  The accuracy of gravitational-wave models of compact binaries has
traditionally been addressed by the mismatch between the model and
numerical-relativity simulations. This is a measure of the overall agreement
between the two waveforms. However, the largest modelling error typically
appears in the strong-field merger regime and may affect subdominant signal
harmonics more strongly. These inaccuracies are often not well characterised by
the mismatch. We explore the use of a complementary, physically motivated tool
to investigate the accuracy of gravitational-wave harmonics in waveform models:
the remnant's recoil, or kick velocity. Asymmetric binary mergers produce
remnants with significant recoil, encoded by subtle imprints in the
gravitational-wave signal. The kick estimate is highly sensitive to the
intrinsic inaccuracies of the modelled gravitational-wave harmonics during the
strongly relativistic merger regime. Here we investigate the accuracy of the
higher harmonics in four state-of-the-art waveform models of binary black
holes. We find that the SEOBNRv4HM_ROM, IMRPhenomHM, IMRPhenomXHM and
NRHybSur3dq8 models are not consistent in their kick predictions. Our results
enable us to identify regions in the parameter space where the models require
further improvement and support the use of the kick estimate to investigate
waveform systematics. We discuss how numerical-relativity kick estimates could
be used to calibrate waveform models further, proposing the first steps towards
kick-based gravitational-wave tuning.
