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thetaramptime: $\tau_\theta >0$

This is used to damp out transient barotropic motions by initializing the value of $\theta$ in the free-surface solver to 1 and ramping it down to the value of $\theta$ specified below. The $\tau_\theta$ parameter determines the timescale, in seconds, over which this is damped, such that, if the value of $\theta$ specified below is given by $\theta_0$, then the value of $\theta$ that is used in the calculations is given by

$\displaystyle \theta(t)$ $\textstyle =$ $\displaystyle \theta_0 + (1-\theta_0)\exp\left(-\frac{t}{\tau_\theta}\right) .$