Drift of Jnr from Snr

Jnr will drift from the state imposed by Snr
Calculations of drift stats are coded into hybrid code
We can measure this drift, e.g.

Average θ_vd difference = Σ |θ_vd^Snr - θ_vd^Jnr|
(Number of grid points)

Bringing Jnr towards Snr

For UKESM, we have to start Snr and Jnr from different UM dumps.

When coupling u, v and θ_vd the average difference in θ_vd looks like the following