Hail Growth Mechanisms in a Colorado Storm: Part II: Hail Formation Processes

Abstract

Hail growth mechanisms in a Colorado multicellular hailstorm were investigated through use of dual-wavelength radar data, three-dimensional wind field data, in situ aircraft measurements and trajectory calculations. Hail production consisted of a transient, pulsating component and a weak, quasi-steady component. Peak hail production resulted when graupel particles and hailstones which grew within the updraft region of a feeder cell were advected in the wind field and fell into the main updraft region where they continued to grow. A minimum in hail production resulted when hail growth was confined to the main updraft region. Recirculation of particles within the main updraft had minor importance in the hail production mechanism. Aggregates of dendritic ice crystals probably comprised most of the hail embyros. These aggregates grew along the forward portion of the storm in a region of precipitation debris associated with decaying cumulus congestus clouds. They were introduced into developing feeder cells and were rapidly converted into graupel and small hail. Water drops, which resulted from the melting of aggregates, comprised some of the embryos of graupel particles which were growing within the main updraft region.