Bimodality in ensemble forecasts of 2 m temperature: identification
International audience Bimodality and other types of non-Gaussianity arise in ensemble forecasts of the atmosphere as a result of nonlinear spread across ensemble members. In this paper, bimodality in 50-member ECMWF ENS-extended ensemble forecasts is identified and characterized. Forecasts of 2 m t...
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ftccsdartic:oai:HAL:insu-03726928v1 2023-11-05T03:45:00+01:00 Bimodality in ensemble forecasts of 2 m temperature: identification Bertossa, Cameron Hitchcock, Peter Degaetano, Arthur Plougonven, Riwal Cornell University New York Laboratoire de Météorologie Dynamique (UMR 8539) (LMD) Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris École normale supérieure - Paris (ENS-PSL) Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS-PSL) Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL) 2021 https://insu.hal.science/insu-03726928 https://insu.hal.science/insu-03726928/document https://insu.hal.science/insu-03726928/file/wcd-2-1209-2021.pdf https://doi.org/10.5194/wcd-2-1209-2021 en eng HAL CCSD Copernicus info:eu-repo/semantics/altIdentifier/doi/10.5194/wcd-2-1209-2021 insu-03726928 https://insu.hal.science/insu-03726928 https://insu.hal.science/insu-03726928/document https://insu.hal.science/insu-03726928/file/wcd-2-1209-2021.pdf BIBCODE: 2021WCD.2.1209B doi:10.5194/wcd-2-1209-2021 info:eu-repo/semantics/OpenAccess ISSN: 2698-4016 Weather and Climate Dynamics https://insu.hal.science/insu-03726928 Weather and Climate Dynamics, 2021, 2, pp.1209-1224. ⟨10.5194/wcd-2-1209-2021⟩ [SDU]Sciences of the Universe [physics] [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology info:eu-repo/semantics/article Journal articles 2021 ftccsdartic https://doi.org/10.5194/wcd-2-1209-2021 2023-10-07T22:59:50Z International audience Bimodality and other types of non-Gaussianity arise in ensemble forecasts of the atmosphere as a result of nonlinear spread across ensemble members. In this paper, bimodality in 50-member ECMWF ENS-extended ensemble forecasts is identified and characterized. Forecasts of 2 m temperature are found to exhibit widespread bimodality well over a derived false-positive rate. In some regions bimodality occurs in excess of 30 % of forecasts, with the largest rates occurring during lead times of 2 to 3 weeks. Bimodality occurs more frequently in the winter hemisphere with indications of baroclinicity being a factor to its development. Additionally, bimodality is more common over the ocean, especially the polar oceans, which may indicate development caused by boundary conditions (such as sea ice). Near the equatorial region, bimodality remains common during either season and follows similar patterns to the Intertropical Convergence Zone (ITCZ), suggesting convection as a possible source for its development. Over some continental regions the modes of the forecasts are separated by up to 15 °C. The probability density for the modes can be up to 4 times greater than at the minimum between the modes, which lies near the ensemble mean. The widespread presence of such bimodality has potentially important implications for decision makers acting on these forecasts. Bimodality also has implications for assessing forecast skill and for statistical postprocessing: several commonly used skill-scoring methods and ensemble dressing methods are found to perform poorly in the presence of bimodality, suggesting the need for improvements in how non-Gaussian ensemble forecasts are evaluated. Article in Journal/Newspaper Sea ice Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) Weather and Climate Dynamics 2 4 1209 1224 |
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Archive ouverte HAL (Hyper Article en Ligne, CCSD - Centre pour la Communication Scientifique Directe) |
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ftccsdartic |
language |
English |
topic |
[SDU]Sciences of the Universe [physics] [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology |
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[SDU]Sciences of the Universe [physics] [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology Bertossa, Cameron Hitchcock, Peter Degaetano, Arthur Plougonven, Riwal Bimodality in ensemble forecasts of 2 m temperature: identification |
topic_facet |
[SDU]Sciences of the Universe [physics] [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology |
description |
International audience Bimodality and other types of non-Gaussianity arise in ensemble forecasts of the atmosphere as a result of nonlinear spread across ensemble members. In this paper, bimodality in 50-member ECMWF ENS-extended ensemble forecasts is identified and characterized. Forecasts of 2 m temperature are found to exhibit widespread bimodality well over a derived false-positive rate. In some regions bimodality occurs in excess of 30 % of forecasts, with the largest rates occurring during lead times of 2 to 3 weeks. Bimodality occurs more frequently in the winter hemisphere with indications of baroclinicity being a factor to its development. Additionally, bimodality is more common over the ocean, especially the polar oceans, which may indicate development caused by boundary conditions (such as sea ice). Near the equatorial region, bimodality remains common during either season and follows similar patterns to the Intertropical Convergence Zone (ITCZ), suggesting convection as a possible source for its development. Over some continental regions the modes of the forecasts are separated by up to 15 °C. The probability density for the modes can be up to 4 times greater than at the minimum between the modes, which lies near the ensemble mean. The widespread presence of such bimodality has potentially important implications for decision makers acting on these forecasts. Bimodality also has implications for assessing forecast skill and for statistical postprocessing: several commonly used skill-scoring methods and ensemble dressing methods are found to perform poorly in the presence of bimodality, suggesting the need for improvements in how non-Gaussian ensemble forecasts are evaluated. |
author2 |
Cornell University New York Laboratoire de Météorologie Dynamique (UMR 8539) (LMD) Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris École normale supérieure - Paris (ENS-PSL) Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS-PSL) Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL) |
format |
Article in Journal/Newspaper |
author |
Bertossa, Cameron Hitchcock, Peter Degaetano, Arthur Plougonven, Riwal |
author_facet |
Bertossa, Cameron Hitchcock, Peter Degaetano, Arthur Plougonven, Riwal |
author_sort |
Bertossa, Cameron |
title |
Bimodality in ensemble forecasts of 2 m temperature: identification |
title_short |
Bimodality in ensemble forecasts of 2 m temperature: identification |
title_full |
Bimodality in ensemble forecasts of 2 m temperature: identification |
title_fullStr |
Bimodality in ensemble forecasts of 2 m temperature: identification |
title_full_unstemmed |
Bimodality in ensemble forecasts of 2 m temperature: identification |
title_sort |
bimodality in ensemble forecasts of 2 m temperature: identification |
publisher |
HAL CCSD |
publishDate |
2021 |
url |
https://insu.hal.science/insu-03726928 https://insu.hal.science/insu-03726928/document https://insu.hal.science/insu-03726928/file/wcd-2-1209-2021.pdf https://doi.org/10.5194/wcd-2-1209-2021 |
genre |
Sea ice |
genre_facet |
Sea ice |
op_source |
ISSN: 2698-4016 Weather and Climate Dynamics https://insu.hal.science/insu-03726928 Weather and Climate Dynamics, 2021, 2, pp.1209-1224. ⟨10.5194/wcd-2-1209-2021⟩ |
op_relation |
info:eu-repo/semantics/altIdentifier/doi/10.5194/wcd-2-1209-2021 insu-03726928 https://insu.hal.science/insu-03726928 https://insu.hal.science/insu-03726928/document https://insu.hal.science/insu-03726928/file/wcd-2-1209-2021.pdf BIBCODE: 2021WCD.2.1209B doi:10.5194/wcd-2-1209-2021 |
op_rights |
info:eu-repo/semantics/OpenAccess |
op_doi |
https://doi.org/10.5194/wcd-2-1209-2021 |
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Weather and Climate Dynamics |
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2 |
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4 |
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1209 |
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1224 |
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1781706224084975616 |