A nonuniform potential vorticity model for frontogenesis is used to study the effects of height-dependent inhomogeneities in the potential temperature and potential vorticity fields at the mesoscale. The model simulation results show that these irregularities induce in the vertical velocity field mesoscale patterns which have considerable vertical extent. It is suggested that these mesoscale inhomogeneities may be possible causes for the high degree of organization of precipitation fields and the formation of middle-level rainbands. ¿American Geophysical Union 1987