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1.
  • Andersson, Andreas, et al. (author)
  • Air-sea gas transfer in high Arctic fjords
  • 2017
  • In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:5, s. 2519-2526
  • Journal article (peer-reviewed)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. (author)
  • Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a High Arctic Fjord During Unstable Very-Close-to-Neutral Conditions
  • 2019
  • In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 170:3, s. 471-488
  • Journal article (peer-reviewed)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. (author)
  • Amplitude modulation of wind turbine sound in cold climates
  • 2020
  • In: Applied Acoustics. - : Elsevier. - 0003-682X .- 1872-910X. ; 158
  • Journal article (peer-reviewed)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. (author)
  • Impact of low-level wind maxima below hub height on wind turbine sound propagation
  • 2020
  • In: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:8, s. 1767-1775
  • Journal article (peer-reviewed)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. (author)
  • Statistics of sea-effect snowfall along the Finnish coastline based on regional climate model data
  • 2020
  • In: Advances in Science and Research. - : COPERNICUS GESELLSCHAFT MBH. - 1992-0628 .- 1992-0636. ; 17, s. 87-104
  • Journal article (peer-reviewed)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. (author)
  • Natural hazards and extreme events in the Baltic Sea region
  • 2022
  • In: Earth System Dynamics. - : Copernicus Publications. - 2190-4979 .- 2190-4987. ; 13:1, s. 251-301
  • Journal article (peer-reviewed)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. (author)
  • Flow over a snow-water-snow surface in the high Arctic, Svalbard : Turbulent fluxes and comparison of observation techniques
  • 2020
  • In: Polar Science. - : Elsevier BV. - 1873-9652 .- 1876-4428. ; 25
  • Journal article (peer-reviewed)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. (author)
  • 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
  • In: Frontiers in Marine Science. - : FRONTIERS MEDIA SA. - 2296-7745. ; 6
  • Research review (peer-reviewed)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. (author)
  • Can weather generation capture precipitation patterns across different climates, spatial scales and under data scarcity?
  • 2017
  • In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7
  • Journal article (peer-reviewed)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 (author)
  • Computational fluid dynamics for dispersion calculation in urban surroundings
  • 2022
  • Doctoral thesis (other academic/artistic)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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12.
  • Burman, Jan, et al. (author)
  • On possibilities to estimate local concentration variations with CFD-LES in real urban environments
  • 2019
  • In: Environmental Fluid Mechanics. - : SPRINGER. - 1567-7419 .- 1573-1510. ; 19:3, s. 719-750
  • Journal article (peer-reviewed)abstract
    • Applied studies with Large Eddy Simulation (LES) of hazardous gas dispersion around buildings in cities have become increasingly feasible due to rapid advancements in computing technology. However, there is little extant literature investigating how each model's results compare with others, as well as their ability to predict near-field dispersion in a real city. In this study, three typical LES sub-grid-scale models are used to simulate gas dispersion, utilizing alternatively constant values and synthetic turbulence at inflow boundaries. The results are compared with data from the Joint Urban 2003 Atmospheric Dispersion Study in Oklahoma City. Flow and turbulence statistics of the simulation is presented at two probe locations, one inside the city-core and one outside. In addition, comparisons with the measured mean concentration and maximum concentration values are conducted. It was found that in the core of the city, simulated turbulence is mainly determined by buildings and their configurations, and is only weakly affected by model type and assumed turbulence at the inflow boundaries. On the other hand, outside and upwind the city center the turbulence set at the inflow boundaries is very important if realistic turbulence statistics is to be achieved. Downstream of the source, all tested models produce similar predictions of maximum concentration values, which in turn are similar to the experimental data. Thus, the results indicate that it could be better to use the LES calculated maximum-concentration instead of the calculated mean-concentration when developing methods for hazard area estimation.
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13.
  • Carlsson, Björn, 1980-, et al. (author)
  • Evaluation of a roughness length model and sea surface properties with data from the Baltic Sea
  • 2010
  • In: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 40:9, s. 2007-2024
  • Journal article (peer-reviewed)abstract
    • The exchange of momentum between the oceans and atmosphere is important for many atmospheric and oceanic processes and is mainly governed by the roughness of sea surface. The roughness can be expressed by a roughness length z0. A roughness length model, based on the concept that z0 is determined by stochastic wave breaking, is presented. The model performance is evaluated using measurements from the Östergarnsholm site, in the Baltic Sea, and pertinent information from other recent investigations. The wave field and the roughness length variations are investigated during various sea state conditions dominated by wind-driven waves. It is found that several parameters, describing the characteristics of the wave field, are dependent on the amount of energy that long waves have relative to the energy of short, wind-driven waves of the sea spectrum (called the swell ratio). The impact of swell ratio on z0 can explain the discrepancies found in various results among relevant investigations. The roughness length model can well reproduce the observed roughness length.
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14.
  • Carlsson, Björn, 1980-, et al. (author)
  • Impact of swell on simulations using a regional atmospheric climate model
  • 2009
  • In: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 61:4, s. 527-538
  • Journal article (peer-reviewed)abstract
    • When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared to under wind sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A three-year simulation and two case studies using the wave field from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and of heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2°C annually on average) while the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg­1). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.
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15.
  • Carlsson, Björn, 1980-, et al. (author)
  • Investigating the effect of a wave-dependent momentum flux in a process oriented ocean model
  • 2009
  • In: Boreal environment research. - 1239-6095 .- 1797-2469. ; 14:1, s. 3-17
  • Journal article (peer-reviewed)abstract
    • New expressions of the drag coefficient were developed using measurements from the Östergarnsholm site in the Baltic Sea. The drag coefficient was significantly lower in the presence of waves travelling faster than the wind (swell). The expressions were implemented in an oceanographic process-oriented model in a 45-year simulation. Since no wave information was included we did an analysis of the potential impact of swell on an ocean model. Current velocity and surface stress were significantly altered during periods with low wind speed but the temperature and the mixing depth in the ocean were not significantly changed. The implementation of the swell effect in a process oriented ocean model is thus of limited importance. There is, however, an indication that for studies of current velocity it is crucial to have a correct description of the drag coefficient.
