ON THE MELLOR-YAMADA TURBULENCE CLOSURE SCHEME - THE SURFACE BOUNDARY-CONDITION FOR Q(2)

A numerical model that uses a level-2 1/2 turbulence closure scheme is used to compare two boundary conditions for the turbulent energy at t he air-sea interface. One boundary condition, the most commonly used, sets the turbulent kinetic energy proportional to the friction velocit y squared, while the other sets the vertical diffusive flux of turbule nt kinetic energy proportional to the friction velocity cubed. The fir st boundary condition arises from consideration (simplification) of th e turbulence closure scheme near boundaries, and the second arises fro m consideration of the influence of surface gravity waves on the trans fer of turbulent kinetic energy from the wind to the water. Simulation s using these two boundary conditions are compared to month-long obser vations of velocity, temperature, and salinity (as shallow as 2 m from the surface) from Knight Inlet, British Columbia, Canada. The circula tion in the inlet is strongly influenced by the wind, tides, and fresh water runoff. The two boundary conditions produce simulations that are different down to a depth of at least 5 m. Somewhat more accurate sim ulations are produced by the second boundary condition. Also, simulati ons using the second boundary condition are more sensitive to variatio ns in the roughness length. Based on the simulations, roughness length s as large as 1 m (or greater) are possible.