The assumption that the draft fraction is negligible incumulus parameterization can be a major source of error inthe parameterized tendencies of thermodynamic quantities.This study estimates the errors arising from this assumptionbased on an analysis of high-resolution cloud-resolvingmodel output.The numerical results reveal a negative errorassociated with the assumption,indicating that themagnitude of the mean subgrid-tendency of moist staticenergy is underestimated when the assumption is employed.The magnitude of the error is proportional to the draftfraction,and is independent of horizontal resolution. Theresults are confirmed by theoretical analyses,which supporta linear relationship between error magnitude and draftfraction, and confirm that the error magnitude isindependent of horizontal resolution. The resultingunderestimates of moist static energy tendency should notbe neglected in the case that the horizontal resolution issufficiently fine to enable the frequent occurrence of largedraft fractions.
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