期刊名称：Journal of Advances in Modeling Earth Systems
电子版ISSN：1942-2466
出版年度：2021
卷号：13
期号：5
页码：e2020MS002359
DOI：10.1029/2020MS002359
出版社：John Wiley & Sons, Ltd.
摘要：In this study, we find significant sensitivity to the choice of time step for the Energy Exascale Earth System Model's atmospheric component, leading to large decreases in the magnitude of cloud forcing when the time step is reduced to 10 s. Reducing the time step size for the microphysics increases precipitation, leading to a drying of the atmosphere and an increase in surface evaporation. This effect is amplified when the microphysics is substepped together with other cloud physics processes. Coupling the model's dynamics and physics more frequently reduces cloud fraction at lower altitudes, while producing more cloud liquid at higher altitudes. Reducing the deep convection time step also reduces low cloud mass and cloud fraction. Together, these results suggest that cloud physics in a global circulation model can depend strongly on time step and, in particular, on the frequency with which cloud-related processes are coupled with each other and with the model dynamics. Plain Language Abstract Computer simulations of the Earth's atmosphere take the state of the atmosphere at one point in time, then predict the state of the atmosphere a short interval of time into the future. The length of this time interval is known as the “time step”. By doing this repeatedly, models can produce a simulated history of the atmosphere for years or even centuries. A smaller time step size requires more computer time but should ideally lead to more accurate results. In this study, we reduce the time step for the atmosphere in the Energy Exascale Earth System Model from half an hour to 10 s. The simulation with a smaller time step has more rain, which removes water from the atmosphere and reduces the fraction of the Earth's surface that is covered by clouds. We also experiment with changing the time step for only some parts of the model and not others. We find that the effects of the time step size on the model are related mostly to the frequency of coupling between processes rather than the time step used for any individual processes.
