Effects of cloud-droplet spectra on the average surface-temperature of ice accreted on fixed cylindrical collectors

An experimental study of the heat balance of a stationary cylindrical collector accreting supercooled water droplets has shown a new dependence on the sizes of the water droplets in the laboratory cloud. In a study in 1967, Macklin and Payne related the steady-state heat-release during accretion to the heat loss by convection and conduction; their equation involved a numerical factor chi for which they assumed a value of 0.28. The present study has shown that chi is a function of the droplet spectrum with values around 0.5 at a velocity of 4 m s(-1) for a mean volume-weighted droplet-diameter of 18 mu m, and that chi approaches 0.3 for droplets greater than 30 mu m diameter. The results have importance to the surface temperatures of riming graupel pellets and hailstones in convective storms which, in the presence of small water-droplets, will be heated to a smaller degree than has been assumed in current models of hailstone growth. Furthermore, the effect of droplet size may influence ice-crystal multiplication and charge transfer in convective clouds, both of which are highly temperature dependent.