A Technique for Investigating Graupel and Hail Development

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  • 1 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

A technique is described for simulating the development of particles in a storm when data on the internal composition and wind-field structures are available. Simulations such as these can be used to investigate particle growth mechanisms.

The method is used to simulate particle development in a hailstorm that occurred in northeastern Colorado on 22 July 1976. Wind fields derived from triple-Doppler radar scans during six periods are used for the calculations and the temporal evolution of the winds during a measurement period is considered. Particles of different types and sizes are initialized at positions throughout the storm at the beginning of the analysis period from radar data using correlations from the in-situ measurements. Particles are “nucleated” within updrafts at later times. Particle growth and sedimentation are calculated according to the habits of particles as they are carried through the storm until they fall to the ground. The liquid water content and drop-size distribution at positions along particle trajectories are calculated from the vertical air velocities. Input parameters of the calculations are varied for the purpose of sensitivity analyses. A data set was compiled of information on positions, sizes, terminal velocities, and other parameters during the development of each of more than 130 000 initialized particles. Information from this data set was compared with available radar, surface and in-situ observations to verify the model inputs and evaluate the simulations.

The calculated manner of particle development in the storm compares favorably with the observed radar, surface and in-situ observations. Several discrepancies between the calculations and the observations are attributed primarily to inadequacies of the wind-field data. Underestimates in the liquid water content could also have accounted for some discrepancies.

Abstract

A technique is described for simulating the development of particles in a storm when data on the internal composition and wind-field structures are available. Simulations such as these can be used to investigate particle growth mechanisms.

The method is used to simulate particle development in a hailstorm that occurred in northeastern Colorado on 22 July 1976. Wind fields derived from triple-Doppler radar scans during six periods are used for the calculations and the temporal evolution of the winds during a measurement period is considered. Particles of different types and sizes are initialized at positions throughout the storm at the beginning of the analysis period from radar data using correlations from the in-situ measurements. Particles are “nucleated” within updrafts at later times. Particle growth and sedimentation are calculated according to the habits of particles as they are carried through the storm until they fall to the ground. The liquid water content and drop-size distribution at positions along particle trajectories are calculated from the vertical air velocities. Input parameters of the calculations are varied for the purpose of sensitivity analyses. A data set was compiled of information on positions, sizes, terminal velocities, and other parameters during the development of each of more than 130 000 initialized particles. Information from this data set was compared with available radar, surface and in-situ observations to verify the model inputs and evaluate the simulations.

The calculated manner of particle development in the storm compares favorably with the observed radar, surface and in-situ observations. Several discrepancies between the calculations and the observations are attributed primarily to inadequacies of the wind-field data. Underestimates in the liquid water content could also have accounted for some discrepancies.

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