Hail Growth in a Three-Dimensional Cloud Model

Jia-Liu Xu National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

A hailstone growth model is developed. The changes in hailstone density due to varying riming densities are considered. The contributions of evaporation to melting as well as wet growth (which cannot be neglected when the cloud temperature is approximately 0°C) are taken into account. A detailed heat balance equation is used in calculating the surface temperature of the hailstone. Detailed terminal velocity equations are applied in this model.

Data of air velocities, liquid water contents, and temperature were obtained from the three-dimensional dynamic model developed by Clark. Embryos were released at each point in a subgrid of 21 × 22 × 17 grid points within the dynamic model domain. Seven runs were made with various parameters: embryo radii of 0.10, 0.20 and 0.25 cm; embryo densities of 0.4 × 103 and 0.9 × 103 kg m−3; and cloud droplet concentrations of 500 and 106 and 1000 × 106 m−3.

Results show that hailstone growth is sensitive to embryo size, cloud droplet concentration and embryo density. The hail growth mostly occurs on its single ascent to the trajectory top in the present model cloud, which has a strong but near vertical updraft. The maximum-sized stones are only 1.5–2.0 cm in diameter. This is quite different from storms with strongly sloping updrafts where much larger stones can result from recirculation.

Abstract

A hailstone growth model is developed. The changes in hailstone density due to varying riming densities are considered. The contributions of evaporation to melting as well as wet growth (which cannot be neglected when the cloud temperature is approximately 0°C) are taken into account. A detailed heat balance equation is used in calculating the surface temperature of the hailstone. Detailed terminal velocity equations are applied in this model.

Data of air velocities, liquid water contents, and temperature were obtained from the three-dimensional dynamic model developed by Clark. Embryos were released at each point in a subgrid of 21 × 22 × 17 grid points within the dynamic model domain. Seven runs were made with various parameters: embryo radii of 0.10, 0.20 and 0.25 cm; embryo densities of 0.4 × 103 and 0.9 × 103 kg m−3; and cloud droplet concentrations of 500 and 106 and 1000 × 106 m−3.

Results show that hailstone growth is sensitive to embryo size, cloud droplet concentration and embryo density. The hail growth mostly occurs on its single ascent to the trajectory top in the present model cloud, which has a strong but near vertical updraft. The maximum-sized stones are only 1.5–2.0 cm in diameter. This is quite different from storms with strongly sloping updrafts where much larger stones can result from recirculation.

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