Intrusive gravity currents and internal gravity wave generation in stratified fluid

Abstract

The excitation of internal gravity waves by fluid intrusions that propagate along the interface between a uniform upper layer and a uniformly stratified lower layer is examined by way of laboratory experiments. Intrusions are generated using a simple lock-release apparatus. Experiments are conducted in which the density gradient of the uniformly stratified layer, the density jump across the interface and the density difference between the lock fluid and the uniform upper layer are varied.

In all cases, the fluid intrusions travelled at a constant speed. The forcing imparted by the generated internal gravity waves did not deform the intrusion head or significantly retard the intrusion's rate of forward advance. For a limited range of density parameters, good agreement was obtained between the experimental data and the two-layer analytical theory of J. Y. Holyer & H. E. Huppert (J. Fluid Mech. vol. 100 (1980), pp. 739–767) which provides estimates for the intrusion speed and depths of penetration into the upper and lower layers. Internal gravity wave excitation is due to the initial collapse of the lock fluid and the forcing imparted by the head of the intrusion. Waves are visualized and their amplitudes measured using ‘synthetic schlieren’.

The vertical flux of horizontal momentum due to internal gravity wave excitation is related to measurable properties of the fluid intrusion. This analysis suggests that outflows produced by tall convective storms that travel along the tropopause may excite non-hydrostatic internal gravity waves in the stratosphere whose momentum flux, at least during the transient generation time, is comparable to that of waves generated by topographic forcing.