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

2. 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

3. Database - historical icing events

4. Icing model WRF model 3 km grid
Continous simulation (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

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

6. Observed wet snow icing6

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

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

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

10. Accretion on glaciers at high elevation/downstream
Modelled wet snow icing – Maximum load
Accretion on glaciers at high elevation/downstream

11. Accretion inland in gently slope of land
Modelled wet snow icing – Maximum load
Accretion inland in gently slope of land

12. Limited accretion in central highland
Modelled wet snow icing – Maximum load
Limited accretion in central highland

13. Low accretion in SW part
Modelled wet snow icing – Maximum load
Low accretion in SW part

14. 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.

15. 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

16. Southeast Iceland
Observed icing
Modelled icing – vertical cylinder

17. Southeast Iceland – Influence of line direction

18. Southeast Iceland
Observed icing

19. 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

20. 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 !