In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The atmosphere convective mass flux is the vertical transport of mass for a field of cumulus clouds or thermals, given by the product of air density and vertical velocity. For an area-average, cell_methods should specify whether the average is over all the area or the area of updrafts and/or downdrafts only. "Updraft" means that the flux is positive in the updward direction (negative downward).