free grads

cdiff

cdiff(expr,dim)

Performs a centered difference operation on expr in the direction specified by dim. The difference is done in the grid space, and no adjustment is performed for unequally spaced grids. The result value at each grid point is the value at the grid point plus one minus the value at the grid point minus one. The dim argument specifies the dimension over which the difference is to be taken, and is a single character: X, Y, Z, or T.

Result values at the grid boundaries are set to missing.

Usage Notes

Examples

1. The cdiff function may be used to duplicate the calculation done by the hcurl function:

   
   

   define dv = cdiff(v,x)
   define dx = cdiff(lon,x)*3.1416/180
   define du = cdiff(u*cos(lat*3.1416/180),y)
   define dy = cdiff(lat,y)*3.1416/180
   display (dv/dx-du/dy)/(6.37e6*cos(lat*3.1416/180))

   The above example assumes an X-Y varying dimension environment. Note that the intrinsic variables lat and lon give results in degrees and must be converted to radians in the calaculation. Also note the radius of the earth is assumed to be 6.37e6 meters thus the U and V winds are assumed to have units of m/sec.

   
   

   Temperature advection can be calculated using the cdiff function as follows:

   
   

   define dtx = cdiff(t,x)

   define dty = cdiff(t,y)

   define dx = cdiff(lon,x)*3.1416/180

   define dy = cdiff(lat,y)*3.1416/180

   display -1*( (u*dtx)/(cos(lat*3.1416/180)*dx) + v*dty/dy )/6.37e6

   where the variable t is temperature, u is the U component of the wind, and v is the V component of the wind.