A Case Study of the Origin of Hail in a Multicell Thunderstorm Using In Situ Aircraft and Polarimetric Radar Data.
An armored T-28 research aircraft made direct observations of the hydrometeors present at approximately the -3°C temperature level in the inflow region of a multicell thunderstorm. During the penetration, both the Colorado State University (CSU) University of Chicago and Illinois State Water Survey (CHILL) 11-cm-wavelength dual-polarization research radar and the Denver, Colorado, Front Range Airport (KFTG) Weather Surveillance Radar-1988 Doppler (WSR-88D) were scanning this storm. Polarimetric radar indications of hail (high reflectivity and low differential reflectivity) appeared near the surface in the echo core adjacent to the aircraft track approximately 6 min after the T-28's inflow transit. Radial velocity data from the KFTG radar were combined with those recorded at CSU CHILL to synthesize the airflow fields in the storm around the time of the T-28 penetration. Hail trajectories were initiated from a location at which the T-28 encountered a burst of approximately 1-cm-diameter, low-density graupel particles within the general storm inflow region. Forward-time trajectory calculations indicated that these graupel particles subsequently grew slightly into small hailstones and ended up within a few kilometers of the near-surface polarimetric radar hail-signature location. Trajectories computed backward in time imply that these hail embryos originated aloft in the forward portion of the echo complex. These are the first quantitative, direct in situ observations of recirculating precipitation becoming embryos for hail development.