GGL mixing parameter change to c_k 0.07

[ifenty]

Concerning the issue of GGL mixing parameter described in this issue: MITgcm/MITgcm#169

We should consider changing the value of c_k to 0.07 to reduce the GGL mixing efficiency to 0.2 from the default 0.285714. This issue was raised by P. Cessi at the ECCO meeting.

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According to Gregg et al., 2018 \gamma should be 0.2.

The current default value of c_k is 0.1. A value of c_k of 0.1 implies a value of \gamma of 0.285714.

For \gamma=0.2 the value of should be c_k=0.07 because:

c_k = 0.5 * gamma * c_ep * P_rt

x = 0.5 * 0.2 * 0.7 * 1
x = 0.07

My vote is to change the default value of c_k to 0.07 because if we only change this in the ECCO setup then this knowledge of the 'proper' \gamma will be lost. The question of whether to carry the old value of c_k in the verification experiments I leave to @jm-c and others. I personally don't like carrying 'bad' parameter values around because when someone new tries to adapat a verification experiment for a new domain they will probably tend to carry these old parameter values around out of ignorance.