| 1. |
- Hartung, Kerstin, 1989-
(författare)
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Paths to improving atmospheric models across scales : The importance of the unresolved scales
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Correct representation of physical processes, the parametrizations, and their interaction with the resolved circulation is crucial for the performance of numerical models. Here, focus is put on understanding model biases and developing tools to alleviate existing biases. Atmospheric blocking can divert the typical atmospheric flow for several days up to weeks and thereby impacts the mean climate of the region experiencing blocking. Models typically underestimate the frequency of atmospheric blocking. Based on results from the global climate model EC-Earth, it is found that the atmospheric model resolution is not strongly influencing the representation of atmospheric blocking once the grid reaches about 80 km grid length in the horizontal. Updating several physical parametrizations, and thereby the model version, is the largest contributor to advancements in simulating atmospheric blocking. The importance of the topography for the large-scale atmospheric flow is further investigated with the reanalysis ERA-Interim by applying a simplified theoretical analysis. It is found that the idealized topographic forcing theory can explain some part of the observed large-scale properties of the flow, though the method does mainly produce relative results. The explained part of the large-scale structure is increased during periods of northwesterly flow and when the flow impinges the mountain ridge almost orthogonally.Small-scale processes acting in air masses transported from midlatitudes to the Arctic are also discussed. Numerical models often struggle with representing the stable conditions in the Arctic and tend to underestimate the downward longwave impact during cloudy conditions. A comparison of single-column models (SCMs) indicates that most models can capture the bimodal longwave distribution which develops from alternating cloudy and clear-sky conditions. SCMs are often used for model development as they allow to decouple the parametrized physical processes from the large-scale environment and enable many parameter sensitivity tests. A new tool is presented which can be used for the development of physical parametrizations in marine and polar conditions. It combines one-dimensional models of the atmosphere and ocean, including sea-ice, into a coupled atmosphere-ocean SCM (AOSCM). The presented setup constitutes an advantage compared to SCMs of one component because the coupling is directly modelled and the interaction between the respective boundary layers does not dependent on prescribed boundary conditions.
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| 2. |
- Avby, Gunilla
(författare)
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Evidence in practice : On knowledge use and learning in social work
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Against the backdrop of the transformations in the entire framing of professional work, social work has come under close scrutiny in many countries, including Sweden. Doubts have been raised about practitioners’ existing knowledge base, and the importance of practitioners engaging in learning and the renewal and extension of professional capacities has been emphasized.The present thesis concerns knowledge use and learning in the daily practices of child investigation work. The aim is to explore processes of knowledge use and learning in practice. The study is based on a mix of qualitative approaches, basically from ethnography, comprising methods such as participant observations, interviews, reflective dialogues and documentary analysis of case data.The main findings demonstrate that investigation work is characterized mainly by the use of practice-based knowledge. Research-based knowledge is predominantly used as a means of explaining a client’s situation or to underpin and legitimize one’s own beliefs and decisions made on other grounds. Professional learning is largely adaptive in character, as the social workers learn to handle tasks in a fairly routinized way on the basis of rules or procedures that draw on existing knowledge in the practice setting.Two conclusions are drawn: First, the use of knowledge in child investigation work bears little resemblance to principles of evidence-based practice. Second, the reproduction of professional knowledge is largely implicit and taken for granted. The study offers insight into the much-discussed topic of putting knowledge into practice, which is of importance to strategies for organizing profes sional learning and knowledgeable practice.
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| 3. |
- Svensson, Jacob, 1978-
(författare)
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Boundary Layer Parametrization in Numerical Weather Prediction Models
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical weather prediction (NWP) and climate models have shown to have a challenge to correctly simulate stable boundary layers and diurnal cycles. This aim of this study is to evaluate, describe and give suggestions for improvements of the descriptions of stable boundary layers in operational NWP models. Two papers are included. Paper I focuses on the description of the surface and the interactions between the surface and the boundary layer in COAMPSR, a regional NWP model. The soil parametrization showed to be of great importance to the structure of the boundary layer. Moreover, it showed also that a low frequency of radiation calculations caused a bias in received solar energy at the surface.In paper II, the focus is on the formulation of the turbulent transport in stable boundary layers. There, an implementation of a diffusion parametrization based on the amount of turbulent kinetic energy (TKE) is tested in a single column model (SCM) version of the global NWP model Integrated Forecast System (IFS). The TKE parametrization turned out to behave similarly as the currently operational diffusion parametrization in convective regimes and neutral regimes, but showed to be less diffusive in weakly stable and stable conditions. The formulations of diffusion also turned out to be very dependent on the length scale formulation. If the turbulence and the gradients of wind temperature and wind are weak, the magnitude of turbulence can enter an oscillating mode. This oscillation can be avoided with the use of a lower limit of the length scale.
