The define command allows you to interactively create a new variable. The syntax is:
define varname = expr
The new variable can then be used in subsequent expressions (it can be used in subsequent define and/or display commands). The new variable is stored in memory, not on disk, so avoid defining variables over large dimension ranges.
The variable is defined to cover the dimension ranges in effect at the time the command is issued. You may define a variable that has from 0 to 4 varying dimensions. The define command is the only case within GrADS where four varying dimensions is valid.
When Z and/or T are varying dimensions, the define command evaluates the expression by stepping through Z and T. In other words, the expression is evaluated within a dimension environment that has fixed Z and T. This will affect how you compose the expression.
When you use a defined variable, data is taken from the variable in a way similar to data taken from a GrADS data file. For example, say you define a four dimensional variable:
set lon -180 0
set lat 0 90
set lev 1000 100
set t 1 10
define temp = rh
After issuing the define command, remember to change the dimension environment so less than 4 dimensions are varying!
set t 5
set lev 500
d temp
The display of the defined variable will display a 2-D slice taken at time 5 and level 500.
If you define a variable that has fixed dimensions, and then later access this variable, the fixed dimensions are treated as "wild cards". The best way to show this is with an example:
set lon -180 0
set lat 0 90
set lev 500
set t 10
define zave = ave(z,t=1,t=30)
The defined variable has two varying dimensions. If we now display this variable (or use it in an expression), the fixed dimensions of the defined variable, namely Z and T, will match ANY Z and T dimension setting:
set t 1
set lev 200
d zave
In the above display, the variable zave would be displayed as it was defined, ie you would get a time average of 500mb heights, even though the level is set to 850.
When the defined variable has varying dimensions, and you have a dimension environment where that dimension is fixed, the proper dimension will be retrieved from the variable:
set lon -180 0
set lat 0 90
set lev 500
set t 10
define temp = z
set lat 40
d temp
In the above example, the defined variable has a varying Y dimension. We then fix the Y dimension to be 40N, and display a 1-D slice. The data from 40N in the defined grid will be accessed. If you then did:
set lat -40
d temp
The data from 40S would be accessed from the defined variable. Since this is beyond the dimensions originally used when the variable was defined, the data would be set to missing.
You can also locally override the dimension environment:
d temp(lat=50)
If that dimension is a varying dimension within the defined variable.If the dimension is a fixed dimension for that variable, the local override will be ignored:
d temp(t=15)
In the above command, the defined variable temp has fixed T, so the t=15 would be ignored.
Note: the define command currently supports only grids.
Once you have defined a grid variables, you may tell GrADS that the new variable is climatological, ie that you wish to treat the time dimension of the new variable in a wild card sense.
The command is:
modify varname <seasonal>
<diurnal>
where the varname is the name of the defined grid (the define command must have been previously used). If the grid is described as seasonal, then it is assumed that the grid contains monthly (or multi-month) means. Note that daily or multi-day means are not yet supported. If diurnal is specified, it is assumed the defined variable contains means over some time period less than a day.
After describing the defined variable as climatological, then the date/times are treated appropriately when data is accessed from the defined variable.
An example. The data set contains 10 years of monthly means:
set lon -180 180
set lat -90 90
set lev 500
set t 1 12
define zave = ave(z,t+0,t=120,1yr)
This define will set up a variable called zave which contains 12 times, each time being the 10 year mean for that month. We are making use here of the fact that the define command loops through a varying time dimension when evaluating the expression, and within the ave function we are making use of the variable time offset of t+0, which uses a start time that is whatever time the define command is using as it loops.
modify zave seasonal
set t 120
d z - zave
The final display will remove the 10 year monthly mean for December from the last December in the data set.