Ice Particle Evolution in the Anvil of a Severe Thunderstorm during CCOPE

Measurements from the National Center for Atmospheric Research Sabreliner aircraft are combined with a multiple Doppler radar synthesis of the wind field to investigate particle growth processes in the anvil region of a severe thunderstorm. The aircraft measurements, obtained in mid- to lower anvil levels at temperatures from −25° to −36°C, show than the size spectra broaden with decreasing altitude, yet ice water content values and other measurements indicate this to be a zone of evaporation. Aggregation—allowing particles to develop to sizes as 1 cm—accounts for this observed growth. Intensification of the storm over the observational period results in the development of increasingly large aggregates. Growth histories and trajectories of anvil particles are calculated using the wind field measurements and are compared to the hydrometeor measurements. Calculations show that particles forming the outer flanks of the anvil initiate in the upwind (relative to the midlevel wind) portions of the updraft and grow larger than those forming the inner anvil region which initiate in the updraft core and further downwind. Thus, the location and size of particles relative to the anvil axis are strongly influenced by the initial particle position. Calculations show that crystal collisions can account for the significant growth through aggregation observed in the anvil.