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Errors and uncertainties in regional climate simulations of rainfall variability over Tunisia: a multi-model and multi-member approach

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Abstract

Temporal and spatial variability of rainfall over Tunisia (at 12 km spatial resolution) is analyzed in a multi-year (1992–2011) ten-member ensemble simulation performed using the WRF model, and a sample of regional climate hindcast simulations from Euro-CORDEX. RCM errors and skills are evaluated against a dense network of local rain gauges. Uncertainties arising, on the one hand, from the different model configurations and, on the other hand, from internal variability are furthermore quantified and ranked at different timescales using simple spread metrics. Overall, the WRF simulation shows good skill for simulating spatial patterns of rainfall amounts over Tunisia, marked by strong altitudinal and latitudinal gradients, as well as the rainfall interannual variability, in spite of systematic errors. Mean rainfall biases are wet in both DJF and JJA seasons for the WRF ensemble, while they are dry in winter and wet in summer for most of the used Euro-CORDEX models. The sign of mean annual rainfall biases over Tunisia can also change from one member of the WRF ensemble to another. Skills in regionalizing precipitation over Tunisia are season dependent, with better correlations and weaker biases in winter. Larger inter-member spreads are observed in summer, likely because of (1) an attenuated large-scale control on Mediterranean and Tunisian climate, and (2) a larger contribution of local convective rainfall to the seasonal amounts. Inter-model uncertainties are globally stronger than those attributed to model’s internal variability. However, inter-member spreads can be of the same magnitude in summer, emphasizing the important stochastic nature of the summertime rainfall variability over Tunisia.

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Acknowledgements

We thank two anonymous reviewers for their comments that remarkably helped improve this study. WRF was provided by the University Corporation for Atmospheric Research website (for more information see http://www.mmm.ucar.edu/wrf/users/download/). ERA-Interim data were provided by the ECMWF. Calculations were performed using HPC resources from PSIUN-CCUB, Université de Bourgogne Franche-Comté (France). Euro-CORDEX data is downloaded from the “Earth System Grid Federation: ESGF” nodes (http://www.euro-cordex.net/060378/index.php.en). We acknowledge Dr. Kirsten Warrach-Sagi from the University of Hohenheim (Germany) for providing the UHOH WRF simulation data. We also acknowledge Lotfi Boughrara from the “DGRE” of Tunisia and the CRU, UDEL, TRMM, GPCC and GPCP datasets providers for providing precipitation records.

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Correspondence to Bilel Fathalli.

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See Fig. 14.

Fig. 14
figure 14

As Fig. 11 but for the Mediterranean domain

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Fathalli, B., Pohl, B., Castel, T. et al. Errors and uncertainties in regional climate simulations of rainfall variability over Tunisia: a multi-model and multi-member approach. Clim Dyn 52, 335–361 (2019). https://doi.org/10.1007/s00382-018-4150-2

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  • DOI: https://doi.org/10.1007/s00382-018-4150-2

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