  The dynamics of the radial envelope of a weak coherent drift wave is
approximately governed by a nonlinear Schr\"odinger equation, which emerges as
a limit of the modified Hasegawa-Mima equation. The nonlinear Schr\"odinger
equation has well-known soliton solutions, and its modulational instability can
naturally generate solitary structures. In this paper, we demonstrate that this
simple model can adequately describe the formation of solitary zonal structures
in the modified Hasegawa-Mima equation, but only when the amplitude of the
coherent drift wave is relatively small. At larger amplitudes, the modulational
instability produces stationary zonal structures instead. Furthermore, we find
that incoherent drift waves with beam-like spectra can also be modulationally
unstable to the formation of solitary or stationary zonal structures, depending
on the beam intensity. Notably, we show that these drift waves can be modeled
as quantumlike particles ("driftons") within a recently developed phase-space
(Wigner-Moyal) formulation, which intuitively depicts the solitary zonal
structures as quasi-monochromatic drifton condensates. Quantumlike effects,
such as diffraction, are essential to these condensates; hence, the latter
cannot be described by wave-kinetic models that are based on the ray
approximation.
