Transcript No Slide Title

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
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.
Introduction
Input/output relationships of ice, firn & snow
•
hydrological budget
Importance
• Catchment hydrology
• Climate change indicator
• Sea level rise
• Global albedo
A simple throughput model
Regional
climate
Local
climate
Energy
balance
Mass balance
Glacier
Response
Geological
Record
Introduction (ctd.)
Principal controls
Winter precipitation and temperature
Summer insolation and temperature
The former controls accumulation
The later controls ablation
Terms
Snow
Firn
Ice
unaltered since deposition ±
wetted snow that has survived more than one summer
no interconnecting passages
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
___________________________________________
The mass balance concept
AS.SN=AEL.V=Ai.IN
if
SN =S.r(snow)
IN =I.r(ice)
where r = density
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 positive the glacier thickens and/or advances
If negative the glacier thins and/or retreats
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
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
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
Accumulation
•
•
•
•
snowfall
rainfall
superimposed ice
regelation ice
Ablation
•
•
•
•
•
•
•
surface melt
basal and englacial melt
evaporation
sublimation
deflation
calving
avalanching
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
Measurement of mass balance
Direct measurement
Hydrologic
Photogrammetric and geodetic
Remote sensing
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
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
Photogrammetric
Sequential digital elevation models
Density correction
Snow fields difficult to map
Hydrologic
Hydrological budget
Precipitation input
Runoff output
Calculate stored water
Depends on reliability of estimates
Remote sensing
eg Satellite interferometry
San Rafael Glacier
DEM
0-2000m
Hrz resolution 15m
Vert resolution 10m
Velocity
dark blue
light blue
green
< 6 cm per day
6-20 cm per day
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).
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
Variability in space and time
Latitude and radiation receipt
• Geographic differences in the proportion of different energy
transfers
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
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
Albedo’s (per cent) for snow and ice
Dry snow
Melting snow
Firn
Clean ice
Slightly dirty ice
Dirty ice
Debris-covered ice
Source: Paterson (1994)
Range
80-97
66-88
43-69
34-51
26-33
15-25
10-15
Mean
84
74
53
40
29
21
12
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
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
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
Hypotheses
Water implicated
Linked cavity mechanism
Subglacial sediment sensitivity
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