Experiments for the gravity currents produced from a two-layer density-stratified buoyancy source in a full-depth, lock-exchange setup with a scaling analysis describing the flow morphologies are presented. In the inertial phase of propagation, the $3/2$ power relationship $x_f^{3/2}=1.5F_I{B}_0^{1/2}(t+t_I)$ robustly applies between the front location $x_f$ and time $t$, where $F_I$ is the Froude number in the inertial phase, $B_0$ is the total released buoyancy, and $t_I$ is the $t$-intercept. We showed that the Froude number in the inertial phase is not a universal constant but depends on the two controlling parameters, namely, the density difference ratio, $R_{\rho }=({\rho }_U-{\rho }_0)/({\rho }_L-{\rho }_0)$, where ${\rho }_U,{\rho }_L$, and ${\rho }_0$ are the densities of the fluids in the upper layer, lower layer, and ambient environment, respectively, and the buoyancy distribution parameter, $R_B=B_U/B_0$, where $B_U$ is the buoyancy in the upper layer. For a given buoyancy distribution parameter, the Froude number in the inertial phase decreases monotonically as the density difference ratio decreases. For a given density difference ratio, the Froude number in the inertial phase has a local minimum as the buoyancy distribution parameter falls in the range of $0.3\lesssim R_B\lesssim 0.5$. When the buoyancy source is homogeneous, the Froude number in the inertial phase has its maximum value at $F_I=1.33\pm 0.02$. The flow morphology is also found to depend on the two controlling parameters. For weakly stratified two-layer heavy fluid, $0.4\lesssim R_{\rho }<1$, mixing between the fluids from the two layers is more immediate. For strongly stratified two-layer heavy fluid, $0<R_{\rho }\lesssim 0.4$, there is less mixing between the layers for flows dominated by the upper layer, $R_B\to 1$, and for flows dominated by the lower layer, $R_B\to 0$. For gravity currents that are produced from a strongly stratified source and dominated by the upper layer, the upper layer may override and outrun the lower layer, which initially takes the lead after the two-layer heavy fluid is released. For gravity currents that are produced from a strongly stratified source and dominated by the lower layer, the lower layer may outrun the upper layer from the outset, resulting in streamwise stratification. Surprisingly, for the gravity currents produced from a strongly stratified source, mixing of fluids from the two layers can be enhanced when the buoyancy distribution parameter falls in the range of $0.3\lesssim R_B\lesssim 0.5$. Such an exceptional observation is now successfully explained by the scaling analysis.

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• Geophysical fluid dynamics