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

Abstract

Ground-based measurements of particle size and fall speed distributions using a Particle Size and Velocity (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 temperatures 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 Ͼ 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 Ն 2.4 mm is 2 m s Ϫ1 with a standard deviation of 0.8 m s Ϫ1 . The large standard deviation is likely related to the coexistence of particles of similar physical size with different percentages of melting. These results suggest that different particle sizes are not required for aggregation since wet-snow particles of the same size can have different fall speeds. Given the large standard deviation of fall speeds in wet snow, the collision efficiency for wet snow is likely larger than that of dry snow. For particle sizes between 1 and 10 mm in diameter within mixed precipitation, rain constituted 1% of the particles by volume within the isothermal layer at 0°C and 4% of the particles by volume for the region just below the isothermal layer where air temperatures rise from 0°to 0.5°C. As air temperatures increased above 0.5°C, the relative proportions of rain versus snow particles shift dramatically and raindrops become dominant. The value of 0.5°C for the sharp transition in volume fraction from snow to rain is slightly lower than the range from 1.1°to 1.7°C often used in hydrological models.

Statistical Characteristics of Raindrop Size Distribution in the Meiyu Season Observed in Eastern China

“…PARSIVEL detects and identifies eight different precipitation types as drizzle, mixed drizzle/rain, rain, mixed rain/snow, snow, snow grains, ice pellets, and hail, according to the WMO, SYNOP, METAR, and NWS weather codes. Yuter et al (2006) confirmed that PARSIVEL can be employed as a weather sensor because of its capability to distinguish between solid and liquid precipitation. Recently, critical evaluations on PARSIVEL measurements were performed by Battaglia et al (2010), Thurai et al (2011).…”

Observational and Synoptic Analyses of the Winter Precipitation Regime Change over Utah

“…In a more recent study, Dai (2008) used 3-hourly weather reports and found that the phase transition from snow to rain over land occurred within a wide range of temperatures from 22 to 48C. Moreover, other studies (Yuter et al 2006;Lundquist et al 2008;Jones 2010) have noted that a 18C wetbulb temperatures around 618C were reasonable limits within which to determine mixed phases of snow and rain.…”

Polarimetric radar observations during an orographic rain event

“…We find fall velocities on the order of −3 m s −1 between approximately 11:00 and 12:00 UTC, which cannot be attributed to dry snow. Based on the fall velocity, magnitude is characteristic for wet snow or graupel (Yuter et al, 2006). Note that the melting layer is below this mesoscale feature.…”