Vertical-Plane Dual-Doppler Radar Observations of Cumulus Toroidal Circulations

Abstract

igh-resolution vertical-plane dual-Doppler velocity data, collected by an airborne profiling cloud radar in transects across nonprecipitating orographic cumulus clouds, are used to examine vortical circulations near cloud top. These vortices are part of a toroidal ring centered at an updraft, usually near the cloud top, and they are essential to cumulus entrainment and dynamics. A large number of transects across toroidal circulations are composited to reveal the typical kinematic structure and associated entrainment patterns. The toroidal ring circulation is ~1 km wide and about half as deep in the sampled clouds (Cu mediocris). The composite flow field shows two nearly symmetric, counterrotating vortices, with a core updraft of ~3 m s?1, consistent vortex-top divergence, two flanking downdrafts of the about same strength, and horizontal (toroidal) vorticity of ~0.03 s?1. Variations with vortex size, age, and ambient shear are examined, and the relative dilution of air in the vortex core is estimated by comparing the liquid water content, estimated from path-integrated power attenuation, with the adiabatic value.