High resolution vertical profiles of polarimetric X-band weather radar observables during snowfall in the Swiss Alps

An X-band polarimetric radar was deployed in the eastern Swiss Alps at an altitude of 2150 m. Radar measurements were supplemented with a set-up of ground-based sensors, in particular three optical disdrometers and one two dimensional video disdrometer, as well as numerous weather stations deployed in an altitude range between 1500 to 3100 m. In addition, integrated water vapor (IWV) data was inferred from a fixed installed GPS ground station. Around 6000 vertical profiles of polarimetric observables, obtained from elevation scans that were performed during two months above the melting layer, were analyzed. First, the behavior of the mean profiles of the reflectivity (Zh), differential reflectivity (Zdr), copolar correlation (ρhv) and specific differential phase shift (Kdp) was interpreted from a microphysical point of view. It was found that the whole evolution of snowflakes, from pristine crystals at temperatures of -30°C to the formation of dendrites around -15°C and finally to large aggregated snow flakes around 0°C is well represented in the polarimetric profiles. In a second step, the profiles were set in relation to different IWV and snow accumulation conditions. It was found that especially the height profile of Zdr behaves differently in conditions of high snow accumulation rates as well as in conditions of elevated IWV as opposed to low water vapor and low snow accumulation conditions. By applying a hydrometeor identification (HID) scheme, the dependence of the abundance of the different hydrometeors to IWV and snow accumulation could be studied. For snow events with high accumulation intensities, reduced Zdr values at high altitudes (> 4 km above the 0°C level) and elevated Zh and Kdp values above around 2.5 km above the 0°C level were found. In these meteorological conditions, the HID schemes also detected a predominant occurrence of graupel particles in the first 0.5 km above the melting layer. Intermediate IWV conditions were identified to favor to process of riming which is a finding that supports the common theories of precipitation formation and growth.