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16.
  • Chen, Deliang, 1961, et al. (author)
  • Summary of a workshop on extreme weather events in a warming world organized by the Royal Swedish Academy of Sciences
  • 2020
  • In: Tellus Series B-Chemical and Physical Meteorology. - : Stockholm University Press. - 1600-0889 .- 0280-6509. ; 72:1
  • Journal article (peer-reviewed)abstract
    • Climate change is not only about changes in means of climatic variables such as temperature, precipitation and wind, but also their extreme values which are of critical importance to human society and ecosystems. To inspire the Swedish climate research community and to promote assessments of international research on past and future changes in extreme weather events against the global climate change background, the Earth Science Class of the Royal Swedish Academy of Sciences organized a workshop entitled 'Extreme weather events in a warming world' in 2019. This article summarizes and synthesizes the key points from the presentations and discussions of the workshop on changes in floods, droughts, heat waves, as well as on tropical cyclones and extratropical storms. In addition to reviewing past achievements in these research fields and identifying research gaps with a focus on Sweden, future challenges and opportunities for the Swedish climate research community are highlighted.
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17.
  • Chen, Sheng, et al. (author)
  • On the First Observed Wave-Induced Stress Over the Global Ocean
  • 2020
  • In: Journal of Geophysical Research - Oceans. - : AMER GEOPHYSICAL UNION. - 2169-9275 .- 2169-9291. ; 125:12
  • Journal article (peer-reviewed)abstract
    • Despite many investigations/studies on the surface wave-induced stress, the global feature of the wave-induced stress has not been obtained previously as that requires a simultaneous observation of wave spectra and wind on a global scale. The China France Oceanography Satellite (CFOSAT) provided an opportunity for the first time to evaluate the global wave-induced stress and its contribution to the total wind stress. In this study, the global spatial distributions of wave-induced stress and its correlated index for August to November in 2019 are presented using the simultaneous ocean surface winds and wave spectra from the CFOSAT. The main results show that the wave-induced stress is fundamentally dependent on the wind and wave fields on a global scale and shows significant temporal and spatial variations. Further analyses indicate that there is an upward momentum flux under strong swells and low wind speeds (below similar to 5 m/s), and an anticorrelation between the dimensionless wave-induced stress and the proportion of swell energy to the total. Finally, the variations of the surface wave induced wind stress are clear asymmetric between northern and southern hemispheres in late summer but symmetric in late fall, which are closely associated with the seasonal changes in large-scale atmospheric circulation.
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19.
  • Dubois, Kévin, 1995-, et al. (author)
  • Technical note : Extending sea level time series for the analysis of extremes with statistical methods and neighbouring station data
  • 2024
  • In: Ocean Science. - : Copernicus Publications. - 1812-0784 .- 1812-0792. ; 20:1, s. 21-30
  • Journal article (peer-reviewed)abstract
    • Extreme sea levels may cause damage and the disruption of activities in coastal areas. Thus, predicting extreme sea levels is essential for coastal management. Statistical inference of robust return level estimates critically depends on the length and quality of the observed time series. Here, we compare two different methods for extending a very short (∼ 10-year) time series of tide gauge measurements using a longer time series from a neighbouring tide gauge: linear regression and random forest machine learning. Both methods are applied to stations located in the Kattegat Basin between Denmark and Sweden. Reasonable results are obtained using both techniques, with the machine learning method providing a better reconstruction of the observed extremes. By generating a set of stochastic time series reflecting uncertainty estimates from the machine learning model and subsequently estimating the corresponding return levels using extreme value theory, the spread in the return levels is found to agree with results derived by more physically based methods.
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20.
  • Emmanuel, George Victor, et al. (author)
  • Role of source terms in parameterizing wave decay in the marginal ice zones
  • 2022
  • In: Ocean Modelling. - : Elsevier. - 1463-5003 .- 1463-5011. ; 180
  • Journal article (peer-reviewed)abstract
    • Wave decay in the marginal ice zones (MIZ) plays a crucial role in shaping the Arctic Ocean behaviours. Parameterizing wave decay in the MIZ is indispensable in climate models for better capturing the climate change in the Arctic. However, current wave decay parameterizations were developed without considering the influences of wave source terms. This study investigated the role and contributions of different source terms in shaping the MIZ wave decay based on sensitivity simulations. Simulation results show that the ice-induced damping term is the main contributor to MIZ wave decay. The wind input source term also plays an important role in the wave evolution in the MIZ. During high wind speeds (wind speed>10 m/s), wind input was found to increase the wave height by about 10% on average (relative to the wave height decayed by ice), irrespective of the sea ice concentration. Wind input contributes up to 30% in high wind speed areas with low sea ice concentration. Meanwhile, the contributions from wind input term to MIZ wave are more important when winds blow from the open water to MIZ. However, the angle between winds and waves is insignificant in shaping contributions from wind input. Low SIC and high wind speed conditions also favour the enhancement of wave spreading. In general, the contributions from nonlinear wave–wave interactions and dissipation source terms were found to play a minor role in shaping MIZ wave decay. However, the role of nonlinear wave–wave interactions can be important in shaping the wave spectrum. Thus, it is suggested to update the model wave decay parameterizations to accommodate the role played by wind input source term, in particular under high wind speed conditions.
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21.
