Size Sorting of Hail in a Three-Dimensional Updraft and Implications for Hail Suppression

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  • 1 National Center for Atmospheric Research, Boulder, Colo. 80307
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Abstract

The size-sorting patterns of hail trajectories are examined in regard to their potential for hail suppression through beneficial competition and the induced lowering of hail trajectories. The trajectories are computed within a three-dimensional velocity field obtained from dual-Doppler radars. The velocity field is assumed to be in a steady state. The cloud microphysical parameters used in the computations are typical of Colorado storms, which are highly continental.

The analysis shows that if seeding is done so as to introduce large numbers of millimeter size ice particles above the −15°C level then hail can be suppressed on one side of this updraft while the amount of large hail growing on the other updraft side is increased. Seeding at low levels with hygroscopic nuclei so as to induce lowering of hail trajectories can be effective in reducing the amount of hail on the ground. However, there is no practical way to achieve total hail suppression in this updraft without the risk of producing more hail in the event of underseeding.

Any updraft characterized by lower level convergence, upper level divergence and vorticity in the flow can be expected to size sort hail in a variety of patterns. Several of these patterns can in some seeding situations produce more large hail on the ground. Seeding to suppress hail in the highly continental cumulus is associated with the risk of producing more hail.

Abstract

The size-sorting patterns of hail trajectories are examined in regard to their potential for hail suppression through beneficial competition and the induced lowering of hail trajectories. The trajectories are computed within a three-dimensional velocity field obtained from dual-Doppler radars. The velocity field is assumed to be in a steady state. The cloud microphysical parameters used in the computations are typical of Colorado storms, which are highly continental.

The analysis shows that if seeding is done so as to introduce large numbers of millimeter size ice particles above the −15°C level then hail can be suppressed on one side of this updraft while the amount of large hail growing on the other updraft side is increased. Seeding at low levels with hygroscopic nuclei so as to induce lowering of hail trajectories can be effective in reducing the amount of hail on the ground. However, there is no practical way to achieve total hail suppression in this updraft without the risk of producing more hail in the event of underseeding.

Any updraft characterized by lower level convergence, upper level divergence and vorticity in the flow can be expected to size sort hail in a variety of patterns. Several of these patterns can in some seeding situations produce more large hail on the ground. Seeding to suppress hail in the highly continental cumulus is associated with the risk of producing more hail.

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