  Monin-Obukhov similarity theory (MOST) is a well-tested approach for
specifying the fluxes when the roughness surfaces are homogeneous. For flow
over waves (inhomogeneous surfaces), phase-averaged roughness length scales are
often prescribed through models based on the wave characteristics and the wind
speed. However, such approaches lack generalizability over different wave ages
and steepnesses due to the reliance on model coefficients tuned to specific
datasets. In this paper, a sea surface-based hydrodynamic drag model applicable
to moving surfaces is developed to model the pressure-based surface drag felt
by the wind due to the waves. The model is based on the surface gradient
approach of \citet{Anderson2010} applicable to stationary obstacles and
extended here to the wind-wave problem. The wave drag model proposed specifies
the hydrodynamic force based on the incoming momentum flux, wave phase speed,
and the surface frontal area. The drag coefficient associated with the
wind-wave momentum exchange is determined based on the wave steepness. The wave
drag model is used to simulate turbulent airflow above a monochromatic wave
train with different wave ages and wave steepnesses. The mean velocity profiles
and model form stresses are validated with available lab-scale experimental
data and show good agreement across a wide range of wave steepnesses and wave
ages. The drag force is correlated with the wave surface gradient and
out-of-phase with the wave height distribution by a factor $\pi/2$ for the
sinusoidal wave-train considered. These results demonstrate that the current
approach is sufficiently general over a wide parameter space compared to wave
phase-averaged models with a minimal increase in computational cost.