  • Engström, Jens, et al. (author)
  • Energy absorption from parks of point-absorbing wave energy converters in the Swedish exclusive economic zone
  • 2020
  • In: Energy Science & Engineering. - : John Wiley & Sons. - 2050-0505. ; 8:1, s. 38-49
  • Journal article (peer-reviewed)abstract
    • In a future energy system based on renewable energy sources, wave energy will most likely play a role due to its high energy potential and low intermittency. The power production from parks of wave energy converters of point absorber type has been extensively studied. This is also the case for the wave energy resource at many coastal areas around the globe. Wave energy has not yet reached a commercial level, and a large variety of technologies exist; therefore, an established method to calculate the technical potential for wave energy has still not been established. To estimate the technical potential of wave energy conversion, some approximations inevitably need to be taken due to the systems high complexity. In this study, a detailed mapping of the wave climate and simulation of large arrays of hydrodynamically cross‐coupled wave energy converters are combined to calculate the technical potential for wave energy conversion in the Swedish exclusive economic zone. A 16‐year wave data set distributed in a 1.1 km × 1.1 km grid is used to calculate the absorbed energy from a park of 200 generic point absorbers. The areas with best potential have an average annual energy absorption of 16 GWh for the selected wave energy park adapted to 1 km2 when using a constant damping, while the theoretical upper bound is 63 GWh for the same area.
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22.
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23.
  • Esters, Leonie, et al. (author)
  • Non-local Impacts on Eddy-Covariance Air–Lake CO2 Fluxes
  • 2021
  • In: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 178:2, s. 283-300
  • Journal article (peer-reviewed)abstract
    • Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.
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24.
  • Golub, Malgorzata, et al. (author)
  • Diel, seasonal, and inter-annual variation in carbon dioxide effluxes from lakes and reservoirs
  • 2023
  • In: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 18:3
  • Journal article (peer-reviewed)abstract
    • Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2 based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 emissions had an average of 25% (range 3%-58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.
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25.
  • Gröger, Matthias, et al. (author)
  • Coupled regional Earth system modeling in the Baltic Sea region
  • 2021
  • In: Earth System Dynamics. - : Copernicus GmbH. - 2190-4979 .- 2190-4987. ; 12:3, s. 939-973
  • Research review (peer-reviewed)abstract
    • Nonlinear responses to externally forced climate change are known to dampen or amplify the local climate impact due to complex cross-compartmental feedback loops in the Earth system. These feedbacks are less well represented in the traditional stand-alone atmosphere and ocean models on which many of today's regional climate assessments rely (e.g., EURO-CORDEX, NOSCCA and BACC II). This has promoted the development of regional climate models for the Baltic Sea region by coupling different compartments of the Earth system into more comprehensive models. Coupled models more realistically represent feedback loops than the information imposed on the region by prescribed boundary conditions and, thus, permit more degrees of freedom. In the past, several coupled model systems have been developed for Europe and the Baltic Sea region. This article reviews recent progress on model systems that allow two-way communication between atmosphere and ocean models; models for the land surface, including the terrestrial biosphere; and wave models at the air-sea interface and hydrology models for water cycle closure. However, several processes that have mostly been realized by one-way coupling to date, such as marine biogeochemistry, nutrient cycling and atmospheric chemistry (e.g., aerosols), are not considered here. In contrast to uncoupled stand-alone models, coupled Earth system models can modify mean near-surface air temperatures locally by up to several degrees compared with their stand-alone atmospheric counterparts using prescribed surface boundary conditions. The representation of small-scale oceanic processes, such as vertical mixing and sea-ice dynamics, appears essential to accurately resolve the air-sea heat exchange over the Baltic Sea, and these parameters can only be provided by online coupled high-resolution ocean models. In addition, the coupling of wave models at the ocean-atmosphere interface allows for a more explicit formulation of small-scale to microphysical processes with local feedbacks to water temperature and large-scale processes such as oceanic upwelling. Over land, important climate feedbacks arise from dynamical terrestrial vegetation changes as well as the implementation of land-use scenarios and afforestation/deforestation that further alter surface albedo, roughness length and evapotranspiration. Furthermore, a good representation of surface temperatures and roughness length over open sea and land areas is critical for the representation of climatic extremes such as heavy precipitation, storms, or tropical nights (defined as nights where the daily minimum temperature does not fall below 20gC), and these parameters appear to be sensitive to coupling. For the present-day climate, many coupled atmosphere-ocean and atmosphere-land surface models have demonstrated the added value of single climate variables, in particular when low-quality boundary data were used in the respective stand-alone model. This makes coupled models a prospective tool for downscaling climate change scenarios from global climate models because these models often have large biases on the regional scale. However, the coupling of hydrology models to close the water cycle remains problematic, as the accuracy of precipitation provided by atmosphere models is, in most cases, insufficient to realistically simulate the runoff to the Baltic Sea without bias adjustments. Many regional stand-alone ocean and atmosphere models are tuned to suitably represent present-day climatologies rather than to accurately simulate climate change. Therefore, more research is required into how the regional climate sensitivity (e.g., the models' response to a given change in global mean temperature) is affected by coupling and how the spread is altered in multi-model and multi-scenario ensembles of coupled models compared with uncoupled ones.
  •  
26.
