| 1. |
- Rutgersson, Anna, 1971-, et al.
(författare)
-
Using land-based stations for air–sea interaction studies
- 2020
-
Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Informa UK Limited. - 0280-6495 .- 1600-0870. ; 72:1, s. 1-23
-
Tidskriftsartikel (refereegranskat)abstract
- In situ measurements representing the marine atmosphere and air-sea interaction are taken at ships, buoys, stationary moorings and land-based towers, where each observation platform has structural restrictions. Air-sea fluxes are often small, and due to the limitations of the sensors, several corrections are applied. Land-based towers are convenient for long-term observations, but one critical aspect is the representativeness of marine conditions. Hence, a careful analysis of the sites and the data is necessary. Based on the concept of flux footprint, we suggest defining flux data from land-based marine micrometeorological sites in categories depending on the type of land influence:1. CAT1: Marine data representing open sea,2. CAT2: Disturbed wave field resulting in physical properties different from open sea conditions and heterogeneity of water properties in the footprint region, and3. CAT3: Mixed land-sea footprint, very heterogeneous conditions and possible active carbon production/consumption.Characterization of data would be beneficial for combined analyses using several sites in coastal and marginal seas and evaluation/comparison of properties and dynamics. Aerosol fluxes are a useful contribution to characterizing a marine micrometeorological field station; for most conditions, they change sign between land and sea sectors. Measured fluxes from the land-based marine station Ostergarnsholm are used as an example of a land-based marine site to evaluate the categories and to present an example of differences between open sea and coastal conditions. At the Ostergarnsholm site the surface drag is larger for CAT2 and CAT3 than for CAT1 when wind speed is below 10m/s. The heat and humidity fluxes show a distinctive distinguished seasonal cycle; latent heat flux is larger for CAT2 and CAT3 compared to CAT1. The flux of carbon dioxide is large from the coastal and land-sea sectors, showing a large seasonal cycle and significant variability (compared to the open sea sector). Aerosol fluxes are partly dominated by sea spray emissions comparable to those observed at other open sea conditions.
|
|
| 2. |
- Tomic, Tajana Tesan, et al.
(författare)
-
MYO5B mutations in pheochromocytoma/paraganglioma promote cancer progression
- 2020
-
Ingår i: PLOS Genetics. - : Public Library of Science. - 1553-7390 .- 1553-7404. ; 16:6
-
Tidskriftsartikel (refereegranskat)abstract
- Identification of additional cancer-associated genes and secondary mutations driving the metastatic progression in pheochromocytoma and paraganglioma (PPGL) is important for subtyping, and may provide optimization of therapeutic regimens. We recently reported novel recurrent nonsynonymous mutations in the MYO5B gene in metastatic PPGL. Here, we explored the functional impact of these MYO5B mutations, and analyzed MYO5B expression in primary PPGL tumor cases in relation to mutation status. Immunohistochemistry and mRNA expression analysis in 30 PPGL tumors revealed an increased MYO5B expression in metastatic compared to non-metastatic cases. In addition, subcellular localization of MYO5B protein was altered from cytoplasmic to membranous in some metastatic tumors, and the strongest and most abnormal expression pattern was observed in a paraganglioma harboring a somatic MYO5B:p.G1611S mutation. In addition to five previously discovered MYO5B mutations, the present study of 30 PPGL (8 previous and 22 new samples) also revealed two, and hence recurrent, mutations in the gene paralog MYO5A. The three MYO5B missense mutations with the highest prediction scores (p.L587P, p.G1611S and p.R1641C) were selected and functionally validated using site directed mutagenesis and stable transfection into human neuroblastoma cells (SK-N-AS) and embryonic kidney cells (HEK293). In vitro analysis showed a significant increased proliferation rate in all three MYO5B mutated clones. The two somatically derived mutations, p.L587P and p.G1611S, were also found to increase the migration rate. Expression analysis of MYO5B mutants compared to wild type clones, demonstrated a significant enrichment of genes involved in migration, proliferation, cell adhesion, glucose metabolism, and cellular homeostasis. Our study validates the functional role of novel MYO5B mutations in proliferation and migration, and suggest the MYO5-pathway to be involved in the malignant progression in some PPGL tumors. © 2020 Tomic et al.
|
|
| 3. |
- Zinke, Julika, 1995-, et al.
(författare)
-
Sea spray emissions from the Baltic Sea : comparison of aerosol eddy covariance fluxes and chamber-simulated sea spray emissions
- 2024
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus Publications. - 1680-7316 .- 1680-7324. ; 24:3, s. 1895-1918
-
Tidskriftsartikel (refereegranskat)abstract
- To compare in situ and laboratory estimates of sea spray aerosol (SSA) production fluxes, we conducted two research campaigns in the vicinity of an eddy covariance (EC) flux tower on the island of Östergarnsholm in the Baltic Sea during May and August 2021. To accomplish this, we performed EC flux measurements for particles with diameters between 0.25 and 2.5 µm simultaneously with laboratory measurements using a plunging jet sea spray simulation chamber containing local seawater sampled close to the footprint of the flux tower. We observed a log-linear relationship between wind speed and EC-derived SSA emission fluxes, a power-law relationship between significant wave height and EC-derived SSA emission fluxes, and a linear relationship between wave Reynolds number and EC-derived SSA emission fluxes, all of which are consistent with earlier studies. Although we observed a weak negative relationship between particle production in the sea spray simulation chamber and seawater chlorophyll-α concentration and a weak positive relationship with the concentration of fluorescent dissolved organic matter in seawater, we did not observe any significant impact of dissolved oxygen on particle production in the chamber.To obtain an estimate of the size-resolved emission spectrum for particles with dry diameters between 0.015 and 10 µm, we combined the estimates of SSA particle production fluxes obtained using the EC measurements and the chamber measurements in three different ways: (1) using the traditional continuous whitecap method, (2) using air entrainment measurements, and (3) simply scaling the chamber data to the EC fluxes. In doing so, we observed that the magnitude of the EC-derived emission fluxes compared relatively well to the magnitude of the fluxes obtained using the chamber air entrainment method as well as the previous flux measurements of Nilsson et al. (2021) and the parameterizations of Mårtensson et al. (2003) and Salter et al. (2015). As a result of these measurements, we have derived a wind-speed-dependent and wave-state-dependent SSA parameterization for particles with dry diameters between 0.015 and 10 µm for low-salinity waters such as the Baltic Sea, thus providing a more accurate estimation of SSA production fluxes.
