| 1. |
- Andersson, Andreas, et al.
(författare)
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Air-sea gas transfer in high Arctic fjords
- 2017
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:5, s. 2519-2526
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Tidskriftsartikel (refereegranskat)abstract
- In Arctic fjords and high-latitude seas, strong surface cooling dominates during a large part of the year, generating water-side convection (w*w) and enhanced turbulence in the water. These regions are key areas for the global carbon cycle; thus, a correct description of their air-sea gas exchange is crucial. CO2-data were measured via the eddy covariance technique in marine Arctic conditions and reveal that water-side convection has a major impact on the gas transfer velocity. This is observed even at wind speeds as high as 9 m s-1, where convective motions are generally thought to be suppressed by wind-driven turbulence. The enhanced air-sea transfer of CO2 caused by water-side convection nearly doubled the CO2uptake, after scaled to open sea conditions the contribution from to the CO2 flux remained as high as 34%; this phenomenon is expected to be highly important for the total carbon uptake in marine Arctic areas.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Olsson, Taru, et al.
(författare)
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Statistics of sea-effect snowfall along the Finnish coastline based on regional climate model data
- 2020
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Ingår i: Advances in Science and Research. - : COPERNICUS GESELLSCHAFT MBH. - 1992-0628 .- 1992-0636. ; 17, s. 87-104
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Tidskriftsartikel (refereegranskat)abstract
- The formation of convective sea-effect snowfall (i.e., snow bands) is triggered by cold air outbreaks over a relatively warm and open sea. Snow bands can produce intense snowfall which can last for several days over the sea and potentially move towards the coast depending on wind direction. We defined the meteorological conditions which statistically favor the formation of snow bands over the north-eastern Baltic Sea of the Finnish coastline and investigated the spatio-temporal characteristics of these snow bands. A set of criteria, which have been previously shown to be able to detect the days favoring sea-effect snowfall for Swedish coastal area, were refined for Finland based on four case study simulations, utilizing a convection-permitting numerical weather prediction (NWP) model (HARMONIE-AROME). The main modification of the detection criteria concerned the threshold for 10 m wind speed: the generally assumed threshold value of 10 ms 1 was decreased to 7 ms(-1). The refined criteria were then applied to regional climate model (RCA4) data, for an 11-year time period (2000-2010). When only considering cases in Finland with onshore wind direction, we found on average 3 d yr(-1) with favorable conditions for coastal sea-effect snowfall. The heaviest convective snowfall events were detected most frequently over the southern coastline. Statistics of the favorable days indicated that the lower 10 m wind speed threshold improved the representation of the frequency of snow bands. For most of the favorable snow band days, the location and order of magnitude of precipitation were closely captured, when compared to gridded observational data for land areas and weather radar reflectivity images. Lightning were observed during one third of the favorable days over the Baltic Sea area.
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| 6. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Natural hazards and extreme events in the Baltic Sea region
- 2022
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Ingår i: Earth System Dynamics. - : Copernicus Publications. - 2190-4979 .- 2190-4987. ; 13:1, s. 251-301
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Tidskriftsartikel (refereegranskat)abstract
- A natural hazard is a naturally occurring extreme event that has a negative effect on people and society or the environment. Natural hazards may have severe implications for human life and can potentially generate economic losses and damage ecosystems. A better understanding of their major causes, probability of occurrence, and consequences enables society to be better prepared to save human lives as well as to invest in adaptation options. Natural hazards related to climate change are identified as one of the Grand Challenges in the Baltic Sea region. Here, we summarize existing knowledge about extreme events in the Baltic Sea region with a focus on the past 200 years as well as on future climate scenarios. The events considered here are the major hydro-meteorological events in the region and include wind storms, extreme waves, high and low sea levels, ice ridging, heavy precipitation, sea-effect snowfall, river floods, heat waves, ice seasons, and drought. We also address some ecological extremes and the implications of extreme events for society (phytoplankton blooms, forest fires, coastal flooding, offshore infrastructure, and shipping). Significant knowledge gaps are identified, including the response of large-scale atmospheric circulation to climate change and also concerning specific events, for example, the occurrence of marine heat waves and small-scale variability in precipitation. Suggestions for future research include the further development of high-resolution Earth system models and the potential use of methodologies for data analysis (statistical methods and machine learning). With respect to the expected impacts of climate change, changes are expected for sea level, extreme precipitation, heat waves and phytoplankton blooms (increase), and cold spells and severe ice winters (decrease). For some extremes (drying, river flooding, and extreme waves), the change depends on the area and time period studied.
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| 7. |
- Sjöblom, Anna, Professor, 1972-, et al.
(författare)
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Flow over a snow-water-snow surface in the high Arctic, Svalbard : Turbulent fluxes and comparison of observation techniques
- 2020
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Ingår i: Polar Science. - : Elsevier BV. - 1873-9652 .- 1876-4428. ; 25
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Tidskriftsartikel (refereegranskat)abstract
- From observations in a High Arctic valley and ice-free fjord in Svalbard during March and April 2013 we show that, while some caution needs to be applied, ordinary slow-response instruments placed over a snow-water-snow surface can be effectively used as a proxy for more sophisticated measuring techniques at complex sites such as leads or a polynyas. The turbulent fluxes of momentum, sensible and latent heat were measured at three locations with a snow-water-snow fetch. At the snow site upwind of the water, the stability was generally stable, the momentum flux small, and the sensible heat flux positive. Over the water however, the internal boundary layer that was formed gave on average an increased vertical gradient in wind speed, temperature, and humidity and turbulent heat fluxes exceeding 400 W m−2. At the snow surface downwind of the water, the conditions were highly variable and all the fluxes were, on average, of very small magnitude. That the behaviour of the internal boundary layers can be highly variable is demonstrated through four case studies. This phenomenon is likely to increase in occurrence with a changing climate.
