| 1. |
- Aldama Campino, Aitor, 1989-
(författare)
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Atmospheric and oceanic circulation from a thermodynamic perspective
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The climate system is continuously transporting and exchanging heat, freshwater, carbon and other tracers in different spatio-temporal scales. Therefore, analysing the system from a thermodynamic or biogeochemical framework is highly convenient. In this thesis the interaction between the ocean and the atmospheric circulation is analysed using thermodynamical and biogeochemical coordinates. Due to the dimensionality of the climate system stream functions are used to reduce this complexity and facilitate the understanding of the different processes that take place. The first half of this thesis, focuses on the interaction between the atmospheric and the ocean circulation from a thermodynamic perspective. We introduce the hydrothermohaline stream function which combines the atmospheric circulation in humidity-potential temperature (hydrothermal) space and the ocean circulation in salinity-temperature coordinates (thermohaline). A scale factor of 7.1 is proposed to link humidity and salinity coordinates. Future scenarios are showing an increase of humidity in the atmosphere due to the increase of temperatures which results in a widening of the hydrothermal stream function along the humidity coordinate. In a similar way, the ocean circulation in the thermohaline space expands along the salinity coordinate. The link between salinity and humidity changes is strongest at net evaporation regions where the gain of water vapour in the atmosphere results in a salinification in the ocean. In addition, the ocean circulation in latitude-carbon space is investigated. By doing so, we are able to distinguish the roles of different water masses and circulation pathways for ocean carbon. We find that the surface waters in the subtropical gyres are the main drivers of the meridional carbon transport in the ocean. By separating the carbon in its different constituents we show that the carbon transported by the majority of the water masses is a result of the solubility pump. The contribution of the biological pump is predominant in the deep Pacific Ocean. The effects of the Mediterranean Overflow Waters on the North Atlantic are discussed in the final part of the thesis.
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| 2. |
- Jönsson, Bror, 1972-
(författare)
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Some Concepts of Estuarine Modeling
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- If an estuarine system is to be investigated using an oceanographic modeling approach, a decision must be made whether to use a simple and robust framework based on e.g. mass-balance considerations, or if a more advanced process-resolving three-dimensional (3-D) numerical model are necessary. Although the former are straightforward to apply, certain fundamental constraints must be fulfilled. 3-D modeling, even though requiring significant efforts to implement, generates an abundance of highly resolved data in time and space, which may lead to problems when attempting to specify the "representative state" of the system, a common goal in estuarine studies.In this thesis, different types of models suitable for investigating estuarine systems have been utilized in various settings. A mass-balance model was applied to investigate potential changes of water fluxes and salinities due to the restoration of a mangrove estuary in northern Colombia. Seiches, i.e. standing waves, in the Baltic Sea were simulated using a 2-D shallow-water model which showed that the dominating harmonic oscillation originates from a fjord seiche in the Gulf of Finland rather than being global. Another study pertaining to the Gulf of Finland used velocity-fields from a 3-D numerical model together with Lagrangian-trajectory analyses to investigate the mixing dynamics. The results showed that water from the Baltic proper is mixed with that from the river Neva over a limited zone in the inner parts of the Gulf. Lagrangian-trajectory analysis was finally also used as a tool to compare mass-balance and 3-D model results from the Gulf of Riga and the Bay of Gdansk, highlighting when and where each method is applicable.From the present thesis it can be concluded that the above described estuarine-modeling approaches not only require different levels of effort for their implementation, but also yield results of varying quality. If oceanographic aspects are to be taken into account within Integrated Coastal Zone Managment, which most likely should be the case, it is therefore important to decide as early as possible in the planning process which model to use, since this choice ultimately determines how much information about the physical processes characterizing the system the model can be expected to provide.
