  A compressible generalization of the Kraichnan model (Phys. Rev. Lett. 72,
1016 (1994)) of passive scalar advection is considered. The dynamical role of
compressibility on the intermittency of the scalar statistics is investigated
for the direct cascade regime. Simple physical arguments suggest that an
enhanced intermittency should appear for increasing compressibility, due to the
slowing down of Lagrangian trajectory separations. This is confirmed by a
numerical study of the dependence of intermittency exponents on the degree of
compressibility, by a Lagrangian method for calculating simultaneous N-point
tracer correlations.
