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Publications (10 of 13) Show all publications
De Luca, P., Messori, G., Wilby, R. L., Mazzoleni, M. & Di Baldassarre, G. (2020). Concurrent wet and dry hydrological extremes at the global scale. Earth System Dynamics, 11(1), 251-266
Open this publication in new window or tab >>Concurrent wet and dry hydrological extremes at the global scale
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2020 (English)In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 11, no 1, p. 251-266Article in journal (Refereed) Published
Abstract [en]

Multi-hazard events can be associated with larger socio-economic impacts than single-hazard events. Understanding the spatio-temporal interactions that characterize the former is therefore of relevance to disaster risk reduction measures. Here, we consider two high-impact hazards, namely wet and dry hydrological extremes, and quantify their global co-occurrence. We define these using the monthly self-calibrated Palmer Drought Severity Index based on the Penman-Monteith model (sc_PDSI_pm), covering the period 1950-2014, at 2.5 degrees horizontal resolution. We find that the land areas affected by extreme wet, dry, and wet-dry events (i.e. geographically remote yet temporally co-occurring wet or dry extremes) are all increasing with time, the trends of which in dry and wet-dry episodes are significant (p value << 0.01). The most geographically widespread wet-dry event was associated with the strong La Nina in 2010. This caused wet-dry anomalies across a land area of 21 million km(2) with documented high-impact flooding and drought episodes spanning diverse regions. To further elucidate the interplay of wet and dry extremes at a grid cell scale, we introduce two new metrics: the wet-dry (WD) ratio and the extreme transition (ET) time intervals. The WD ratio measures the relative occurrence of wet or dry extremes, whereas ET quantifies the average separation time of hydrological extremes with opposite signs. The WD ratio shows that the incidence of wet extremes dominates over dry extremes in the USA, northern and southern South America, northern Europe, north Africa, western China, and most of Australia. Conversely, dry extremes are more prominent in most of the remaining regions. The median ET for wet to dry is similar to 27 months, while the dry-to-wet median ET is 21 months. We also evaluate correlations between wet-dry hydrological extremes and leading modes of climate variability, namely the El Nina-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multi-decadal Oscillation (AMO). We find that ENSO and PDO have a similar influence globally, with the former significantly impacting (p value < 0.05) a larger area (18.1 % of total sc_PDSI_pm area) compared to the latter (12.0 %), whereas the AMO shows an almost inverse pattern and significantly impacts the largest area overall (18.9 %). ENSO and PDO show the most significant correlations over northern South America, the central and western USA, the Middle East, eastern Russia, and eastern Australia. On the other hand, the AMO shows significant associations over Mexico, Brazil, central Africa, the Arabian Peninsula, China, and eastern Russia. Our analysis brings new insights on hydrological multi-hazards that are of relevance to governments and organizations with globally distributed interests. Specifically, the multi-hazard maps may be used to evaluate worst-case disaster scenarios considering the potential co-occurrence of wet and dry hydrological extremes.

Place, publisher, year, edition, pages
COPERNICUS GESELLSCHAFT MBH, 2020
National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-408112 (URN)10.5194/esd-11-251-2020 (DOI)000518771000001 ()
Funder
Swedish Research Council, 2016-03724
Available from: 2020-04-04 Created: 2020-04-04 Last updated: 2020-04-04Bibliographically approved
Faranda, D., Messori, G. & Yiou, P. (2020). Diagnosing concurrent drivers of weather extremes: application to warm and cold days in North America. Climate Dynamics, 54(3-4), 2187-2201
Open this publication in new window or tab >>Diagnosing concurrent drivers of weather extremes: application to warm and cold days in North America
2020 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 54, no 3-4, p. 2187-2201Article in journal (Refereed) Published
Abstract [en]

A fundamental challenge in climate science is decomposing the concurrent drivers of weather extremes in observations. Achieving this can provide insights into the drivers of individual extreme events as well as into possible future changes in extreme event frequencies under greenhouse forcing. In the present work, we exploit recent results from dynamical systems theory to study the co-variation and recurrence statistics of different atmospheric variables. Specifically, we present a methodology to quantify the recurrences of bivariate variables and the coupling between distinct univariate variables in terms of their joint recurrences. The coupling is defined by a parameter which varies according to the chosen variables, season, and domain and can be understood in terms of the underlying physics of the atmosphere. For suitably chosen variables, this approach enables to decompose the different drivers of weather extremes. Here, we compute the above metrics for near-surface temperature and sea level pressure, and use them to study warm or cold days over North America. We first identify states where temperature is strongly or weakly coupled to the large-scale atmospheric circulation, and then elucidate the interplay between coupling and the occurrence of temperature extremes.

