| 1. |
- Gutiérrez-Loza, Lucía, 1989-
(författare)
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On mechanisms controlling air-sea gas exchange
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon is essential to the Earth’s system functioning, playing a major role in physical and biogeochemical processes in the atmosphere, the terrestrial biosphere, and the oceans. The concentration of carbon-based greenhouse gases in the atmosphere, such as carbon dioxide (CO2) and methane (CH4), has been increasing since the industrial era. Therefore, assessing the redistribution of these greenhouse gases between the Earth’s reservoirs has become essential for understanding the current climate system and modelling future climate scenarios.The oceans are a component of the global carbon cycle, and their role as sinks and sources of greenhouse gases has significant implications for the Earth’s climate. The gas exchange between the atmosphere and the ocean is driven by the concentration difference in these two reservoirs. However, the turbulent processes in the layers adjacent to the ocean surface control the efficiency of the transport.This thesis investigates mechanisms controlling the air–sea gas exchange using direct measurements of CO2 and CH4 fluxes from the Östergarnsholm station in the Central Baltic Sea. The gas exchange of both gases is found to have a strong variability at time scales from sub-hourly to inter-annual. The region is found to be a net source of CH4, with both the concentration gradient and wind as controlling mechanisms. In the case of the CO2 fluxes, the variability is strongly modulated by local processes such as sea spray and water-side convection, as well as precipitation. Interestingly, an asymmetric effect is observed, with these processes enhancing the upward transport of CO2 but not the downward flux. Furthermore, a model-based sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms on the air-sea gas exchange at a regional scale. The results show that water-side convection, precipitation, and surfactants strongly modulate the spatio-temporal variability of the CO2 fluxes in the Baltic Sea.
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| 2. |
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| 3. |
- Eriksson, Ola, 1979-
(författare)
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Numerical Computations of Wakes Behind Wind Farms : A tool to study Farm to Farm interaction
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- More and larger wind farms are planned offshore. As the most suitable sites to build are limited these new wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel long distances. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area.This thesis contains numerical studies of these long distance wakes. In these studies Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for the study of farm to farm interaction. Parameter studies are setup to analyze how to best use the model for the study of long distance wakes with regards to 1) numerical and physical parameters in the model, 2) the extension of the domain and turbulence as well as the characteristics of the flow far downstream and 3) the downstream development of turbulence with different combinations of wind shear and turbulence level.Using an initial simulation setup a wind farm was studied and preliminary results were obtained. These results were subsequently improved upon by applying the simulation setup adjustments indicated in the parameter studies. A comparison with a mesoscale model was also done. The mesoscale model was shown to be relevant for studies of long distance wakes in another study comparing LES and WRF. Finally an idealized farm to farm interaction case was studied with focus on the impact of including the Coriolis force in the simulations.Combining LES with a mesoscale model is of interest to study further.
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| 4. |
- Andersson, Andreas, et al.
(författare)
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Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a High Arctic Fjord During Unstable Very-Close-to-Neutral Conditions
- 2019
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 170:3, s. 471-488
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Tidskriftsartikel (refereegranskat)abstract
- Eddy-covariance measurements made in the marine atmospheric boundary layer above a high Arctic fjord (Adventfjorden, Svalbard) are analyzed. When conditions are unstable, but close to neutral −0.1 < z/L < 0, where z is the height, and L is the Obukhov length, the exchange coefficient for sensible heat CH is significantly enhanced compared with that expected from classical surface-layer theory. Cospectra of the vertical velocity component (w) and temperature (T) reveal that a high-frequency peak develops at f ≈ 1 Hz for z/L > − 0.15. A quadrant analysis reveals that the contribution from downdrafts to the vertical heat flux increases as conditions become close to neutral. These findings are the signature of the evolving unstable very-close-to-neutral (UVCN) regime previously shown to enhance the magnitude of sensible and latent heat fluxes in the marine surface layer over the Baltic Sea. Our data reveal the significance of the UVCN regime for the vertical flux of the carbon dioxide (CO2) concentration (C). The cospectrum of w and C clearly shows how the high-frequency peak grows in magnitude for z/L > − 0.15, while the high-frequency peak dominates for z/L > − 0.02. As found for the heat flux, the quadrant analysis of the CO2 flux shows a connection between the additional small-scale turbulence and downdrafts from above. In contrast to the vertical fluxes of sensible and latent heat, which are primarily enhanced by the very different properties of the air from aloft (colder and drier) during UVCN conditions, the increase in the air–sea transfer of CO2 is possibly a result of the additional small-scale turbulence causing an increase in the water-side turbulence. The data indicate an increase in the gas-transfer velocity for CO2 for z/L > − 0.15 but with a large scatter. During the nearly 2 months of continuous measurements (March–April 2013), as much as 36% of all data are associated with the stability range −0.15 < z/L < 0, suggesting that the UVCN regime is of significance in the wintertime Arctic for the air–sea transfer of heat and possibly also CO2.
