Why Glen’s Flow Rate Factor A is Tied to Temperature and the Implications of Treating it as Constant in Glaciology

The Glen Flow Law

The Glen Flow Law is an important mathematical relationship used in glaciology to describe the deformation of ice. It relates the strain rate of ice to the applied stress through a power law relationship. The flow law is expressed as

ε̇ = A(τ)τ^n

where ε̇ is the strain rate, τ is the deviatoric stress, and A is the flow rate factor, which is a function of temperature. The exponent n is typically assumed to be 3.

The flow factor A represents the sensitivity of the ice to deformation. It is a measure of how easily ice can deform for a given stress. The flow factor depends on many factors, including temperature, crystal orientation, impurities, and strain rate. Of these factors, temperature is one of the most important.

The Effect of Temperature on Flow Factor A

Flow factor A is a function of temperature, and it has been found that A is proportional to an exponential function of temperature. The relationship is described by the following equation:

A = A₀ exp(Q/RT)

where A₀ is a constant, Q is the activation energy for ice deformation, R is the gas constant, andT is the absolute temperature in Kelvin. This equation shows that the flow factor increases exponentially with temperature, meaning that ice is more sensitive to deformation at higher temperatures.
The activation energy Q is an important parameter characterizing the energy barrier for ice deformation. It is related to the energy required to break the bonds between ice molecules and move them to new positions. Q is generally found to be about 60 kJ/mol for ice, which is similar to the activation energy for other crystalline materials.

The exponential relationship between A and temperature has been confirmed by laboratory experiments and field observations. For example, studies have shown that the flow factor of glacial ice increases by a factor of 2-3 over a temperature range of 0 to -10°C. This means that a small change in temperature can have a large effect on ice deformation.

Why we assume A to be constant

Although A is temperature dependent, it is often assumed to be constant in glaciological models. This is because the temperature profile of glaciers and ice sheets typically changes very slowly over time, and therefore the temperature dependence of A can be considered a relatively small effect.

In addition, assuming A to be constant simplifies the mathematical relationships used in glaciological models, making them easier to solve and interpret. In many cases, the assumption of a constant A has been found to produce results that are consistent with observations.
However, it is important to note that assuming A to be constant may not always be accurate, especially in cases where there are significant changes in temperature over time. For example, if a glacier experiences a rapid warming event, the temperature dependence of A may become more important, and assuming a constant A could lead to inaccurate predictions of glacier deformation.

In addition, recent studies have suggested that there may be more complex temperature dependencies of A that are not captured by the simple exponential relationship described above. These studies have proposed alternative functional forms for A that better capture the observed temperature dependence of glacier deformation.

In summary, while it is known that the flow factor A is temperature dependent, it is often assumed to be constant in glaciological models due to the slow changing nature of glacier temperatures and the mathematical simplifications this assumption provides. However, it is important to recognize that this assumption may not always be correct and that more complex temperature dependencies of A may exist.

Conclusion

Understanding the temperature dependence of the flow factor A is important for accurately predicting the deformation of glaciers and ice sheets. The exponential relationship between A and temperature has been confirmed by laboratory experiments and field observations, and it has been found that ice is more sensitive to deformation at higher temperatures. However, despite its temperature dependence, A is often assumed to be constant in glaciological models due to the slow changing nature of glacier temperatures and the mathematical simplifications this assumption provides. While this assumption may be correct in many cases, it is important to recognize its limitations and potential inaccuracies, particularly in cases where there are significant changes in temperature over time. Ongoing research is helping to refine our understanding of the temperature dependence of A and to develop more accurate and complex mathematical relationships for glaciological modeling. By continuing to improve our understanding of the underlying physics and processes that govern glacier deformation, we can better predict the behavior of glaciers and ice sheets in a changing climate and inform strategies for managing the impacts of these changes on our planet.

FAQs

1. What is the Glen flow law?

The Glen flow law is an important mathematical relationship used in glaciology to describe the deformation of ice. It relates the strain rate of ice to the applied stress through a power-law relationship.

2. What is the flow rate factor A?

The flow rate factor A represents the sensitivity of the ice to deformation. It is a measure of how easily ice can deform under a given stress.

3. How is the flow rate factor A related to temperature?

The flow rate factor A is a function of temperature, and it has been found that A is proportional to an exponential function of temperature. The relationship is described by the equation A = A₀ exp(Q/RT), where A₀ is a constant, Q is the activation energy for ice deformation, R is the gas constant, and T is the absolute temperature in Kelvin.

4. Why is the temperature dependence of the flow rate factor A important?

The temperature dependence of the flow rate factor A is important for accurately predicting the deformation of glaciers and ice sheets. The exponential relationship between A and temperature has been confirmed by laboratory experiments and field observations, and it has been found that ice is more sensitive to deformation at higher temperatures.

5. Why is the flow rate factor A often assumed to be constant?

The flow rate factor A is often assumed to be constant in glaciological models because the temperature profile of glaciers and ice sheets is typically very slow to change over time. Additionally, assuming A to be constant simplifies the mathematical relationships used in glaciological models, making them easier to solve and interpret. In many cases, assuming a constant A has been found to produce results that are consistent with observations.

6. What are the limitations of assuming A to be constant?

Assuming A to be constant may not always be accurate, especially in cases where there are significant changes in temperature over time. For example, if a glacier experiences a rapid warming event, the temperature dependence of A may become more important and assuming a constant A could lead to inaccurate predictions of glacial deformation. Furthermore, recent studies have suggested that there may be more complex temperature dependencies of A that are not captured by the simple exponential relationship.

7. How can ongoing research improve our understanding of the temperature dependence of A?

Ongoing research is helping to refine our understanding of the temperature dependence of A and to develop more accurate and complex mathematical relationships for glaciological modeling. By continuing to improve our understanding of the underlying physics and processes governing glacial deformation, we can better predict the behavior of glaciers and ice sheets in a changing climate and inform strategies for managing the impacts of these changes on our planet.