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dt: $\Delta t$

Time step. Assuming $\theta>0.5$, stability is limited by

\Delta t < C\left[\frac{\min(D_g)}{\max(\vert U\vert)},\frac...
\right] ,

where $c_i$ is the internal wave speed, $C$ is the relevant Courant number, and $D_g$ is the minimum Voronoi distance. Note that this assumes first-order upwind is used for advection of momentum. If central-differencing is used, stability is further constrained by the grid Peclet number (see Section 7.1.51 for details).