Impacts of shallow convection on the simulation of the tropical precipitation diurnal cycle

Abstract

With the aim to examine the role of shallow convection in the simulation of the tropical precipitation diurnal cycle in boreal winter during 1998–2009, two simulations are carried out using the National Center for Atmospheric Research Community Atmosphere Model Version 3.0 (NCAR CAM3.0) model: a control run using standard model settings and a sensitive run (no shallow convection run, NSC) in which the shallow convection is disabled below 700 hPa and between 20°S and 20°N. Evaluation of the mean state shows that the spatial pattern of precipitation amount, and the precipitation frequency and intensity are reasonably reproduced in the control run in comparison to that of Tropical Rainfall Measuring Mission (TRMM)-3B42 data. However, the lack of shallow convection in the NSC run significantly affects precipitation distribution due to the changes of precipitation frequency. On diurnal scale, shallow convection affects the simulation of the mean state of tropical precipitation diurnal cycle in different ways over different underlying surfaces. The mechanisms dominating the changes in the precipitation diurnal cycle in two typical domains, the tropical western Pacific (10°S–10°N, 150°–180°E) and the tropical South America (20°S–10°N, 70°–40°W), are further investigated. The control run shows a better skill in representing the diurnal precipitation cycle over tropical South America, with an afternoon peak. Analysis of moist static energy (MSE) change shows that the amplitude of convection is weaker in the NSC run, because the changes of the solar radiation and diffusion disturbance tend 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. Nonetheless, the amplitude of the diurnal cycle is too strong in the NSC run, because the change of solar radiation leads to a stronger contrast between radiative warming in the day and cooling at night without the shallow convection.