  Recent numerical simulations of dynamo action resulting from rotating
convection have revealed some serious problems in applying the standard picture
of mean field electrodynamics at high values of the magnetic Reynolds number,
and have thereby underlined the difficulties in large-scale magnetic field
generation in this regime. Here we consider kinematic dynamo processes in a
rotating convective layer of Boussinesq fluid with the additional influence of
a large-scale horizontal velocity shear. Incorporating the shear flow enhances
the dynamo growth rate and also leads to the generation of significant magnetic
fields on large scales. By the technique of spectral filtering, we analyse the
modes in the velocity that are principally responsible for dynamo action, and
show that the magnetic field resulting from the full flow relies crucially on a
range of scales in the velocity field. Filtering the flow to provide a true
separation of scales between the shear and the convective flow also leads to
dynamo action; however, the magnetic field in this case has a very different
structure from that generated by the full velocity field. We also show that the
nature of the dynamo action is broadly similar irrespective of whether the flow
in the absence of shear can support dynamo action.
