Surface Wave-Aerodynamic Roughness Length Model for Air-Sea Interactions

A novel method for determining the equivalent hydrodynamic surface roughness length induced by ocean waves is proposed and evaluated. Termed the Surface Wave Aerodynamic Roughness Length (SWARL) model, this approach integrates instantaneous wave-surface elevation maps with an airflow-dependent Reynolds number to estimate aerodynamic roughness. The model captures pressure drag by conceptualizing the local airflow relative to wave phase speed as an idealized inviscid flow over inclined surfaces (Ayala et al., Bound. Layer Met. 2024). Contributions from viscous effects and unresolved small-scale ripples are modeled via established equilibrium formulations. SWARL predictions are validated against an extensive set of more than 300 datasets, encompassing both monochromatic and broad-spectrum wave conditions. Comparisons demonstrate that SWARL significantly improves agreement with experimental and observational data relative to existing parametrization techniques, specifically when wave-surface characteristics are known. For data with wave fields that are not fully characterized (such as field data), the model yields predictions with accuracy similar to existing models. These findings emphasize that including detailed flow physics and extensive wave-field characterization in the modeling of ocean wave surface roughness can provide significant improvements in roughness-length-based modeling of air-sea interactions