Figure A1.

Sketch of our general coarse‐graining methodology in panel (a) and for cloud area fraction in panel (b). We picture a vertical slice through two grid columns. For simplicity we assume that the grid boxes all have the same depth. The greenly hatched area depicts a coarse‐scale grid box V. Panel (a): Due to our approximation the weight α _2,3 for the value in grid box S _2,3 is 1/6 and therefore larger than it were without the sequential horizontal and vertical coarse‐graining steps. Panel (b): In the vertical range of V we vertically coarse‐grain cloud cover values according to a maximum overlap assumption before we coarse‐grain in the horizontal.