# Vertical velocity

### From GHER

The subroutine real3u distinguishes between 3 vertical velocities:

- real vertical velocity
- vertical velocity relative to a fix reference
- geometric vertical velocity
- vertical velocity relative of coordinate system (which moves due to the movement of the free surface)
- upwelling vertical velocity
- vertical velocity relative to a the vertical coordinate system = HU3II/HU3II

The three velocities are related by:

real vertical velocity = geometric vertical velocity + upwelling vertical velocity