|
|
| 4. |
- Esters, Leonie, et al.
(författare)
-
Non-local Impacts on Eddy-Covariance Air–Lake CO2 Fluxes
- 2021
-
Ingår i: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 178:2, s. 283-300
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 5. |
- Gutiérrez Loza, Lucia, et al.
(författare)
-
Measurement of air-sea methane fluxes in the Baltic Sea using the eddy covariance method
- 2019
-
Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 7
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 6. |
- Gutiérrez Loza, Lucia, et al.
(författare)
-
On physical mechanisms enhancing air-sea CO2 exchange
- 2022
-
Ingår i: Biogeosciences. - : European Geosciences Union (EGU). - 1726-4170 .- 1726-4189. ; 19:24, s. 5645-5665
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 7. |
- Hallgren, Christoffer, et al.
(författare)
-
Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
-
Tidskriftsartikel (refereegranskat)abstract
- Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
|
|
| 8. |
|
|
| 9. |
- Högström, Ulf, et al.
(författare)
-
Surface Stress over the Ocean in Swell-Dominated Conditions during Moderate Winds
- 2015
-
Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 72:12, s. 4777-4795
-
Tidskriftsartikel (refereegranskat)abstract
- Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform(FLIP) and the Air–Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to , where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves.A model for prediction of the friction velocity from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result.The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor and that CD can attain values several times larger than predicted by wind speed–only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.
|
|
| 10. |
- Högström, Ulf, et al.
(författare)
-
The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions
- 2018
-
Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 75:8, s. 2579-2588
-
Tidskriftsartikel (refereegranskat)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
|
|
| 11. |
|
|
| 12. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Influences of surface gravity waves on atmospheric boundary layer structure and fluxes
- 2012
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric models are strongly dependent on the turbulent exchange of momentum and scalars at the air-sea interface. Surface gravity waves have been shown to influence the exchange process, but these effects are often poorly represented or neglected in weather and climate models. A better understanding of the mechanisms behind wave-related turbulent exchange can be reached from direct measurements and high-resolution numerical modelling (Sullivan et. al. 2008, 2010). Using large eddy simulations (LES) and field measurements we investigate and show how surface gravity waves affect atmospheric boundary layer turbulence and fluxes. Previous studies have shown that the atmospheric response to fast, long waves propagating away from their generation area, referred to as swell, can become different from conventional shear-driven boundary layers influenced by atmospheric stability. We have here used the LES model from Sullivan et. al. (2008) to examine the dynamics of the marine boundary layer under influence of swell waves (Nilsson et. al. 2012). This model has the capability to resolve a moving sinusoidal wave at its lower boundary and is used to study the atmospheric influence caused by an idealized dominant swell wave and its effects on turbulent flux. The modelling results show that wave-induced motions leads to altered mean wind profiles and increased turbulence length scales in dominant swell conditions. Also turbulent fluxes are affected by the presence of swell. For a more detailed understanding of the mechanisms we investigate vertical momentum flux using a multiresolution analysis technique (Vickers and Mahrt 2003). Preliminary results indicate that upward directed momentum flux is often closely related to the vertical wind variance. In most atmospheric situations this upward momentum flux is however compensated by a larger downward directed momentum flux related to shear-induced streamwise oriented wind streaks. In low wind speed situations with dominant swell waves such motion are however modulated and in extreme cases the upward directed momentum flux can even exceed the downward directed flux, causing a net upward Reynolds-averaged flux. In addition to numerical simulations we have carried out multiresolution analysis on measured turbulence signals for different atmospheric conditions to characterise air-sea interaction during swell in comparison to other boundary layer processes that are also present over flat terrain and growing sea conditions. Field measurements from several sites are used in the analysis; among them is data from our main observational site Östergarnsholm located on a small flat island in the Baltic Sea. This site has been used in several previous studies of air-sea interaction, coastal meteorology and studies of the atmospheric response to surface waves. The analysis of the measurements support the LES results in that upward momentum flux is related to the vertical wind variance and in low wind speed situations this can dominate the total net flux. For growing sea conditions with higher wind speed and stronger shear the net flux becomes downward, however, due to a much larger downward directed flux component. One of our long-term research goals is to better understand the role of surface wave processes, then include such effects in large-scale atmospheric models, and evaluate the sensitivity of the atmospheric circulation on such modelling attempts. Similar to the effects of wave-induced mixing in the presence of non-breaking surface waves for the oceanic circulation (Qiao et. al. 2004), we find that swell waves may be responsible for increased mixing of the atmospheric boundary layer. This wave-induced mixing can in near-neutral