| 1. |
- De Luca, Paolo, et al.
(författare)
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Dynamical systems theory sheds new light on compound climate extremes in Europe and Eastern North America
- 2020
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 146:729, s. 1636-1650
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Tidskriftsartikel (refereegranskat)abstract
- We propose a novel approach to the study of compound extremes, grounded in dynamical systems theory. Specifically, we present the co‐recurrence ratio (α), which elucidates the dependence structure between variables by quantifying their joint recurrences. This approach is applied to daily climate extremes, derived from the ERA‐Interim reanalysis over the 1979–2018 period. The analysis focuses on concurrent (i.e., same‐day) wet (total precipitation) and windy (10 m wind gusts) extremes in Europe and concurrent cold (2 m temperature) extremes in Eastern North America and wet extremes in Europe. Results for wet and windy extremes in Europe, which we use as a test‐bed for our methodology, show that α peaks during boreal winter. High α values correspond to wet and windy extremes in northwestern Europe, and to large‐scale conditions resembling the positive phase of the North Atlantic Oscillation (NAO). This confirms earlier findings which link the positive NAO to a heightened frequency of extratropical cyclones impacting northwestern Europe. For the Eastern North America–Europe case, α extremes once again reflect concurrent climate extremes – in this case cold extremes over North America and wet extremes over Europe. Our analysis provides detailed spatial information on regional hotspots for these compound extreme occurrences, and encapsulates information on their spatial footprint which is typically not included in a conventional co‐occurrence analysis. We conclude that α successfully characterises compound extremes by reflecting the evolution of the associated meteorological maps. This approach is entirely general, and may be applied to different types of compound extremes and geographical regions.
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| 2. |
- Dey, Dipanjan, et al.
(författare)
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Identification of the atmospheric water sources and pathways responsible for the East Asian summer monsoon rainfall
- 2024
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - 0035-9009 .- 1477-870X. ; 150:759, s. 763-775
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Tidskriftsartikel (refereegranskat)abstract
- The East Asian summer monsoon rainfall provides water security and socio-economic benefit for over 20% of the global population. However, the sources of this rainfall and how it is carried to the East Asian landmass are still uncertain. To address this, atmospheric water sources and pathways associated with the East Asian summer rainfall are identified and quantified in this study using atmospheric water trajectories, calculated with a novel Lagrangian framework. Evaporated water from the East Asian landmass is found to be the major contributor to East Asian rainfall, amounting to local recycling. The results further indicated that the south Indian Ocean is a major non-local source for rainfall over southern East Asia during June to August. The role of the south Indian Ocean as a source of atmospheric water is one of the major findings of the study and would help in better understanding and predicting the East Asian summer rainfall. Evaporated waters from the Pacific Ocean (particularly the far-west Pacific Ocean) dominate the non-local contribution to precipitation over northern East Asia during June to September and over southern East Asian rainfall during September. The spatial structure of the East Asian rainfall is reported to be determined by the atmospheric waters that are evaporated and transported from the non-local sources. The role of the north Indian Ocean and the South Asian landmass as a source of water for East Asian precipitation is minimal and restricted to southern East Asia. The cross-equatorial Somali jet and equatorial trade winds associated with the western North Pacific subtropical high are important pathways for East Asian precipitation sourced over the south Indian Ocean and the Pacific Ocean respectively. In contrast, minor roles are attributed to the Bay of Bengal as a source, and midlatitude westerlies as a transport pathway, for East Asian precipitation. Randomly chosen atmospheric water trajectories were obtained from the backward tracing of the East Asian summer rainfall. These trajectories are a small subset of a total of 5 million trajectories. The green dots are the starting (net precipitation) positions and the blue dots indicate the ending locations (net evaporation) of the trajectories. Basins were defined as the south Indian Ocean (SIO), north Indian Ocean (NIO), South Asia (SA), East Asia (EA), Pacific Ocean (PAC), Atlantic Ocean (ATL). Atmospheric water sources and pathways associated with the East Asian summer rainfall are identified and quantified in this study using a novel Lagrangian framework. The results show the following: (1) Evaporated water from the East Asian landmass is found to be the major contributor to East Asian rainfall, amounting to local recycling. (2) The south Indian Ocean is a major non-local source for rainfall over southern East Asia during June-August. Evaporated waters from the Pacific Ocean dominate the non-local contribution to precipitation over northern East Asia during June-September. (3) The spatial structure of the East Asian rainfall is reported to be determined by the atmospheric waters that are evaporated and transported from the non-local sources. Minor roles are attributed to the Bay of Bengal as a source, and midlatitude westerlies as a transport pathway, for East Asian precipitation.image
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| 3. |
- Ekman, Annica M. L., 1972-, et al.