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| 5. |
- Arnqvist, Johan, 1985-
(författare)
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Mean Wind and Turbulence Conditions in the Boundary Layer above Forests
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.
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| 6. |
- Carlsson, Björn, 1980-
(författare)
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Implementation and Analysis of Air-Sea Exchange Processes in Atmosphere and Ocean Modelling
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To understand and to predict the weather and climate, numerical models are important tools and it is crucial that the controlling processes are described correctly. Since 70% of the global surface is covered with water the description how the ocean and atmosphere communicates has a considerable impact. The ocean–atmosphere exchange occurs through transport of momentum (friction) and heat, governed by turbulent eddies. The sea surface is also an important source of turbulence in both directions. The scales of the turbulent eddies cannot be resolved in ocean and climate models. Therefore, the turbulent exchanges have to be related to mean variables, such as wind speed and temperature differences. By using measurements, new methods to describe the air–sea exchange during two specific processes were developed. These processes are the so-called UVCN-regime (Unstable Very Close to Neutral stratification) and swell, i.e. waves which are not produced by the local wind. These processes were included in an ocean model and in a regional atmospheric climate model and the impact was investigated.The UVCN-regime enhances the heat transport significantly during the autumn and winter months in the ocean model. This results in a shallower well-mixed surface layer in the ocean. Wind-following swell reduces the surface friction, which is very important for the atmosphere. Some secondary effects in the climate model are reduced low-level cloud cover and reduced precipitation by more than 10% over sea areas. Locally and for short periods the impact is large. It is important to include the UVCN-regime and the swell impact in models, to make simulations more reliable.
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| 7. |
- Conrady, Kristina, et al.
(författare)
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Amplitude modulation of wind turbine sound in cold climates
- 2020
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Ingår i: Applied Acoustics. - : Elsevier. - 0003-682X .- 1872-910X. ; 158
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Tidskriftsartikel (refereegranskat)abstract
- Amplitude modulation is assumed to be a major annoyance factor of wind turbine sound. However, studies on the generation of amplitude modulation and the impact of atmospheric conditions on amplitude modulation are limited, especially in cold climates. Long-term acoustic and meteorological measurements in the vicinity of a wind farm in northern Sweden show a dependence of the occurrence of amplitude modulation on wind direction and atmospheric stability. The occurrence of amplitude modulation is highest for crosswinds from southwest, compared with the other wind directions. Moreover, the occurrence of amplitude modulation is clearly linked to atmospheric stability and highest for very stable conditions. The impact of atmospheric stability is supported by analyses of wind shear, the wind speed gradient close to the surface and the bulk Richardson number. Amplitude modulation is more likely during winter than during summer and more likely during night and early morning than during noon and early afternoon.
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| 8. |
- Conrady, Kristina, et al.
