✨ Feature Request

As described in CF conventions

Motivation

From @matthew-mizielinski. Considering how to represent hybrid height over glaciers, where the orography moves/changes over time.

Additional context

@pp-mo expects problems with FF or PP loading, as it would require a specific sequence of merge steps.

Please add additional verbose information in this section e.g., references, screenshots, listings etc

### Tasks
- [ ] https://github.com/SciTools/iris/issues/6162
- [ ] https://github.com/SciTools/iris/issues/6163
- [ ] https://github.com/SciTools/iris/issues/6203
- [ ] https://github.com/SciTools/iris/issues/6204