The mixing processes involving organized large-scale motion and chaotic small-scale motions across a sharp density interface were investigated experimentally via a pH-sensitive, laser-induced fluorescence technique in a water tank. This nonintrusive technique allows one to distinguish fluid that has been molecularly mixed from that which has been merely stirred. A turbulent round jet impinged from above on the sharp density interface over a flow Reynolds number of 2500 less than or equal to Re less than or equal to 25,000 and a flow Richardson number of 0 less than or equal to Ri less than or equal to 5 based on the local jet scales at the interface. It was found that at large Re, molecular mixing first occurs at the perimeter of the jet front, forming a mixed layer, in contrast to the case of a jet in a uniform environment, where engulfment occurs in the back of the large vortical structure. In the case of relatively weak stratification approaching the atmospherically relevant situations, as the jet penetrates the density interface and continues to advance, the mixed layer develops into a complex reverse jet that ejects backward along the sides of the original jet core. Surprisingly, the latter remains little mixed. This seems to explain why undiluted cloudbase air has been found at all levels within cumulus clouds during aircraft penetration. At moderate stratification, a mixing transition was observed across which the mixed layer thickness changed by an order of magnitude, showing that Re also plays a role in mixing in stratified flows. With stronger stratification, the jet front barely penetrates the interface. A physical model is presented to explain explicitly the jet transport and the mixing transition across the density interface. These results reveal that the commonly held view that entrainment at the density interface is dependent only on Ri needs to be reconsidered.