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| 4. |
- Woods, Cian, 1986-
(författare)
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The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis examines the connections between atmospheric circulation in the high-latitudes, northward moisture transport, and Arctic climate variability and change during winter. An event based approach is taken by objectively defining phenomena termed “moisture intrusions” -- filamentary flows of anomalously moist air which originate at 70°N and cross the entire Arctic basin. They typically emanate from within the poleward advecting branches of mid-latitude cyclones held in place by blocking patterns to the east. Moisture intrusions contribute only a minority of the total northward moisture transport at 70°N, yet drive a significant proportion of the inter annual variability in surface temperature and downward longwave radiation over the entire polar cap. A positive trend in the frequency of these events, in response to a moistening of the atmosphere, is shown to have driven approximately 45% and 35% of the observed warming and sea ice decline in the Barents Sea during Dec-Jan over the past two decades. Moisture intrusions act to erode the temperature inversion and thus contribute to bottom amplified warming even in the absence of sea ice loss. Negative sea ice anomalies induced by intrusions persist for up to weeks at a time -- promoting upward turbulent heat fluxes and further bottom amplified warming. Systematic biases in the statistics of moisture intrusions are discovered in the CMIP5 models. The biases are predominantly a result of misrepresentation of the intense moisture fluxes and are almost entirely due to biases in the meridional velocity. Moisture intrusion biases explain only about 17% of the temperature bias in the Atlantic sector. The predicted biases, while small in amplitude, are very highly correlated with the true biases in the models however, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback. A analysis of the uncertainties in computed turbulent air-sea flux (TASF) climatologies arising due to the parameterisation of bulk formulae is also presented. TASF climatologies are computed over a series of sensitivity experiments using surface state variables from ERA-Interim. The largest source of uncertainty is related to the computation of the transfer coefficients and hence the choice of bulk algorithm itself. The majority of parameter approximations have small impacts when tested individually, but can lead to large disagreements when implemented in tandem.
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| 5. |
- Frey, Lena, 1986-
(författare)
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Aerosol-cloud-radiation interactions in global climate models
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds can reflect, absorb and re-emit radiation, thereby inducing a cooling or warming effect on the climate. However, the response of clouds to a changing climate is highly uncertain and the representation of clouds in state-of-the-art climate models remains a key challenge for future climate projections. Factors contributing to this uncertainty include processes on the microphysical scale involving aerosol particles with the size of just a few nanometers to micrometers. This thesis focuses on the representation of aerosol-cloud-radiation interactions in global climate models. Using idealized experiments from a model-intercomparison project with different anthropogenic aerosol forcings, it was found that both sulfate and non-sulfate aerosols yield an increase in cloud albedo in five regions of subtropical marine stratocumulus clouds. The changes in cloud albedo in the models were driven by changes in the cloud droplet number concentration and liquid water content. Further, it was found that the microphysical coupling of underlying aerosol-cloud interactions in models seems to dominate on the monthly timescale in subtropical marine stratocumulus regions, which can not be confirmed in observations. Quantifying the effect of aerosols on cloud properties in observations remains challenging. In addition, comparisons with satellite retrievals and the global climate model NorESM showed that this model is not able to capture elevated aerosol above cloud, seen in observations in two regions of marine stratocumulus clouds. Sensitivity experiments revealed that the model is most sensitive to the aerosol emissions, convection and wet scavenging in terms of the vertical aerosol distribution. Finally, the representation of aerosol absorption in global climate models was investigated. It was found that most of the models underestimate absorption by aerosols in a focus domain in Asia. Sensitivity studies with NorESM give rise to variations that lie within the large inter-model diversity.