Place, publisher, year, edition, pages
SPRINGER, 2020
Keywords
Weather extremes, Analogues, Climate dynamics, Dynamical systems theory
National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-407728 (URN)10.1007/s00382-019-05106-3 (DOI)000514856500019 ()
Available from: 2020-03-27 Created: 2020-03-27 Last updated: 2020-03-27Bibliographically approved
Hochman, A., Alpert, P., Kunin, P., Rostkier-Edelstein, D., Harpaz, T., Saaroni, H. & Messori, G. (2020). The dynamics of cyclones in the twentyfirst century: the Eastern Mediterranean as an example. Climate Dynamics, 54(1-2), 561-574
Open this publication in new window or tab >>The dynamics of cyclones in the twentyfirst century: the Eastern Mediterranean as an example
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2020 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 54, no 1-2, p. 561-574Article in journal (Refereed) Published
Abstract [en]

The Mediterranean region is projected to be significantly affected by climate change through warming and drying. The Eastern Mediterranean (EM) is particularly vulnerable since the bulk of the precipitation in the region is associated with a specific circulation pattern, known as Cyprus Low (CL). Here, we study the influence of increased greenhouse gases on the average properties and dynamics of CLs, using a regional semi-objective synoptic classification. The classification is applied to NCEP/NCAR reanalysis data for the present day (1986-2005) as well as to eight CMIP5 models for the present day and for the end of the century (2081-2100; RCP8.5). This is complemented by a dynamical systems analysis, which is used to investigate changes in the dynamics and intrinsic predictability of the CLs. Finally, a statistical downscaling algorithm, based on past analogues, is applied to eighteen rain stations over Israel, and is used to project precipitation changes associated with CLs. Significant changes in CL properties are found under climate change. The models project an increase in CL meridional pressure gradient (0.5-1.5 hPa/1000 km), which results primarily from a strong increase in the pressure over the southern part of the study region. Our results further point to a decrease in CL frequency (- 35%, as already noted in an earlier study) and persistence (- 8%). Furthermore, the daily precipitation associated with CL occurrences over Israel for 2081-2100 is projected to significantly reduce (- 26%). The projected drying over the EM can be partitioned between a decrease in CL frequency ( 137 mm year(-1)) and a reduction in CL-driven daily precipitation ( 67 mm year(-1)). The models further indicate that CLs will be less predictable in the future.

Place, publisher, year, edition, pages
SPRINGER, 2020
Keywords
Cyprus low, Cyclone predictability, Climate change, Cyclone dynamics, Synoptic classification, Dynamical systems, Statistical downscaling, Daily precipitation
National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-408475 (URN)10.1007/s00382-019-05017-3 (DOI)000493693700001 ()
Funder
Swedish Research Council, 2016-03724
Available from: 2020-04-08 Created: 2020-04-08 Last updated: 2020-04-08Bibliographically approved
Hochman, A., Alpert, P., Harpaz, T., Saaroni, H. & Messori, G. (2019). A new dynamical systems perspective on atmospheric predictability: Eastern Mediterranean weather regimes as a case study. Science Advances, 5(6), Article ID eaau0936.
Open this publication in new window or tab >>A new dynamical systems perspective on atmospheric predictability: Eastern Mediterranean weather regimes as a case study
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2019 (English)In: Science Advances, E-ISSN 2375-2548, Vol. 5, no 6, article id eaau0936Article in journal (Refereed) Published
Abstract [en]

The atmosphere is a chaotic system displaying recurrent large-scale configurations. Recent developments in dynamical systems theory allow us to describe these configurations in terms of the local dimension-a proxy for the active number of degrees of freedom-and persistence in phase space, which can be interpreted as persistence in time. These properties provide information on the intrinsic predictability of an atmospheric state. Here, this technique is applied to atmospheric configurations in the eastern Mediterranean, grouped into synoptic classifications (SCs). It is shown that local dimension and persistence, derived from reanalysis and CMIP5 models' daily sea-level pressure fields, can serve as an extremely informative qualitative method for evaluating the predictability of the different SCs. These metrics, combined with the SC transitional probability approach, may be a valuable complement to operational weather forecasts and effective tools for climate model evaluation. This new perspective can be extended to other geographical regions.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2019
National Category
Meteorology and Atmospheric Sciences Climate Research
Identifiers
urn:nbn:se:uu:diva-390649 (URN)10.1126/sciadv.aau0936 (DOI)000473798500004 ()31183396 (PubMedID)
Funder
Swedish Research Council, 2016-03724
Available from: 2019-08-13 Created: 2019-08-13 Last updated: 2019-08-13Bibliographically approved
Messori, G., Ruiz-Perez, G., Manzoni, S. & Vico, G. (2019). Climate drivers of the terrestrial carbon cycle variability in Europe. Environmental Research Letters, 14(6), Article ID 063001.
Open this publication in new window or tab >>Climate drivers of the terrestrial carbon cycle variability in Europe
2019 (English)In: Environmental Research Letters, ISSN 1748-9326, E-ISSN 1748-9326, Vol. 14, no 6, article id 063001Article, review/survey (Refereed) Published
Abstract [en]