(författare)
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Impact of low-level wind maxima below hub height on wind turbine sound propagation
- 2020
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Ingår i: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:8, s. 1767-1775
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of the effect of low-level wind maxima (LLWM) below hub height on sound propagating from wind turbines has been performed at a site in northern Sweden. The stably stratified boundary layer, which is typical for cold climates, commonly features LLWM. The simplified concept for the effects of refraction, based on the logarithmic wind profile or other approaches where the wind speed is continuously increasing with height, is often not applicable there. Long-term meteorological measurements in the vicinity of a wind farm were therefore used to identify LLWM. Sound measurements were conducted simultaneously to the meteorological measurements. LLWM below hub height decrease the sound level close to the surface downwind of the wind farm. This effect increases with increasing strength of the LLWM. The occurrence of LLWM as well as strength and height of the LLWM are dependent on the wind direction.
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| 9. |
- Conrady, Kristina
(författare)
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Wind Turbine Sound in Cold Climates
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increase in the number of wind turbines (WTs) in populated areas in cold climates increases the number of people potentially being affected by WT sound. Outdoor sound propagation is strongly dependent on meteorological conditions, however, limitations in the knowledge exist regarding the implications of meteorological conditions in cold climates. Long-term acoustic and meteorological measurements were conducted in the vicinity of two wind farms in northern Sweden, to investigate the effect of snow and low-level wind maxima on WT sound, to analyse the occurrence of amplitude modulation and to evaluate selection methods for WT sound measurements. Different selection methodologies were applied to the acoustical data. The simplest method only includes a minimum rotational frequency of the WTs, while the most comprehensive method additionally includes criteria based on spectral resemblance, temporal variation of the sound level, amplitude modulation and wind speed. The effect of snow on WT sound depends on the snow quality. Snow on trees lowers the sound level by ca. 2 dBA. Low-level wind maxima below hub height reduce the sound level near the surface. Since this effect is increasing with increasing strength of the low-level wind maximum, the WT sound is assumed to be partly trapped above the low-level wind maximum. Amplitude modulation was shown to be dependent on atmospheric stability and was most common for very stable conditions. Moreover, a clear difference between the occurrences of amplitude modulation for the two crosswind sectors was observed. The choice of selection method needs to be taken into account when comparing different studies since it affects the results and conclusions. The studies emphasise to include the effects of individual meteorological conditions of a site in the formulation of guidelines on WT sound.
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| 10. |
- Dingwell, Adam, 1987-
(författare)
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Dispersion modelling of volcanic emissions
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gases and particles released by volcanoes pose a serious hazard to humans and society. Emissions can be transported over long distances before being reduced to harmless concentrations. Knowing which areas are, or will be, exposed to volcanic emissions is an important part inreducing the impact on human health and society. In this thesis, the dispersion of volcanic emissions is studied using a set of atmospheric models.The work includes contribution to the development of the Lagrangian Particle Dispersion Model FLEXPART-WRF. Three case studies have been performed, one studying potential ash emissions from potential future eruptions on Iceland, a second covering SO2 emissions from Mt. Nyiragongo in D.R. Congo, and a third studying the SO2 emission rate of the Holuhraun eruption (Iceland) in 2014–2015.The first study covers volcanic ash hazard for air traffic over Europe. Three years of meteorological data are used to repeatedly simulate dispersion from different eruption scenarios. The simulations are used to study the probability of hazardous concentrations in ash in European airspace. The ash hazard shows a seasonal variation with a higher probability of efficient eastward transport in winter, while summer eruptions pose a more persistent hazard.In the second study, regional gas exposure around Mt. Nyiragongo is modelled using flux measurements to improve the description of the emission source. Gases are generally transported to the north-west in June–August and to the south-west in December–January. A diurnal variation due to land breeze around lake Kivu contributes to high concentrations of SO2 along the northern shore during the night. Potentially hazardous concentrations are occasionally reached in populated areas in the region, but mainly during the nights.The third study uses inverse dispersion modelling to determine the height and emission rates based on traverse measurements of the plume at 80–240 km from the source. The calculated source term yields better agreement with satellite observations compared to commonly used column sources.The work in this thesis presents improvements in dispersion modelling of volcanic emissions through improved models, more accurate representation of the source terms, and through incorporating new types of measurements into the modelling systems.
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