  The interaction between planetary waves and an arbitrary zonal flow is
studied from a phase-space viewpoint. Using the Wigner distribution, a
planetary wave Vlasov equation is derived that includes the contribution of the
mean flow to the zonal potential vorticity gradient. This equation is applied
to the problem of planetary wave modulational instability, where it is used to
predict a fastest growing mode of finite wavenumber. A wave-mean flow numerical
model is used to test the analytical predictions, and an intuitive explanation
of modulational instability and jet asymmetry is given via the motion of
planetary wavepackets in phase space.
