Dual-Doppler and Multiparameter Radar Observations of a Bow-Echo Hailstorm

Patrick C. Kennedy Department of Atmospheric Science, Colorado State University, fort Collins, Colorado

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Steven A. Rutledge Department of Atmospheric Science, Colorado State University, fort Collins, Colorado

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Abstract

During the afternoon and evening hours of 23 May 1991 a hail-producing multicellular severe thunderstorm developed near Denver, Colorado, and tracked eastward for more than 100 km. Along this path, hailstone diameters of 2–7 cm (0.75–2.75 in.) were reported at several points. The storm was observed by both the CSU-CHILL (CHL) and NCAR Mile High (MUR) 10-cm Doppler radars. The general echo morphology evolved by way of cyclic, discrete new cell formation near an outflow boundary moving ahead of the storm's forward flank. As this new cell growth occurred, the shape of the storm's most intense core also evolved in a periodic fashion. On four separate occasions these cores briefly assumed a bow shape with peak reflectivity values of 65–70 dBZ. The evolution of one such bow echo was examined by a series of six CHL-MHR dual-Doppler analyses. The resultant airflow patterns suggested that the core reflectivity structure was deformed into the bowlike configuration by updraft-induced flow field perturbations. During the period covered by the dual-Doppler analyses, dual polarization measurements made by the CSU-CHILL radar were used to infer hail characteristics by placing the differential reflectivity (ZDR) and zero-lag cross correlation between horizontally and vertically polarized echoes [ρHV(0)] observations in the context of the synthesized wind fields. These polarimetric data suggest that the areal coverage of the hail and the diameter of the hailstones both maximized during the bow-echo phase.

Abstract

During the afternoon and evening hours of 23 May 1991 a hail-producing multicellular severe thunderstorm developed near Denver, Colorado, and tracked eastward for more than 100 km. Along this path, hailstone diameters of 2–7 cm (0.75–2.75 in.) were reported at several points. The storm was observed by both the CSU-CHILL (CHL) and NCAR Mile High (MUR) 10-cm Doppler radars. The general echo morphology evolved by way of cyclic, discrete new cell formation near an outflow boundary moving ahead of the storm's forward flank. As this new cell growth occurred, the shape of the storm's most intense core also evolved in a periodic fashion. On four separate occasions these cores briefly assumed a bow shape with peak reflectivity values of 65–70 dBZ. The evolution of one such bow echo was examined by a series of six CHL-MHR dual-Doppler analyses. The resultant airflow patterns suggested that the core reflectivity structure was deformed into the bowlike configuration by updraft-induced flow field perturbations. During the period covered by the dual-Doppler analyses, dual polarization measurements made by the CSU-CHILL radar were used to infer hail characteristics by placing the differential reflectivity (ZDR) and zero-lag cross correlation between horizontally and vertically polarized echoes [ρHV(0)] observations in the context of the synthesized wind fields. These polarimetric data suggest that the areal coverage of the hail and the diameter of the hailstones both maximized during the bow-echo phase.

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