  • Gutiérrez Loza, Lucia, et al. (author)
  • Air–sea CO2 exchange in the Baltic Sea—A sensitivity analysis of the gas transfer velocity
  • 2021
  • In: Journal of Marine Systems. - : Elsevier. - 0924-7963 .- 1879-1573. ; 222
  • Journal article (peer-reviewed)abstract
    • Air–sea gas fluxes are commonly estimated using wind-based parametrizations of the gas transfer velocity. However, neglecting gas exchange forcing mechanisms – other than wind speed – may lead to large uncertainties in the flux estimates and the carbon budgets, in particular, in heterogeneous environments such as marginal seas and coastal areas. In this study we investigated the impact of including relevant processes to the air–sea CO2 flux parametrization for the Baltic Sea. We used six parametrizations of the gas transfer velocity to evaluate the effect of precipitation, water-side convection, and surfactants on the net CO2 flux at regional and sub-regional scale. The differences both in the mean CO2 fluxes and the integrated net fluxes were small between the different cases. However, the implications on the seasonal variability were shown to be significant. The inter-annual and spatial variability were also found to be associated with the forcing mechanisms evaluated in the study. In addition to wind, water-side convection was the most relevant parameter controlling the air–sea gas exchange at seasonal and inter-annual scales. The effect of precipitation and surfactants seemed negligible in terms of the inter-annual variability. The effect of water-side convection and surfactants resulted in a reduction of the downward fluxes, while precipitation was the only parameter that resulted in an enhancement of the net uptake in the Baltic Sea.
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27.
  • Gutiérrez Loza, Lucia, et al. (author)
  • Measurement of air-sea methane fluxes in the Baltic Sea using the eddy covariance method
  • 2019
  • In: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 7
  • Journal article (peer-reviewed)abstract
    • Methane (CH4) is the second-most important greenhouse gas in the atmosphere having a significant effect on global climate. The ocean-particularly the coastal regions-have been recognized to be a net source of CH4, however, the constraints on temporal and spatial resolution of CH4 measurements have been the limiting factor to estimate the total oceanic contributions. In this study, the viability of micrometeorological methods for the analysis of CH4 fluxes in the marine environment was evaluated. We present 1 year of semi-continuous eddy covariance measurements of CH4 atmospheric dry mole fractions and air-sea CH4 flux densities at the Ostergarnsholm station at the east coast of the Gotland Island in the central Baltic Sea. The mean annual CH4 flux density was positive, indicating that the region off Gotland is a net source of CH4 to the atmosphere with monthly mean flux densities ranging between -0.1 and 36 nmol m(-2)s(-1). Both the air-water concentration gradient and the wind speed were found to be crucial parameters controlling the flux. The results were in good agreement with other measurements in the Baltic Sea reported in the MEMENTO database. Our results suggest that the eddy covariance technique is a useful tool for studying CH4 fluxes and improving the understanding of air-sea gas exchange processes with high-temporal resolution. Potentially, the high resolution of micrometeorological data can increase the understanding of the temporal variability and forcing processes of CH4 flux.
  •  
28.
  • Gutiérrez-Loza, Lucía (author)
  • Mechanisms controlling air-sea gas exchange in the Baltic Sea
  • 2020
  • Licentiate thesis (other academic/artistic)abstract
    • Carbon plays a major role in physical and biogeochemical processes in the atmosphere, the biosphere, and the ocean. CO2 and CH4 are two of the most common carbon-containing compounds in the atmosphere, also recognized as major greenhouse gases. The exchange of CO2 and CH4 between the ocean and the atmosphere is an essential part of the global carbon cycle. The exchange is controlled by the air–sea concentration gradient and by the efficiency of the transfer processes. The lack of knowledge about the forcing mechanisms affecting the exchange of these climate-relevant gases is a major source of uncertainty in the estimation of the global oceanic contributions. Quantifying and understanding the air–sea exchange processes is essential to constrain the estimates and to improve our knowledge about the current and future climate. In this thesis, the mechanisms controlling the air–sea gas exchange in the Baltic Sea are investigated.The viability of micrometeorological techniques for CH4 monitoring in a coastal environment is evaluated. One year of semi-continuous measurements of air–sea CH4 fluxes using eddy covariance measurements suggests that the method is useful for CH4 flux estimations in marine environments. The measurements allow long-term monitoring at high frequency rates, thus, capturing the temporal variability of the flux. The region off Gotland is a net source of CH4, with both the air–sea concentration gradient and the wind as controlling mechanisms.A sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms controlling the air–sea CO2 exchange in the Baltic Sea. This analysis shows that the spatio-temporal variability of CO2 fluxes is strongly modulated by water-side convection, precipitation, and surfactants. The effect of these factors is relevant both at regional and global scales, as they are not included in the current budget estimates.
  •  
29.
  • Gutiérrez Loza, Lucia, et al. (author)
  • On physical mechanisms enhancing air-sea CO2 exchange
  • 2022
  • In: Biogeosciences. - : European Geosciences Union (EGU). - 1726-4170 .- 1726-4189. ; 19:24, s. 5645-5665
  • Journal article (peer-reviewed)abstract
    • Reducing uncertainties in the air–sea CO2 flux calculations is one of the major challenges when addressing the oceanic contribution in the global carbon balance. In traditional models, the air–sea CO2 flux is estimated using expressions of the gas transfer velocity as a function of wind speed. However, other mechanisms affecting the variability in the flux at local and regional scales are still poorly understood. The uncertainties associated with the flux estimates become particularly large in heterogeneous environments such as coastal and marginal seas. Here, we investigated the air–sea CO2 exchange at a coastal site in the central Baltic Sea using nine years of eddy covariance measurements. Based on these observations we were able to capture the temporal variability of the air–sea CO2 flux and other parameters relevant for the gas exchange. Our results show that a wind-based model with similar pattern to those developed for larger basins and open sea condition can, on average, be a good approximation for k. However, in order to reduce the uncertainty associated to these averages and produce reliable short-term k estimates, additional physical processes must be considered. Using a normalized gas transfer velocity, we identified conditions associated to enhanced exchange (large k values). During high and intermediate wind speeds (above 6–8 m s−1),conditions on both sides of the air–water interface were found to be relevant for the gas exchange. Our findings further suggest that at such relatively high wind speeds, sea spray is an efficient mechanisms for air–sea CO2 exchange. During low wind speeds (<6 m s−1), water-side convection was found to be a relevant control mechanism. The effect of both sea spray and water-side convection on the gas exchange showed a clear seasonality with positive fluxes (winter conditions) being the most affected.
