  We extend the model of surface granular flow proposed in \cite{bcre} to
account for the effect of an external `wind', which acts as to dislodge
particles from the static bed, such that a stationary state of flowing grains
is reached. We discuss in detail how this mechanism can be described in a
phenomenological way, and show that a flat bed is linearly unstable against
ripple formation in a certain region of parameter space. We focus in particular
on the (realistic) case where the migration velocity of the instability is much
smaller than the grains' velocity. In this limit, the full dispersion relation
can be established. We find that the critical wave vector is of the order of
the saltation length. We provide an intuitive interpretation of the
instability.
