Snow microphysical processes in orographic turbulence revealed by cloud radar and in situ snowfall camera observations

Abstract

Turbulence influences snow microphysics and precipitation formation, while simultaneously degrading polarimetric radar measurements through broadening of the canting angle distribution. We investigate these interactions in the Colorado Rocky Mountains, where an orographic turbulent layer consistently forms in the lee of Gothic Mountain during precipitation events. To isolate microphysical signals from turbulence-induced artifacts, we apply a novel framework contrasting radar observations above and below the turbulent layer. The dataset combines polarimetric W-band and collocated Ka-band radar measurements with surface in situ observations from the Video In Situ Snowfall Sensor (VISSS). All observations were collected during the CORSIPP project, part of the ARM SAIL campaign (winter 2022/2023). Aggregation is identified as a dominant process within the turbulent layer, occurring primarily between -12 and -15 °C. It is responsible for reflectivity (Ze) increase of up to 20 dBZkm-1 and reduction of the mean particle fall velocity. Enhanced KDP and sZDRmax further suggest secondary ice production through ice-collisional fragmentation, generating anisotropic splinters. Riming also occurs frequently, with Ze increases up to 15 dBZkm-1 and systematically increasing mean particle fall velocity. Riming inside the turbulent layer was often observed at temperatures below -10 °C, indicating the presence of supercooled liquid at cold conditions. Statistical analysis revealed that the turbulent layer is frequently collocated with supercooled liquid water layers near the Gothic Mountain summit. Our findings demonstrate how radar polarimetry may be safely used to investigate microphysical processes inside a turbulent layer and highlight the impact of orographic turbulence on snow microphysics and precipitation enhancement.