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30.
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31.
  • Högström, Ulf, et al. (author)
  • The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions
  • 2018
  • In: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 75:8, s. 2579-2588
  • Journal article (peer-reviewed)abstract
    • Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U 5 4ms21 . For the lowest observed wind speeds U , 2.4 m s21 , the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height Hsd in the range 0.6 , Hsd , 1.4 m. For Hsd , 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed
  •  
32.
  • Janzon, Erik (author)
  • Local Effects On Icing Forecasts for Wind Power In Cold Climate
  • 2022
  • Licentiate thesis (other academic/artistic)abstract
    • This thesis will examine the local effects of land cover on icing forecasts. In Paper I, a single column model was used to test the sensitivity of icing forecasts to land cover fraction. Here, the ice accretion forecast was found to be highly sensitive to the wind magnitude response to the surface roughness. Diabatic effects related to the surface albedo played a secondary role, significant in cases with strong solar irradiance. Paper II examined the impact of 2-dimensional patterns of land cover heterogeneity on the effective surface roughness and blending height using large eddy simulation over a diurnal cycle of solar irradiance. The blending height--or the elevation at which the atmospheric response to the underlying land cover becomes horizontally homogeneous--has been proposed as a guide for coupling numerical weather models to surface parameterizations. In stable conditions, when the atmospheric boundary layer height was shallow, the blending height over surfaces with large heterogeneity length scale was found to be much lower than that of analytical models from previous studies. A new formula for a dynamic blending height was proposed taking this effect into account. The effective surface roughness was found to decrease with increasing land cover heterogeneity. The wind power response from an idealized wind turbine with 80-meter hub height to the effective surface roughness was shown, with a positive response in wind power with increasing land cover heterogeneity. The wind power response was smaller and less systematic with wind turbines extending above the blending height, further highlighting the utility of an accurate formulation for this variable.
  •  
33.
  • Janzon, Erik, et al. (author)
  • Modelling the effects of surface heterogeneity on the internal boundary layer during a diurnal cycle
  • Other publication (other academic/artistic)abstract
    • To characterize the effects of subgrid surface heterogeneity, the blending height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction (NWP) models. Previous modelling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, Large Eddy Simulations (LES) are used to examine the response of the blending height and effective surface roughness to two-dimensional chessboard patterns of alternating high and low vegetation given a diurnal cycle of solar irradiance. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. As the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced.
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34.
  • Janzon, Erik, et al. (author)
  • Modelling the flow response to surface heterogeneity during a semi-idealized diurnal cycle
  • 2023
  • In: Journal of Applied Meteorology and Climatology. - : American Meteorological Society. - 1558-8424 .- 1558-8432. ; 62:4, s. 511-527
  • Journal article (peer-reviewed)abstract
    • To characterize the effects of subgrid surface heterogeneity, the blending height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction (NWP) models. Previous modelling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, Large Eddy Simulations (LES) are used to examine the response of the blending height and effective surface roughness to \reva{tiled land cover heterogeneity, or a two-dimensional chessboard pattern }of alternating high and low vegetation given a diurnal cycle of solar irradiance \revg{in subarctic conditions}. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. As the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced.
  •  
35.
  • Janzon, Erik, et al. (author)
  • Single Column Model Simulations of Icing Conditions in Northern Sweden : Sensitivity to Surface Model Land Use Representation
  • 2020
  • In: Energies. - : MDPI. - 1996-1073. ; 13:16
  • Journal article (peer-reviewed)abstract
    • In-cloud ice mass accretion on wind turbines is a common challenge that is faced by energy companies operating in cold climates. On-shore wind farms in Scandinavia are often located in regions near patches of forest, the heterogeneity length scales of which are often less than the resolution of many numerical weather prediction (NWP) models. The representation of these forests-including the cloud water response to surface roughness and albedo effects that are related to them-must therefore be parameterized in NWP models used as meteorological input in ice prediction systems, resulting in an uncertainty that is poorly understood and, to the present date, not quantified. The sensitivity of ice accretion forecasts to the subgrid representation of forests is examined in this study. A single column version of the HARMONIE-AROME three-dimensional (3D) NWP model is used to determine the sensitivity of the forecast of ice accretion on wind turbines to the subgrid forest fraction. Single column simulations of a variety of icing cases at a location in northern Sweden were examined in order to investigate the impact of vegetation cover on ice accretion in varying levels of solar insolation and wind magnitudes. In mid-winter cases, the wind speed response to surface roughness was the primary driver of the vegetation effect on ice accretion. In autumn cases, the cloud water response to surface albedo effects plays a secondary role in the impact of in-cloud ice accretion, with the wind response to surface roughness remaining the primary driver for the surface vegetation impact on icing. Two different surface boundary layer (SBL) forest canopy subgrid parameterizations were tested in this study that feature different methods for calculating near-surface profiles of wind, temperature, and moisture, with the ice mass accretion again following the wind response to surface vegetation between both of these schemes.
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36.
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37.
  • Katsidoniotaki, Eirini, et al. (author)
  • Response of Point-Absorbing Wave Energy Conversion System in 50-Years Return Period Extreme Focused Waves
  • 2021
  • In: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 9:3
  • Journal article (peer-reviewed)abstract
    • This work evaluates the survivability of a point-absorbing wave energy converter at sea states along and inside the 50-year environmental contour for a selected-site in North Sea, by utilizing CFD simulations. Focused wave groups based on NewWave theory are used to model the extreme waves. The numerical breaking waves have been previously predicted by the analytical breaking criterion, showing that the latter provides an accurate estimate for the breaking state. The forces on key components of the device and the system’s dynamics are studied and compared. Slamming loads are identified in the interaction with extreme waves, particularly with breaking waves, and compared with the analytical formulas for slamming estimation as suggested by industrial standards. Considering the extreme wave characteristics, the accompanied phenomena and the resulting WEC’s response, this work contributes to the identification of the design-waves given the environmental contour of the selected site. The top-left side of the contour is identified as the more critical area as it consists of steep and high waves inducing significant nonlinear phenomena, resulting in high loads.
