Radar Echo Structure, Air Motion and Hail Formation in a Large Stationary Multicellular Thunderstorm

Abstract

Observations from aircraft, Doppler radars, surface mesonetwork, upper air network and surveillance radar are used to describe the structure and evolution of a large rain and hailstorm that occurred on 22 June 1976 in northeastern Colorado. In its early stage, the storm was unicellular in nature and was moving northward somewhat slower than the middle-level environmental winds. Once significant outflow developed beneath the storm, new cells started their growth over the southeastwardly progressing outflow boundary and then advected northward and interacted with the now nearly motionless main updraft. For about 45 min the storm exhibited characteristics of the steady, vaulted supercell, though overall it was still clearly multicellular in nature. During this stage nearly 2.5?109 kg s?1 of air and 2?107 kg s?1 of moisture were being processed through cloud base within the vault updraft. Precipitation that grew to millimeter size within the small (about 5 km diameter) cells served as embryos for continued growth to hailstones as big as 3 cm diameter within the main updraft associated with the weak echo vault. Calculated ice particle growth trajectories in the nearly steady phase indicated that recycling of precipitation particles within a single updraft was not possible, however, as all particles followed simple paths northward across and to the sides of the main updraft core, failing out as rain, graupel or hail on the north and west sides of the updraft. Small particles that entered the main updraft where speeds exceeded 15 m s?1 were exhausted into the anvil as 5?8 mm graupel.