The Two Diurnal Modes of Tropical Upward Motion

This study describes a new mechanism governing the diurnal variation of vertical motion in tropical oceanic heavy rainfall zones, such as the intertropical convergence zone. In such regions, the diurnal heating of widespread anvil clouds due to shortwave radiative absorption enhances upward motion in these upper layers in the afternoon. This radiatively driven ascent promotes an afternoon maximum of anvil clouds, indicating a diurnal cloud-radiative feedback. The opposite occurs at nighttime: While rainfall exhibits a dominant peak at night-early morning, the boundary layer rooted upward motion and latent heating tied to this peak are forced to be more bottom heavy by the nighttime anomalous radiative cooling at upper levels. This mechanism therefore favors the stratiform top-heavy heating mode during daytime and suppresses it nocturnally. These diurnal circulation signatures arise from microphysical-radiative feedbacks that manifest on the scales of organized deep convection, which may ultimately impact the daily mean radiation budget.