Lecture 2: glacier mass balance Flashcards
What is mass balance a function of?
Elevation, climate, topography and hypsometry.
What is hypsometry?
The distribution of terrain over elevation range. (May influence glacier response to climate change as sensitivity to summer melt is highest in low elevation zones).
The antarctic peninsula has warmed by 6 times the global average in the last century, what has this caused?
Collapse of floating ice shelves due to high surface melting and enhanced bottom melting.
What are the four processes of accummulation?
Wind drift, snow fall, avalanching and meltwater freezing (riming or superimposed ice)
Put these in order of declining altitude - Wet snow zone, Dry snow zone, Superimposed ice zone, percolation zone.
Wet snow zone, percolation zone, wet snow zone, superimposed ice zone.
Advance and retreat of glaciers is not necessarily under climatic control. What are the the controls on calving?
Longitudinal stretching due to friction and water pressure.
Water depth at the calving margin relative to ice thickness.
Net balance is the difference between two consecutive annual mass balance minima. What are the two variations on this?
Specific net balance - the net change in mass at one point
Cumulative mass balance - the running total of net balances since measurements began.
The himilayas are characterised by what type of glacial regime, whilst what type includes tropical glaciers?
Himilayas - summer accumulation
Tropical glaciers - year round ablation
Rates of accumulation and ablation change with altitude. This can be modelled by the mass balance gradient. How? What does this tell us?
Plots specific net balance against altitude. High accumulation/ablation gradients indicate high through-flow. Links climatic controls on acc/abl with glacier behaviour.
How can mass balance be measured directly?
Using pits to measure accumulation and fixed stakes to measure ablation at the end of each accumulation/ablation season, usually along the glacier centre line. The mass can then be multiplied by the area of the glacier.
How can mass balance be measured climatically?
Ablation can be estimated using the degree day or energy balance equation
The energy balance equation gives the energy available to melt ice as the sum of what four quantities of energy?
The shortwave radiation flux, the longwave radiation flux, sensible heat transfer and latent heat transfer.
The degree day method uses meteorological parameters to estimate ‘positive degree days’. What are these? And what is the equation?
Days when ice should melt (>0 degrees).
M = a(Ta-Tb) where…
a=degree day factor, Ta=average daily temp, Tb= base temp
Which properties of glaciers influence degree days?
topography and aspect
Remote sensing of glaciers captures aerial photos and what else?
Airborne laser / radar altimetry to measure elevation.
Which two mass balance monitoring techniques can be used on small glaciers?
direct and hydrological
What is the hydrological method of measuring net balance?
Net balance = precipitation - (runoff+evaporation)
Variables have to be estimated over entire basin.
Satellite remote sensing and ground validation of this needed to determine Greenland and Antarctic mass balance, as what difficulties arise?
- Extensive on ground measurements are impossible
- Local areas of thinning have to be balanced against mass gain in other regions
- Error bars can be significant
Other than when calculating volume change from satellite measurements, what 3 things do you need in order to determine mass balance?
- Mass accumulation
- Ice discharge from ice streams / outlet glaciers
- Surface melt.
West Antarctic has experienced 59% mass loss in 10 years. Where has this loss been concentrated and what process is associated with this?
Concentrated along outlet glaciers. Fast flow can cause ungrounding of glaciers (thinning and collapse of ice shelves)
What is the most significant control on glacier speed?
The ungrounding of an ice plain upstream of grounding line.
Thermal forcing from the ocean causes how much ice shelf bottom melting?
10m/yr/degree
