Atmospheric Vortices in Shallow Convection

Abstract

Observations of funnel clouds over Port Phillip Bay, Victoria, Australia, indicate that they occur during outbreaks of cool air from the Southern Ocean advecting over the relatively warm bay waters. These clouds act as tracers for shallow convection vortices with dynamics similar to large dust devils. The related phenomena of waterspouts and tornadoes differ from these vortices by requiring deep convection and downdraft and updraft interactions associated with rain processes. Deardorff (1978a) suggests that a necessary condition for the formation of dust devils is −/L of the order of 100 or more, where h is the convective boundary layer height and L the Obukhov length. Calculations of −/L over the bay and over land for the days of observation are consistent with this suggestion. They indicate that significant rotation may occur at −/L as low as 50. This information, if confirmed, may make it possible to use boundary layer numerical models to forecast likely conditions of dust devil occurrence over mesoscale regions, which would be of benefit to pilots of light aircraft and helicopters.