conditions with swell act to cause similarity to more convective atmospheric states (Nilsson et. al. 2012). A simplified parameterization with an inclusion of a wave-field dependent mixing length formulation has therefore been suggested and implemented in a regional climate model coupled to a wave model (Rutgersson et. al. 2012). In conclusion we believe a better understanding of surface wave processes is needed to build model parameterizations for the marine atmospheric boundary layer. We have therefore continued with new analysis of field measurements and LES using a multiresolution technique to investigate the atmospheric response to the surface waves that separates the atmospheric and oceanic boundary layers. This new investigation reveals wave signatures in the atmospheric variables and distinguishes some of the effects that surface gravity waves causes for the near surface turbulence structure and fluxes. Nilsson, E. O., A. Rutgersson, A.-S. Smedman and P. P. Sullivan. 2012. Convective boundary layer structure in the presence of wind-following swell. Quarterly Journal of Royal Meteorological Society. In press Rutgersson, A., E. Nilsson and R. Kumar. 2012. Introducing surface waves in a coupled wave-atmosphere regional climate model: Impact on atmospheric mixing length. Journal of Geophysical Research – Oceans. Accepted Sullivan P., J. McWilliams and T. Hristov. 2010. Large-Eddy Simulations of high wind marine boundary layers above a spectrum of resolved waves. 19th AMS symposium on Boundary Layers and Turbulence. Sullivan PP, JB. Edson, T. Hristov and JC. McWilliams. 2008. Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves. J. Atmos. Sci. 65:1225-1244. Qiao F., Y. Yuan, Y. Yang, Q. Zheng, C. Xia and J. Ma, 2004. Wave-induced mixing in the upper ocean: Distribution and application to a global ocean circulation model. Geophys. Res. Lett. 31, Ll1303. Vickers D., and Mahrt L. 2003. The Cospectral Gap and Turbulent Flux Calculations. Journal of Atmospheric and Oceanic Technology 20: 660-672.
|
|
| 13. |
|
|
| 14. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Turbulent momentum flux characterisation using extended multiresolution analysis
- 2014
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 140, s. 1715-1728
-
Tidskriftsartikel (refereegranskat)abstract
- The variability of turbulent momentum flux in neutral and unstable atmospheric boundary layers is characterised by analysing surface-layer measurements and data from large-eddy simulations (LES). The method involves multiresolution (MR) decomposition of vertical wind and advected variables into eddy fluctuations on different scales. It provides a measure of the amount of flux variability that stems from same-scale correlations and from combinations of different scale eddy fluctuations. Combining two analysis methods enabled MR component cospectra to be introduced in order to study the contribution of downward and upward flux on different scales. These component cospectra were used to investigate at which scales most of the upward and downward momentum flux occurs. By using MR spectra, cospectra, and flow visualisation this investigation provides insights into turbulence structure and fluxes in neutral and unstable stratification. It is shown that most of the flux variability in the lower part of the boundary layer can be characterised as a combination of larger scale streamwise elongated horizontal wind streaks and smaller scale vertical wind fluctuations. These streaks are found to account for a large part of downward momentum flux at relatively large, energy-containing scales. Most of the upward momentum flux is found to occur at smaller scales. This can be interpreted as showing that upward momentum flux in these conditions is caused by the generation of smaller scale secondary motions when larger scale turbulence elements break down and dissipate. Differences in the height dependence of turbulence structure and momentum flux for neutral and unstably stratified conditions are also investigated and related to the existence of wind streaks and horizontal rolls in these different conditions.
|
|
| 15. |
- Osterwalder, S., et al.
(författare)
-
Critical Observations of Gaseous Elemental Mercury Air-Sea Exchange
- 2021
-
Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 35:8
-
Tidskriftsartikel (refereegranskat)abstract
- Air-sea exchange of gaseous elemental mercury (Hg-0) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg-0 flux measurements to validate parameterizations of the Hg-0 transfer velocity contributes to this uncertainty. We measured the Hg-0 flux on the Baltic Sea coast using micrometeorological methods (gradient-based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg-0 (mean +/- 1 sigma = 13.5 +/- 3.5 ng m(-3); ca. 10% of total Hg) compared to the atmosphere (1.3 +/- 0.2 ng m(-3)). The Hg-0 flux calculated using the gas exchange model ranged from 0.1-1.3 ng m(-2) h(-1) (10th and 90th percentile) over the course of the campaign (May 10-June 20, 2017) and showed a distinct diel fluctuation. The mean coastal Hg-0 fluxes determined with the two gradient-based approaches and REA were 0.3, 0.5, and 0.6 ng m(-2) h(-1), respectively. In contrast, the mean open sea Hg-0 flux measured with REA was larger (6.3 ng m(-2) h(-1)). The open sea Hg-0 flux indicated a stronger wind speed dependence for the Hg-0 transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg-0 transfer velocity is more consistent with gases that have less water solubility than CO2 (e.g., O-2). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air-sea Hg exchange.
|
|
| 16. |
|
|
| 17. |
- Rutgersson, Anna, 1971-, et al.
(författare)
-
Natural hazards and extreme events in the Baltic Sea region
- 2022
-
Ingår i: Earth System Dynamics. - : Copernicus Publications. - 2190-4979 .- 2190-4987. ; 13:1, s. 251-301
-
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.
|
|
| 18. |
|
|
| 19. |
|
|
| 20. |
- Svensson, Nina, 1988-, et al.
