Radial intrusions from turbulent plumes in uniform stratification

Laboratory experiments investigate the radial spread of an intrusion created by a turbulent forced plume in uniformly stratified ambient fluid. The flow evolution is determined as it depends upon the ambient buoyancy frequency, N, and the source momentum and buoyancy fluxes, M0 and F0, respectively. The plume reaches its maximum vertical extent, Zm, collapses back upon itself as a fountain and then spreads radially outwards at its neutral buoyancy depth, Zs, where the intrusion has the same density as the ambient. Through theory and experiments we determine that Zs = f(σ )Hp, in which Hp = M03/4F0−1/2, σ = (M0N/F0), and f(σ ) ∝ σ−3/8 for σ 50 and f(σ ) ∝ σ−1/4 for σ 50. In the inertia-buoyancy regime the intrusion front advances in time approximately as Rf ∝ t3/4, consistent with models assuming a constant buoyancy flux into the intrusion. Where the intrusion first forms, at radius R1, its thickness h1 is approximately constant in time. The thickness of the intrusion as a whole, h(r, t), adopts a self-similar shape of the form h/h1 [(Rf − r)/(Rf − R1)], with p 0.55 ± 0.03. The comparison of these results to large volcanic plumes penetrating into and spreading in the stratosphere is discussed. C © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869119]