Numerical Experiments Examining the Mechanism of Diurnal Variation of Tropical Convection

Abstract

A regional atmospheric model was used to clarify the mechanisms that drive the diurnal cycle of tropical convection over the western Pacific Ocean. The diurnal cycle of convection was simulated using data from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Mechanisms suggested by previous investigators were confirmed through comparison experiments. The diurnal variation in precipitation was simulated without the effect of horizontal differences in radiative cooling between cloudy and clear regions. Horizontal differences of radiative cooling are not the main factor driving diurnal variation. Direct radiative cooling, and cloud radiative cooling effects destabilize the atmosphere during the night. Numerical experiments that reduced cloud radiative cooling demonstrated the importance of the cloud radiative cooling effect rather than the direct radiative cooling effect. Moreover, in the standard experiment, strong cloud radiative cooling appeared at both upper levels and at the top of the boundary layer. The experiment demonstrated that both the upper level cloud radiative cooling, and the boundary layer cloud radiative cooling, are important in driving the diurnal variation of convection. Boundary-layer cloud radiative cooling destabilizes the low layer atmosphere and promotes the development of shallow convection to deep convection. Diurnal variation of water vapor also plays a role in driving diurnal variation of convection. Moisture is transported upward from the boundary layer at night when radiative cooling of the boundary-layer cloud is intensified.