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A locally time-invariant metric for climate model ensemble predictions of extreme risk

Published online by Cambridge University Press:  07 July 2023

Mala Virdee*
Affiliation:
Department of Computer Science and Technology, University of Cambridge, Cambridge, United Kingdom
Markus Kaiser
Affiliation:
Department of Computer Science and Technology, University of Cambridge, Cambridge, United Kingdom Monumo Ltd., Cambridge, United Kingdom
Carl H. Ek
Affiliation:
Department of Computer Science and Technology, University of Cambridge, Cambridge, United Kingdom
Emily Shuckburgh
Affiliation:
Department of Computer Science and Technology, University of Cambridge, Cambridge, United Kingdom
Ieva Kazlauskaite
Affiliation:
Department of Engineering, University of Cambridge, Cambridge, United Kingdom
*
Corresponding author: Mala Virdee; Email: mv490@cam.ac.uk

Abstract

Adaptation-relevant predictions of climate change are often derived by combining climate model simulations in a multi-model ensemble. Model evaluation methods used in performance-based ensemble weighting schemes have limitations in the context of high-impact extreme events. We introduce a locally time-invariant method for evaluating climate model simulations with a focus on assessing the simulation of extremes. We explore the behavior of the proposed method in predicting extreme heat days in Nairobi and provide comparative results for eight additional cities.

Information

Type
Methods Paper
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2023. Published by Cambridge University Press
Figure 0

Figure 1. The locally time-invariant skill metric $ \mathrm{\mathcal{L}} $ is used to compare a simulated time series $ A $ to a reference time series $ B $. Instead of calculating the pairwise least-squares error, we propose adding a slack in either direction for reference points with which each simulated data point can be matched. In this illustration, a slack of one time step in either direction is added, as represented by the green shapes. We then find an optimal bipartite matching $ \pi $ that minimizes the sum of distances between the time series. On the left, data points are compared out of order in overlapping windows to calculate distance between $ A $ and $ B $. On the right, we emphasize that the bipartite matching enforces the constraint that no data point in either time series can be used twice.

Figure 1

Table 1. Results from six multi-model ensemble methods for Nairobi, evaluated against ERA5 reference data. For each method, the predicted number of extreme heat days n in the train and test periods, and RMSE for daily mean temperature predictions for these extreme heat days are shown. The locally time-invariant skill $ {\mathrm{\mathcal{L}}}^{15} $ for predicted temperature for extreme heat days in the test period is also shown.

Figure 2

Figure 2. Left: Sample daily mean temperature time series from Nairobi for a period where observed daily average temperatures exceed historical 90th quantile threshold for several consecutive days, showing individual ensemble members, ERA5 reference, multi-model mean baseline, Bayesian model averaging (BMA) (a), and BMA ($ {\pi}_{15} $) (c) predictions. The shaded region indicates the $ \pm $2 standard deviations from BMA predictions. The dotted vertical line indicates the date of cross-section shown right. Right: Cross-section of BMA (b) and BMA ($ {\pi}_{15} $) (d) predictive distributions and individual BMA-weighted ensemble members for 1 day. In this example, BMA ($ {\pi}_{15} $) has assigned greater weight to a model that predicted a higher temperature.

Figure 3

Figure 3. Evaluation of six multi-model ensemble methods for experiments across nine cities, showing (a): RMSE for predicting daily average temperature for all days; and (b): RMSE for predicting daily average temperature for extreme heat days.

Figure 4

Figure 4. Summary of rankings of six multi-model ensemble methods for experiments across nine cities, ranked by (a): RMSE in predicting daily average temperature for all days; (b): RMSE in predicting daily average temperature for extreme heat days; and (c): absolute error for predicting number of extreme heat days. In each case, the best-performing method is rank $ 1 $.

Figure 5

Figure 5. Climate model weights calculated from five Bayesian model averaging methods for experiments from nine cities.

Figure 6

Figure 6. Distributions of daily average temperature for test period from each general circulation model simulation and the ERA5 reference for nine cities.

Supplementary material: PDF

Virdee et al. supplementary material

Appendices A-B

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