Gravity currents in cold fresh water.

We consider surface gravity currents in fresh water where the temperatures of the current and the ambient are on opposite sides of the temperature of maximum density. Buoyancy reversal may occur in the current, due to entrainment of ambient water to produce a mixture that is denser than the ambient. Using an empirical parametrisation of entrainment in lock-release gravity currents, the distance travelled and time taken before the current is arrested due to buoyancy reversal are calculated as functions of the initial temperatures. This is done for two-dimensional and axisymmetric geometries, with a free surface and with a no-slip lid. The distance travelled and the speed of the current both increase with increasing initial buoyancy, but the distance is limited by loss of fluid from the head of the current to its tail; the time taken depends on the balance between these effects. There is greater entrainment under a no-slip lid than a free surface, so gravity currents generally travel further in the latter case.