Increased aerodynamic roughness owing to surfzone foam

Drag coefficients Cd obtained through direct eddy covariance estimates of the wind stress were observed at four different sandy beaches with dissipative surfzones along the coastline of Monterey Bay, California. The measured surfzone Cd (~2 × 10−3) is twice as large as open-ocean estimates and consistent with recent estimates of Cd over the surfzone and shoaling region. Owing to the heterogeneous nature of the near shore consisting of nonbreaking shoaling waves and breaking surfzone waves, the surfzone wind stress source region is estimated from the footprint probability distribution derived for stable and unstable atmospheric conditions. An empirical model developed for estimating the Cd for open-ocean foam coverage dependent on wind speed is modified for foam coverage owing to depth-limited wave breaking within the surfzone. A modified empirical Cd model for surfzone foam predicts similar values as the measured Cd and provides an alternative mechanism to describe roughness.