# Temperature index model calibrated on traditional MB data#

The first mass balance (MB) model ever implemented in OGGM is an extended version of the temperature-index model presented by Marzeion et al., (2012).

While the equation governing the mass balance is that of a traditional temperature-index model, our special approach to calibration requires that we spend some time describing it.

The monthly mass balance $$B_i$$ at elevation $$z$$ is computed as:

$B_i(z) = P_i^{Solid}(z) - \mu ^{*} \, max \left( T_i(z) - T_{Melt}, 0 \right) + \epsilon$

where $$P_i^{Solid}$$ is the monthly solid precipitation, $$T_i$$ the monthly temperature and $$T_{Melt}$$ is the monthly mean air temperature above which ice melt is assumed to occur (-1°C per default). Solid precipitation is computed out of the total precipitation. The fraction of solid precipitation is based on the monthly mean temperature: all solid below temp_all_solid (default: 0°C) and all liquid above temp_all_liq (default: 2°C), linear change in between.

The parameter $$\mu ^{*}$$ indicates the temperature sensitivity of the glacier, and it needs to be calibrated. $$\epsilon$$ is a residual, to be determined at the calibration step.

OGGM needs to compute the temperature and precipitation at the altitude of the glacier grid points. The default is to use a fixed lapse rate of -6.5K km $$^{-1}$$ and no gradient for precipitation.

## Calibration#

We will start by making two observations:

• the sensitivity parameter $$\mu ^{*}$$ is depending on many parameters, most of them being glacier-specific (e.g. avalanches, topographical shading, cloudiness…).

• the sensitivity parameter $$\mu ^{*}$$ will be affected by uncertainties and systematic biases in the input climate data.

As a result, $$\mu ^{*}$$ can vary greatly between neighboring glaciers. The calibration procedure introduced by Marzeion et al., (2012) and implemented in OGGM makes full use of these apparent handicaps by turning them into assets.

The calibration procedure starts with glaciers for which we have direct observations of the annual specific mass balance SMB. We use the WGMS FoG (shipped with OGGM) for this purpose.

For each of these glaciers, time-dependent “candidate” temperature sensitivities $$\mu (t)$$ are estimated by requiring that the average specific mass balance $$\overline{B_{31}}$$ is equal to zero. $$\overline{B_{31}}$$ is computed for a 31-year period centered around the year $$t$$ and for a constant glacier geometry fixed at the RGI date (e.g. 2003 for most glaciers in the European Alps).

In [1]: example_plot_mu_ts()  # the code for these examples is posted below


Around 1900, the climate was cold and wet. As a consequence, the temperature sensitivity required to maintain the 2003 glacier geometry is high. Inversely, the recent climate is warm and the glacier must have a small temperature sensitivity in order to preserve its 2003 geometry.

Note that these $$\mu (t)$$ are just hypothetical sensitivities necessary to maintain the glacier in equilibrium in an average climate at the year $$t$$. We call them “candidates”, since one (or more) of them is likely to be close to the “real” sensitivity of the glacier.

This is when the mass balance observations come into play: each of these candidates is used to compute the mass balance during the period were we have observations. We then compare the model output with the expected mass balance and compute the model bias:

In [2]: example_plot_bias_ts()  # the code for these examples is posted below


The residual bias is positive when $$\mu$$ is too low, and negative when $$\mu$$ is too high. Here, the residual bias crosses the zero line twice. The two dates where the zero line is crossed correspond to approximately the same $$\mu$$ (but not exactly, as precipitation and temperature both have an influence on it). These two dates at which the $$\mu$$ candidates are close to the “real” $$\mu$$ are called $$t^*$$ (the associated sensitivities $$\mu (t^*)$$ are called $$\mu^*$$). For the next step, one $$t^*$$ is sufficient: we pick the one which corresponds to the smallest absolute residual bias.

At the glaciers where observations are available, this detour via the $$\mu$$ candidates is not necessary to find the correct $$\mu^*$$. Indeed, the goal of these computations are in fact to find $$t^*$$, which is the actual value interpolated to glaciers where no observations are available.

The benefit of this approach is best shown with the results of a cross-validation study realized by Marzeion et al., (2012) and confirmed by OGGM:

This substantial improvement in model performance is due to several factors:

• the equilibrium constraint applied on $$\mu$$ implies that the sensitivity cannot vary much during the last century. In fact, $$\mu$$ at one glacier varies far less in one century than between neighboring glaciers, because of all the factors mentioned above. In particular, it will vary comparatively little around a given year $$t$$ : errors in $$t^*$$ (even large) will result in small errors in $$\mu^*$$.

