Doppler frequency shift in ocean wave measurements: Frequency downshift of a fixed spectral wave number component by advection of wave orbital velocity

Because the Jacobian connecting wave number to frequency conversion is nonlinear, in the presence of background waves even without a mean current a wave number spectral component does not appear at the expected intrinsic frequency in the frequency spectrum measured by a stationary probe. The advection of the wave number component by the orbital current of background waves produces a net downshift in the encounter frequency. Comparison with laboratory measurements shows that the effect is important in low‐wind conditions. As wind speed increases, the frequency downshift due to orbital advection becomes less recognizable.