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| 8. |
- Steinhoff, Tobias, et al.
(författare)
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Constraining the Oceanic Uptake and Fluxes of Greenhouse Gases by Building an Ocean Network of Certified Stations : The Ocean Component of the Integrated Carbon Observation System, ICOS-Oceans
- 2019
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Ingår i: Frontiers in Marine Science. - : FRONTIERS MEDIA SA. - 2296-7745. ; 6
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Forskningsöversikt (refereegranskat)abstract
- The European Research Infrastructure Consortium "Integrated Carbon Observation System" (ICOS) aims at delivering high quality greenhouse gas (GHG) observations and derived data products (e.g., regional GHG-flux maps) for constraining the GHG balance on a European level, on a sustained long-term basis. The marine domain (ICOS-Oceans) currently consists of 11 Ship of Opportunity lines (SOOP - Ship of Opportunity Program) and 10 Fixed Ocean Stations (FOSs) spread across European waters, including the North Atlantic and Arctic Oceans and the Barents, North, Baltic, and Mediterranean Seas. The stations operate in a harmonized and standardized way based on community-proven protocols and methods for ocean GHG observations, improving operational conformity as well as quality control and assurance of the data. This enables the network to focus on long term research into the marine carbon cycle and the anthropogenic carbon sink, while preparing the network to include other GHG fluxes. ICOS data are processed on a near real-time basis and will be published on the ICOS Carbon Portal (CP), allowing monthly estimates of CO2 air-sea exchange to be quantified for European waters. ICOS establishes transparent operational data management routines following the FAIR (Findable, Accessible, Interoperable, and Reusable) guiding principles allowing amongst others reproducibility, interoperability, and traceability. The ICOS-Oceans network is actively integrating with the atmospheric (e.g., improved atmospheric measurements onboard SOOP lines) and ecosystem (e.g., oceanic direct gas flux measurements) domains of ICOS, and utilizes techniques developed by the ICOS Central Facilities and the CP. There is a strong interaction with the international ocean carbon cycle community to enhance interoperability and harmonize data flow. The future vision of ICOS-Oceans includes ship-based ocean survey sections to obtain a three-dimensional understanding of marine carbon cycle processes and optimize the existing network design.
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| 9. |
- Breinl, Korbinian, et al.
(författare)
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Can weather generation capture precipitation patterns across different climates, spatial scales and under data scarcity?
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Stochastic weather generators can generate very long time series of weather patterns, which are indispensable in earth sciences, ecology and climate research. Yet, both their potential and limitations remain largely unclear because past research has typically focused on eclectic case studies at small spatial scales in temperate climates. In addition, stochastic multi-site algorithms are usually not publicly available, making the reproducibility of results difficult. To overcome these limitations, we investigated the performance of the reduced-complexity multi-site precipitation generator TripleM across three different climatic regions in the United States. By resampling observations, we investigated for the first time the performance of a multi-site precipitation generator as a function of the extent of the gauge network and the network density. The definition of the role of the network density provides new insights into the applicability in data-poor contexts. The performance was assessed using nine different statistical metrics with main focus on the inter-annual variability of precipitation and the lengths of dry and wet spells. Among our study regions, our results indicate a more accurate performance in wet temperate climates compared to drier climates. Performance deficits are more marked at larger spatial scales due to the increasing heterogeneity of climatic conditions.
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| 10. |
- Burman, Jan
(författare)
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Computational fluid dynamics for dispersion calculation in urban surroundings
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased knowledge on dispersion processes in urban environment will enhance the ability in the society to handle events where releases of toxic substances can occur. Also, the ability to increase preparedness at locations where such events potentially can emerge. Can Computational Fluid Dynamics (CFD) models contribute to increased knowledge and what type of models are most suitable considering dispersion in urban environment? CFD-models can simulate almost any scenario but urban scales are still computationally de-manding. Simplifications of the basic equations are needed. Mainly two methods to do this is feasible, namely Reynolds Averaged Navier-Stokes models (RANS) and Large Eddy Simula-tion models (LES). These methods are commonly used for hydrodynamic flow studies. In this thesis the eddy viscosity hypothesis is implemented and used in all turbulence models. The scenarios studied includes flow and dispersion past objects at the side of a road, flow over buildings, dispersion in urban environments and on synthetic stochastic boundary condi-tions. The basic flow around objects assume that the turbulence is realistically modelled. In RANS the flow is steady state while the turbulence is fully modelled. In LES only the smallest turbulent eddies are modelled while the flow is resolved in time. In the urban environment tur-bulent fluctuations have the dimension of the buildings and the wind speed. Thus, it is important that these fluctuations are correctly described for the purpose of the simulation with a CFD-model.The results show that CFD can replace the real world in well specified scenarios when stud-ying certain aspects, like effects from objects in the path of the dispersion and effects of atmos-pheric stability. However, the simulations of dispersion in urban environments show that RANS and LES models can produce quite unequal results regarding hazard area estimation. When comparing LES results to data from full scale experiments, it is clear that LES-models have fundamental ability to handle effects found in real life. Here can be mentioned dispersion paths and maximum values which are important when estimating the extent of hazard areas. On the other hand specific temporal fluctuations can hardly be predicted, only statistics.Finally, by using synthetic inflow boundary conditions, realistic representation of spectra of turbulent kinetic energy is enforced in areas with low and sparse buildings. In the high rise building area the building interaction with the flow develops a turbulent urban sub layer that is not much influenced from the inflow boundary. By studying synthetic forcing as a part of the boundary conditions together with a stably stratified boundary layer, more tools to simulate the real world events are examined.
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