           domain on which diffusion is to be performed
             the size of the domain is determined by the size ofthe input
             i.e, longitude index ranges from is to is + size(t,1) -1
                   latitude index ranges from js to js + size(t,2) -1

         delt -- real
           time step , seconds
           if using leapfrog step, set equal to 2*dt = time(i+1) - time(i-1)

         u -- real, dimension(:,:,:)   zonal wind (m/sec)
         v -- real, dimension(:,:,:)   meridional wi