at the free surface.
This is the linear dynamic free-surface condition,
which is exactly satisfied at the wave crest in
Figure 3.5, whereas there is a higher-order error
under the wave trough. By “higher-order error”
we mean that the error is approximately proportional
to (ζa/λ)n, where n ≥ 2. This means
that linear theory is correct to O(ζa/λ), where
O( ) means order of magnitude. We should note
that the dynamic pressure −ρ∂ϕ/∂t half a wavelength
down in the fluid is only 4% of its value
at z = 0.