Rayleigh-Taylor instability in rotating volcanic umbrellas

We study the shape of an expanding volcanic umbrella that is turbulent and is rotating about a vertical axis though the center of the umbrella. We argue that the centrifugal forces associated with the rotation of the umbrella trigger a turbulent variant of the Rayleigh--Taylor instability. As a manifestation of this instability, the edge of the umbrella becomes wavy. As a test case, we consider the wavy umbrella from the climactic eruption of Mount Pinatubo on June 15, 1991.