Chaff Tagging for Tracking the Evolution of Cloud Parcels

Abstract

A means to mark and track a moving and mixing volume within a cloud is useful for estimating the transport and dispersion of seeding material or other aerosols and thus for determining where and when the microphysical effects of the aerosols should occur.  A proven approach has been to release and track chaff into a cloud and track it with circular polarization radar to estimate the rates of dispersion, loft, and dilution of ingested aerosol, as well as the filling of the cloud volume.  The chaff, an effective tracer, is separable from the cloud because it strongly depolarizes the radar's signal.  A primary disadvantage of this cloud-volume-filling approach is that radar measurements of the microphysical evolution of the tracked cloud parcels may be masked by the chaff itself.  To measure transport and dispersion in the cloud and simultaneously unambiguously examine the microphysical evolution in the same volume, a refined approach was devised to tag but not fill the cloud volume with chaff. Several experimental designs for chaff tagging are proposed for stratiform or gravity-wave clouds and for cumuliform clouds.  The tagging technique will be more difficult to apply in convective clouds and is not yet tested.  However, a "parallel lines" experiment designed for stratiform clouds was tested with hygroscopic seeding in a wave cloud with intrusions of convective cloud.  The chaff lines were tracked for about one hour despite some ingestion by convective elements.  This demonstrated that a seeded cloud volume could be bracketed and tracked effectively.  A hygroscopic seeding signature, anticipated to appear as a line of enhanced reflectivity along a seeded flight path between the chaff lines, was not detected.  A "tagged circle" experiment with silver iodide seeding was conducted in a precipitation wave cloud.  A seeded circle was tagged with arcs of chaff, allowing it to be tracked efficiently.  A seeding effect was indicated along the circular seeded path between the chaff arcs as signatures of enhanced reflectivity and values of depolarization indicative of pristine, new ice crystals.  The results of these experiments demonstrate the potential of the chaff tagging technique.