may wish to produce output for printing or incorporating graphics in a text file. GrADS is able to produce output files in two different ways. Simple output When you have a graphic displayed that you want to print, enter the command printim d:/filename.gif gif white This will produce a gif-file with black and white colours reversed (this is the best option for printing). Choosing black instead of white will produce an image that looks like what you have on screen (i.e. with a black background, requires much toner!!). Advanced output 6
If you image contains colours that you want to transform into shades of grey, then use the following procedure. Produce a file with the extension.gmf, which stands for GrADS Meta File. Do this by entering the command enable print d:/filename.gmf This enables the print command, and directs print command output to the file given. Any existing contents of this file will be lost (i.e. it will be overwritten). When you have a graphic displayed that you want to print, enter the command print This will copy the vector instructions used to create the current display into the output file in a GrADS metacode format. You must close the output file either by quitting GrADS (quit command) or the reinit command or by entering: disable print At this point a.gmf-file has been produced and is available in the d:/ directory. The PC version of GrADS contains a viewer for these file types. Double clicking the selected.gmf-file opens this file in the GrADS metafile viewer. The viewer shows the selected file with black and white colures reversed. Now the coloured graphic can be turned into a grey graphic by clicking the appropriate icon in the toolbar at the top. Next, then result can (1) be printed directly from the viewer or (2) be saved as a.wmf-file (a Windows meta file). Windows meta files can be read by most graphical packages and can also be inserted directly in most word processors (e.g. Microsoft Word). GrADS also provides utilities which create postscript files from.gmf-files. These utilities will not be discussed here. 1.6 More GrADS capabilities 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. The GrADS documentation can be found on your computer in the form of a number of html-files describing all possibilities of this visualization tool. One of the most important possibilities not described in this tutorial is the GrADS scripting language, which is of great help in reducing the amount of tedious typing of commands. See the module for reference and exercises. 7