The terrestrial biosphere is a key component of the global carbon cycle and is heavily influenced by climate. Climate variability can be diagnosed through metrics ranging from individual environmental variables, to collections of variables, to the so-called climate modes of variability. Similarly, the impact of a given climate variation on the terrestrial carbon cycle can be described using several metrics, including vegetation indices, measures of ecosystem respiration and productivity and net biosphere-atmosphere fluxes. The wide range of temporal (from sub-daily to paleoclimatic) and spatial (from local to continental and global) scales involved requires a scale-dependent investigation of the interactions between the carbon cycle and climate. However, a comprehensive picture of the physical links and correlations between climate drivers and carbon cycle metrics at different scales remains elusive, framing the scope of this contribution. Here, we specifically explore how climate variability metrics (from single variables to complex indices) relate to the variability of the carbon cycle at sub-daily to interannual scales (i.e. excluding long-term trends). The focus is on the interactions most relevant to the European terrestrial carbon cycle. We underline the broad areas of agreement and disagreement in the literature, and conclude by outlining some existing knowledge gaps and by proposing avenues for improving our holistic understanding of the role of climate drivers in modulating the terrestrial carbon cycle.

Keywords
carbon cycle, climate, Europe, vegetation, soil
National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-387989 (URN)10.1088/1748-9326/ab1ac0 (DOI)000469809700001 ()
Funder
Swedish Research Council, 2016-06313Swedish Research Council, 2016-04146Swedish Research Council, 2016-03724Swedish Research Council Formas, 2018-01820Swedish Research Council Formas, 2018-00968
Available from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-06-27Bibliographically approved
Vichi, M., Eayrs, C., Alberello, A., Bekker, A., Bennetts, L., Holland, D., . . . Toffoli, A. (2019). Effects of an Explosive Polar Cyclone Crossing the Antarctic Marginal Ice Zone. Geophysical Research Letters, 46(11), 5948-5958
Open this publication in new window or tab >>Effects of an Explosive Polar Cyclone Crossing the Antarctic Marginal Ice Zone
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2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 11, p. 5948-5958Article in journal (Refereed) Published
Abstract [en]

Antarctic sea ice shows a large degree of regional variability, which is partly driven by severe weather events. Here we bring a new perspective on synoptic sea ice changes by presenting the first in situ observations of an explosive extratropical cyclone crossing the winter Antarctic marginal ice zone (MIZ) in the South Atlantic. This is complemented by the analysis of subsequent cyclones and highlights the rapid variations that ice-landing cyclones cause on sea ice: Midlatitude warm oceanic air is advected onto the ice, and storm waves generated close to the ice edge contribute to the maintenance of an unconsolidated surface through which waves propagate far into the ice. MIZ features may thus extend further poleward in the Southern Ocean than currently estimated. A concentration-based MIZ definition is inadequate, since it fails to describe a sea ice configuration which is deeply rearranged by synoptic weather. Plain Language Summary The extent of Antarctic sea ice is characterized by large regional variations that are in stark contrast with the alarming decreasing trends found in the Arctic. This is partly due to the presence of severe weather events, like extratropical cyclones travelling through the Southern Ocean and reaching the marginal ice zone (MIZ). The MIZ is a region where the ocean, atmosphere, and sea ice processes are closely interlinked. We provide direct evidence of how winter polar cyclones rearrange the MIZ and how their effects extend into the ice-covered region as far as the Antarctic continent. We present the first observations of large ice drift, ice concentration, and temperature changes as an explosively deepening cyclone crosses the MIZ. This case study is complemented by analysis of subsequent but more frequent storms that confirms how storminess in the Southern Ocean maintains a sea ice surface that is less compact, more mobile, and more extended than previously anticipated. Our results urge the scientific community to revise the current definition of the MIZ and improve its representation in models to better include the role of polar cyclones in detecting Antarctic sea ice trends.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2019
National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-392884 (URN)10.1029/2019GL082457 (DOI)000477616200035 ()
Funder
Swedish Research Council, 201603724The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), SA2017-7063
Available from: 2019-09-24 Created: 2019-09-24 Last updated: 2019-09-24Bibliographically approved
Scher, S. & Messori, G. (2019). Generalization properties of feed-forward neural networks trained on Lorenz systems. Nonlinear processes in geophysics, 26(4), 381-399
Open this publication in new window or tab >>Generalization properties of feed-forward neural networks trained on Lorenz systems
2019 (English)In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 26, no 4, p. 381-399Article in journal (Refereed) Published
Abstract [en]

