Studies of the spatial variability of alpine snowfall using an X-band polarimetric radar

A mobile polarimetric X-band radar (MXPol) was deployed in Davos, Switzerland in collaboration with SLF (WSL - Institute for Snow and Avalanche Research). MXPol was installed in the beginning of October 2009 and collected valuable and continuous information of the winter seasons of 2009/2010 and 2010/2011. Additionally, local measurements of snow accumulation on the ground were made using two Airborne Laser-Scanning (ALS) and Terrestrial Laser (TLS). The present work focuses on the Wannengrat area where the measurements were collocated. This configuration makes possible the comparison of the variability of snowfall (as seen by the radar) and in snow accumulation (from laser scans) over the entire winter seasons. A better understanding of these variabilities is a key step to improve snow pack modeling for water resources and avalanche prediction. Several sub-domains related to the topography and wind pattern are considered. The seasonal spatial structure of snowfall is analyzed and its variability from event to event as well. The differences between the spatial variability of snowfall (a few hundred meters above ground level) seen by the radar and the spatial variability in snow accumulation indicate that there are other processes involved in snow distribution (ie. transport by wind), taking place very close or at the ground level.

Seasonal small-scale spatial variability in alpine snowfall and snow accumulation

Alexis Berne, Michael Lehning, Marc Schneebeli Zeugin, Danny Eddy Scipion Castillo

[1] In mountainous regions, snow accumulation on the ground is crucial for mountain hydrology and water resources. The present study investigates the link between the spatial variability in snowfall and in snow accumulation in the Swiss Alps. A mobile polarimetric X-band radar deployed in the area of Davos (Switzerland) collected valuable and continuous information on small-scale precipitation for the winter seasons of 2009/2010 and 2010/2011. Local measurements of snow accumulation were collected with airborne laser-scanning for the winters of 2007/2008 and 2008/2009. The spatial distribution of snow accumulation exhibits a strong interannual consistency that can be generalized over the winters in the area. This unique configuration makes the comparison of the variability in total snowfall amount estimated from radar and in snow accumulation possible over the diverse winter periods. As expected, the spatial variability, quantified by means of the variogram, is shown to be larger in snow accumulation than in snowfall. However, the variability of snowfall is also significant, especially over the mountain tops, leads to preferential deposition during snowfall and needs further investigation. The higher variability at the ground is mainly caused by snow transport.

American Geophysical Union2013

From the Clouds to the Ground - Snow Precipitation Patterns in Complex Terrain

Alexis Berne, Nikola Besic, Marco Gabella, Franziska Gerber, Michael Lehning, Varun Sharma

Snow accumulation patterns are determined by many different processes from ice crystal nucleation in clouds to snow redistribution by wind and avalanches. In between, snow precipitation undergoes different dynamical and microphysical processes, such as ice crystal growth, aggregation and riming, which determine the growth of particles and thereby influence the intensity and structure of the snowfall event. In alpine terrain the interaction of different processes and the topography may lead to orographic enhancement and preferential deposition of precipitation. To better understand the relative importance of different pre-depositional processes on snow precipitation we analyze three snowfall events between January and March 2016 in Davos (CH). Snow precipitation patterns from MeteoSwiss operational weather radar measurements on Weissfluhgipfel and very high resolution Weather Research and Forecasting (WRF) model simulations show reasonable agreement with measurements from automatic weather stations. Overall, radar precipitation patterns show higher small scale variability compared to WRF simulations. Two-dimensional autocorrelation maps and variograms of precipitation patterns reveal a strong dependence of radar and WRF precipitation on topography and the prevailing wind direction, which emphasizes the importance of large-scale orographic enhancement on regional scales and small-scale topography-wind interactions on a mountain ridge scale.

2018

Radar measurements of blowing snow off a mountain ridge

Josué Etienne Gehring, Wolf Hendrik Huwald, Michael Lehning, Benjamin Andreas Walter

Modelling and forecasting wind-driven redistribution of snow in mountainous regions with its implications on avalanche danger, mountain hydrology or flood hazard is still a challenging task often lacking in essential details. Measurements of drifting and blowing snow for improving process understanding and model validation are typically limited to point measurements at meteorological stations, providing no information on the spatial variability of horizontal mass fluxes or even the vertically integrated mass flux. We present a promising application of a compact and low-cost radar system for measuring and characterizing larger-scale (hundreds of metres) snow redistribution processes, specifically blowing snow off a mountain ridge. These measurements provide valuable information of blowing snow velocities, frequency of occurrence, travel distances and turbulence characteristics. Three blowing snow events are investigated, two in the absence of precipitation and one with concurrent precipitation. Blowing snow velocities measured with the radar are validated by comparison against wind velocities measured with a 3D ultra-sonic anemometer. A minimal blowing snow travel distance of 60–120m is reached 10–20% of the time during a snow storm, depending on the strength of the storm event. The relative frequency of transport distances decreases exponentially above the minimal travel distance, with a maximum measured distance of 280 m. In a first-order approximation, the travel distance increases linearly with the wind velocity, allowing for an estimate of a threshold wind velocity for snow particle entrainment and transport of 7.5–8.8m/s, most likely depending on the prevailing snow cover properties. Turbulence statistics did not allow a conclusion to be drawn on whether low-level, low-turbulence jets or highly turbulent gusts are more effective in transporting blowing snow over longer distances, but highly turbulent flows are more likely to bring particles to greater heights and thus influence cloud processes. Drone-based photogrammetry measurements of the spatial snow height distribution revealed that increased snow accumulation in the lee of the ridge is the result of the measured local blowing snow conditions.

2020

Radar measurements of blowing snow off a mountain ridge

Josué Etienne Gehring, Wolf Hendrik Huwald, Michael Lehning, Benjamin Andreas Walter

2020