Effects of time step size on the simulation of tropical climate in NCAR-CAM3

Abstract

This paper describes the effects of time step on the simulation of tropical climate in the NCAR-Community Atmosphere Model version 3 (CAM3). A set of multi-year integrations are carried out in a real-planet framework using actual land–ocean distribution and observed sea surface temperature. Over the tropics there is an increase in total rainfall with a decrease in time step size. Using a lower time step, there is a decrease in the convective component of rainfall, however, the stratiform component increases, and more than compensates the decrease in the former, thus leading to a higher total rainfall. A decrease in time step leads to an increase in the frequencies of moderate, and heavy rainfall categories, which is responsible for the increase in time mean total rainfall over the tropics. Also, the spatial distribution of rainfall becomes more realistic during both summer and winter seasons. In regard to the simulation of equatorial waves, it is found that a lower time step leads to a reduction in the speed of Kelvin waves. The latent heating profile becomes more bottom-heavy with a reduction in time step size, which potentially leads to slower Kelvin waves. Finally, additional experiments conducted in an aqua-planet framework show a consistent and systematic change in the analyzed variables with change in time step, and hence confirm the robustness of the results across modeling frameworks.