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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 12
Geosci. Model Dev., 10, 4563-4575, 2017
https://doi.org/10.5194/gmd-10-4563-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Geosci. Model Dev., 10, 4563-4575, 2017
https://doi.org/10.5194/gmd-10-4563-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Model description paper 15 Dec 2017

Model description paper | 15 Dec 2017

A method to encapsulate model structural uncertainty in ensemble projections of future climate: EPIC v1.0

Jared Lewis1, Greg E. Bodeker1, Stefanie Kremser1, and Andrew Tait2 Jared Lewis et al.
  • 1Bodeker Scientific, 42 Russell Street, Alexandra, 9320, New Zealand
  • 2National Institute of Water and Atmospheric Research, Wellington, New Zealand

Abstract. A method, based on climate pattern scaling, has been developed to expand a small number of projections of fields of a selected climate variable (X) into an ensemble that encapsulates a wide range of indicative model structural uncertainties. The method described in this paper is referred to as the Ensemble Projections Incorporating Climate model uncertainty (EPIC) method. Each ensemble member is constructed by adding contributions from (1) a climatology derived from observations that represents the time-invariant part of the signal; (2) a contribution from forced changes in X, where those changes can be statistically related to changes in global mean surface temperature (Tglobal); and (3) a contribution from unforced variability that is generated by a stochastic weather generator. The patterns of unforced variability are also allowed to respond to changes in Tglobal. The statistical relationships between changes in X (and its patterns of variability) and Tglobal are obtained in a training phase. Then, in an implementation phase, 190 simulations of Tglobal are generated using a simple climate model tuned to emulate 19 different global climate models (GCMs) and 10 different carbon cycle models. Using the generated Tglobal time series and the correlation between the forced changes in X and Tglobal, obtained in the training phase, the forced change in the X field can be generated many times using Monte Carlo analysis. A stochastic weather generator is used to generate realistic representations of weather which include spatial coherence. Because GCMs and regional climate models (RCMs) are less likely to correctly represent unforced variability compared to observations, the stochastic weather generator takes as input measures of variability derived from observations, but also responds to forced changes in climate in a way that is consistent with the RCM projections. This approach to generating a large ensemble of projections is many orders of magnitude more computationally efficient than running multiple GCM or RCM simulations. Such a large ensemble of projections permits a description of a probability density function (PDF) of future climate states rather than a small number of individual story lines within that PDF, which may not be representative of the PDF as a whole; the EPIC method largely corrects for such potential sampling biases. The method is useful for providing projections of changes in climate to users wishing to investigate the impacts and implications of climate change in a probabilistic way. A web-based tool, using the EPIC method to provide probabilistic projections of changes in daily maximum and minimum temperatures for New Zealand, has been developed and is described in this paper.

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The Ensemble Projections Incorporating Climate model uncertainty (EPIC) method uses climate pattern scaling to expand a small number of daily maximum and minimum surface temperature projections into an ensemble that captures the structural uncertainty between climate models. The method is useful for providing projections of changes in climate to users wishing to investigate the impacts of climate change in a probabilistic and computationally efficient way.
The Ensemble Projections Incorporating Climate model uncertainty (EPIC) method uses climate...
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