A Study of Ocean Surface Roughness Using Field Observations

Ocean surface roughness is a key boundary condition to simulating and predicting the ocean-atmosphere coupled system on a global scale. Deriving an accurate model for ocean roughness is complicated by the nonlinear interaction of wind and surface waves. Large-scale models have parameterized roughness solely in terms of the tangential wind stress, however this is a well-known oversimplification of the physics. Using a high quality marine atmospheric boundary layer data set collected from a unique ocean platform, the ocean surface roughness, z₀, will be directly calculated and compared to state-of-the-art empirical roughness models. Surface roughness was directly calculated using the eddy covariance, profile, and bulk methods from the observations collection aboard FLIP, the Floating Instrumentation Platform, during a 2017 Southern California ocean expedition. Preliminary analysis of the direct measurements revealed both u_*^-1and u_*² dependence of z₀ as the wind forcing approached 0; conventional models predict only the u_*^-1 form in the low wind regime. Values of z₀ at higher wind stresses follow the expected dependence on u_*, but are significantly lower than predicted from empirical relations. This presentation will showcase a comparison of observed and predicted roughness using both wind- and sea state dependent models. The physical meaning behind the low u_* divergence in the observed z₀ will be discussed.