University of Alberta
Edmonton, Alberta, Canada
Interfacial gravity currents, or intrusions, may occur naturally along an inversion in the atmosphere as a consequence, for example, of cold thunderstorm outflows or due to rapid mixing by the collision of two fronts. Depending on the relative density of the upper and lower layers and the density of the intrusion, solitary waves or bores may be generated ahead of the current, as demonstrated in laboratory experiments (e.g. Rottman and Simpson (1989)). Less well studied is the dynamics of trapped internal waves in the wake of the gravity current. In laboratory experiments we show that, if the interfacial thickness is small compared with the width of the gravity current, the excitation of trapped internal waves is weak. However, if the interfacial thickness is comparable to, but smaller than the width of the gravity current, large amplitude trapped internal waves are excited. In most experiments these waves are sinuous and propagate with horizontal phase speeds much slower than that of the intrusion. These large amplitude waves extract significant momentum from the current, and in some circumstances, the current itself is observed to stop propagating and a solitary wave is generated. Theory of a three-layer fluid is presented to explain this transitional behaviour in terms of a resonant coupling between the gravity current and trapped internal waves.