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Using nudging to improve global-regional dynamic consistency in limited-area climate modeling: What should we nudge?

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Abstract

Regional climate modelling sometimes requires that the regional model be nudged towards the large-scale driving data to avoid the development of inconsistencies between them. These inconsistencies are known to produce large surface temperature and rainfall artefacts. Therefore, it is essential to maintain the synoptic circulation within the simulation domain consistent with the synoptic circulation at the domain boundaries. Nudging techniques, initially developed for data assimilation purposes, are increasingly used in regional climate modeling and offer a workaround to this issue. In this context, several questions on the “optimal” use of nudging are still open. In this study we focus on a specific question which is: What variable should we nudge? in order to maintain the consistencies between the regional model and the driving fields as much as possible. For that, a “Big Brother Experiment”, where a reference atmospheric state is known, is conducted using the weather research and forecasting (WRF) model over the Euro–Mediterranean region. A set of 22 3-month simulations is performed with different sets of nudged variables and nudging options (no nudging, indiscriminate nudging, spectral nudging) for summer and winter. The results show that nudging clearly improves the model capacity to reproduce the reference fields. However the skill scores depend on the set of variables used to nudge the regional climate simulations. Nudging the tropospheric horizontal wind is by far the key variable to nudge to simulate correctly surface temperature and wind, and rainfall. To a lesser extent, nudging tropospheric temperature also contributes to significantly improve the simulations. Indeed, nudging tropospheric wind or temperature directly impacts the simulation of the tropospheric geopotential height and thus the synoptic scale atmospheric circulation. Nudging moisture improves the precipitation but the impact on the other fields (wind and temperature) is not significant. As an immediate consequence, nudging tropospheric wind, temperature and moisture in WRF gives by far the best results with respect to the Big-Brother simulation. However, we noticed that a residual bias of the geopotential height persists due to a negative surface pressure anomaly which suggests that surface pressure is the missing quantity to nudge. Nudging the geopotential has no discernible effect. Finally, it should be noted that the proposed strategy ensures a dynamical consistency between the driving field and the simulated small-scale field but it does not ensure the best “observed” fine scale field because of the possible impact of incorrect driving large-scale field.

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Acknowledgments

We are thankful to R. Laprise for fruitful discussion. This research has received funding from the ANR-MEDUP project, GIS “Climat-Environnement-Société” MORCE-MED project, and through ADEME (Agence de l’Environnement et de la Maîtrise de l’Energie) contract 0705C0038. It was also supported by the IPSL group for regional climate and environmental studies. This work also contributes to the HyMeX program (HYdrological cycle in The Mediterranean EXperiment) through INSU-MISTRALS support and the Med-CORDEX program (A COordinated Regional climate Downscaling EXperiment—Mediterranean region).

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  1. Laboratoire de Météorologie Dynamique, CNRS, Institut Pierre Simon Laplace, Ecole Polytechnique, 91128, Palaiseau Cedex, France

    Hiba Omrani, Philippe Drobinski & Thomas Dubos

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  1. Hiba Omrani
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  2. Philippe Drobinski
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Correspondence to Hiba Omrani.

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Omrani, H., Drobinski, P. & Dubos, T. Using nudging to improve global-regional dynamic consistency in limited-area climate modeling: What should we nudge?. Clim Dyn 44, 1627–1644 (2015). https://doi.org/10.1007/s00382-014-2453-5

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  • DOI: https://doi.org/10.1007/s00382-014-2453-5

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