Observations of particle size distributions of single crystal and aggregate frozen hydrometeors

Abstract

Currently, Greenland ice sheet mass loss is increasing at a rate near 1000 Gt yr-1, making the ice sheets top contributors to current sea level rise. Errors in constraining the mass balance of the Antarctic and Greenland ice sheets are primarily from measurement of frozen precipitation. In this study, we use the Multi-Angle Snowflake Camera (MASC) to capture images of falling snow in Boulder, Colorado. The MASC contains three cameras positioned on adjacent edges of a decagon, which provides one of the first instruments capable of collecting three-dimensional information of frozen hydrometeors. We created autonomous image analysis software designed specifically to measure snowflake particles from image triplets recorded by the MASC. After processing snowflake measurements are organized and presented in particle size distributions, which show the expected number of particles with a given size. Particle size distributions for precipitation have previously been presented as exponential distributions that vary with precipitation rate (mass/time). We found evidence indicating the shape of the distribution to exhibit a dependence on the presence of aggregation. Our observations of non-aggregate particles follow a log-normal distribution, while aggregates adhere to an exponential distribution. Therefore, we propose snowfall size distributions to be a combination of log-normal and exponential size distributions depending on the relative fraction of aggregated versus non-aggregated particles.