The Growth of Snow in Winter Storms:. An Airborne Observational Study

Abstract

In studies of precipitation growth, comparisons between theory and observation are difficult because of the problem in obtaining a complete 4-dimensional (space and time) description of the kinematic, thermodynamic and microphysical properties of the atmosphere. A new flight plan has been devised which permits one to observe the height evolution of snow-size spectra in a reference frame where the effects of horizontal gradients and temporal changes are minimized. The flight plan, termed the advecting spiral descent (ASD), requires an aircraft to start aloft in a mesoscale precipitation area and then spiral downward in a constant bank angle, descending at approximately the mean fallspeed of snow. Qualitative comparisons between ASD observations and particle growth theory suggest that snow evolves through at least three stages characterized by deposition, aggregation and breakup. The breakup process serves to limit the number of large snow particles and interacts with aggregation to produce a limiting value of the slope of the snow-size spectrum.