Natural Variability during Snowfall: Observations of Snowflake Microstructure and Calculations of Corresponding Snowfall Scattering Properties

Abstract:

Significant progress has been achieved in approximating snowflakes and ice-cloud particles by increasingly more realistic and detailed shape models and in calculating associated scattering properties crucial to snowfall remote sensing. The applied approximations of the snowflake microstructure applied for the scattering calculations, however, are still based on few available field measurement data, often integrated over many individual snow storms, and only include several microstructural properties that cannot fully capture the natural variability during snowfall, e.g. different degrees of riming or aggregate snowflakes formed from more than one distinct ice crystal habit. In this study, (i) the natural variability of key microstructural properties during snowfall is quantified for individual snow storms based on high-resolution multi-view snowflake imaging data collected with the Multi-Angle Snowflake Camera (MASC) at Alta ski area (Alta, UT), and (ii) the corresponding variability in snowflake scattering properties is calculated. In addition to snowflake size, orientation and aspect ratio, 'particle complexity' (specifying snowflake perimeter and brightness variations in the MASC snowflake images) is included in the presented approach, yielding a quantitative and objective measure of characteristic snowflake microstructure, including crystal habit and degree of riming, important for realistically modelling snowfall scattering properties. The aim is to present an analysis of the impact of the observed natural microstructural variability on the derived snowflake scattering properties and ultimately on the snowfall radar reflectivity integrated over the obtained variability of snowflake microstructure and scattering properties.