Square of the normalized instability growth rate, Γ², as a function of ${\overline{\delta }}_1^2$ for the simulation runs of subset 1 (see Fig. 9). The dashed line shows the best-fit power law for runs in the global mode regime (${\overline{\delta }}_1^2>{10}^4$; circles): ${\mathrm{\Gamma }}^2\propto 1/{\overline{\delta }}_1^{4.1}$. The dotted line indicates the best fit for runs in the moderately stratified regime with WKB-type modes (triangles in the range ${10}^3<{\overline{\delta }}_1^2<{10}^4$): ${\mathrm{\Gamma }}^2\propto 1/{\overline{\delta }}_1^{1.3}$. The shaded gray and blue areas mark, respectively, the stable and global mode regimes as identified in Fig. 9. The thick horizontal bars associated with each data point indicate the range of ${\overline{\delta }}_1$ values attained during the linear phase of the instability growth (see text for details). For the unstratified case and the first five stratified runs, these bars are smaller than the symbol size.