Yes, what I want is what Paul suggests: just sum_wz without the final division.
I don’t need a weighted sum, I just want to get the sum of all of the values – in my case it is gridded dataset of total lightning strokes – within a 1500km radius of each grid node. The quantity I want to sum should not be multiplied by a weight, I want the total to just be the gross number of strokes (not per unit area).