• the equilibrium constraint will also imply that systematic biases in temperature and precipitation (no matter how large) will automatically be compensated by all $$\mu (t)$$, and therefore also by $$\mu^*$$. In that sense, the calibration procedure can be seen as a empirically driven downscaling strategy: if a glacier is here, then the local climate (or the glacier temperature sensitivity) must allow a glacier to be there. For example, the effect of avalanches or a negative bias in precipitation input will have the same impact on calibration: $$\mu^*$$ should be reduced to take these effects into account, even though they are not resolved by the mass balance model.

The most important drawback of this calibration method is that it assumes that two neighboring glaciers should have a similar $$t^*$$. This is not necessarily the case, as other factors than climate (such as the glacier size) will influence $$t^*$$ too. Our results (and the arguments listed above) show however that this is an approximation we can cope with.

In a final note, it is important to mention that this procedure is primarily a calibration method, and as such it can be statistically scrutinized (for example with cross-validation). It can also be noted that the MB observations play a relatively minor role in the calibration: they could be entirely avoided by fixing a $$t^*$$ for all glaciers in a region (or even worldwide). The resulting changes in calibrated $$\mu^*$$ will be comparatively small (again, because of the local constraints on $$\mu$$). The MB observations, however, play a major role for the assessment of model uncertainty.

## Regional calibration#

New in version 1.4!

As of version 1.4, we now also offer to calibrate the mass balance at the regional level (RGI regions) based on geodetic mass balance products ([Zemp_et_al_2019] or [Hugonnet_et_al_2021], see oggm.workflow.match_regional_geodetic_mb()). This is done by correcting (shifting) the residual for each glacier ($$\epsilon$$ in the equation above) by a constant value so that the regional estimates match the observations. This is not applied per default, as it might lead to unrealistic results at the single glacier scale (but it is very useful for global studies). An overview of the regional shifts for all regions and presently available set-ups can be assessed via this notebook. In the preprocessed directories, it is available under the path index match_geod.

## References#

Chimani_et_al_2012

Chimani, B., Matulla, C., Böhm, R., Hofstätter, M.: A new high resolution absolute Temperature Grid for the Greater Alpine Region back to 1780, Int. J. Climatol., 33(9), 2129–2141, DOI 10.1002/joc.3574, 2012.

Harris_et_al_2010

Harris, I., Jones, P. D., Osborn, T. J., & Lister, D. H. (2014). Updated high-resolution grids of monthly climatic observations - the CRU TS3.10 Dataset. International Journal of Climatology, 34(3), 623–642. https://doi.org/10.1002/joc.3711

Hugonnet_et_al_2021

Hugonnet, R., McNabb, R., Berthier, E. et al. Accelerated global glacier mass loss in the early twenty-first century, Nature 592, 726–731 (2021). https://doi.org/10.1038/s41586-021-03436-z

Zemp_et_al_2019

Zemp, M., Huss, M., Thibert, E., Eckert, N., McNabb, R., Huber, J., Barandun, M., Machguth, H., Nussbaumer, S. U., Gärtner-Roer, I., Thomson, L., Paul, F., Maussion, F., Kutuzov, S. and Cogley, J. G.: Global glacier mass changes and their contributions to sea-level rise from 1961 to 2016, Nature, 568(7752), 382–386, doi:10.1038/s41586-019-1071-0, 2019.

## Implementation details#

If you had the courage to read until here, it means that you have concrete questions about the implementation of the mass balance model in OGGM. Here are some more details:

• the mass balance in OGGM is computed from the altitudes and widths of the flowlines grid points (see Glacier flowlines). The easiest way to let OGGM compute the mass balance for you is to use the core.massbalance.PastMassBalance.

• the interpolation of $$t^*$$ is done with an inverse distance weighting algorithm (see tasks.local_t_star())

• if more than one $$t^*$$ is found for some reference glaciers, than the glaciers with only one $$t^*$$ will determine the most likely $$t^*$$ for the other glaciers (see tasks.compute_ref_t_stars())

• yes, the temperature gradients and the precipitation scaling factor will have an influence on the results, but it is small since any change will automatically be compensated by $$\mu^*$$. We are currently quantifying these effects more precisely.