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38.
  • Lansö, Anne Sofie, et al. (author)
  • The influence of short-term variability in surface water pCO2 on modelled air-sea CO2 exchange
  • 2017
  • In: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 69
  • Journal article (peer-reviewed)abstract
    • Coastal seas and estuarine systems are highly variable in both time and space and with their heterogeneity difficult to capture with measurements. Models are useful tools in obtaining a better spatiotemporal coverage or, at least, a better understanding of the impacts such heterogeneity has in driving variability in coastal oceans and estuaries. A model-based sensitivity study is constructed in this study in order to examine the effects of short-term variability in surface water pCO(2) on the annual air-sea CO2 exchange in coastal regions. An atmospheric transport model formed the basis of the modelling framework for the study of the Baltic Sea and the Danish inner waters. Several maps of surface water pCO(2) were employed in the modelling framework. While a monthly Baltic Sea climatology (BSC) had already been developed, the current study further extended this with the addition of an improved near-coastal climatology for the Danish inner waters. Furthermore, daily surface fields of pCO(2) were obtained from a mixed layer scheme constrained by surface measurements of pCO(2) (JENA). Short-term variability in surface water pCO(2) was assessed by calculating monthly mean diurnal cycles from continuous measurements of surface water pCO(2), observed at stationary sites within the Baltic Sea. No apparent diurnal cycle was evident in winter, but diurnal cycles (with amplitudes up to 27 mu atm) were found from April to October. The present study showed that the temporal resolution of surface water pCO(2) played an influential role on the annual air-sea CO2 exchange for the coastal study region. Hence, annual estimates of CO2 exchanges are sensitive to variation on much shorter time scales, and this variability should be included for any model study investigating the exchange of CO2 across the air-sea interface. Furthermore, the choice of surface pCO(2) maps also had a crucial influence on the simulated air-sea CO2 exchange.
  •  
39.
  • Leijon, Jennifer (author)
  • Wave Powered Desalination
  • 2018
  • Licentiate thesis (other academic/artistic)abstract
    • The aim of this licentiate thesis is to investigate opportunities to produce fresh-water and electricity using marine renewable energy sources, such as wave power and marine current power, for water scarce areas. It summarizes the ongoing work within the field and suggests some possibilities of future re-search. Specifically, the desalination process reverse osmosis has been dis-cussed in combination with the wave energy concept designed at Uppsala Uni-versity for a site by the Kenyan coast. A review on wave powered desalination systems was presented, and the wave climate of the Kenyan coast was reana-lyzed and discussed with respect to the wave power and desalination applica-tion. Also, the magnetic circuit of the linear generator was investigated, as well as the control of the system, to enhance its sustainability. Moreover, the marine current energy converter designed at Uppsala University was investi-gated for desalination purposes. Only literature studies and simulations were performed, and the research would benefit from experimental work.
  •  
40.
  • MacIntyre, Sally, et al. (author)
  • Turbulence in a small boreal lake: Consequences for air-water gas exchange
  • 2021
  • In: Limnology and Oceanography. - : WILEY. - 0024-3590 .- 1939-5590. ; 66:3, s. 827-854
  • Journal article (peer-reviewed)abstract
    • The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (epsilon) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface epsilon varied from 10(-8) to 10(-6) m(2) s(-3) for the 0-4 m s(-1) winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10(-3) m(2) s(-1) on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L-N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using epsilon agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s(-1), a condition that can lead to elevated near-surface epsilon and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes.
  •  
41.
  • Mahrt, Larry, et al. (author)
  • Sea-Surface Stress Driven by Small-Scale Non-stationary Winds
  • 2020
  • In: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 176:1, s. 13-33
  • Journal article (peer-reviewed)abstract
    • A number of studies have indicated that non-stationarity of the wind field over the sea significantly disrupts the equilibrium between the wind, stress, and wave fields. However, no studies have systematically examined the impact of non-stationarity using a large dataset. Here, we examine the effect of non-stationarity of the wind field on the stress by analyzing six years of turbulent flux data from the Östergarnsholm site. On average, the impact of the non-stationarity on the stress and drag coefficient becomes important for wind speeds lessthan about 6 ms-1. Differences between the acceleration and deceleration stages are revealed by phase shifting the calculation of the non-stationarity with respect to the calculation of the stress and drag coefficient. The time structure of events is examined by selecting samples of large increases and decreases of the wind speed. Analyses needed for further progress are noted.
  •  
42.
  • Mahrt, Larry, et al. (author)
  • The sea surface heat flux at a coastal site
  • 2022
  • In: Journal of Physical Oceanography. - : American Meteorological Society. - 0022-3670 .- 1520-0485. ; 52:12, s. 3297-3307
  • Journal article (peer-reviewed)abstract
    • We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea-surface flux due to vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 km to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability, but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underly the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.
  •  
43.
  • Mahrt, Larry, et al. (author)
  • Vertical divergence of the atmospheric momentum flux near the sea surface at a coastal site
  • 2021
  • In: Journal of Physical Oceanography. - Boston MA, USA : American Meteorological Society. - 0022-3670 .- 1520-0485. ; 51:11, s. 3529-3537
  • Journal article (peer-reviewed)abstract
    • Motivated by previous studies, we examine the underestimation of the sea surface stress due to the stress divergence between the surface and the atmospheric observational level. We analyze flux measurements collected over a 6-yr period at a coastal tower in the Baltic Sea encompassing a wide range of fetch values. Results are posed in terms of the vertical divergence of the stress scaled by the stress at the lowest observational level. The magnitude of this relative stress divergence increases with increasing stability and decreases with increasing instability, possibly partly due to the impact of stability on the boundary layer depth. The magnitude of the relative stress divergence increases modestly with decreasing wave age. The divergence of the heat flux is not well correlated with the divergence of the momentum flux evidently due to the greater influence of advection on the temperature. Needed improvement of the conceptual framework and needed additional measurements are noted.