Neural networks are able to approximate chaotic dynamical systems when provided with training data that cover all relevant regions of the system's phase space. However, many practical applications diverge from this idealized scenario. Here, we investigate the ability of feed-forward neural networks to (1) learn the behavior of dynamical systems from incomplete training data and (2) learn the influence of an external forcing on the dynamics Climate science is a real-world example where these questions may be relevant: it is concerned with a non-stationary chaotic system subject to external forcing and whose behavior is known only through comparatively short data series. Our analysis is performed on the Lorenz63 and Lorenz95 models. We show that for the Lorenz63 system, neural networks trained on data covering only part of the system's phase space struggle to make skillful short-term forecasts in the regions excluded from the training Additionally, when making long series of consecutive forecasts, the networks struggle to reproduce trajectories exploring regions beyond those seen in the training data, except for cases where only small parts are left out during training. We find this is due to the neural network learning a localized mapping for each region of phase space in the training data rather than a global mapping. This manifests itself in that parts of the networks learn only particular parts of the phase space. In contrast, for the Lorenz95 system the networks succeed in generalizing to new parts of the phase space not seen in the training data. We also find that the networks are able to learn the influence of an external forcing, but only when given relatively large ranges of the forcing in the training These results point to potential limitations of feed-forward neural networks in generalizing a system's behavior given limited initial information. Much attention must therefore be given to designing appropriate train-test splits for real-world applications.

National Category
Meteorology and Atmospheric Sciences Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-397963 (URN)10.5194/npg-26-381-2019 (DOI)000495389400001 ()
Funder
Swedish Research Council, 2016-03724
Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06Bibliographically approved
Scher, S. & Messori, G. (2019). How Global Warming Changes the Difficulty of Synoptic Weather Forecasting. Geophysical Research Letters, 46(5), 2931-2939
Open this publication in new window or tab >>How Global Warming Changes the Difficulty of Synoptic Weather Forecasting
2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 5, p. 2931-2939Article in journal (Refereed) Published
Abstract [en]

Global warming projections point to a wide range of impacts on the climate system, including changes in storm track activity and more frequent and intense extreme weather events. Little is however known on whether and how global warming may affect the atmosphere's predictability and thus our ability to produce accurate weather forecasts. Here, we combine a state-of-the-art climate and a state-of-the-art ensemble weather prediction model to show that, in a business-as-usual 21st century setting, global warming could significantly change the predictability of the atmosphere, defined here via the expected error of weather predictions. Predictability of synoptic weather situations could significantly increase, especially in the Northern Hemisphere. This can be explained by a decrease in the meridional temperature gradient. Contrarily, summertime predictability of weekly rainfall sums might significantly decrease in most regions.

Plain Language Summary

Due to the chaotic nature of the atmosphere, it is impossible to make weather forecasts that are completely accurate. Therefore, all weather forecasts are inherently uncertain to a certain degree. However, this uncertainty-and thus the "difficulty" of making good forecastsis not the same for all forecasts. This opens up the highly important question whether global warming will affect the difficulty of weather forecasts. Due to the enormous socioeconomic importance of accurate weather forecasts, it is essential to know whether climate change adaption policies also need to take into account potential changes in the difficulty and accuracy of weather forecasts. We show that in a warmer world, it will be easier to predict fields such as temperature and pressure. Contrarily, it will be harder to make accurate precipitation forecasts, which might strongly affect both disaster prevention and rainfall-dependent industries such as the energy sector, all of which heavily rely on accurate precipitation forecasts. Additionally, we show that the uncertainty of predictions of pressure fields is to a large extent controlled by fluctuations in the temperature difference between the North Pole and the equator. This is a new and important insight into the fundamentals of weather forecast uncertainty.

