On vortex formation in multicell convective clouds in a shear-free environment

Abstract

The evolution of multicell convective clouds in a shear-free environment investigated with a high resolution explicit microphysical cloud model. After developing from initial thermal perturbations, three clouds attract each other in their low-level inflow circulations and merge to form a single convective cell. An intriguing feature of the simulation is that rotation (vertical vorticity) is generated in the merging clouds without the agency of the Coriolis force or an ambient wind shear. This vorticity is manifested as a counter-rotating vortex pair on the periphery of each of the cloud's updrafts. An analysis of the cloud's vorticity budget reveals that the vortex pairs are produced by the tilting of the clouds' baroclinically-generated azimuthal vorticity into the vertical by the updrafts. Since vertical vorticity cannot be generated by a single axisymmetric cloud growing in a shear-free environment, the vortex pairs are a direct result of cloud-cloud interactions. These interactions enable each cloud to tap into its own reservoir of (horizontal) vorticity. Once generated, the vertical vorticity is amplified by the stretching mechanism in the cloud updraft and advected upward.