The role of large-scale convective organization for tropical high cloud amount

Tropical high clouds are closely coupled to deep convection, but local cloud amount and convective mass flux are nonlinearly related. We use the Geophysical Fluid Dynamics Laboratory atmosphere-only model AM2 forced with idealized sea surface temperature (SST) perturbations to study the sensitivity of high clouds to the large-scale distribution of convection. Increasing/decreasing the SST contrast between convective and nonconvective regions decreases/increases the tropical deep convective area, and warming of convective areas decreases the tropical average convective mass flux (〈m_c〉). In all experiments, fractional high cloud amount changes are less than fractional changes in 〈m_c〉. High cloud amount is half as sensitive as expected from the climatological average cloud amount, as a function of convective mass flux, due to strong compensation from nonconvective high clouds. The latter results from changes in relative humidity related to the change in 〈m_c〉. This effect renders high cloud amount remarkably robust to perturbations, though radiative effects of convective and nonconvective clouds will differ.