Forecasting and modelling ice layer formation on the snowpack due to freezing precipitation in the Pyrenees

In the Pyrenees, freezing precipitation at high elevations is quite frequent in winter, leading to the formation of an ice layer on the surface of the snowpack. It may cause many accidents amongst mountaineers and skiers, with sometimes more fatalities per winter than avalanches. Such events are not predicted by current operational systems for snow and avalanche hazard forecasting. A crowd-sourced database of surface ice layer occurrences was first built up, using reports from Internet mountaineering and ski-touring communities, to mitigate the lack of observations from conventional observation networks. Simple diagnostics of freezing precipitation were then developed, based on the cloud water content and 2-m temperature forecast by the Numerical Weather Prediction model AROME, operating at 2.5-km resolution. An evaluation over five winters gave a probability of detection reaching 81% and a false alarm ratio of 23% compared to occurrences reported in the observation database. A new modelling of ice formation on the surface of the snowpack due to impinging supercooled water was added to the detailed snowpack model Crocus. It was combined with the atmospheric diagnostic of freezing precipitation. Resulting snowpack simulations over five winters captured the formation of the main observed ice layers. The performance of the diagnostic associated with the ice formation modelling was assessed for the event of 5-6 January 2012, with altitudinal and spatial distributions of the ice layer matching the observations. These simple methods enable to forecast the occurrence of surface ice layer formations and to simulate their evolution within the snowpack, even if an accurate estimation of the amount of freezing precipitation remains the main challenge.