Sensitivity of plume dynamics to the parameterization of vertical mixing

A three-dimensional primitive equation, baroclinic numerical model incorpor ating a range of turbulence closure schemes is used to investigate the effe cts of vertical diffusion of momentum and density upon the spread of a fres hwater plume, with particular reference to the Ebro plume. Initial calculat ions show that there are some differences in the horizontal spread and vert ical mixing of the plume when diffusion coefficients are computed from a tw o-equation turbulence energy model compared with a one-equation model. To u nderstand results from the turbulence energy models, the sensitivity of the plume dynamics to variations in the coefficient of vertical eddy viscosity and diffusivity is also considered, with increases in these parameters hav ing a significant effect upon the cross-shore and along-shore spread of the plume. Also, increasing these parameters changes the plume characteristics from supercritical to subcritical and reduces the occurrence of meandering and baroclinic instability along the plume's off-shore edge. However, diff erences in the southerly spread (the direction of Kelvin wave propagation i n the model) of the plume (although not its northerly spread) produced by c hanges in diffusion coefficients are small compared with the influence of c hanges in the bottom slope, upon the along-shore southerly spread of the pl ume, which moves in the direction of Kelvin wave propagation in the near co astal region. Results from the series of calculations are used as a guide i n experimental design, with reference to a planned experiment in the Ebro r egion involving a coastal HF Radar deployment, as well as off-shore measure ments. Calculations suggest that surface current measurements from a coasta l HF Radar, together with a detailed survey of the density field associated with the plume, may be an appropriate, although indirect, means of determi ning suitable mixing coefficients to use in plume discharge problems. Detai led measurements of water depth variation will also be required. Copyright (C) 1999 John Wiley & Sons, Ltd.