Observations of Precipitation Size and Fall Speed Characteristics within Coexisting Rain and Wet

Ground-based measurements of particle size and fall speed distributions using a Particle Size and Ve-locity (PARSIVEL) disdrometer are compared among samples obtained in mixed precipitation (rain and wet snow) and rain in the Oregon Cascade Mountains and in dry snow in the Rocky Mountains of Colorado. Coexisting rain and snow particles are distinguished using a classification method based on their size and fall speed properties. The bimodal distribution of the particles ’ joint fall speed–size characteristics at air tem-peratures from 0.5 ° to 0°C suggests that wet-snow particles quickly make a transition to rain once melting has progressed sufficiently. As air temperatures increase to 1.5°C, the reduction in the number of very large aggregates with a diameter 10 mm coincides with the appearance of rain particles larger than 6 mm. In this setting, very large raindrops appear to be the result of aggregrates melting with minimal breakup rather than formation by coalescence. In contrast to dry snow and rain, the fall speed for wet snow has a much weaker correlation between increasing size and increasing fall speed. Wet snow has a larger standard deviation of fall speed (120%–230 % relative to dry snow) for a given particle size. The average fall speed for observed wet-snow particles with a diameter 2.4 mm is 2 m s1 with a standard deviation of 0.8 m s1. The large standard deviation is likely related to the coexistence of particles of similar physical size wit