Impacts of Shallow Convection on the Simulation of the Tropical Precipitation Diurnal Cycle

The role of shallow convection in the simulation of the tropical precipitation diurnal cycle in boreal winter during 1998-2009 is examined. Two simulations are carried out using the NCAR CAM3.0 model: a control run using standard model settings, and a sensitive run (NSC) in which the shallow convection is disabled below 700 hPa and between 20°S–20°N. Evaluation of the mean state, the spatial pattern of precipitation amount, and the precipitation frequency and intensity are reasonably reproduced in the control run compared to the results of TRMM-3B42. However, the lack of shallow convection in the NSC run directly affects precipitation distribution, attributed to changes of precipitation frequency. On a diurnal scale, shallow convection can affect the simulation of the mean state of tropical precipitation diurnal cycle in different ways over different underlying surfaces. The mechanisms of the changes in the precipitation diurnal cycle in two typical domains, the tropical western Pacific (10°S–10°N, 150°E–180°E) and the tropical South America (20°S–10°N, 70°W–40°W), are further investigated. The control run shows better skill representing the diurnal precipitation cycle over tropical South America, with an afternoon peak. Corresponding to the changes of moist static energy (MSE), the amplitude of convection is weaker in the NSC run, since the changes of the solar radiation and diffusion disturbance act to suppress the deep convection that follows a shallow convection. Over the tropical western Pacific, the simulated diurnal precipitation cycles with an early morning peak are associated with the recharge–discharge paradigm in both runs. However, the amplitude of the diurnal cycle is too strong in the NSC run, since the change of solar radiation leads to stronger contrast between radiative warming in the day and cooling at night if without shallow convection.