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44.
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45.
  • Maldonado, Tito, 1983-, et al. (author)
  • Interannual variability of the midsummer drought in Central America and the connection with sea surface temperatures
  • 2016
  • In: 3Rd International Conference On El Nino. - : Copernicus GmbH. ; , s. 35-50
  • Conference paper (peer-reviewed)abstract
    • The midsummer drought (MSD) in Central America is characterised in order to create annual indexes representing the timing of its phases (start, minimum and end), and other features relevant for MSD forecasting such as the intensity and the magnitude. The MSD intensity is defined as the minimum rainfall detected during the MSD, meanwhile the magnitude is the total precipitation divided by the total days between the start and end of the MSD. It is shown that the MSD extends along the Pacific coast, however, a similar MSD structure was detected also in two stations in the Caribbean side of Central America, located in Nicaragua. The MSD intensity and magnitude show a negative relationship with Nino 3.4 and a positive relationship with the Caribbean low-level jet (CLLJ) index, however for the Caribbean stations the results were not statistically significant, which is indicating that other processes might be modulating the precipitation during the MSD over the Caribbean coast. On the other hand, the temporal variables (start, minimum and end) show low and no significant correlations with the same indexes. The results from canonical correlation analysis (CCA) show good performance to study the MSD intensity and magnitude, however, for the temporal indexes the performance is not satisfactory due to the low skill to predict the MSD phases. Moreover, we find that CCA shows potential predictability of the MSD intensity and magnitude using sea surface temperatures (SST) with leading times of up to 3 months. Using CCA as diagnostic tool it is found that during June, an SST dipole pattern upon the neighbouring waters to Central America is the main variability mode controlling the inter-annual variability of the MSD features. However, there is also evidence that the regional waters are playing an important role in the annual modulation of the MSD features. The waters in the PDO vicinity might be also controlling the rainfall during the MSD, however, exerting an opposite effect at the north and south regions of Central America.
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46.
  • Maldonado, Tito, 1983-, et al. (author)
  • Regional precipitation estimations in Central America using the Weather Research and Forecast model
  • 2018
  • In: Revista de biologia tropical. - : REVISTA DE BIOLOGIA TROPICAL. - 0034-7744 .- 2215-2075. ; 66:1, s. S231-S254
  • Journal article (peer-reviewed)abstract
    • Using the regional climate model WRF, and the NCEP-NCAR Reanalysis Project data as boundary and initial conditions. regional precipitation for Central America was estimated by means of the dynamical downscaling technique for two selected periods: January 2000 and September 2007. Four-nested domains, d01, d02. d03 and d04 with a grid-resolution of 90 km, 30 km, 10 km. and 3.3 km respectively. were configured over this region. The runs were reinitialized every 5 days with 6 h of spin-up time for adjustment of the model. A total of eight experiments (four per month) were tested in order to study: a) two important Cumulus Parameterization Schemes (CPS): Kain-Fritsch (KF) and Grell-Devenyi (GD); and b) the physical interaction between nested domains (one- and two-way nesting), during each simulated month. The modeled precipitation was in agreement with observations for January 2000, and also captured the mean climate features of rainfall concerning magnitude, and spatial distribution, such as the particular precipitation contrast between the Pacific and the Caribbean coast. Outputs of the coarse domains (d01, d02, and d03) for September 2007 revealed differences between experiments within the domains when a visual comparison of the spatial distribution was made. However, for the inner grid (d04), all the experiments, showed a similar spatial distribution and magnitude estimation, mainly in those runs using one-way nesting configuration. The results for the month of September differed substantially with the observations, which could be related to associated deficiencies in the boundary condition that do not reproduce well the transition periods from warm to cold ENSO episodes for the selected periods of study. In all the experiments, the KF scheme calculated more precipitation than the GD scheme and it was associated to the ability of the GD scheme to reproduce spotty but intense rainfall, and apparently, this scheme was reluctant to activate, showing frequent events of low intensity rain. However, when rainfall did develop, it was very intense. Also, the time series did not replicate specific precipitation events. Thus, the 5-days integration period used in this study was not enough to reproduce short-period precipitation events. Finally, physical interaction issues between the nested domains were reflected in discontinuities in the precipitation field, which have been associated with mass field adjustment in the CPS.
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47.
  • Maldonado, Tito, 1983-, et al. (author)
  • The early rainy season in Central America : the role of the tropical North Atlantic SSTs
  • 2017
  • In: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 37:9, s. 3731-3742
  • Journal article (peer-reviewed)abstract
    • We explored the relationship between the precipitation anomalies during May to June as the first peak of the rainy season in the Pacific slope of Central America, and sea surface temperature (SST) fluctuations in the surrounding oceans, using canonical correlation analysis (CCA). With this approach, we studied variations in total precipitation, frequency of rainy days and the monthly occurrence of days with rainfall above (below) the 80th (20th) percentile, due to changes in the nearby SST. Composites of the sea-level pressure (SLP), geopotential heights (200 hPa), relative humidity (700 hPa), horizontal moisture flux and wind at 850 hPa were estimated to provide a dynamical analysis. The composites are calculated using the information obtained with CCA. In addition, we used a general circulation model forced with fixed SST to explore the sensitivity of the model to the SST patterns found using CCA. The results show that the SST over the tropical North Atlantic controls the precipitation fluctuations at interannual scales, due to its connection with the tropical upper tropospheric trough. Warmer (colder) temperatures result in SLP below normal in the Caribbean region, associated with an increase in the heights at 200 hPa. This vertical configuration reduces the wind shear between 850 and 200 hPa and increases the input of humidity to mid-levels, creating favourable conditions for deep convection, and favouring the generation of tropical cyclone activity. In the Pacific, a positive anomalous low-level moisture flux is observed from the ocean to the continental parts of the region and may enhance the formation of mesoscale systems. The classic prediction schemes show a lead time of 1 or 2 months; this is an advantage for climate services operative work. The atmospheric model outcomes replicate the main results found in the composite analysis, reflecting its potential use for model output statistics predictive schemes.
