Offshore spreading of buoyant bulge from numerical simulations and laboratory experiments

The ability of a three-dimensional hydrodynamic model to reproduce buoyant water entering a coastal sea at laboratory scales of O[1 cm] is studied using Regional Ocean Modeling System (ROMS). ROMS is typically used for geophysical scale simulations. Inflowing water forms a growing anti-cyclonic buoyant bulge and coastal current. Available laboratory data is from a rotating circular basin experiment. The numerical domain is a rectangular basin with three open boundaries and a straight inflow channel for freshwater discharge. Altogether 11 pairs of laboratory-numerical simulation runs are analyzed. Three additional simulations are made to study the influence of ambient salinity. Rotation rate, ambient salinity and inflow rate-including oscillatory inflow as a proxy for tides, is varied. The present study concentrates on comparison of the bulge offshore front. Development of a bulge and downcoast coastal current was observed in all experiments. Two phases of bulge spreading are identified. An initial rapid spreading phase lasts 0.3-0.7 rotation periods and a following slow expansion that lasts until the end of the simulation. The shift from first phase to second coincides with the formation of the coastal current. Bulge front spreading agrees well with inflow Kelvin number. When K>1/K<1, the model underestimates/overestimates the bulge offshore reach. Physical processes of discharged water are altered in the inflow estuary before the water enters the main basin. With estuary wide/narrow in comparison to the deformation radius resulting with non-uniform outflow profile. These differences however do not notably alter the spreading during the second phase. Bulge front spreading is scaled with various non-dimensional parameters and best scaling is achieved during the first phase for laboratory simulation with internal radius and numerical bulge spreading with the bulge Rossby radius. During the second phase both scale with with the bulge Rossby radius. The numerical bulge expands at a steady rate of 0.10cm s-1 and laboratory bulge at 0.11cm s-1.