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James E. Dye
and
John C. Willett

Abstract

A study of two long-lived Florida anvils showed that reflectivity >20 dBZ increased in area, thickness, and sometimes magnitude at the midlevel well downstream of the convective cores. In these same regions electric fields maintained strengths >10 kV m−1 for many tens of minutes and became quite uniform over tens of kilometers. Millimetric aggregates persisted at 9–10 km for extended times and distances. Aggregation of ice particles enhanced by the strong electric fields might have contributed to reflectivity growth in the early anvil, but is unlikely to explain observations farther out in the anvil. The enhanced reflectivity and existence of small, medium, and large ice particles far out into the anvil suggest that an updraft was acting, perhaps in weak convective cells formed by instability generated from the evaporation and melting of falling ice particles. It is concluded that charge separation must have occurred in these anvils, perhaps at the melting level but also at higher altitudes, in order to maintain fields >10 kV m−1 at 9–10 km for extended periods of time over large distances. The authors speculate that charge separation occurred as a result of ice–ice particle collisions (without supercooled water being present) via either a noninductive or perhaps even an inductive mechanism, given the observed broad ice particle spectra, the strong preexisting electric fields, and the many tens of minutes available for particle interactions. The observations, particularly in the early anvil, show that the charge structure in these anvils was quite complex.

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Timothy J. Lang
,
Steven A. Rutledge
,
James E. Dye
,
Martin Venticinque
,
Pierre Laroche
, and
Eric Defer

Abstract

Concurrent measurements from the CSU-CHILL multiparameter Doppler radar, the Office National d’Etudes et de Recherches Aérospatiales VHF lightning interferometer, and the National Lightning Detection Network, obtained during phase A of the Stratosphere–Troposphere Experiments: Radiation, Aerosols, Ozone (STERAO-A) field project, provided a unique dataset with which to study the relationships between convective storm microphysics and associated lightning. Two storms have been examined in detail in this study: 10 and 12 July 1996. Both storms were long lived, existing in some form for over 4 h apiece, and produced very low cloud-to-ground (CG) lightning flash rates, in particular negative CG flash rates (generally <1 min−1 and often no CG flashes for periods ranging from 10 to almost 30 min), during all or a portion of their lifetimes while simultaneously producing relatively high intracloud (IC) flash rates (>30 min−1 at peak). For both storms, radar reflectivity intensity and the production of hail were anticorrelated with the production of significant negative cloud-to-ground lightning. These observations are shown to be consistent with an elevated charge hypothesis and suggest a possible way of correlating updraft speed, hail, and storm severity to CG and IC flash rates.

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