nfast: Number of barotropic timesteps needed to compute time-averaged barotropic variables centered at time level n+1 ndtfast: Number of barotropic timesteps between each baroclinic timestep.

I am confused with the time-stepping in ROMS.

For example, I set dt = 150s, ndtfast = 30 in ocean.in, so I got dtfast = 5s, and I also got nfast = 42 computed by set_weights.F. In main3d.F, there are 42 barotropic time-steps for each dt, rather than 30 steps. SO my question is that are there 12 time steps(M*-M) double counted for each dt? For the next baroclinic step n+2, the barotropic step starts from m=M or from m=M* on the figure attached?

Could anyone give me a further explain? Thank you very much!