A numerical study of the diurnal cycle of tropical oceanic convection

Idealized two-dimensional cloud-resolving numerical modeling was conducted to investigate the diurnal variability of deep tropical oceanic convection. The model was initialized with a horizontally homogeneous atmosphere upon which a uniform and time-independent large-scale forcing was imposed. The underlying surface was assumed to be an open ocean with a constant sea surface temperature. Emphasis was on two distinct regimes: (a) highly organized squall-line-like convection in strong ambient shear and (b) less organized nonsquall cloud clusters without ambient shear. A pronounced diurnal cycle was simulated for the highly organized case; convective activity and intensity attained a maximum around predawn and a minimum in the late afternoon. A similar diurnal variability was obtained for the less organized case and was characterized by more precipitation during the night and early morning and less precipitation in the afternoon and evening. The modeled diurnal variation was primarily attributed to the direct interaction between radiation and convection, whereas the cloud-cloud-free differential heating mechanism played a secondary role. When the radiative effect of clouds was excluded, a diurnal cycle was still present. Moreover, the cloud radiative forcing had a negative influence on precipitation/convective activity, in contrast with general circulation modeling results.