On the origin of rotation derived from super rapid scan satellite imagery at the cloud tops of severe deep convection

Abstract

Severe thunderstorms routinely exhibit adjacent maxima and minima in cloud-Top vertical vorticity (CTV) downstream of overshooting tops within flow fields retrieved using sequences of fine-Temporal-resolution (1-min) Geostationary Operational Environmental Satellite (GOES)-R series imagery. Little is known about the origin of this socalled CTV couplet signature, and whether the signature is the result of flow-field derivational artifacts. Thus, the CTV signature s relevance to research and operations is currently ambiguous. Within this study, we explore the origin of nearcloud-top rotation using an idealized supercell numerical model simulation. Employing an advanced dense optical flow algorithm, image stereoscopy, and numerical model background wind approximations, the artifacts common with cloud-Top flow-field derivation are removed from two supercell case studies sampled by GOES-R imagers. It is demonstrated that the CTV couplet originates from tilted and converged horizontal vorticity that is baroclinically generated in the upper levels (above 10 km) immediately downstream of the overshooting top. This baroclinic generation would not be possible without a strong and sustained updraft, implying an indirect relationship to rotationally maintained supercells. Furthermore, it is demonstrated that CTV couplets derived with optical flow algorithms originate from actual rotation within the storm anvils in the case studies explored here, though supercells with opaque above-Anvil cirrus plumes and strong anvil-level negative vertical wind shear may produce rotation signals as an artifact without quality control. Artifact identification and quality control is discussed further here for future research and operations use.