The conditions on and l at the free surface are
where is the friction velocity at the surface .
Equation (59) assumes that surface waves do not contribute to the turbulence near the free surface. This is only a convenient approximation useful for most situations. However, when the upper mixed layer is shallow and winds are light, the surface wave contribution cannot be ignored, and can be included by parameterizing the kinetic energy flux due to waves and prescribing that instead at the free surface. Similarly l can be prescribed as a function of the significant wave height. Nevertheless, wave-induced turbulence dies off quite rapidly with depth and is seldom important at depths of more than a few meters.