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Assessing Glacier Volume & Mass ∆ (GRACE (Gravity Recover &…
Assessing Glacier Volume & Mass ∆
Active vs Passive
Active methods
i.e. radar, laser
Instrument beams radiation down to surface from satellite
Passive RS
Instrument picks up reflected solar radiation
Type of
Mass Lost?
Mass lost - snow or firn or ice ? (thus what's relavent density?)
= Important consideration esp wrt potential sealevel ∆
Glacier properties
Thickness: i.e. height, surface elevation
Length/area/extent
Intensity Spectra
Amount of each wavelength of radiation
received at Earth surface from the sun
Reflectance Spectra
Ratio of given wavelength of radiation
received (incident) vs reflected
Allows albedo calculation
Albedo
= % incident light at given wavelength reflected
Electromagnetic radiation
Radiation from sun comprises components of a range of wavelengths (a spectrum)
Different land types have different reflectance spectra
Light from sun = shortwave radiation
Glacier Height ∆ Field Msmt
Field msmt = v labour intensive
Only 37 glaciers have a 40 yr time series measured
Methods (on individual glaciers)
Snow density msmt
Snow/ice stake readings
Easier alternative = remote sensing elevation ∆
Volume
∆ Msmt
Volume ∆ attributed to surface elevation ∆
Convert elevation ∆ to mass by multiplying by density
∴ Must assign density to lost material (snow/firn/ice?)
Glacier Length ∆ Field Msmt
Glacier terminus position
= monitored routinely on ~ 500 glaciers (of 100,000s)
Long record (data begins late 1800s)
Methods:
Aerial photography
Satellite imaging
Maps
Photos / paintings
Dated moraines
Band
Analysis
What?
LANDSAT - satellite imagery
Frequency bands extracted
from imagery
Each band receives unique wavelength
range of electro mag rad
∴ Chosen radiation wavelength/intensities
= returned with photo analysis
Using Bands to Extract Snow/Ice/Firn
Locating ice = problematic at times
Algorithms in Landsat can give false appearance of natural colour
Scientific / robust method
= use reflectance bands
i.e. NSDI
Normalised Snow Difference Index
NDSI = Normalised difference of 2 bands
one in visible range
one in near-infrared or shortwave
Used to map snow / ice
1. Subtract band 5 image from band 2
pixel by pixel i.e. (Band2-Band5)
2. Normalise for limited light/effect of dust
i.e. divide by sum of 2 bands
NDSI = (band2-band5) / (band2+band5)
Benefits
Ads distinction between snow + cloud/veg
Snow is:
highly reflective in visible
highly absorptive in IR / shortwave
Whereas
cloud remains highly reflective in both parts of spectrum
Mass ∆
Glacier dimension ∆ --> Mass loss
Glaciers can ∆ mass via both surface area & thickness ∆
Mass Loss Quantification
Mass = volume x density, thus need to know:
Volume ∆
Density of material lost
Ice density ~ 900 kg/m3
Snow/firn ~ 600 g/m3
GRACE
Gravity Recover & Climate Experiment
Measures variations in Earth's gravity field over time
Resolution: coarse
(~300km)
Other mass ∆s also affect record
Limited to major ice sheets