NEXT WEEK. Tues: Please read Martinson et al 1981, at least through section 4. Paragraph about projects due 5/21 by

Transcription

1 NEXT WEEK Tues: Please read Martinson et al 1981, at least through section 4 Paragraph about projects due 5/21 by

2 Where is the coastline? Surface Air Temperature Jan 1999

3 Where is the sea ice edge? Surface Air Temperature Jan 1999

4 Surface Air Temperature Jan 1999

5 Where is the coastline? Surface Air Temperature Jul 1999

6 Where is the sea ice edge? Surface Air Temperature Jul 1999

7 Surface Air Temperature Jul 1999

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9 Lindsay and Zhang 2005

10 Oden ice breaker Photo by Chris Linder, WHOI

11 Photo by C Haas

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16 Salt is in the brine pockets/channels/ pores Photo by B. Light

17 Alloy = material composed of two or more molecules. Often behaves unlike either element alone. It may be complete solid or partial solid/liquid. Sea ice is a combination of NaCl and H2O. At normal temperatures on Earth it is a partial solid/liquid. ALL of the NaCl is in the liquid!!! Thus ice is an alloy of pure ice and brine.

18 Alloy = material composed of two or more molecules. Often behaves unlike either element alone. It may be complete solid or partial solid/liquid. Sea ice is a combination of NaCl and H2O. At normal temperatures on Earth it is a partial solid/liquid. ALL of the NaCl is in the liquid!!! Thus ice is an alloy of pure ice and brine. The concentration of salt affects the freezing point. Normally we assume the temperature of the ice/brine combination is at the BRINE FREEZING POINT, or LIQUIDUS. This assumption is known as local thermodynamic equilibrium (LTE). Eutectic materials (like sea ice) exhibit this behavior. Sea ice is also known as a reactive porous medium because its ice/brine proportion changes with temperature. Brine pockets are pores.

19 Alloy = material composed of two or more molecules. Often behaves unlike either element alone. It may be complete solid or partial solid/liquid. Sea ice is a combination of NaCl and H2O. At normal temperatures on Earth it is a partial solid/liquid. ALL of the NaCl is in the liquid!!! Thus ice is an alloy of pure ice and brine. The concentration of salt affects the freezing point. Normally we assume the temperature of the ice/brine combination is at the BRINE FREEZING POINT, or LIQUIDUS. This assumption is known as local thermodynamic equilibrium (LTE). Eutectic materials (like sea ice) exhibit this behavior. Sea ice is also known as a reactive porous medium because its ice/brine proportion changes with temperature. Brine pockets are pores. Think of salting icy roads. The ice melts just enough to dilute the brine until it reaches the melting point, more salt makes more melt. The local in LTE means the temperature need not be isothermal except just at the interface of ice and brine, which is at the liquidus temperature. The temperature also need not be in steady state, but changes slowly enough to maintain the liquidus condition. Again think of salting a road.

20 X-ray Computed Tomography Images by Dan Pringle In Golden et al 2007 Porosity or pore fraction is φ

21 North Pole Web Cam 2003 July 4 July 25 Toward the end of July, pond coverage decreased markedly (Fig. 4) while radiation and air temperatures were still at their normal summer values. A possible explanation is that the ice had become sufficiently porous for the melt water ponds to drain by percolation. Untersteiner on the NOAA Arctic Theme Page.

22 Porous nature of sea ice causes: Space for algae, chemistry, sea water cycling, gas to pass through, etc We think sea ice plays a role in cycling carbon, sulfur, halogen (ozone), etc. Many open questions. Sea ice rots from inside out Pore structure affects deformation (virtually unstudied)

23 Idealized binary alloy eutectic diagram T, Temperature Pure Brine Phase Ice + Brine Solidus line Ice + solid salt Brine + solid salt S br, Brine Salinity Liquidus is the freezing point of the brine, given its salinity. Solidus is the point where the salt solidifies. Between them is a slurry of ice and brine. Liquidus temperature is approximately linear in S br

24 Idealized binary alloy eutectic diagram T, Temperature Pure Brine Phase Ice + Brine Solidus line Ice + solid salt Brine + solid salt S br, Brine Salinity Why does ice interior T vary seasonally? 1) absorbed solar radiation in the ice interior 2) conduction of heat

25 T = -15 C Size of channels, or porosity, grows with temperature Because the brine is always at the freezing point, or liquidus temperature T = -5 C T = - µ S br T = -2 C S br = Salinity in the brine channels T = Temperature in C Photos by B. Light

26 Sea Ice Salinity Evolution Salinity = average of brine salinity and ice salinity (the latter is essentially zero!) High permeability at base of young ice, interior is too cold Malmgren (1927) Permeability increases by 1-2 orders of magnitude in melt season

27 Brine pockets are internal melt. Their presence reduces energy needed to melt sea ice compared to fresh ice. q = energy of melting (aka enthalpy) a function of T and S Lo = latent heat of fusion for fresh ice Bitz and Lipscomb, 1999

28 Bitz and Lipscomb (1999) conserving is a model treating brine pockets via q(t,s) (as in two slides ago); non-cons. is like the Maykut and Untersteiner model, which messes up brine pocket physics a bit; Semtner 3L is a model where brine pockets are approx. by taking all solar radiation that penetrates into the ice into a reservoir used to prolong surf. at 0 C in fall; Semtner 0L ignores brine pockets and allows no sunlight to penetrate into ice. Models from Thorndike (1992) will be discussed in a moment.

29 Maykut and Untersteiner (1971) ran this model on the UW mainframe Bitz and Lipscomb (1999) called it non-cons. because MU71 had q=ρl o despite treating brine pockets in heat equation above. BL99 showed for ice q=q(t,s)

30 Seasonal evolution of sea ice surface temperature and thickness Maykut and Untersteiner Model Toy Model Thorndike (1992)

31 Some assumptions and simplifications: - Ignore sensible and latent heat fluxes, radiation absorbed at upper surface - Linearize outgoing LW radiation - F SW = 0 in winter (half the year) - F SW 200 W/m 2 in summer (half the year) - Linear temperature profile through ice - Heat capacity and conductivity constant - T b is always at the freezing point - Ignore salinity, snow, melt ponds, dynamics, ridging - Atmospheric and oceanic heat fluxes constant - Boring ocean (mixed layer only, no brine rejection)

32 Winter (cooling and growing) cooling happens fast (~10 days) (11)

33 Winter (cooling and growing) T s as a function of h while growing (10)

34 Winter (cooling and growing) T s as a function of h while growing (10) growth rate of ice (13)

35 Winter (cooling and growing) growth rate of ice growth rate (m/yr) thickness (m)

36 Winter (cooling and growing) growth rate of ice T s as a function of h while growing growth rate (m/yr) T s ( C) thickness (m) thickness (m)

37 Winter (cooling and growing) growth rate of ice T s as a function of h while growing growth rate (m/yr) 10*k T s ( C) 10*k thickness (m) k/10 thickness (m) k/10

38 Winter (cooling and growing) growth rate of ice thickness over time (assuming continuous growth) growth rate (m/yr) thickness (m) thickness (m) years

39 Summer (warming and melting) warming happens fast (~20 days) (16)

40 Summer (warming and melting) melt rate of ice (T s = T b = 0) (17) (melt rate independent of h)

41 Numerical solutions (F SW = 200 W/m 2 )

42 Numerical solutions (F SW = 200 W/m 2 )

43 Numerical solutions (F SW = W/m 2 )

44 Numerical solutions (F SW = W/m 2 )

45 Numerical solutions (F SW = W/m 2 )

46 Numerical solutions (F SW = W/m 2 )

47 Numerical solutions (F SW = W/m 2 )

48 Numerical solutions (F SW = W/m 2 )