EXPERIMENTAL CLOSURE OF THE HEAT AND MASS-TRANSFER THEORY OF SPHEROIDAL HAILSTONES

Hailstone growth experiments were performed in a vertical icing wind t unnel using 2-cm oblate ice spheroids (axis ratio of 0.67) mounted on a gyrator system. The liquid water content ranged from 1 to 5 g m(-3), air temperature from -21 degrees to -3 degrees C, air speed from 9 to 24 m s(-1),and air pressure from 40 to 100 kPa. Icing time, ice and w ater mass of the hailstone deposit, and final major and minor axis dia meters were measured to determine the accretion of supercooled droplet s from the air flow. An infrared imaging system was used to measure lo cal and mean hailstone surface temperatures. These experiments allowed calculation of the last two unknowns in the heat and mass transfer eq uations for spheroidal hailstones: the net collection efficiency, E(ne t) = 0.59 K-0.15 (over a Stokes parameter range of 6 less than or equa l to K less than or equal to 18), and the Nusselt number, Nu = 0.15 Re -0.69 (over a Reynolds number range of 13 000 less than or equal to Re less than or equal to 50 000, that is, freely falling hailstones with diameters of similar to 1 to similar to 3 cm). The net collection eff iciency results are consistent with previous investigations. The Nusse lt number for spheroids, a measure of heat transfer by convection and conduction, with its built-in shape, chi, and roughness factor, theta, is 45%-65% larger than Nu for smooth spheres and 25%-45% larger than Nu for rough, melting spherical hailstones. Approximately 20% of the i ncrease is due to particle shape and the remainder to roughness. These results can be used to improve computer models of convective storms. In addition, the fourfold increase in Nusselt number with increasing l iquid water content previously reported is attributed in this study to uncertainties caused by a combination of shape change, shedding, and low ice fraction at air temperatures close to 0 degrees C.