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| 3. |
- Mårtensson, Sebastian, 1981-
(författare)
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Ridged sea ice modelling in climate applications
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work aims to increase our understanding of the nature of large scale features of sea ice from a dynamics point of view.Sea ice plays an important part in the exchange of heat and humidity between sea and air and thus is an important component of the climate system. Its physical presence also directly impacts the various forms of life such as diatoms, polar bears and humans alike.The dynamics of sea ice affect both weather and climate, through the large scale drift in the Arctic from the Siberian coast towards Fram Strait, through creation of cracks in the ice called leads or polynyas, and through ridging and other mechanical deformations of ice floes.In this work, we have focused on modelling of ridged ice for a number of reasons. Direct observations of the internal ice state is very difficult to perform and in general, observations of sea ice are either sparse or of limited information density. Ridged ice can be seen as the memory of high ice stress events, giving us a view on these highly dynamic events. Ridging is of major importance for the ice thickness distribution, as the thickest ice can only be formed through mechanical processes. Further, ridged ice is of direct interest for anyone conducting shipping through seasonal or perennial ice covered seas as it can form impenetrable barriers or in extreme even cases crush a ship caught within the ice pack.To this end, a multi-category sea ice model, the HELsinki Multi category Ice model (HELMI), was implemented into the Rossby Centre Ocean model (RCO). HELMI has explicit formulations for ridged and rafted ice, as well as sub-grid scale ice thickness distribution (a feature shared with other multi category models) and an ice strength based on energetics. These features give RCO better representation of sub-grid scale physics and gives us the possibility to study the deformed ice in detail.In paper I we look at the change in behaviour in the Arctic as the ice becomes more mobile, leading to a slight increase in modelled ridged ice volume in the central Arctic, despite a general trend of a decreasing ice cover.Paper II takes us to the Baltic Sea and the possibilities of modelling ridge ice concentration with a statistical model.In Paper III we investigate how the diminishing ice cover in future scenarios affects the biological activity in the Baltic Sea.Finally Paper IV investigates how the ice stress and the internal ice force can be interpreted in terms of ice compression on the ship scale.
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| 4. |
- Ridderstedt, Ivan, 1990-
(författare)
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Improving the Efficiency of Public Procurement : Empirical evidence using micro-level contract data
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Among OECD countries, close to one-third of government expenditure is allocated using public procurement. This implies that the skill of public authorities in handling the procurement format has a significant impact on how much the public sector accomplish in relation to their goals and responsibilities. The aim of this thesis is to provide new insights into how public procurers can achieve more for the money by improving the design of their auctions and contracts. Specifically, the thesis examines how the efficiency can be influenced by the following three aspects: the method of bid evaluation (the award mechanism), whether items are procured separately or grouped into larger contracts, and variations among procurers in the activities and quantities they specify in a given case.The thesis includes four essays that analyze detailed contract information from Swedish procurements of road reinvestments and public bus services. For a comprehensive sample of the examined contracts, information has been collected from procurement documents and compiled into rich microdata sets. The use of microdata has enabled detailed reviews of how the procurements have been designed as well as statistical analysis of associations between the procurer’s choices and the outcome. While the essays primarily focus on results and implications regarding their respective research questions, the kappa provides a more general discussion about quantitative microanalysis in the field of procurement. This reflects a complementary aim of the thesis, which is to contribute to well-informed decisions regarding whether and how contracting authorities should facilitate analyses of this kind.Two of the essays investigate to what extent and how the Swedish national infrastructure manager Trafikverket can improve its efficiency in procurements of highway pavement replacement. Trafikverket spends approximately EUR 200 million on these works annually. Hence, if better procurement methods could lower the cost by 5% without reducing the quality, about EUR 10 million would be freed up annually for additional road maintenance or other uses. Both essays indicate that even greater efficiency gains can be achieved through improved dissemination of best practices. The results support Trafikverket's policy to bundle similar and adjacent road work but imply that the contracts are generally not large enough to fully benefit from the economies of scale. A conclusion is that it is motivated for Trafikverket to consider the scope for efficient bundling when deciding on the timing of these pavement replacement measures.The other two essays examine how the Swedish regional Public Transport Authorities (PTAs) have implemented the scoring rule award mechanism and its efficiency. This alternative to the lowest price and quality-only mechanisms is promoted within the EU procurement directive and is dominant in many countries. However, the literature on how well public agencies implement this policy is scarce and fragmented. One of the essays shows that scoring rules are common in Swedish procurements of public bus services, but also that there are significant differences between the PTAs in whether and how they have applied this award mechanism. Several implementations have unsound and likely unintended properties, in the light of economic theory. The findings suggest that