Anvils produced by vigorous tropical convection contribute significantly to the earth’s radiation balance, and their radiative properties depend largely on the concentrations and sizes of the ice particles that form them. These microphysical properties are determined to an important extent by the fate of supercooled droplets, with diameters from 3 to about 20 microns, lofted in the updrafts. The present study addresses the question of whether most or all of these droplets are captured by ice particles or if they remain uncollected until arriving at the -38C level where they freeze by homogeneous nucleation, producing high concentrations of very small ice particles that can persist and dominate the albedo.
Aircraft data of ice particle and water droplet size distributions from seven field campaigns at latitudes from 25N to 11S are combined with a numerical model in order to examine the conditions under which significant numbers of supercooled water droplets can be lofted to the homogeneous nucleation level. Microphysical data were collected in pristine to heavily dust-laden maritime environments, isolated convective updrafts, and tropical cyclone updrafts with peak velocities reaching 25 m s21. The cumulative horizontal distance of incloud sampling at temperatures of -20C and below exceeds 50 000 km. Analysis reveals that most of the condensate in these convective updrafts is removed before reaching the -20C level, and the total condensate continues to diminish linearly upward. The amount of condensate in small (<50 micron in diameter) droplets and ice particles, however, increases upward, suggesting new droplet activation with an appreciable radiative impact. Conditions promoting the generation of large numbers of small ice particles through homogeneous ice nucleation include high concentrations of cloud condensation nuclei (sometimes from dust), removal of most of the water substance between cloud base and the -38C levels, and acceleration of the updrafts at mid- and upper levels such that velocities exceed 5–7 m s-1.
