Since you have a curvilinear grid you need the underlying area of the grid cells to do this properly. You then take the smoothed area weighted variable and divide it by the smoothed area.
The denominators of the transforms cancel and you are left with an area weighted smooth.
Other smoothing operators will work too (I think @SBN is probably better for this but it's up to you)..
The long way is to use the @SHF transform and appropriate weights but that's a pain.
On 17/01/15 03:55, Paul Goddard wrote: