Glacier Thermodynamics: Ice Temperature and Heat Transfer Processes

Transcription

1 Glacier Thermodynamics: Ice Temperature and Heat Transfer Processes ESS431: Principles of Glaciology ESS505: The Cryosphere Wednesday, 10/24 Ben Hills

2 Today s Objectives: Why do we care about ice temperature? Cover the fundamentals of heat transfer. What are the important time/length scales for heat transfer in ice? What is the general thermal structure of a glacier or an ice sheet? What heat transfer processes are important in which regions?

3 Importance of Ice Temperature Background on Heat Transfer Ice Sheet Thermal Structure

4 Why Ice Temperature? 1. Ice Deformation (Viscosity) " = $ % & ' $ % = $ ( ) *+, -. Joughin et al. (2010)

5 Why Ice Temperature? 1. Ice Deformation (Viscosity) 2. Basal Sliding Credit: Paul D. Bons, Ilka Weikusat

6 Why Ice Temperature? 1. Ice Deformation (Viscosity) 2. Basal Sliding 3. Paleoclimate

7 Why Ice Temperature? 1. Ice Deformation (Viscosity) 2. Basal Sliding 3. Paleoclimate 4. Drilling

8 Why Ice Temperature? 1. Ice Deformation (Viscosity) 2. Basal Sliding 3. Paleoclimate 4. Drilling 5. Geophysics (Radar and Seismic)

9 Why Ice Temperature? 1. Ice Deformation (Viscosity) 2. Basal Sliding 3. Paleoclimate 4. Drilling 5. Geophysics (Radar and Seismic) 6. Mass Balance mm.w.e. van den Broeke et al. (2011)

10 Importance of Ice Temperature Background on Heat Transfer Ice Sheet Thermal Structure

11 Background on Heat Transfer Laws of Thermodynamics 1. Energy Conservation: Energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. Δ" = $ & 2. Entropy The entropy of any isolated system always increases. Isolated systems spontaneously evolve towards thermal equilibrium the state of maximum entropy of the system. Δ' 0

12 Background on Heat Transfer Modes of Heat Transfer 1. Conduction - Transfer of heat via direct molecular collision.

13 Background on Heat Transfer Modes of Heat Transfer 1. Conduction - Transfer of heat via direct molecular collision. Fourier s Law:! = $ & $ is the thermal conductivity of the material Hot Heat Flux! Cold

14 Background on Heat Transfer Modes of Heat Transfer 1. Conduction - Transfer of heat via direct molecular collision. 2. Convection - Thermal energy is carried by a moving fluid. dt dt = & ( &

15 Background on Heat Transfer Modes of Heat Transfer 1. Conduction - Transfer of heat via direct molecular collision. 2. Convection - Thermal energy is carried by a moving fluid. 3. Radiation - The emission of electromagnetic waves carries energy away from a thermal body.

16 Background on Heat Transfer 3. Radiation - The emission of electromagnetic waves carries energy away from a thermal body. Stefan-Boltzmann Law:! = #$ %

17 The Heat Equation ( #$ /!(#$)!& = ( ) ( ) + / * *,-* 1 1+dx!(#$)!& = 0( 01 + / Heat Capacity Definition $(4) = Incompressibility and Total Derivative!#!& 2! #2 3!& = / = 0 04 #2 3 0& + u = Total Derivative /!4!& = 04 0& & = 04 0& & = < u / #2 3

18 The Heat Equation for Ice!"!# = % ' " ) " + 1 ( ) / Diffusion (mostly vertical) Advection (all 3 directions) Sources (see figure)

19 Characteristic Diffusion Length 8 = 2 %#!"!# = % ' " ) " + 1 ( ) / Copper m 2 /s Iron m 2 /s Air m 2 /s Stainless Steel m 2 /s Ice m 2 /s Water m 2 /s

20 Peclet Number!"!# = % ' " ) " + 1 ( ) / Gulf Stream Pe~10 12 Ice Sheet (horizontal) Pe~10 5 Solid earth (mantle convection) Pe~10 3 Ice Sheet (vertical) Pe~ = ): % Near-surface Ice Sheet Pe~10-1 Granite Outcrop Pe<<1

21 Importance of Ice Temperature Background on Heat Transfer Ice Sheet Thermal Structure

22 Thermal Classifications of Glaciers Temperate High Accumulation High Melt Fast Moving Franz Josef Glacier New Zealand Cold Low Accumulation Low Melt Slow Moving McMurdo Dry Valleys Antarctica Polythermal Possibly bridges climate zones (i.e. cold at top and warm at bottom) Includes both ice sheets Grand Pacific Glacier Glacier Bay, Alaska

23 Regions Boundary Conditions 1. Surface 2. Bed 3. Ice Divide 4. Downstream of Divide Advection Frictional Heating 5. Fast-Flowing Ice 6. Ablation Zone Latent Heating

24 1) Surface!"!# = % ' " ) " + 1 ( ) / Dominated by conduction with the overlying atmosphere. Melting becomes important in the ablation and percolation zones.

25 1) Surface

26 1) Surface Skin Depth is period dependent

27 1) Surface Hills et al. (2018)

28 1) Surface Hills et al. (2018)

29 2) Bed!"!# = % ' " ) " + 1 ( ) / Heat sources from the friction of ice sliding over bedrock and from geothermal heating. Basal melting can add to the total mass balance, but is mostly important for its influence on sliding.

30 2) Bed Pressure Melting Point Frozen Melted Geothermal Flux MacGregor et al. (2016)

31 3) Ice Divide!"!# = % ' " ) " + 1 ( ) / Ice flow is purely vertical. The only source term here is firn compaction which is small compared to shear strain or latent heating.

32 3) Ice Divide Air temperature variations through time Geothermal flux

33 4) Downstream of Divide!"!# = % ' " ) " + 1 ( ) / Ice starts moving downhill, so the surface boundary warms at the lapse rate. Horizontal advection is typically faster than diffusion.

34 4) Downstream of Divide Jakobshavn Isbræ Luthi et al. (2002), J.Glac. Iken et al. (1993), J.Glac.

35 5) Fast-Flowing Ice!"!# = % ' " ) " + 1 ( ) / Ice streams represent a sharp boundary in ice velocity, so the shear margins at their boundary have very high strain rates. Some argue that the strain rate is high enough that a significant portion of the ice column is temperate.

36 5) Fast-Flowing Ice &! = ( () * ( + =, ( - ) Margin Ice Stream Margin! "$! "#

37 5) Fast-Flowing Ice Harrison et al. (1998), JGlac. Engelhardt (2004), Ann. Glac.

38 6) Ablation Zone!"!# = % ' " ) " + 1 ( ) / Much liquid water available at the surface. If water finds a pathway into the ice (through fractures) it can refreeze and release the latent heat to warm the ice.

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42 6) Ablation Zone Latent Heat Breaking Bonds

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44 " = $ %& /m 0.04 /m

45 0.01 /m /m 0.04 /m

46 Hypothesis: Differential advection creates horizontal temperature gradients.

47 50 years Crevasse Field

48 Summary a b c d!"!# = % ' " u " +, -.

49 More Reading Cuffey and Paterson (2010) Chapter 9 van der Veen (2013) Chapter 6 Hooke (2005) Chapter 6 Zotikov (1986) Thermodynamics of Glaciers (Aschwanden, 2014; McCarthy Summer School Notes)