The importance of convection in the generation of atmospheric internal gravity waves (IGW) is well established. Beres, Alexander & Holton (2002) identified three principal mechanisms by which the action of convective elements may excite IGW: the ``mechanical oscillator effect'' which involves the oscillatory deflection to the boundary of a stratified layer by updrafts and downdrafts, the ``obstacle effect'', in which a horizontal mean flow is perturbed by a ``fluidic'' barrier, and the ``deep heating effect'', which is related to the thermal forcing associated with latent heat release within a convective storm. In the present experimental study, we propose a fourth, indirect mechanism wherein IGW are generated by the outflow from a convective storm that propagates as a fluid intrusion along the tropopause.

Experiments are performed in which a fluid intrusion is released along the interface between a uniformly stratified fluid and a uniform fluid. The strength of stratification, the density difference across the interface as well as that between the fluid intrusion and the uniform fluid are varied. IGW characteristics, including their amplitudes, are determined using ``synthetic schlieren.''

IGW are noted in all but the most weakly stratified cases. Their amplitude increases with the intrusion's depth of penetration into the stratified layer. The characteristic angle of IGW propagation is nearly constant however, with values between 42 - 51 degrees. The intrusion's velocity and depth of penetration into the upper and lower layers are well predicted by the theory of Holyer and Huppert (1980) over a limited range of densities. Since this theory assumes both layers to be uniform, we conclude that the behaviour of the fluid intrusion is in certain cases independent of the strength of stratification.