Mass Balance and Modelling Revision

Why is it important to study the mass balance of glaciers?

Glaciers around the world are shrinking and they are the world’s most sensitive natural water towers; changes to melt pose challenges to resource management, avalanche forecasting, glacier dynamics, hydrology and hydrochemistry

What is glacier mass balance?

The change in mass for all or a part of a glacier over a specific time period calculated as the sum of accumulation and ablation over a specific time period

How is glacier mass balance measured?

This can be measured through direct measurements in snowsuits using ablation stakes, geodetic measures include photographs and satellites and gravimetric methods use the GRACE satellites to compare changes to the Earth’s gravitational field

How is glacial mass balance changing as a result of climate change?

Climate changes cause the equilibrium line to shift and mass balance changes based on the climate sensitivity of the individual glacier as a result of unique snow and ice characteristics

Is the degree-day model empirically or physically based?

Empirically based

How does the degree-day model calculate mass balance change?

Glacier melt is a function of the degree-day factor for ice and snow per positive degree day; this assumes a linear relationship between melt and air temperature which simplifies complex processes

How do degree-day values vary across glaciers and how is this measured?

Degree-day factors vary across glaciers are are impacted by elevation, solar radiation and topography; ice has a higher DDF than snow due to a higher albedo and this can be measured using snow limiter outflow, ablation stakes or computation

Where was variation in DDFs proven?

Across glaciers in Western China where elevation and topography were found to be key factors in impacting DDFs

Where was a degree-day model used to model mass balance and what did this show?

This was used to model the melt of 5 Swiss glaciers and showed that the largest mass balance changes occurred at the snout, precipitation increases were also shown to be able to partially offset melt and there was a strong relation between sensitivity and altitude

What are the limitations of degree-day models?

There are limited parameters for these models and they are based on empirical relations rather than physical processes; they also lack temporal and spatial resolution

Why are degree-day models the most commonly used?

There is a large availability of air temperature data, easy extrapolation, good model performance and computational simplicity

Is the energy balance model empirically or physically based?

Physically based

How does the energy balance model seek to model mass balance?

This measures the mass loss of the glacier based on all energy fluxes including surface energy balance, external meteorological conditions, the topography of the glacier, latent heat, refreezing etc.

What fluxes are proven to be the most important for melt?

Radiation, followed by surface heat flux and only a small amount supplied by latent heat

Where was energy balance modelling used and what did this show?

This was used at Haut Glacier D’Arolla and it was found that a key condition was the roughness of surface ice that changed albedo and the shading effect of surrounding topography which impacted the surface heat flux

What are the drawbacks of energy balance models?

They require lots of data meaning there is a large amount of equipment and maintenance needed; this means they can be effective at small scales but the need for large volumes of data makes them hard to scale up

What did the hybrid model attempt to improve?

Wanted to improve the temporal and spatial resolution of the degree-day model but without using as many parameters as the energy-balance model

How does the hybrid model model mass balance changes?

The temperature-index hybrid model uses the classical degree-day model supplemented by clear sky potential (clouding) and local topography to impact global radiation

Where was the hybrid model used and what did this show?

This was used at Storglaciaren to model hourly melt rates, resulting in large diurnal cycles of melt as well as showing that the south-facing slopes had much higher melt rates than north-facing ones due to the impact of local topography and cloudiness