Scale-Interaction Implications for the Waterspout Life Cycle. II

Abstract

The high formation frequency and previously documented life cycle of Florida Keys' waterspouts result from energy and angular momentum cascades through five scales of atmospheric motion, namely the funnel, spiral, cumulus, cloud-line and synoptic scales. The funnel scale appears to be the most complicated in that pronounced horizontal and vertical flow asymmetries in the spray vortex and funnel-cloud walls have been quantitatively analyzed. Waterspouts are apparently a two-cell type of radial vertical circulation. The spiral scale of motion is first defined by sea surface spiral patterns during stage 2 of the waterspout life cycle. The spiral scale is important because it marks the primary waterspout growth phase and gives the first visual evidence of boundary layer rotation and inflow. The single-cumulus scale is important in providing updrafts and shower outflow for concentrating larger-scale vorticity. Motions on the cloud-line scale are perhaps most crucial for waterspout formation. Less than 5% of all 1969–70 waterspouts were spawned by isolated cumulus clouds. Individual shower cells in a cloud-line protect one another from dilution by entrainment, and shower outflows interact to further concentrate the weak large-scale positive vorticity. In some cases the shear across shower-induced wind-shift lines may be the vorticity source for waterspouts. Finally, the fifth interacting scale of motion, the synoptic scale, is shown to exert a controlling influence on convective cloud-line developments in the Lower Keys.