Microphysical Properties of Snowfall in the Swiss Alps as derived from Collocated Multi-Angle Snowflake Camera and W-band Cloud Profiler Measurements

Remote sensing of snowfall remains a challenge because of the rich variety of shapes, sizes and other properties that snowflakes and ice crystals can adopt in the atmosphere. Snowfall rate can be estimated from weather radar using relationships between solid hydrometeors microstructure (size, shape, mass) and their scattering properties. These relationships are difficult to estimate and remain largely uncertain, mainly due to the natural variability in these properties. In order to better characterize the microphysics of snowfall as well as improve its quantitative estimation using remote sensing, it is essential to document the microstructural properties of individual falling snowflakes. The Multi-Angle Snowflake Camera (MASC) is a ground based snowflake imager and is among the few instruments able to provide such information.

In this study, we utilized a new supervised classification method applied on pictures recorded with a MASC in order to classify observed particles into 6 distinct hydrometeor classes (columnar crystals, planar crystals, combination of columnar and planar crystals, aggregates, graupels and small particles) and estimate their degree of riming on a continuous scale ranging from zero (no riming) to one (graupel). The classification is performed on more than 8 months of MASC data collected in the Swiss Alps. The outcome is in turn used to investigate important microstructural properties of falling snowflakes (particle size, aspect ratio, shape complexity, orientation, fallspeed) and refine the relationships between them (e.g. shape-size, fallspeed-size) as a function of the hydrometeor type and degree of riming. Discrepancies between the microstructural properties observed whether if the MASC is deployed in a still environment (within a Double-Fence Intercomparison Reference) or in open air will be discussed.

From January 2017 to June 2017, the MASC and a 94 GHz cloud profiling radar (RPG-FMCW-94) were deployed in the Swiss Alps at an elevation of 2372 m asl. A special feature of RPG-FMCW-94 is that it includes a passive radiometer channel centered at 89 GHz which can be used to estimate the integrated liquid water path (LWP) in the column above the radar and therefore provides relevant information for the identification and quantification of supercooled liquid water in the clouds. Preliminary analysis of the riming as observed by these instruments will also be presented.