(författare)
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Influence of horizontal resolution and complexity of aerosol–cloud interactions on marine stratocumulus and stratocumulus-to-cumulus transition in HadGEM3-GC3.1
- 2023
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - 0035-9009 .- 1477-870X. ; 755:149, s. 2049-2066
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Tidskriftsartikel (refereegranskat)abstract
- Stratocumulus (Sc) clouds and stratocumulus-to-cumulus transitions (SCTs) are challenging to represent in global models and they contribute to a large spread in modeled subtropical cloud feedbacks. We evaluate the impact of increasing the horizontal model resolution (∼135, 60 and 25 km, respectively) and increasing the complexity of the aerosol–cloud interaction parameterization (interactive versus non-interactive at medium resolution) on springtime subtropical marine Sc properties and SCTs in the atmosphere-only version of HadGEM3-GC3.1. No significant impact on the spatial location of the SCT could be found between the different model versions. Increasing horizontal resolution led to small but significant increases in liquid water content and a stronger (more negative) shortwave (SW) cloud radiative effect (CRE), in particular over the southern-hemisphere Sc regions. However, for two out of the four studied regions, the stronger SW CRE also brought the model outside the range of satellite-derived values of the SW CRE. Applying non-interactive aerosols instead of interactive aerosols also led to significantly higher liquid water content and a stronger SW CRE over the southern-hemisphere Sc regions, while over the northern-hemisphere Sc regions, a competition between a substantial increase in the cloud droplet number concentration and small changes in the liquid water content resulted in a weaker SW CRE or non-significant changes. In general, using interactive instead of non-interactive aerosol–cloud interactions brought the model closer to satellite-retrieved mean values of the SW CRE. Our results suggest that increasing the horizontal resolution or the complexity of the aerosol–cloud parameterization has a small but statistically significant effect on the SW CRE of marine Sc, in particular over regions with high liquid water content. For these regions, the effect of introducing non-interactive versus interactive aerosol–cloud interactions is about as large as increasing the horizontal resolution from medium to high.
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| 4. |
- Fitch, Anna C.
(författare)
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Improving stratocumulus cloud turbulence and entrainment parametrizations in OpenIFS
- 2022
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 148:745, s. 1782-1804
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Tidskriftsartikel (refereegranskat)abstract
- A series of modifications to the stratocumulus cloud turbulence and entrainment parametrizations in the Open Integrated Forecasting System (OpenIFS) model are performed in a Lagrangian single-column model framework to determine whether they are able to improve the representation of the stratocumulus-topped boundary layer and its transition to cloud break-up. Two very different regimes are studied and compared with large-eddy simulations: a subtropical marine stratocumulus-to-cumulus transition case driven by a warming sea-surface temperature, and an Arctic air-mass transformation case where warm and moist air is advected over sea ice during wintertime. The addition of nonlocal mixing driven by stratocumulus cloud-top cooling for stable and dry convective boundary-layer types is found to sustain the cloud deck for longer in the Arctic case, while it dissipates around one day earlier in the original OpenIFS. The assumption that cloud-top-driven mixing reaches the surface, forming a coupled boundary layer, is relaxed. The resulting eddy diffusivity profile is altered such that mixing at cloud top may be enhanced relative to cloud base, promoting decoupling. Decoupling in the subtropical boundary layer is further promoted when the cloud-top turbulent velocity scale is based on the integral of the in-cloud buoyancy flux, accounting for the turbulence generated by latent heat release within the cloud, which increases cloud-top entrainment further relative to cloud base. Some long-standing model issues persist, such as underestimation of the subtropical boundary-layer deepening rate, which leads to a cold and moist bias in the boundary layer. In the Arctic, the boundary layer is generally too well mixed and warm at the surface. The degree of decoupling is also underestimated in both cases. However, the modifications discussed here lead to some improvement in OpenIFS.
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| 5. |
- Fries, Johan, et al.
(författare)
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Key parameters for droplet evaporation and mixing at the cloud edge
- 2021
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 147:737, s. 2160-2172
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of liquid water in ice-free clouds determines their radiative properties, a significant source of uncertainty in weather and climate models. Evaporation and turbulent mixing cause a cloud to display large variations in droplet number density, but quite small variations in droplet size (Beals et al., Science, 2015, vol. 350, pp. 87-90). However, direct numerical simulations of the joint effect of evaporation and mixing near the cloud edge predict quite different behaviours, and how to reconcile these results with the experimental findings remains an open question. To infer the history of mixing and evaporation from observational snapshots of droplets in clouds is challenging, because clouds are transient systems. We formulated a statistical model that provides a reliable description of the evaporation-mixing process as seen in direct numerical simulations and allows us to infer important aspects of the history of observed droplet populations, highlighting the key mechanisms at work and explaining the differences between observations and simulations.
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| 6. |
- Hartung, Kerstin, et al.
(författare)
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Diagnosing topographic forcing in an atmospheric dataset : The case of the North American Cordillera
- 2020
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 146:726, s. 314-326
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Tidskriftsartikel (refereegranskat)abstract
- It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large-scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single-layer models. The goal of this study is to design and use a simple diagnostic of the large-scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific-North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking.
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| 7. |
- Hoskins, Brian, et al.
