  In this paper the Rayleigh-Taylor instability (RTI) along the density
interfaces of gravity-current fronts is analyzed. Both the location and the
spanwise wavenumber of the most unstable mode determined by the local
dispersion relation agree with those of the strongest perturbation obtained
from numerical simulations, suggesting that the original formation mechanism of
lobes and clefts at the current front is RTI. Furthermore, the predictions of
the semi-infinite RTI model, i.e. the original dominating spanwise wavenumber
of the Boussinesq current substantially depends on the Prandtl number and has a
1/3 scaling law with the Grashof number, are confirmed by the three-dimensional
numerical simulations.