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| 6. |
- Li, Xiang-Yu (李翔宇), 1990-
(författare)
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Droplet growth in atmospheric turbulence : A direct numerical simulation study
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This Ph.D. thesis examines the challenging problem of how turbulence affects the growth of cloud droplets in warm clouds. Droplets grow by either condensation or collision. Without turbulence, the condensation process driven by a uniform supersaturation field is only efficient when droplets are smaller than 15 μm (radius). Gravitational collision becomes effective when the radius of droplets is larger than 50 μm. The size gap of 15–50 μm, in which neither condensation nor collision processes dominate droplet growth, has puzzled the cloud microphysics community for around 70 years. It is the key to explaining the rapid warm rain formation with a timescale of about 20 minutes. Turbulence has been proposed to bridge this size gap by enhancing droplet growth processes, and thereby, to explain rapid warm rain formation. Since cloud–climate interaction is one of the greatest uncertainties for climate models, the fundamental understanding of rapid warm rain formation may help improve climate models.The condensational and collisional growth of cloud droplets in atmospheric turbulence is essentially the problem of turbulence-droplet interaction. However, turbulence alone is one of the unresolved and most challenging problems in classical physics. The turbulence–droplet interaction is even more difficult due to its strong nonlinearity and multi-scale properties in both time and space. In this thesis, Eulerian and Lagrangian models are developed and compared to tackle turbulence–droplet interactions. It is found that the Lagrangian superparticle model is computationally less demanding than the Eulerian Smoluchowski model.The condensation process is then investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations with droplets modeled as Lagrangian particles. With turbulence included, the droplet size distribution is found to broaden, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower droplet size distributions. Furthermore, the time evolution of droplet size distributions is observed to strongly depend on the Reynolds number and only weakly on the mean energy dissipation rate. Subsequently, the effect of turbulence on the collision process driven by both turbulence and gravity is explored. It is found that the droplet size distribution depends moderately on the mean energy dissipation rate, but is insensitive to the Reynolds number. Finally, the effect of turbulence on the combined condensational and collisional growth is investigated. In this case, the droplet size distribution is found to depend on both the Reynolds number and the mean energy dissipation rate. Considering small values of the mean energy dissipation rate and high Reynolds numbers in warm clouds, it is concluded that turbulence enhances the condensational growth with increasing Reynolds number, while the collision process is indirectly affected by turbulence through the condensation process. Therefore, turbulence facilitates warm rain formation by enhancing the condensation process, which predominantly depends on the Reynolds number.
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| 7. |
- Svensson, Ola, 1967-
(författare)
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Nämnda ting men glömda : Ortnamn, landskap och rättsutövning
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dissertation describes the names related to justice and places in the landscape where justice was administered, applying an interdisciplinary perspective with place names as the chief source material.One aim is to collect and describe place names in Skåne designating or indirectly associated with meeting places and districts of the court, and to study the named places. The study covers many different periods, but especially the Middle Ages and the transition from the Late Iron Age to the Middle Ages. The analysis raises questions such as: Was there continuity in judicial sites between prehistoric and historic times? How old are the hundreds (härader)? Is there a spatial link between judicial sites and other central functions such as cult, markets, or rulers’ estates?The work is permeated by material-based onomastic research in combination with current perspectives in text research, historical geography, and archaeology. Nine case studies are conducted to describe the interaction between place, linguistic expression, and meaning.The study demonstrates the existence of a large corpus of names reflecting the early administration of justice. Most of the many field names which contain ting ‘court’ and galge ‘gallows’ can be related to the actual administration of justice. The medieval sites where courts assembled and people were executed stand out in particular, but in many cases these have prehistoric roots. Both unbroken continuity and the reuse of earlier places of assembly may be assumed. Close to sites with names indicating the administration of justice there are also landscape features with names that grant epic and mythical status to the locale. The special quality of these places was handed down, incorporated in larger narratives, based on changing ideas and circumstances in different periods.The landscape of the hundred courts (häradsting) is archaic, magnificent and mythical, and shared, qualities that contributed to the maintenance and legitimation of judicial practice. A division into a general, public judicial sphere and a more limited and exclusive sphere can be seen. In the medieval exercise of justice this division is manifested in two different judicial districts – härad and birk – but the phenomenon can be traced back to the Late Iron Age. The study also problematizes a traditional image of the names of the hundreds.
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