Investigation of the effect of the time step on the physics–dynamics interaction in CAM5 using an idealized tropical cyclone experiment.

To understand the effect of the time step on the physics–dynamics interaction in a model, we used an idealized tropical cyclone test to evaluate the sensitivities to the physics time step in the Community Atmosphere Model Version 5 (CAM5). The investigated time steps were 450, 900 and 1800 s at a resolution of 1°, and 225, 450, 900 and 1800 s at a resolution of 0.25° in the corresponding ensemble simulations. We found that the intensity and precipitation of the simulated tropical cyclone and the physics parameterizations are fairly sensitive to the time step. These sensitivities are affected by the dynamical core and the physics–dynamics coupling strategy and vary with the horizontal resolution. In low-resolution runs, the intensity of the simulated tropical cyclone varies little with physics time step in the finite volume (FV) dynamical core, but it tends to weaken with decreasing time steps in the spectral element (SE) dynamical core. The horizontal circulation of the tropical cyclone in both the FV and SE simulations increases as the length of the time step decreases in high-resolution runs, where large-scale condensation dominates. The sensitivities in the physical parameterizations to time step play an important role in regulating the impact of time step on the physics–dynamics interaction, especially in high-resolution simulations. Compared with the sequential coupling approach (ftype1) with a sudden adjustment at each physics time step in the SE core, the dribbling coupling strategy (ftype0) that adjusts the state more gradually weakens the effect of the physical parameterizations. [ABSTRACT FROM AUTHOR]