Simulation of the Wind-Forced Near-Surface Circulation in Knight Inlet: A Parameterization of the Roughness Length

Abstract

Month-long observations of along-channel velocity made close to the surface of Knight Inlet are used with a numerical model to estimate the roughness length z0 on the water side of the air?sea interface. In analogy with a very common parameterization for z0 on the air side of the air?sea interface, z0 is parameterized in the numerical model as z0 = au2?/g where u? = (τ/?)1/2 is the friction velocity, g is the acceleration due to gravity, τ is the wind stress, ? is the density of water, and a is an empirical constant. It is found that a ≈ O(105) for the dataset from Knight Inlet, a value four orders of magnitude larger than the value commonly used to estimate z0 on the air side of the air?sea interface. When compared to empirical estimates of the significant wave height Hs, it is found that z0 ≈ O(Hs). Further evidence is provided that a numerical model that uses the Mellor?Yamada level 2.5 turbulence closure scheme can simulate the near-surface, wind-forced circulation quite well.