Radiative–Convective Processes in Simulated Diurnal Variations ofTropical Oceanic Convection

Abstract

This paper presents an analysis of the diurnal variation of tropical oceanic convection and its associated energy cycle as simulated by an anelastic cumulus ensemble model. The model includes subgrid turbulence, cloud microphysics, and radiative transfer processes. In two experiments designed to simulate the diurnal cycles in large-scale disturbed (A1) and undisturbed conditions (A4) over the tropical western Pacific warm pool, the model produces diurnal variations that are in general agreement with observations. In A1, a time-independent SST and mean ascending motion are prescribed in the model. The model generates a diurnal cycle with positive (negative) rainfall anomalies during the night (day), and the maximum (minimum) rainfall near 0200 (1300?1400) local time. In A4, a diurnally varying SST is prescribed in the model and the domain-averaged vertical velocity is constrained to be zero. The simulated diurnal variations still have a nocturnal rainfall maximum but with a weaker magnitude and a secondary peak in the afternoon. The model is then used to investigate the radiative effects of clouds that has been suggested as the cause of the nocturnal rainfall maximum by many studies. Two experiments (A2 and A3) are performed with the model, in which a time-independent SST is prescribed and the domain-averaged vertical velocity is constrained to be zero. The only difference between the two experiments is that the cloud?radiation interaction is suppressed in A3. The results show that despite the significant difference in total cloudiness/rainfall due to the difference in mean vertical motion between A2 and A1, and the difference in cloud?radiative forcing between A2 and A3, the simulated diurnal cycles in all three experiments show a dominant nocturnal rainfall maximum. The results support the suggestion by Sui et al. that the nocturnal rainfall maximum is related to more (less) available precipitable water in the night (day) due to the diurnal radiative cooling/heating cycle.