(författare)
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The detailed dynamics of the June–August Hadley Cell
- 2020
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : John Wiley & Sons. - 1477-870X .- 0035-9009. ; 146:727, s. 557-575
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Tidskriftsartikel (refereegranskat)abstract
- The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for the reduction of tropical temperature gradients to a value such that the implied zonal winds are realisable. At the heart of the theory is the notion of angular momentum conservation in the upper branch of the Hadley Cells. Eddy mixing associated with extra‐tropical systems is invoked to give continuity at the edge of the Hadley Cell and to reduce the subtropical jet by a factor of 3 or more to those observed. In this paper a detailed view is presented of the dynamics of the June–August Hadley Cell, as given by ERA data for the period 1981–2010, with an emphasis on the dynamics of the upper branch. The steady and transient northward fluxes of angular momentum have a very similar structure, both having a maximum on the equator and a reversal in sign near 12°S, with the transient flux merging into that associated with eddies on the winter sub‐tropical jet. In the northward absolute vorticity flux, the Coriolis torque is balanced by both the steady and transient fluxes. The overturning circulations that average to give the Hadley Cell are confined to specific longitudinal regions, as are the steady and transient momentum fluxes. In these regions, both intra‐seasonal and synoptic variations are important. The dominant contributor to the Hadley Cell is from the Indian Ocean and W Pacific regions, and the maxima in OLR variability and meridional wind in these regions have a characteristic structure associated with the Westward‐moving Mixed Rossby‐Gravity wave. Much of the upper tropospheric motion into the winter hemisphere occurs in filaments of air from the summer equatorial region. These filaments can reach the winter sub‐tropical jet, leading to the strengthening of it and of the eddies on it, implying strong tropical‐extratropical interaction.
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| 8. |
- Marcheggiani, Andrea, et al.
(författare)
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The life cycle of meridional heat flux peaks
- 2022
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 148:744, s. 1113-1126
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Tidskriftsartikel (refereegranskat)abstract
- Covariance between meridional wind and air temperature in the lower troposphere quantifies the poleward flux of dry static energy in the atmosphere; in the midlatitudes, this is primarily realised by baroclinic weather systems. It is shown that strong covariance between temperature and meridional wind results from both enhanced correlation and enhanced variance, and that the two evolve according to a distinct temporal structure akin to a life-cycle. Starting from a state of low correlation and variance, there is first a gradual build-up to modal growth at constant, high correlation, followed by a rapid decay at relatively low correlation values. This life-cycle evolution is observed most markedly over oceanic regions, and cannot be explained on purely statistical grounds. We find that local peaks of meridional heat flux are not exclusively linked to the action of individual weather systems and can affect the atmospheric circulation on larger length-scales through wave propagation along waveguides.
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| 9. |
- Messori, Gabriele, et al.
(författare)
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A spatial model for return values of warm extremes in the high Arctic
- 2020
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 146:733, s. 3865-3876
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Tidskriftsartikel (refereegranskat)abstract
- Wintertime warm extremes in the high Arctic, primarily associated with intrusions of moist airmasses from the midlatitudes, have occurred with an ostensibly high frequency in recent years. Here, we compute the temperature anomaly return values associated with such events for long return times. Our approach, which we term space-time maxima-exposure (STM-E), improves on conventional extreme value estimates performed on a single-location basis by explicitly taking into account the spatial structure of the moisture intrusions driving the temperature extremes. We specifically show that the STM-E approach provides spatially smoother return value estimates, a lower uncertainty in the parameters of the extreme value distribution fits and mitigates the single-location underestimation of the magnitude of return values of temperature extremes.
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| 10. |
- Moon, Woosok, et al.
(författare)
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Baroclinic instability and large-scale wave propagation in a planetary-scale atmosphere
- 2022
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 148:743, s. 809-825
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Tidskriftsartikel (refereegranskat)abstract
- Midlatitude atmospheric variability is dominated by the dynamics of the baroclinically unstable jet stream, which meanders and sheds eddies at the scale of the Rossby deformation radius. The eddies interact with each other and with the jet, affecting the variability on a wide range of scales, but the mechanisms of planetary-scale fluctuations of the jet are not well understood. Here, we develop a theoretical framework to explore the stability of planetary-scale motions in an idealized two-layer model of the atmosphere. The model is based on a combination of vertical shear and the Sverdrup relation, providing the dynamic link between the two layers, with meridional eddy heat fluxes parameterized as a diffusive process with the memory of past baroclinicity of the jet. We find that a planetary-scale instability exists if the vertical shear of the jet does not exceed a particular threshold. The inclusion of the eddy-memory effect enables westward or eastward propagation of planetary waves relative to the barotropic mean flow. Importantly, we find growing planetary waves that propagate slowly westward or are stationary, which could have important implications for the formation of atmospheric blocking events. Our theoretical results suggest that, with ongoing polar amplification due to global warming and the corresponding reduction of the vertical shear of the mean wind, the background conditions for the growth of planetary-scale waves via planetary-scale baroclinic instability are becoming more favorable.
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