Not all physics computations are conducted at every time step of integration. PCCM2 in its tested configuration (T42, 20-minute time steps, hourly radiation, and twice daily absorptivity and emissivity calculation) does a representative execution of physics over the course of a 12-hour simulation. It is representative in the sense that the time spent computing physics will contain cost components for all physics modules in proportions that are representative of long runs of the model. We can classify the time steps into the following categories:

- a.
- Radiation time step with calculations of emissivity (ems) and absorptivity (abs): All physics computations are conducted at this step. This step includes the calculation of emissivities and absorptivities (RADABS and RADEMS subroutines) for the longwave radiation. These calculations are conducted once every 12 hours in the model. We shall term this type of time-step as ``A''.
- b.
- Radiation time step without emissivity and absorptivity: The longwave radiation calculation does not include the computation of emissivities and absorptivities. All calculations for shortwave radiation are conducted. These calculations are done once every hour of integration. This category of time steps is denoted type ``B''.
- c.
- No-radiation time step: Only convection, diffusion, surface
fluxes, and
gravity wave drag are calculated during this time step. All
time steps other than the radiation time steps are
of this category, type ``C.''

The computational time required for a composite, or average, time step is

which represents the cost of 1 time step with radiation and abs/ems calculations, 11 others with radiation but without abs/ems, and the remainder without radiation or abs/ems averaged over the 36-step period between type-A time steps (12 simulation hours). To characterize the effect of physics load imbalance on model performance as a whole over long simulations, we discuss the overall load and imbalance in terms of this average time step. Subsequently, we detail the individual contributions to this overall imbalance from each computational module making up CCM physics during the three different types of time step.

Wed Dec 7 03:37:14 CST 1994