Keywords
ensemble forecasts, climate change, forecast uncertainty, synoptic meteorology
National Category
Meteorology and Atmospheric Sciences Climate Research
Identifiers
urn:nbn:se:uu:diva-381820 (URN)10.1029/2018GL081856 (DOI)000462612900066 ()
Funder
Swedish Research Council, 2016-03724Swedish National Infrastructure for Computing (SNIC)
Available from: 2019-04-24 Created: 2019-04-24 Last updated: 2019-04-24Bibliographically approved
Faranda, D., Sato, Y., Messori, G., Moloney, N. R. & Yiou, P. (2019). Minimal dynamical systems model of the Northern Hemisphere jet stream via embedding of climate data. Earth System Dynamics, 10(3), 555-567
Open this publication in new window or tab >>Minimal dynamical systems model of the Northern Hemisphere jet stream via embedding of climate data
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2019 (English)In: Earth System Dynamics, ISSN 2190-4979, E-ISSN 2190-4987, Vol. 10, no 3, p. 555-567Article in journal (Refereed) Published
Abstract [en]

We derive a minimal dynamical systems model for the Northern Hemisphere midlatitude jet dynamics by embedding atmospheric data and by investigating its properties (bifurcation structure, stability, local dimensions) for different atmospheric flow regimes. The derivation is a three-step process: first, we obtain a 1-D description of the midlatitude jet stream by computing the position of the jet at each longitude using ERA-Interim. Next, we use the embedding procedure to derive a map of the local jet position dynamics. Finally, we introduce the coupling and stochastic effects deriving from both atmospheric turbulence and topographic disturbances to the jet. We then analyze the dynamical properties of the model in different regimes: one that gives the closest representation of the properties extracted from real data; one featuring a stronger jet (strong coupling); one featuring a weaker jet (weak coupling); and one with modified topography. Our model, notwithstanding its simplicity, provides an instructive description of the dynamical properties of the atmospheric jet.

Place, publisher, year, edition, pages
COPERNICUS GESELLSCHAFT MBH, 2019
National Category
Climate Research
Identifiers
urn:nbn:se:uu:diva-395460 (URN)10.5194/esd-10-555-2019 (DOI)000486705600001 ()
Funder
EU, European Research Council, 338965-A2C2Swedish Research Council, 2016-03724EU, Horizon 2020, A2C2 (338965)
Available from: 2019-10-31 Created: 2019-10-31 Last updated: 2019-10-31Bibliographically approved
Lembo, V., Messori, G., Graversen, R. & Lucarini, V. (2019). Spectral Decomposition and Extremes of Atmospheric Meridional Energy Transport in the Northern Hemisphere Midlatitudes. Geophysical Research Letters, 46(13), 7602-7613
Open this publication in new window or tab >>Spectral Decomposition and Extremes of Atmospheric Meridional Energy Transport in the Northern Hemisphere Midlatitudes
2019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 13, p. 7602-7613Article in journal (Refereed) Published
Abstract [en]

The atmospheric meridional energy transport in the Northern Hemisphere midlatitudes is mainly accomplished by planetary and synoptic waves. A decomposition into wave components highlights the strong seasonal dependence of the transport, with both the total transport and the contributions from planetary and synoptic waves peaking in winter. In both winter and summer months, poleward transport extremes primarily result from a constructive interference between planetary and synoptic motions. The contribution of the mean meridional circulation is close to climatology. Equatorward transport extremes feature a mean meridional equatorward transport in winter, while the planetary and synoptic modes mostly transport energy poleward. In summer, a systematic destructive interference occurs, with planetary modes mostly transporting energy equatorward and synoptic modes again poleward. This underscores that baroclinic conversion dominates regardless of season in the synoptic wave modes, whereas the planetary waves can be either free or forced, depending on the season. Plain Language Summary The atmospheric heat transport from low to high latitudes is the main mechanism through which the climate reequilibrates the latitudinally uneven absorption of solar radiation. The atmospheric transport is fueled by instabilities driven by the presence of temperature differences between low and high latitudes and acts in such a way to reduce such gradient. This is one of the main stabilizing mechanisms of the climate system. In this work, we investigate how motions of different spatial scales contribute to atmospheric heat transports in the Northern Hemisphere. We discover that the relative importance of synoptic and planetary scale atmospheric motions is different in summer and winter. Our analysis delves into the analysis of events associated with extreme heat transport toward high latitudes, where we see a compensating mechanism between synoptic and planetary atmospheric motions. We further study days characterized by very large and very small (or even negative) heat transport toward the high latitudes. These "extreme events" are driven by complex interactions between the different scales. Our results are relevant for elucidating basic dynamical and thermodynamical properties of the atmosphere and can be used to benchmark the performance of climate models.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2019
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-391942 (URN)10.1029/2019GL082105 (DOI)000476960100061 ()
Funder
Swedish Research Council, 2016-03724EU, Horizon 2020, 727852The Research Council of Norway, 280727
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2020-03-25Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2032-5211

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