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Jefrey Stith, John Scala, Roger Reinking, and Brooks Martner

Abstract

The results from three methods for studying transport and dispersion in cumuli are compared. These three methods include two tracer techniques and a numerical simulation. The tracers, SF6 and radar chaff, were simultaneously released below the base of a convective storm. The SF6 was measured in situ by two research aircraft and the chaff was followed using TRACIR (tracking air with circular-polarized radar), a method that measures the circular depolarization ratio (CDR) of the chaff, which is much stronger than that of most hydrometeors. TRACIR allows the CDR signal from the chaff to be measured and traced even when the reflectivity from the chaff is much less than that from the cloud. The behavior of the two-tracer release was compared with the trajectories of air from a two-dimensional simulation of the storm, using a nonhydrostatic cloud model, the National Aeronautics and Space Administration/Goddard Cumulus Ensemble Model. By combining information from the three techniques, their individual shortcomings are alleviated, and a more complete documentation of transport and dispersion is provided.

The tracers were followed during a 32-min period as they were transported 6 km vertically by the storm at an average rate of 2.6 m s−1. This was within the ranges of the vertical transport rate of trajectories in the model simulation. The maximum updraft speed measured by the aircraft was 18 m s−1, which agreed well with the maximum updraft in the simulators of 20 m s−1. Both the simulation and the chaff observations show that portions of the released material were incorporated into the cloud and other portions were not.

The main area of downward transport was located in the lower third of the simulated cloud where the rainfall was the heaviest. Major downdrafts were not found in the upper regions of the storm where the aircraft were sampling. The simulation suggests that the precipitation-induced downdraft played the major role in determining the trajectories of air from the cloud base, at least at the mature stage of the storm. Interactions between cloud-base air and downdrafts took place in the lower third of the storm where the bulk of the precipitation was located.

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