DEVELOPMENT OF A POINT PLUME IN THE PRESENCE OF BACKGROUND ROTATION

This paper describes a laboratory study on the evolution of a point tu rbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume ini tially evolves as if there is no rotation. However, the rotational eff ects become important after the plume descends a vertical distance h(c 1)approximate to 3.3(B/Omega(3))(1/4) for a normalized time Omega t(c1 )approximate to 2.4, whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here Omega is the rate of background rotation and B is the specific bu oyancy flux of the plume. The rotational effects inhibit the lateral g rowth of the plume at a time Omega t(c2)approximate to 5.5, when the m aximum plume width is b(c)approximate to 1.4(B/Omega(3))(1/4). Thereaf ter, the vertical descent continues and the plume evolves into a cylin drical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after t raveling a fluid depth of H, the plume deflects, propagates horizontal ly, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case of H<h(c1) show that this instability is ini tiated at a horizontal length scale proportional to the Rossby deforma tion radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies form ed by the barotropic instability of the surface rim current, thus prod ucing heton-like structures. The influence of the diameter do of the p lume on the flow evolution is also studied, and it is shown that plume s with aspect ratio h/d(0)<12 (where h is the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explai n the results. Some geophysical applications of the study are also dis cussed. (C) 1998 American Institute of Physics.