Скачать презентацию Snow Ice Climate I Energy and

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Snow / Ice / Climate I Energy and Mass n n “The Essence of Glaciology” Processes of: accumulation – precipitation n ablation – melt, sublimation, calving n n n Transformation of snow firn ice n n wind & avalanche can affect either takes time – depends on mass, temperature, etc Balance of acc & abl energy budget

Energy and Mass n n The annual energy budget of a glacier is the sum of inputs minus the sum of outputs ± changes in storage. The annual mass budget of a glacier is the specific (at-a-point) budget times the area to which it applies, summed across the entire glacier: n Bn = Σ(1 -i) (bni x Ai)

Energy Budget n INPUTS n Solar (short-wave) radiation n Long-wave radiation n Conduction (air) n Conduction (ground) n Convection (air) [sensible] n Latent heat n Condensation, freezing n OUTPUTS Reflection [albedo] Long-wave Conduction Convection Latent heat Evaporation, melt ? ? energy from sliding/friction, water flow ?

Energy Balance? n n Varies with position on a glacier, time of day, season, cloud cover, wind … Convection often estimated by difference (assuming balance) “Balance” implies no change in storage (temperature) Studies are rare because of difficulty.

Examples of Energy Budgets

Specific Mass Budget – Stratigraphic n n Most commonly, End Of Summer to EOS Uses old snow / firn / ice as a marker

Specific Mass Budget Protocols n n Stakes = aluminum conduit melted into ice Winter balance (bw) n n Summer balance (b. S) n n bw = depth of snow x density (= “water equivalent”) bs = bn – bw (accumulation area) bs = bw + lost ice times 0. 917 (ablation area) Firn line, bn = 0 Equilibrium line altitude (ELA), bn = 0

Specific Mass Budget Trends n n n Accumulation often increases slightly with increasing altitude above the ELA. @ ELA, bn = 0 Ablation increases rapidly with decreasing altitude below the ELA.

Specific Mass Budget with Climate n n n “Accumulation gradient” = Δmassacc/Δelevation = mm. H 2 O/melevation “Ablation gradient” = Δmassabl /Δelevation = mm. H 2 O/melevation “Activity gradient” = gradient @ ELA Maritime = high activity gradient Continental = low A. G.

Specific Mass Budget with Climate n n n “Accumulation gradient” = Δmassacc/Δelevation = mm. H 2 O/melevation “Ablation gradient” = Δmassabl /Δelevation = mm. H 2 O/melevation “Activity gradient” = gradient @ ELA Maritime = high activity gradient Continental = low A. G.

Why is the ablation gradient >> the accumulation gradient? n n Accumulation = f (precip) Ablation = f (melt) n n Melt = f (T, albedo) n snow ~ 0. 9 n ice ~0. 5 n debris ~ 0. 2, BUT can also insulate other reasons?

Specific Mass Budget with Time n n n Remarkably consistent! Shape = f (climate) Position = f (weather)

Snow / Ice / Climate II Snowlines – Space and Time n Snowlines and their many definitions n n Estimating bn = 0 Contemporary controls on snowlines n local climate / weather and topography Spatial variability n Temporal variability n Pleistocene snowlines and climates n

Snowlines I – Cirque Floors n n Permanent snowfields? No – glaciers! Cirque floor elevations Maximum erosion at minimum size n Problems = size, timing n

Snowlines II – Lateral Moraines n Highest laterals = initiation of deposition [discuss more with “glacier flow” ? ] n Problem = postglacial slope erosion/removal

Snowlines III – Glaciation Threshold n n True “snowline” Problems = many Area? n Topography? n Summits > glacier elevations n

Snowlines IV – THAR n n n Toe-headwall altitude ratio Requires reconstruction Assumes known “correct” ratio – 40%?

Snowlines V – AAR n Accumulation area ratio n Requires complete reconstruction n Assumes correct ratio. 55–. 60–. 65 ? n [topo map method]

Snowline Comparisons n Meierding (1982) CO Front Range n tried many ratios n n Locke (1990) Montana n small glaciers n s. d. ~ 350 m n CF (n=12) LM (45) GT (13) THAR (24) AAR (24) 3161 m 3188 3388 3161 (40%) 3163 (65%) CF (n=400) 2347 m LM (321) 2121 THAR (330) 2355 (40%) AAR (264) 2353 (65%)

ELA = representative? n Many studies say so! n e. g. , Sutherland (1984) n ELA balance represents average winter balance for entire glacier n Measure once – use a lot!

Glacial Climates n Glaciers exist only in a narrow range of climates = f(winter ppt and summer T) n = f(P, T, and continentality n

Glacial Climates

Controls on Snowlines I – Latitude n Latitude ≈ temperature (treeline) n n Latitude ≠ precipitation (snowlines) n n highest near equator saddle near equator Weak gradients (<1 m/km)

Controls II – Continentality n n n Lowest near moisture source Higher inland Strong gradients n up to 10 m/km

No Hem Glaciers n n n Latitude? Continentality Ocean currents n Local precipitation

Temporal Resolution n Glaciers respond at annual to decadal scales

Temporal Inconsistency n n Not all glaciers respond similarly Not even glaciers in the same region!

Temporal Inconsistency n n Not all glaciers respond similarly Not even glaciers in the same region!

Wahrhaftig and Birman, 1965 n Sierra Nevada n Note effects of subtropical high & rain shadow Pleistocene Snowlines I

Pleistocene Snowlines II n n US West (Porter et al. 1983) Effects of: Sub. T High n Westerlies n Storm tracks n Orography n

Pleistocene Snowlines II

Montana Climate & Glaciers n n Glaciers Inferred air mass movement Residuals Inferred causes

MT/ID Paleoclimate n n Complex pattern! More detailed than modern weather stations and SNOTEL sites!

Spatial Resolution n n Humlum (1985) West Greenland Local data are consistent Needs no smoothing High resolution!