Effect of time step size in MM5 simulations of a mesoscale convective system

Results from simulations of a mesoscale convective system (MCS) that occurr ed in a weakly forced large-scale environment show significant sensitivity to the time step size chosen for model integration. As the time step is dec reased, the movement and longevity of the simulated MCS depart further from the observations. It is found that the time step-dependent formulation of the numerical diffusivity is responsible for a major part of this sensitivi ty in the simulated MCS. The changes contributed by truncation errors may o r may not be negligible, depending on the physics schemes used in the simul ations. This suggests that the choice of a specific linearly stable model t ime step, in combination with the definition of the numerical diffusivity, may at times be as important as the choice of the basic model configuration .