  We study the dynamics of seeded plasma blobs and depletions in an (effective)
gravitational field. For incompressible flows the radial center of mass
velocity of blobs and depletions is proportional to the square root of their
initial cross-field size and amplitude. If the flows are compressible, this
scaling holds only for ratios of amplitude to size larger than a critical
value. Otherwise, the maximum blob and depletion velocity depends linearly on
the initial amplitude and is independent of size. In both cases the
acceleration of blobs and depletions depends on their initial amplitude
relative to the background plasma density, is proportional to gravity and
independent of their cross-field size. Due to their reduced inertia plasma
depletions accelerate more quickly than the corresponding blobs. These scaling
laws are derived from the invariants of the governing drift-fluid equations and
agree excellently with numerical simulations over five orders of magnitude. We
suggest an empirical model that unifies and correctly captures the radial
acceleration and maximum velocities of both blobs and depletions.
