Processes of Hydrometeor Development in Oklahoma Convective Clouds

Abstract

This study employs in situ measurements to examine cloud conditions in which hydrometeors develop in mature Oklahoma convective clouds and to develop hypotheses as to how they formed. The measurements were made from penetrations on six days using a T-28 aircraft. Values of the maximum vertical velocity W in cells ranged from 5 to 35 m s−1, and the liquid water content (LWC) up to 6 gmminus;3;LWCs are usually less than adiabatic. Drops are found primarily in strong updrafts at T/>−8°c. Graupel are present in low concentrations in the strong updrafts and in moderate concentrations in the weak to intermediate updrafts. Planar and needle ice crystals and aggregates are present in copious concentrations in regions of low LWC and W. Strong evidence exists for production of secondary ice crystals (SICS) through a Hallett and Mossop type of mechanism involving cloud droplets >24μm in diameter. Particle growth calculations are used in conjunction with the measurements to infer the processes of formation of drops, graupel and hail, and secondary ice crystals. Most drops of diameters <500μm found at temperatures below 0°C are inferred to form through coalescence growth and most of diameters >500μm through shedding from growing and/or melting graupel and hail. Embryos of hailstones are found to develop to 1 cm in diameter most rapidly from millimetric size drops produced from shedding and from aggregates of planar ice-crystals. Most growth of particles to 1 cm hailstones occurs in the regions of intermediate values of LWC (1-2 gm−3) and W (5–15 m sminus;1) at temperatures higher than −20°C. In these regions, moderate concentrations of ice particles can develop over appreciable periods and depletion of the liquid water content due to collection by ice particles is minimal. The regions of high LWC and W are found to be the least conducive to SIC production. Initially, most SICs come from riming of aggregates in clouds which develop embedded within cloud layers and from frozen drops in clouds which develop in isolation. The SICs themselves are found to produce abundant SICs in regions of low LWC and W. Secondary ice crystal production is found to be more copious in embedded than in isolated clouds.