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A Numerical Experiment on the Growth and Feedback Mechanisms of Hailstones in a One-Dimensional Steady-State Model Cloud

R. ListDept. of Physics, University of Toronto, Canada

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R. B. CharltonDept. of Physics, University of Toronto, Canada

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P. I. ButtulsDept. of Physics, University of Toronto, Canada

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Abstract

Calculations are made on the growth of hailstone embryos of given size and concentration which are injected into a one-dimensional steady-state updraft, and grow while ascending, the updraft obeying the condition that ρVs is a constant. The growth was found to have a considerable effect on the free water content of the cloud due to depletion by the growing particles. The hailstones of this model generally reach biggest sizes if their concentration is low and if the embryos are as big as possible. Embryos of 5 mm diameter can grow to 2.5–3.0 cm in diameter within 8–12 min if the conditions are right.

It is further shown that thermal feedback is of great importance in calculating the cloud temperature since it greatly affects buoyancy and icing conditions; in this case, the frictional heating of the falling hydrometeors has to be included along with the heat of fusion. The buoyancy is investigated because it is necessary to decide which set of input parameters for the growth curves and the free water contents distributions is reasonable. For those hailclouds where hailstones grow while ascending, it may be concluded that the biggest updrafts do not necessarily produce the biggest hailstones. The icing conditions of the growing particles turned out to be such that the outermost layers of the biggest stones always grow non-spongy.

Abstract

Calculations are made on the growth of hailstone embryos of given size and concentration which are injected into a one-dimensional steady-state updraft, and grow while ascending, the updraft obeying the condition that ρVs is a constant. The growth was found to have a considerable effect on the free water content of the cloud due to depletion by the growing particles. The hailstones of this model generally reach biggest sizes if their concentration is low and if the embryos are as big as possible. Embryos of 5 mm diameter can grow to 2.5–3.0 cm in diameter within 8–12 min if the conditions are right.

It is further shown that thermal feedback is of great importance in calculating the cloud temperature since it greatly affects buoyancy and icing conditions; in this case, the frictional heating of the falling hydrometeors has to be included along with the heat of fusion. The buoyancy is investigated because it is necessary to decide which set of input parameters for the growth curves and the free water contents distributions is reasonable. For those hailclouds where hailstones grow while ascending, it may be concluded that the biggest updrafts do not necessarily produce the biggest hailstones. The icing conditions of the growing particles turned out to be such that the outermost layers of the biggest stones always grow non-spongy.

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