(författare)
-
A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface
- 2017
-
Ingår i: Advances in Meteorology. - : Hindawi Limited. - 1687-9309 .- 1687-9317. ; 2017
-
Tidskriftsartikel (refereegranskat)abstract
- Streaky structures of narrow (8-9 km) high wind belts have been observed from SAR images above the Baltic Sea during stably stratified conditions with offshore winds from the southern parts of Sweden. Case studies using the WRF model and in situ aircraft observations indicate that the streaks originate from boundary layer rolls generated over the convective air above Swedish mainland, also supported by visual satellite images showing the typical signature cloud streets. The simulations indicate that the rolls are advected and maintained at least 30-80 km off the coast, in agreement with the streaks observed by the SAR images. During evening when the convective conditions over land diminish, the streaky structures over the sea are still seen in the horizontal wind field; however, the vertical component is close to zero. Thus advected feature from a land surface can affect the wind field considerably for long times and over large areas in coastal regions. Although boundary layer rolls are a well-studied feature, no previous study has presented results concerning their persistence during situations with advection to a strongly stratified boundary layer. Such conditions are commonly encountered during spring in coastal regions at high latitudes.
|
|
| 21. |
- Wu, Lichuan, et al.
(författare)
-
A review of surface swell waves and their role in air-sea interactions
- 2024
-
Ingår i: Ocean Modelling. - : Elsevier. - 1463-5003 .- 1463-5011. ; 190
-
Forskningsöversikt (refereegranskat)abstract
- Swell waves, characterized by the long wavelength components generated by distant weather systems or storms, exert a significant influence on various air-sea interaction processes, thereby impacting weather and climate systems. Over recent decades, substantial progress has been achieved in comprehending the dynamics of swell waves and their implications for air-sea interactions. This paper presents a comprehensive review of advancements and key findings concerning surface swell waves and their interactions with the atmosphere. It encompasses a range of topics, including wave growth theory, the effects of swell waves on air-sea momentum, heat, and mass fluxes, as well as their influence on atmospheric turbulence and mixed layer processes. The most important characteristics of the swell impact (where it differs from wind sea conditions) are the wave-induced upward component of the surface stress leading to alteration of total surface stress, generation of a low-level wind maxima or changed wind profile and change of scale and behaviour of turbulence properties (turbulence kinetic energy and integral length scale). Furthermore, the paper explores the modelling of swell dissipation, the integration of swell influences in weather and climate models, and the broader climatic implications of surface swell waves. Despite notable advances in understanding swell processes, persistent knowledge gaps remain, underscoring the need for further research efforts, which are outlined in the paper.
|
|
| 22. |
|
|
| 23. |
- Dubois, Kévin, 1995-, et al.
(författare)
-
Technical note : Extending sea level time series for the analysis of extremes with statistical methods and neighbouring station data
- 2024
-
Ingår i: Ocean Science. - : Copernicus Publications. - 1812-0784 .- 1812-0792. ; 20:1, s. 21-30
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 24. |
- Engström, Jens, et al.
(författare)
-
Energy absorption from parks of point-absorbing wave energy converters in the Swedish exclusive economic zone
- 2020
-
Ingår i: Energy Science & Engineering. - : John Wiley & Sons. - 2050-0505. ; 8:1, s. 38-49
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 25. |
- Grip, Carl-Erik, et al.
(författare)
-
Application of Pinch Analysis in an Integrated Steel Plant in Northern Sweden
- 2013
-
Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 53:7, s. 1202-1210
-
Tidskriftsartikel (refereegranskat)abstract
- The energy system in a modern integrated steel plant is a complicated network of units exchangingenergy and matter with each other. System studies using process integration tools are important to avoidsub-optimization. At the steel plant in Luleå such studies have been carried out using a MILP-based mathematicalprogramming tool (reMIND), mainly because of its inherent flexibility for handling combinedflows and reactions of both matter and chemical, thermal and mechanical energy. There are, however,areas where the energy system is dominated by creation, transport and exchange of thermal energy, andwhere pinch analysis can be expected to be a valuable tool. For this reason a pinch targeting study wascarried out for the plant site of the integrated steel plant in Luleå. The coke plant and the iron making/steelmaking plant were both studied with three ambition levels of possible improvements. The study confirmedthat pinch analysis is a powerful tool for targeting energy savings in areas where thermal energyflows dominate the local energy system, e.g., the gas cleaning area at the coke plant. The study also indicatedthat a connection between the energy systems in the coke plant and the iron making/steelmakingwould be valuable. This is not 100% feasible because of distance, but, a common steam net could add adegree of flexibility.
|
|
| 26. |
- Guo Larsén, Xiaoli, et al.
(författare)
-
Climate Change and Offshore Wind Energy in the Baltic Sea
- 2024
-
Ingår i: Oxford Research Encyclopedia of Climate Science.
