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Glacier mass and energy balance

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Glacier mass and energy balance

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  1. Glacier mass and energy balance 1. Introduction 2. The mass balance concept • Accumulation and ablation • Mass balance years 3. Snow metamorphism 4. Measurement of mass balance 5. Glacier energy balance • Energy balance equations • Geographic variability of energy balance terms 6. Glacier movement • Responses to changes in mass balance • Glacier surges

  2. References Benn, D.I. and Evans, D.J.A. (1998) Glaciers and glaciation. Chapter 2. Sugden, D.E. and John, B.S. (1976) Glaciers and Landscape. Edward Arnold, London Chapter 3 Glacier Systems Bennett, M.R. and Glasser, N.F. (1996) Glacial Geology: Ice Sheets and Landforms. Wiley, Chichester. Chapter 3 p 29-37.

  3. Introduction Input/output relationships of ice, firn & snow • hydrological budget Importance • Catchment hydrology • Climate change indicator • Sea level rise • Global albedo

  4. A simple throughput model Regional climate Local climate Energy balance Mass balance Glacier Response Geological Record

  5. Introduction (ctd.) Principal controls Winter precipitation and temperature Summer insolation and temperature The former controls accumulation The later controls ablation Terms Snow unaltered since deposition ± Firn wetted snow that has survived more than one summer Ice no interconnecting passages

  6. Typical densities Substance Density (kg m3) ___________________________________________ New snow 50-70 Damp new snow 100-200 Settled snow 200-300 Depth hoar 100-300 Wind packed snow 350-400 Firn 400-830 Very wet snow and firn 700-800 Glacier ice 830-910 Water 1000 Basal ice 900-c1200 ___________________________________________

  7. The mass balance concept AS.SN=AEL.V=Ai.IN where AS = accumulation zone area SN =snow (liquid equivalent) AEL = area of the equilibrium line section V = mean annual velocity Ai = area of ablation zone IN = ice (liquid equivalent) if SN =S.r(snow) IN =I.r(ice) where r = density If positive the glacier thickens and/or advances If negative the glacier thins and/or retreats

  8. A balance year Begins in late summer or autumn End of winter season • late spring or summer ablation>accumulation End of balance year • when accumulation>ablation Net balance is given by: bn= bw+bs or bn= ct-at where bn = net balance bw = winter balance bs = summer balance ct = total accumulation over a year at = total ablation over a year

  9. Summer balance Melted snow and ice lost Measured by a network of stakes Temperate glaciers • most ice lost by melt and runoff • ablation stakes and density corrections Cold glaciers • refreezing of meltwater • density change • heat input during refreezing • superimposed ice • internal accumulation

  10. The equilibrium line Temperate glaciers • Edge of the previous winter snowline after the end of summer • aka firn line Polar glaciers • Boundary between glacial ice of the of the ablation zone and superimposed ice • Winter snowline lies above the equilibrium line

  11. Accumulation • snowfall • rainfall • superimposed ice • regelation ice Ablation • surface melt • basal and englacial melt • evaporation • sublimation • deflation • calving • avalanching

  12. Snow metamorphism The dry snow zone Settling, packing change 0.4-550kg m-3 Changes in crystal size and shape Deformation, compression Water Melting, transportation, refreezing Increase in grain size Superimposed ice Internal accumulation Depth hoar Temperature gradient metamorphism Evaporation at depth, condensation further up Coarse grained firn

  13. Measurement of mass balance Direct measurement Hydrologic Photogrammetric and geodetic Remote sensing

  14. Direct measurement Most common: sampling ablation & accumulation at sites over the glacier • 20 per km2 recommended • 1 per 1000 km2 in practice on large ice masses Winter balance • establishing snow pits at different elevations • density measurements • combine with a network of probed snow depths

  15. Boas Glacier, Baffin Island • 35 density measurements • mean = 0.328 g.cm-3 ± 0.04 • 21 probed cross-sections • mean = 1.289m ± 0.32 • bw = 0.328 x 1.289 = 0.422 (mm H2O) Area integration: • glacier area = 1.45 x 106 m2 • bw= 1.45x106 x 0.422 = 61x104 m3 H2O

  16. Photogrammetric Sequential digital elevation models Density correction Snow fields difficult to map

  17. Hydrologic Hydrological budget Precipitation input Runoff output Calculate stored water Depends on reliability of estimates

  18. San Rafael Glacier DEM 0-2000m Hrz resolution 15m Vert resolution 10m Velocity dark blue < 6 cm per day light blue 6-20 cm per day green 20-45 cm per day yellow 45-85 cm per day orange 85-180 cm per day red >180 cm (accurate to within 5 mm per day). Remote sensing eg Satellite interferometry

  19. General energy balance equation Qs¯+QL¯+Qc+Ql+Qe=Qm+QE+Qs­+QL­+QC where Qs = Short-wave radiation QL = Long-wave radiation Qc = Heat gained from condensation Ql = Eddy transfer of sensible heat Qe = Heat gained by refreezing or melt Qm = Heat used to melt ice or snow QE = Heat used for sublimation QC = Heat conducted into the or ice and used to raise temperature

  20. Variability in space and time Latitude and radiation receipt • Geographic differences in the proportion of different energy transfers

  21. Variability in space and time Latitude and radiation receipt • Geographic differences in the proportion of different energy transfers Major heat sources Qs, QL, Qc, Ql • High latitude and high altitude glaciers net radiation impt. • Wide geographic scatter of sites net radiation & sensible heat transfer impt. • Coastal high latitudes latent heat transfer important

  22. Major heat sinks Qm, QE, QC • Melting dominant (Qm) - temperate glaciers • Sublimation dominant (QC) - polar arid • Sublimation & condensation co-dominant (QC & QC) - some polar and subpolar ice masses

  23. Glacier movement Responses to changes in mass balance Direct responses and lagged responses Scale dependency • Big glaciers respond more slowly than small glaciers • transmission time Velocity dependency • ice temperature • bed gradient • glacier hydrology Directional dependency • Negative responses fast • Positive responses slow

  24. eg West Coast/East Coast of NZ Typical response times • Valley glaciers 10-60 yrs • East Antarctic ice sheet 2,500-5,000yrs Kinematic waves • Development of bulges that travel down glacier 3-5 times faster than average velocity

  25. Transient flow phenomena: surging Major perturbations in steady state flow Characteristics: • rapid advances of the terminus over short periods • Long periods of quiescent behaviour (years to centuries) • Short periods of rapid advance and ice motion (months to a few years) • Terminus advance • Flattening of upper profile • Steepening of lower profile • Crevassing

  26. Hypotheses Water implicated Linked cavity mechanism Subglacial sediment sensitivity

  27. Glacier energy balance Mass balance vs. energy balance Simplified heat balance: FT=Fr+Fc+Fl+Fp+Ff where FT = Total heat content of the snow/ice Fr = Radiative heat flux Fc = Sensible heat flux Fl = Latent heat flux Fp = Heat flux from precipitation Ff = Heat content from freezing