Implementing frictional damping of waves under sea-ice in the Arctic MFC wave model

Transcription

1 Implementing frictional damping of waves under sea-ice in the Arctic MFC wave model Øyvind Saetra, Ana Carrasco and Graig Sutherland MET - Research and Development Department Division for Ocean and Ice

2 Outline The ARC MFC Wave model Introduction of sea-ice damping term in the wave model Attenuation models for wave ice interaction A simple case study a 5-day forecast starting 13/ UTC Use of observations? Conclusions

3 The ARC MFC Wave Model WAM model on 8 km resolution rotated spherical grid Forced with ECMWF wind, and wave spectra at open boundaries Runs daily at 06 UTC to to produce 10 day forecasts Daily 5-days forecasts are provided through the Copernicus CMEMS portal

4 Introducing the wave damping by sea-ice F + (C g F )=(1 C ice )(S inp +S nl +S dis )+C ice S ice t xi C ice Ice concentration S ice = αt F

5 Wave attenuation - scattering Kohout and Meylan (2008) Conservative no dissipation but increase in directional spread of wave energy Function of floe size distribution Minimal scattering for floe diameters less than 2-3 times the incoming wavelength Computations require ensemble averages of possible combinations of reflections and transmissions of wave energy Complicated to estimate for 3D wave field

6 Wave attenuation - dissipation Dissipation due to Rayleigh drag of surface ice layer (Robinson and Palmer, 1990). Requires finding physically meaningful roots of dispersion relation. Dissipation due to visco(elastic) layer [impermeable ice layer] (Keller, 1998; Wang and Shen, 2010). Requires root finding of dispersion relation. More complicated than Robinson and Palmer (1990). Dissipation due to turbulent boundary layer in water (Weber, 1987; Liu and Mollo-Christensen, 1988; Kohout et al., 2011) Dissipation due to viscous drag in a mushy (permeable) ice layer (Sutherland, 2017)

7 Dissipation models Robinson and Palmer (1990) (RP) Dissipation is Rayleigh drag of surface velocity (Γ is tuned to observations) κ=k +i(αsp /2) (F κ ⁴+ρ(g h^ ω) i ω Γ) κ=ρ ω2 Liu and Mollo-Christensen (1988) (LMC), Weber (1987) α sp = k νω 2Cg 2 Kohout et al. (2011) α sp =C d H k 2 Dissipation occurs in turbulent boundary layer under the ice Dissipation due to ice bottom roughness (approximated to give exponential attenuation) Sutherland (2017) 1 α sp= M frac hice ω k 4Cg Dissipation due to friction in permeable ice matrix. Same as Weber (1987), but viscosity limited by 1 depth of mushy layer ν= 2 ω( M 2 frac hice )

8 Damping coefficients (spatial)

9 E-folding distance (4% of energy)

10 RP-Damping, ICETH=0.5 m

11 RP: 14 October UTC

12 RP: 16 October UTC

13 Reduction of wave height Hs 0=4 E=3 m T = s E= E0 e α sp L E 0=( Hs0 / 4)²= m ² 5 1 α sp= m = m ² L=125 km Hs=1.09 m

14 Ice cover from ARC MFC TOPAZ

15 RP: 16 October UTC

16 Difference: 16 Octobewr 2017 = 4 UTC

17 RP-LMC: 16 Octobewr 2017 = 4 UTC

18 RP-LMC: Difference in wave period

19 Future Work Observation - Decide on acase study - Decide on which attenuation model to use - Model tuning Coupled models - Interaction with sea-ice model - Interaction with ocean circulation and armopheric model

20 Thank you!