Wet snow icing Comparing simulated accretion with observational experience Árni Jón Elíasson Sigurjón Páll Ísaksson Hálfdán Ágústsson Egill Thorsteins

Scope Wet snow icing Modelling of wet snow accretion Overview of observed of wet snow accretion Directional dependency of accretion Qualitative analysis of accretion model performance

Database - historical icing events

Icing model WRF model 3 km grid Continous simulation 1994-2014 (21 years) Thompson microphyscs scheme Cylindrical icing model Sticking efficiency acc. to Nygaard et.al. 2013 Vertical cylinder as well as horizontal cylinder Ice shedding if t>3°C or no accretion for 24h Constant density of 700 kg/m3, typical for Iceland

Observed wet snow icing - Má ekki stækka kortið svo það fylli alveg síðu?

Observed wet snow icing 6

Modelled wet snow icing – Maximum load 1994-2014 - Vertical cylinder

Modelled wet snow icing – Maximum load 1994-2014 Accretion upstream

Modelled wet snow icing – Maximum load 1994-2014 Accretion downstream

Accretion on glaciers at high elevation/downstream Modelled wet snow icing – Maximum load 1994-2014 Accretion on glaciers at high elevation/downstream

Accretion inland in gently slope of land Modelled wet snow icing – Maximum load 1994-2014 Accretion inland in gently slope of land

Limited accretion in central highland Modelled wet snow icing – Maximum load 1994-2014 Limited accretion in central highland

Low accretion in SW part Modelled wet snow icing – Maximum load 1994-2014 Low accretion in SW part

Remarks Wet snow accretion has a very complex spatial distribution in complex terrain as well as a high regional variability The most severe icing is usually relatively close to the coast Load can be quite high ( > 15 kg/m ) The loads central highlands experience low loads in the icing model, especially in the precipitation shadow north of the glaciers.

Remarks High wet snow icing loads inland are often associated with a relatively gentle upwards slope of the land, or where there is an upstream mountain barrier. Extreme wet snow maxima are found at the foot of high mountains and massifs in parts of the country. According to the model most of the severe accretion in higher altitude occurs on glaciers, associated with high precipitation and often on their downstream side. Southwest-Iceland has relatively little wet snow accretion except in localized areas

Southeast Iceland Observed icing Modelled icing – vertical cylinder

Southeast Iceland – Influence of line direction

Southeast Iceland Observed icing

Concluding remarks In general a good correlation between areas with observed & modelled icing Conservative to use vertical cylinder model Important to consider actual line line direction compared to main icing direction Icing model may predict too high loading at high accretion values and is critically dependent on the relevant atmospheric parameters and their variability

Concluding remarks In 21 years it was in some cases one accretion period exceeding others Sensitivity analysis by, shifting temperature and liquid water content may give additional information Thank you for your attention !