Spurious rollover of wave attenuation rates in sea ice caused by noise
in field measurements
The effects of instrument noise on estimating the spectral attenuation
rates of ocean waves in sea ice are explored using synthetic
observations in which the true attenuation rates are known explicitly.
The spectral shape of the energy added by noise, relative to the
spectral shape of the true wave energy, is the critical aspect of the
investigation. A negative bias in attenuation that grows in frequency is
found across a range of realistic parameters. This negative bias
decreases the observed attenuation rates at high frequencies, such that
it can explain the rollover effect commonly reported in field studies of
wave attenuation in sea ice. The published results from four field
experiments are evaluated in terms of the noise bias, and a spurious
rollover (or flattening) of attenuation is found in all cases.
Remarkably, the wave heights are unaffected by the noise bias, because
the noise bias occurs at frequencies that contain only a small fraction
of the total energy.