-
Forskningsöversikt (refereegranskat)abstract
- Wind energy is becoming an essential part of the energy system in the Baltic Sea region (BSR). There has been a tremendous development of offshore wind energy in the early 21st century in this region, and the plan for further growth in the coming years is ambitious. The development and implementation of offshore wind energy is a complex process involving many physical and sociopolitical aspects. These aspects have their own characteristics in the BSR. Therefore, they have their unique impact and constraints on the regional development and implementation of the strategic energy technology (SET) plans. This includes implementing next-generation wind turbine technology, offshore wind farms and system integration, floating offshore wind and wind energy industrialization, wind energy operation, maintenance and installation, ecosystems, social impact and human capital agendas, and basic wind energy sciences.Climate change is an important issue to address in relation to future development. Among the questions that may arise are: How would climate change affect the wind resource, extreme wind, and several meteorological and oceanic variables relevant to the offshore wind energy sector? What does this effect imply for the development of offshore wind energy in the BSR?It is encouraging to acknowledge that there have been numerous relevant, good quality, pertinent studies on the subject of the BSR, and many more are ongoing. It is also inspiring to see that in the wind energy sector, there are already many technologies, methods, and tools that are sufficiently mature, and many of them, together with lessons learned through studies in other offshore regions, can be applied to support the urgent and extensive scale development of offshore wind in the BSR.
|
|
| 27. |
- Gurell, Jonas, 1981-, et al.
(författare)
-
The FERRUM project : Transition probabilities for forbidden lines in [Fe II] and experimental metastable lifetimes
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 508:1, s. 525-529
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Accurate transition probabilities for forbidden lines are important diagnostic parameters for low-density astrophysical plasmas. In this paper we present experimental atomic data for forbidden [Fe II] transitions that are observed as strong features in astrophysical spectra. Aims. We measure lifetimes for the 3d(6)((3)G)4s a (4)G(11/2) and 3d(6)((3)D)4s b (4)D(1/2) metastable levels in Fe II and experimental transition probabilities for the forbidden transitions 3d(7) a (4)F(7/2,9/2)-3d(6)((3)G)4s a (4)G(11/2). Methods. The lifetimes were measured at the ion storage ring facility CRYRING using a laser probing technique. Astrophysical branching fractions were obtained from spectra of Eta Carinae, obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. The lifetimes and branching fractions were combined to yield absolute transition probabilities. Results. The lifetimes of the a (4)G(11/2) and the b (4)D(1/2) levels have been measured and have the following values, tau = 0.75 +/- 0.10 s and tau = 0.54 +/- 0.03 s respectively. Furthermore, we have determined the transition probabilities for two forbidden transitions of a (4)F(7/2,9/2)-a (4)G(11/2) at 4243.97 and 4346.85 angstrom. Both the lifetimes and the transition probabilities are compared to calculated values in the literature.
|
|
| 28. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
-
Mg-doped Al0.85Ga0.15N layers grown by hot-wall MOCVD with low resistivity at room temperature
- 2010
-
Ingår i: PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS. - : John Wiley and Sons, Ltd. - 1862-6254 .- 1862-6270. ; 4:11, s. 311-313
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the hot-wall MOCVD growth of Mg-doped AlxGa1-xN layers with an Al content as high as x similar to 0.85. After subjecting the layers to post-growth in-situ annealing in nitrogen in the growth reactor, a room temperature resistivity of 7 k Omega cm was obtained indicating an enhanced p-type conductivity compared to published data for AlxGa1-xN layers with a lower Al content of x similar to 0.70 and a room temperature resistivity of about 10 k Omega cm. It is believed that the enhanced p-type conductivity is a result of reduced compensation by native defects through growth conditions enabled by the distinct hot-wall MOCVD system.
|
|
| 29. |
- Katsidoniotaki, Eirini, et al.
(författare)
-
Response of Point-Absorbing Wave Energy Conversion System in 50-Years Return Period Extreme Focused Waves
- 2021
-
Ingår i: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 9:3
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 30. |
- Lundberg, Valeria, 1984, et al.
(författare)
-
Converting a kraft pulp mill into a multi-product biorefinery: techno-economic analysis of a case mill
- 2014
-
Ingår i: Clean Technologies and Environmental Policy. - : Springer Science and Business Media LLC. - 1618-954X .- 1618-9558. ; 16:7, s. 1411-1422
-
Tidskriftsartikel (refereegranskat)abstract
- In this case study, we investigated the conversion of an existing Swedish kraft pulp mill to the production of dissolving pulp, with export of electricity, lignin, and a hemicellulose stream suitable for upgrading. By increasing the level of heat integration of the mill, it was possible to achieve self-sufficiency in terms of steam and to produce significant amounts of excess steam. The excess steam could facilitate the integration of a lignin separation plant or be used for power generation. The production of dissolving pulp requires a higher input of wood that is required for the same level of pulp production as is achieved with kraft pulp. For the studied mill, the batch digester was the main limitation for pulp production. Nevertheless, if the digester capacity was increased, then the level of pulp production could be maintained. In addition, the recovery boiler, causticization plant, and evaporation plant had sufficient capacities for preserving the same production level upon conversion, and could easily be upgraded to a certain degree through relatively simple measures for an increase in pulp production. However, increasing pulp production beyond that limit required extensive upgrades or investments in new equipment, which negatively affected annual earnings. Annual earnings were found to be also dependent upon the level of heat integration, type of by-product, and the costs for lignin and electricity. However, our results suggest that the optimal process configuration is more dependent upon other factors, such as the long-term vision of the company and policy instruments.
|
|
| 31. |
- Lundberg, Valeria, 1984, et al.
