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Dynamical-Microphysical Evolution of a Convective Storm in a Weakly-Sheared Environment. Part II: Airflow and Precipitation Trajectories from Doppler Radar Observations

L. Jay MillerNational Center for Atmospheric Research, Boulder, CO 80307

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James E. DyeNational Center for Atmospheric Research, Boulder, CO 80307

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Brooks E. MartnerDepartment of Atmospheric Science, University of Wyoming, Laramie 82071

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Abstract

Measurements from three Doppler radars of air motion and observations of the environment and storm reflectivity structure, supplemented by aircraft measurements of precipitation and cloud particles, are used to establish the dynamical framework for precipitation development in a convective storm that grew in a weakly-sheared wind environment. The moderately intense, evolving storm consisted of a series of cells that developed in late afternoon on 25 July 1976 in southeastern Wyoming. The storm, which moved along the sub-cloud wind direction, had a persistent but unsteady updraft region on its right forward flank. This updraft region consisted of several small convective elements with two or more intense updraft cores evident at all times. Middle-level flow around the updraft region eventually resembled obstacle flow with downdrafts located on the flanks and in the wake of the updraft. This storm-wide, organized circulation apparently allowed precipitation particles to reenter an updraft and grow for periods longer than would have been possible if all their growth had occurred in a single ascent within an updraft core of 10 to 20 m s−1 speeds. Such vertical motions would have carried particles to cloud top in 5 to 10 min, a growth period too short to account for the observed millimeter-size particles in the updraft. This storm lasted for more than one hour and produced hail particles as large as 9 mm diameter that were observed at cloud base by aircraft.

Abstract

Measurements from three Doppler radars of air motion and observations of the environment and storm reflectivity structure, supplemented by aircraft measurements of precipitation and cloud particles, are used to establish the dynamical framework for precipitation development in a convective storm that grew in a weakly-sheared wind environment. The moderately intense, evolving storm consisted of a series of cells that developed in late afternoon on 25 July 1976 in southeastern Wyoming. The storm, which moved along the sub-cloud wind direction, had a persistent but unsteady updraft region on its right forward flank. This updraft region consisted of several small convective elements with two or more intense updraft cores evident at all times. Middle-level flow around the updraft region eventually resembled obstacle flow with downdrafts located on the flanks and in the wake of the updraft. This storm-wide, organized circulation apparently allowed precipitation particles to reenter an updraft and grow for periods longer than would have been possible if all their growth had occurred in a single ascent within an updraft core of 10 to 20 m s−1 speeds. Such vertical motions would have carried particles to cloud top in 5 to 10 min, a growth period too short to account for the observed millimeter-size particles in the updraft. This storm lasted for more than one hour and produced hail particles as large as 9 mm diameter that were observed at cloud base by aircraft.

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