No need to use callbacks...just set the operator timings to Continuous.
Note that once you start animating the terrain operators, you need to be mindful of how all their blending modes interact. So for example, if you initialize your terrain and then add a Terrain Noise operator (whose blending mode default is 'Add')...by setting its timing to Continuous, it will additive apply changes to the terrain over time....whereas, if you change the blending mode to "Set", the changes will no longer add on top of each other.
If you are animating parameters that don't need to be history dependent (ex: you're not trying to make cumulative changes over time to the terrain, like in a liquid sim)...you can also experiment with setting the flow's simulation mode to "history-independent". This is a new experimental setting that was added specifically for this purpose - so you don't need to simulate unnecessary frames for non-cumulative terrain animations.
Note that once you start animating the terrain operators, you need to be mindful of how all their blending modes interact. So for example, if you initialize your terrain and then add a Terrain Noise operator (whose blending mode default is 'Add')...by setting its timing to Continuous, it will additive apply changes to the terrain over time....whereas, if you change the blending mode to "Set", the changes will no longer add on top of each other.
If you are animating parameters that don't need to be history dependent (ex: you're not trying to make cumulative changes over time to the terrain, like in a liquid sim)...you can also experiment with setting the flow's simulation mode to "history-independent". This is a new experimental setting that was added specifically for this purpose - so you don't need to simulate unnecessary frames for non-cumulative terrain animations.