(författare)
-
Enlarging the Product Portfolio of a Kraft Pulp Mill via Hemicellulose and Lignin Separation – Process Integration Studies in a Case Mill
- 2013
-
Ingår i: Chemical Engineering Transactions. - 2283-9216 .- 2283-9216. ; 35, s. 127-132
-
Tidskriftsartikel (refereegranskat)abstract
- Increased energy and raw material prices along with contracting markets for kraft pulp, have highlighted the need for the pulp industry to enlarge their traditional product portfolio with new value-added products via the implementation of biorefinery concepts. In this paper, we have investigated potentials for enlarging the product portfolio of a kraft pulp mill by extracting hemicellulose prior to digesting and in this way, converting to dissolving pulp production.A case study has been performed on a Swedish kraft pulp mill, in order to evaluate the consequences of the original mill configuration, level of heat integration and choice of by-products produced have on theoverall profitability of the mill upon conversion to dissolving pulp production. For the mill studied, the batch digester is identified to be the bottleneck for both kraft and dissolving pulp production. If the digester capacity is increased by e.g. purchasing new effects, the pulp production could be maintained.The results from the energy study indicate that dissolving pulp production is more heat demanding than kraft production. However, by increasing the heat integration of the mill, it is possible to, not only become self-sufficient in terms of steam, but also to produce significant amounts of excess steam. The steam excess facilitates integration of a lignin separation plant or can be used for power generation.The net annual profit was evaluated for varying prices of electricity and lignin. For the economic conditions studied, lignin separation was always better than power generation, if lignin can be priced as oil.
|
|
| 32. |
- Mahrt, Larry, et al.
(författare)
-
Sea-Surface Stress Driven by Small-Scale Non-stationary Winds
- 2020
-
Ingår i: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 176:1, s. 13-33
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 33. |
- Mahrt, Larry, et al.
(författare)
-
The sea surface heat flux at a coastal site
- 2022
-
Ingår i: Journal of Physical Oceanography. - : American Meteorological Society. - 0022-3670 .- 1520-0485. ; 52:12, s. 3297-3307
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 34. |
- Mahrt, Larry, et al.
(författare)
-
Vertical divergence of the atmospheric momentum flux near the sea surface at a coastal site
- 2021
-
Ingår i: Journal of Physical Oceanography. - Boston MA, USA : American Meteorological Society. - 0022-3670 .- 1520-0485. ; 51:11, s. 3529-3537
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 35. |
- Molinder, Jennie, et al.
(författare)
-
Probabilistic Forecasting of Wind Turbine Icing Related Production Losses Using Quantile Regression Forests
- 2021
-
Ingår i: Energies. - BASEL, SWITZERLAND : MDPI. - 1996-1073. ; 14:1
-
Tidskriftsartikel (refereegranskat)abstract
- A probabilistic machine learning method is applied to icing related production loss forecasts for wind energy in cold climates. The employed method, called quantile regression forests, is based on the random forest regression algorithm. Based on the performed tests on data from four Swedish wind parks available for two winter seasons, it has been shown to produce valuable probabilistic forecasts. Even with the limited amount of training and test data that were used in the study, the estimated forecast uncertainty adds more value to the forecast when compared to a deterministic forecast and a previously published probabilistic forecast method. It is also shown that the output from a physical icing model provides useful information to the machine learning method, as its usage results in an increased forecast skill when compared to only using Numerical Weather Prediction data. A potential additional benefit in machine learning for some stations was also found when using information in the training from other stations that are also affected by icing. This increases the amount of data, which is otherwise a challenge when developing forecasting methods for wind energy in cold climates.
|
|
| 36. |
- Nilsson, Daniel, 1983-
(författare)
-
Doping of high-Al-content AlGaN grown by MOCVD
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The high-Al-content AlxGa1-xN, x > 0.70, is the principal wide-band-gap alloy system to enable the development of light-emitting diodes operating at the short wavelengths in the deep-ultraviolet, λ < 280 nm. The development of the deep-ultraviolet light-emitting diodes (DUV LEDs) is driven by the social and market impact expected from their implementation in portable units for water disinfection and based on the damaging effect of the deep-ultraviolet radiation on the DNA of various microorganisms. Internationally, intense research and technology developments occur in the past few years, yet, the external quantum efficiency of the DUV LEDs is typically below 1%.One of the main material issues in the development of the DUV LEDs is the achievement of n- and ptype doped layers of high-Al-content AlxGa1-xN with low resistivity, which is required for the electrical pumping of the diodes. The doping process, however, becomes significantly more complex with increasing the Al content and the resistivity value can be as high as 101-102 Ω cm for n-type AlN doped by silicon, and 107-108 Ω cm for p-type AlN doped by magnesium.The present study is therefore focused on gaining a better understanding of the constraints in the doping process of the high-Al-content AlxGa1-xN alloys, involving mainly the silicon dopant. For this purpose, the epitaxial growth of the high-Al-content AlxGa1-xN and AlN by the implementation of the distinct hot-wall MOCVD is developed in order to achieve layers of good structural and morphological properties, and with low content of residual impurities, particularly oxygen and carbon. Substitutional point defects such as ON and CN may have a profound impact on the doping by their involvement in effects of n-type carrier compensation. The