  •  
48.
  • Maldonado, Tito, et al. (author)
  • The role of the meridional sea surface temperature gradient in controlling the Caribbean low-level jet
  • 2017
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:11, s. 5903-5916
  • Journal article (peer-reviewed)abstract
    • The Caribbean low-level jet (CLLJ) is an important modulator of regional climate, especially precipitation, in the Caribbean and Central America. Previous work has inferred, due to their semiannual cycle, an association between CLLJ strength and meridional sea surface temperature (SST) gradients in the Caribbean Sea, suggesting that the SST gradients may control the intensity and vertical shear of the CLLJ. In addition, both the horizontal and vertical structure of the jet have been related to topographic effects via interaction with the mountains in Northern South America (NSA), including funneling effects and changes in the meridional geopotential gradient. Here we test these hypotheses, using an atmospheric general circulation model to perform a set of sensitivity experiments to examine the impact of both SST gradients and topography on the CLLJ. In one sensitivity experiment, we remove the meridional SST gradient over the Caribbean Sea and in the other, we flatten the mountains over NSA. Our results show that the SST gradient and topography have little or no impact on the jet intensity, vertical, and horizontal wind shears, contrary to previous works. However, our findings do not discount a possible one-way coupling between the SST and the wind over the Caribbean Sea through friction force. We also examined an alternative approach based on barotropic instability to understand the CLLJ intensity, vertical, and horizontal wind shears. Our results show that the current hypothesis about the CLLJ must be reviewed in order to fully understand the atmospheric dynamics governing the Caribbean region.
  •  
49.
  • Meier, Markus, et al. (author)
  • An Earth System Science Program for the Baltic Sea Region
  • 2014
  • In: EOS. - 0096-3941 .- 2324-9250. ; 95:13, s. 109-110
  • Journal article (peer-reviewed)abstract
    • From Russia in the east to Sweden, Denmark, and Germany in the west, reaching south to the tips of the Czech Republic, Slovakia, and Ukraine, the Baltic Sea watershed drains nearly 20% of Europe (see Figure 1). In the highly populated south, the temperate climate hosts intensive agriculture and industry. In the north, the landscape is boreal and rural. In the Baltic Sea itself, complex bathymetry and stratification patterns as well as extended hypoxic and anoxic deep waters add to the diversity. Yet in recent history, the differences across the Baltic Sea region have been more than physical: In the mid-20th century, the watershed was split in two.
  •  
50.
  • Müller, Jens Daniel, et al. (author)
  • Cyanobacteria net community production in the Baltic Sea as inferred from profiling pCO2 measurements
  • 2021
  • In: Biogeosciences. - : European Geosciences Union (EGU). - 1726-4170 .- 1726-4189. ; 18:17, s. 4889-4917
  • Journal article (peer-reviewed)abstract
    • Organic matter production by cyanobacteria blooms is a major environmental concern for the Baltic Sea, as it promotes the spread of anoxic zones. Partial pressure of carbon dioxide (pCO2) measurements carried out on Ships of Opportunity (SOOP) since 2003 have proven to be a powerful tool to resolve the carbon dynamics of the blooms in space and time. However, SOOP measurements lack the possibility to directly constrain depth-integrated net community production (NCP) in moles of carbon per surface area due to their restriction to the sea surface. This study tackles the knowledge gap through (1) providing an NCP best guess for an individual cyanobacteria bloom based on repeated profiling measurements of pCO2 and (2) establishing an algorithm to accurately reconstruct depth-integrated NCP from surface pCO2 observations in combination with modelled temperature profiles.Goal (1) was achieved by deploying state-of-the-art sensor technology from a small-scale sailing vessel. The low-cost and flexible platform enabled observations covering an entire bloom event that occurred in July–August 2018 in the Eastern Gotland Sea. For the biogeochemical interpretation, recorded pCO2 profiles were converted to C∗T, which is the dissolved inorganic carbon concentration normalised to alkalinity. We found that the investigated bloom event was dominated by Nodularia and had many biogeochemical characteristics in common with blooms in previous years. In particular, it lasted for about 3 weeks, caused a C∗T drawdown of 90 µmol kg−1, and was accompanied by a sea surface temperature increase of 10 ∘C. The novel finding of this study is the vertical extension of the C∗T drawdown up to the compensation depth located at around 12 m. Integration of the C∗T drawdown across this depth and correction for vertical fluxes leads to an NCP best guess of ∼1.2 mol m−2 over the productive period.Addressing goal (2), we combined modelled hydrographical profiles with surface pCO2 observations recorded by SOOP Finnmaid within the study area. Introducing the temperature penetration depth (TPD) as a new parameter to integrate SOOP observations across depth, we achieve an NCP reconstruction that agrees to the best guess within 10 %, which is considerably better than the reconstruction based on a classical mixed-layer depth constraint.Applying the TPD approach to almost 2 decades of surface pCO2 observations available for the Baltic Sea bears the potential to provide new insights into the control and long-term trends of cyanobacteria NCP. This understanding is key for an effective design and monitoring of conservation measures aiming at a Good Environmental Status of the Baltic Sea.
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