Radar and in-situ observations of rimed precipitation in alpine snowfall

Riming is a turbulent processes occurring in mixed-phase environments and it is among the most effective mechanisms that increase the mass of precipitation. Multiple remote sensing and in-situ instruments are necessary to properly document the interaction between ice crystals and liquid cloud droplets leading in the end to rimed precipitation. The cloud and aerosol characterization experiment (CLACE) is a series of campaigns conducted in the Swiss Alps by research groups from many countries, focused on aereosol-cloud activation and ice nucleation in mixed-phase clouds. During CLACE 2014, the X-band Doppler dual polarization radar (MXPol) of the Environmental Remote Sensing laboratory (LTE) of EPFL, Lausanne, was deployed at an altitude of 2065 m at the Kleine Scheidegg pass. At the Jungfraujoch observatory (3581 m) multiple cloud particle probes and particle imagers were installed. These sensors sampled 13 cloud and precipitation events during the exceptionally active months of January and February 2014.

The comparison of in-situ, radar observations, and snow accumulation measurements indicate that riming is a recurring ingredient for a significant accumulation of snow. MXPol observations allow to formulate hypothesis about shape, density, type and concentration of the ice particles constituting snowfall while the in-situ probes provide the content of ice and liquid water in mixed phase clouds as well as particle images. The vertical structure of rimed precipitation is characterized by enhanced specific differential phase shift Kdp associated either to rimed anisotropic crystals in smaller concentration or to secondary ice produced in much larger concentration. Below the level of Kdp enhancement, reflectivity is increasing while all the polarimetric signatures were disappearing, suggesting aggregation and further riming as dominant processes.

Riming was usually leading to rapid depletion of the available supercooled liquid water (SLW) in the clouds and the main events were usually of short duration (up to 3h). In one case, however, SLW availability was observed for many hours leading to statistically significant accumulations of snow. The mechanism behind this enhancement was found to be a turbulent layer associated to a wind shear, stable in time above the radar location, related to the passage of a cold front. The turbulence within the layer was promoting production of SLW, riming and aggregation and secondary ice production (probably originating from ice to ice impact), leading to efficient fallout of water mass.

This study brings new insight into the relation between riming and snow accumulation, explanations of some frequently documented polarimetric signatures in snow as well as the description of the main mechanism, related to wind shear, that led to significant accumulation of snow in a specific case.