process temperature can be set from 1000 °C and up to 1400 °C in the present study, which is a principal advantage in order to optimize the material properties of the high-Al-content AlxGa1-xN and AlN. The epitaxial growth of the high-Alcontent AlxGa1-xN and AlN is largely performed on 4H-SiC substrates motivated by (i) the lattice mismatch of ~ 1% along the basal plane (the smallest among other available substrates including Si and sapphire), (ii) the good thermal conductivity of 3.7 W cm-1 K-1, which is essential to minimize the self-heating during the operation of any light-emitting diode, and (iii) the limited access to true-bulk AlN wafers. The Si doping is investigated over a large range of [Si] ~ 1×1017 cm-3 - 1×1020 cm-3. Only the high doping range of [Mg] ~ (1-3)×1019 cm-3 is targeted motivated by the large thermal ionization energy of this common acceptor (from 200 meV in GaN to about 630 meV in AlN). The material characterization involves extensive implementation of atomic force microscopy (AFM), x-ray diffraction (XRD), cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), capacitancevoltage measurements, as well as measurements of the conductivity of the layers by contactless microwave-based technique. The possibility to perform electron paramagnetic resonance (EPR) measurements on the Si-doped high-Al-content AlxGa1-xN is essential in order to establish any effect of self-compensation of the shallow donor state of silicon through the related so-called DX state. The EPR measurements corroborate the study of the incorporation kinetics of silicon and oxygen at various process temperatures and growth rates.The outcome of this study is accordingly summarized and presents our understanding for (i) the complex impact of silicon and oxygen on the n-type conductivity of Al0.77Ga0.23N, which is the alloy composition at which a drastic reduction of the n-type conductivity of high-Al-content AlxGa1-xN is commonly reported (paper 1); (ii) the strain and morphology compliance during the intentional doping by silicon and magnesium, and its correlation with the resistivity in the highly doped layers of Al0.82Ga0.18N alloy composition (paper2); (iii) the n-type conductivity of highly-Si-doped Al0.72Ga0.28N layers as bound by the process temperature (paper 3); and (iv) the shallow donor or DX behavior of the Si dopant in conductive AlxGa1-xN layers, 0.63 ≤ x ≤ 1 (paper 4). It is noted that the measured n-type conductivity in reference layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition is on par with the state-of-the-art values, i.e. ≤ 0.05 Ω cm, and 0.012 Ω cm, respectively. A room-temperature resistivity of 7 kΩ cm is measured in Mg-doped layers of Al0.85Ga0.15N alloy composition, which is superior to the state-of-art values (paper 5). The performance of the transport properties of the high-Al-content AlxGa1-xN layers is expected to improve with improvement of their material quality. This can be achieved by improvement of the crystalline quality of the AlN-on-SiC template and by the implementation of true-bulk AlN substrates. The AlN heteroepitaxial growth at the process temperatures of 1100-1200 °C is therefore investigated (paper 6). The lattice constants, structural and optical properties of true-bulk, homoepitaxial and heteroepitaxial AlN material grown at high process temperatures of up to 1400 °C is further reported (paper 7).
|
|
| 37. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Assessment of Extreme and Metocean Conditions in the Swedish Exclusive Economic Zone for Wave Energy
- 2020
-
Ingår i: Atmosphere. - : MDPI AG. - 2073-4433. ; 11:3
-
Tidskriftsartikel (refereegranskat)abstract
- Here, accessibility to near-shore and offshore marine sites is evaluated based on wave and ice conditions. High-resolution third-generation wave model results are used to examine the operation and maintenance conditions for renewable energy sources with a focus on wave energy. Special focus is given to the wave field and ice characteristics for areas within the Swedish Exclusive Economic Zone including analysis of return levels for extreme values for significant wave height, which provides guidance for dimensioning wave energy converters. It is shown that the number of weather windows and accessibility are influenced by distance from the coast and sea-ice conditions. The longest waiting periods for the closest weather window that is available for Operation and Maintenance (O&M) is in ice-free conditions shown to be strongly correlated with the fetch conditions. The sheltered Baltic Sea is shown to have very high accessibility if marine infrastructure and vessels are designed for access limits of significant wave height up to 3 m. In the northern basins, the waiting periods increase significantly, if and when the ice-conditions are found to be critical for the O&M activity considered. The ice-conditions are examined based on compiled operational sea-ice data over a climatic time period of 34 years. The results are location specific for the Swedish Exclusive Economic Zone, but the analysis methods are transferable and applicable to many other parts of the world, to facilitate assessment of the most promising areas in different regions.
|
|
| 38. |
|
|
| 39. |
|
|
| 40. |
|
|
| 41. |
|
|
| 42. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Convective boundary-layer structure in the presence of wind-following swell
- 2012
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 138:667, s. 1476-1489
-
Tidskriftsartikel (refereegranskat)abstract
- The marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind-following swell and various stability states has been investigated using large-eddy simulation (LES) data. The dominant energy-containing motions in the near-neutral atmospheric boundary layer over flat terrain are known to be dominated by near-ground shear-induced regions of high- and low-speed flow. Windfields and momentum fluxes from LES for swell-dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary-layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave-induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length-scales during wind-following swell.