best practices are not spread effectively between the organizations. The second essay on this theme exploits the property that one PTA alternated between lowest price and a performance focused variant of scoring rules and, equally important, could provide data on a relevant and reliable outcome measure. Based on 30 months of observations on monitored punctuality, performance is not found to be better in contracts awarded using scoring rules. The results suggest that the promotion of scoring rules is not accompanied with adequate guidance on when and how to implement it efficiently.The kappa highlights how microdata at the contract level enable research that considers both the ‘uniqueness’ of each procurement and random variation. In the analyses of road reinvestments, the detailed data allows us to consider several important characteristics of the contracts and the treated road segments. Similarly, the analysis of bus punctuality considers several aspects of the traffic assignment. In this way, quantitative microanalysis can provide reliable quantifications of how costs and quality are affected by both the procurer's actions, while considering the external conditions. Many of these associations are too complex to be reliably assessed by an individual civil servant, even if it is highly knowledgeable and experienced. However, the thesis emphasizes that even quantitative analysis can yield misleading results if the data material is not good enough, for example, if the sample is too small or relevant aspects are not captured.A central argument in the thesis is that the standardized and rigid procurement process creates particularly excellent conditions for collecting useful procurement data. However, the potential for this has largely remained untapped by the public agencies themselves. Instead, these data have primarily been compiled within individual research projects, where even the collection of tendering documents has been a notorious obstacle. In Sweden, Trafikverket's information management and cost control have repeatedly been criticized. However, Trafikverket is far from the ‘worst in class’ in these matters, and the criticism mainly reflects the agency's position as one of Sweden's largest procurers. An overall conclusion of the thesis is that public agencies in general should take a more active role in improving how procurement data are collected and managed. Of particular importance is the establishment of well-structured archiving processes for procurement documents, along with systems to track changes in both costs and content after a contract is awarded. For smaller authorities, it is reasonable that this development is coordinated or driven by a centralized function, whereas Trafikverket is well positioned to take a leading role in finding and disseminating best practices. Ultimately, the responsibility for instructing and overseeing improvements in this area lies with the governing bodies of the public agencies, given that the agencies do not set their own agendas.
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| 5. |
- Westerling, Lars, 1943-
(författare)
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Interaction of Cylindrical Penetrators with Ceramic and Electromagnetic Armour
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Armoured vehicles have traditionally used steel armour as protection against penetrators such as projectiles and shaped charge jets. The latter produce a thin stretching metal jet, usually of copper, with a tip velocity of about 7-8 km/s. In order to obtain more weight-efficient solutions, there is a search for lighter materials and other protection techniques. In this thesis, ceramic and electromagnetic armours are studied. Ceramic materials are lighter than steel, and their high compressive strength makes them useful as armour materials. Electromagnetic armour consists of two metal plates connected to an electric power supply capable of delivering a strong current pulse. A conductive penetrator passing through both plates is destroyed by the effects of the resulting current.Tests of the ceramic armour materials alumina and boron carbide were performed with reverse impact technique, which signifies that a target assembly (ceramic confined in a metal cylinder) was launched by a gun towards a projectile placed in front of the gun barrel. By this technique yaw was eliminated, but the geometric scale had to be very small. Therefore, we studied scaling laws for ceramic armour through a series of tests with direct impact technique and projectile diameters from 2 to 10 mm. The small scale has the advantage that flash X-ray photography can be used to photograph the projectile inside the ceramic target. The phenomenon of interface defeat or dwell was also demonstrated. It signifies that the ceramic, at least for a short time, can withstand the impact pressure so that the projectile just flows out onto the target surface. A transition velocity, above which dwell does not occur, was determined. Simulations were performed with the continuum-dynamic code Autodyn and by use of a model for the brittle ceramic materials by Johnson and Holmquist. The simulations reasonably well represented the penetration behaviour above the transition velocity. They also did below, if under this condition the ceramic model was forced to remain undamaged.The performance of electromagnetic armour was tested against a shaped charge jet. The jet was registered with shadowgraph flash X-ray photography between the plates and after passing through the plates. The current through the jet and the voltage over the plates were also registered. The current caused heating leading to melting and even vaporization. The magnetic Lorentz force compressed the jet radially, and as this effect increases with decreasing jet radius, instability may arise. Explosions in the compressed regions resulted in a fragmented jet with disk-shaped fragments which are less effective penetrators than the elongated fragments obtained in the absence of current. We also performed a theoretical study, in which the penetrator was subjected to small elastic strains only and the current was constant. The magnetic field was determined by FFT, and the stresses due to the Lorentz force were calculated with a semi-analytical method. The velocity skin effect was demonstrated.
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