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

TL;DR

This paper introduces the SWARL model, which predicts ocean surface roughness length by incorporating detailed wave physics and flow characteristics, showing improved accuracy over previous models especially with well-characterized wave data.

Contribution

The paper presents a novel surface wave-aerodynamic roughness length model that integrates flow physics and wave surface data for better air-sea interaction predictions.

Findings

SWARL model outperforms prior models with well-characterized data.

Inclusion of detailed flow physics improves roughness length predictions.

Model achieves comparable accuracy to existing methods with limited data.

Abstract

A new model to evaluate the equivalent hydrodynamic length or surface roughness, z0, of ocean waves is developed and tested. The proposed Surface Wave-Aerodynamic Roughness Length (SWARL) model requires maps of the wave surface height at consecutive times and the air flow characteristic Reynolds number as inputs. Pressure drag is accounted for by approximating the relative velocity in a frame moving with the local wave phase-speed assuming ideal inviscid ramp flow (Ayala et al. 2024). Drag from viscous and unresolved ripples is modeled using the standard equilibrium model. The SWARL model is tested using over 300 datasets for monochromatic and broad-spectrum wave surfaces. The model-predicted z0 and drag coefficients are compared to measured values, as well as commonly used wave parametrization methods found in the literature. For datasets with well-characterized surfaces, the proposed model shows significantly better agreement with data compared to prior models. For data that did not include a full characterization of the wave fields (typically field data), the model yields predictions with accuracy similar to prior models. Results highlight that including detailed flow physics and extensive wave-field characterization in the modeling of z0 can provide significant improvements in roughness-length based modeling of air-sea interactions.