|
|
| 43. |
|
|
| 44. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Evaluating humidity and sea salt disturbances on CO2 flux measurements
- 2018
-
Ingår i: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 35, s. 859-875
-
Tidskriftsartikel (refereegranskat)abstract
- Global oceans are an important sink of atmospheric carbon dioxide (CO2). Therefore, understanding the air-sea flux of CO2 is a vital part in describing the global carbon balance. Eddy covariance (EC) measurements are often used to study CO2 fluxes from both land and ocean. CO2 are usually measured with infrared absorption sensors, which at the same time measure water vapor. Studies have shown that presence of water vapor fluctuations in the sampling air potentially result in erroneous CO2 flux measurements due to cross-sensitivity of the sensor. Here we compare measured CO2 fluxes from both enclosed path Li-Cor 7200 sensors and open-path Li-Cor 7500 instruments from an inland measurement site and a marine site. We also introduce new quality control criteria based upon a Relative Signal Strength Indicator (RSSI). The sampling gas in one of the Li-Cor 7200 instruments was dried by means of a multi-tube diffusion dryer so that the water vapor fluxes were close to zero. With this setup we investigated the effect that cross-sensitivity of the CO2 signal to water vapor can have on the CO2 fluxes. The dryer had no significant effect on the CO2 fluxes. We tested the hypothesis that the cross-sensitivity effect is caused by hygroscopic particles such as sea salt by spraying a saline solution on the windows of the Li-Cor 7200 instruments during the inland field test. Our results confirm earlier findings that sea salt contamination can affect CO2 fluxes significantly and confirm earlier findings, that drying the sampling air for the gas analyzer is an effective method to reduce this signal contamination.
|
|
| 45. |
|
|
| 46. |
- Nilsson, Erik, 1983-, et al.
(författare)
-
Från mätt vind till vindklimat, Normalårskorrigering
- 2009
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Metoder för normalårskorrigering har analyserats, d.v.s. att uppskatta den klimatologiska medelvinden på en plats utgående från uppmätt vind under en kortare period tillsammans med en långtidsreferens. Dessutom har den osäkerhet som kvarstår i de erhållna normalårskorrigerade medelvindarna studerats. Som långtidsreferens har NCEP/NCAR-återanalysdata använts. Mer precist den geostrofiska vinden beräknad utgående från höjden till 850 hPa tryckytan, vilket motsvarar den horisontella lufttrycksgradienten på ca 1300 m höjd. Även vindhastigheten på 850 hPa-nivån har använts vid analyserna. Återanalysdata skapas av en global numerisk datormodell för atmosfären genom att assimilera alla tillgängliga meteorologiska observationer i modellen och köra den i 6 timmar, varefter nya data assimileras. Den databas som skapas är global med en rumslig upplösning på 2.5° i longitud och latitud och en tidsupplösning på 6 timmar. Tre olika metoder för estimering av långtidsmedel baserat primärt på den geostrofiska vinden som långtidsreferens har utvärderats. Data från 8 platser med tillgång till mätningar av vind under 12-34 år har använts. En av platserna (Näsudden) har mätningar på 75 meters höjd och i allmänhet bör störst vikt läggas vid resultaten på den platsen i jämförelse med de andra platserna där mätningarna gjorts på 10 meters höjd vilket medför större risk för mycket lokala småskaliga effekter på mätningarna, svåra att fånga i korrektionerna. De olika metodernas svagheter och styrkor återspeglas dock ofta även i datamaterial från 10 m. Således kan dessa platser användas föratt få ett jämförelsematerial att testa normalårskorrigeringsmetoderna på. Resultaten visar att man bör använda minst ett års mätningar för att fånga åtminstone en naturlig årscykel på mätplatsen och därmed minska osäkerheten i normalårskorrigeringen. Att använda mer än ett års mätningar minskar naturligtvis osäkerheten ytterligare men inte lika kraftigt som under det första året. Vid utvärdering av hur bra vi kan estimera den klimatologiska medelvinden över 15 år för Näsudden utgående från en kortare period med mätningar kan det konstateras att de tre metoderna ger likartade resultat när man använder data från 1 år eller mer. Den kvarvarande osäkerheten i estimaten är relativt stor med en 5 % risk att den normalårskorrigerade medelvinden ligger mer än 0.42 m/s ifrån den’sanna’ klimatologiska medelvinden när man använder 1 års mätningar. Efter 2 års mätningar ligger motsvarande osäkerhet på 0.32 m/s. Man kan dock konstatera för Näsuddenmaterialet att normalårskorrigeringen har minskat osäkerheten till ungefär hälften jämfört med om vi endast medelvärdesbildar mätdata utan att tillämpa någon normalårskorrektion. Betydelsen av val av långtidsreferens har undersökts för Näsudden, 75 m höjd, genom att istället för geostrofisk vind använda faktisk vind på 850 hPa nivån i en eller flera av återanalysmodellens beräkningspunkter. Resultaten visar att vissa val av enskilda punkter med modellerad vind som referens visserligen kan ge mindre osäkerhet i estimeringen men att andra punkter samtidigt kan ge sämre resultat jämfört med valet av geostrofisk vind som referens. Korrelationen mellan mätplats och referens är inte vägledande. Perioder med relativt god korrelation garanterar inte små medelfel. En period med lägre korrelationskoefficient kan ge mindre medelfel än en period med hög korrelation. Genom att välja ut 4 till mätplatsen närbelägna punkter med modellerad vind i 850 hPa-ytan och bilda ett ytmedelvärde över dessa och på så vis skapa en ny långtidsreferens erhålls dock en referens som är likvärdig med den geostrofiska vinden.
|
|
| 47. |
|
|
| 48. |
- Nilsson, Erik, 1983-
(författare)
-
Large eddy simulation of the atmosphere in the presence of surface waves
- 2011
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The marine atmospheric boundary layer is influenced by a myriad of both large- and small-scale physical processes. Ocean waves constitute the boundary between processes in the ocean and in the atmosphere. Typically two types of waves on the ocean surface are distinguished from each other: wind sea and swell. When waves are being generated and grow they are called wind sea and when they propagate away from their generation area they are called swell waves. Air-sea interaction processes such as energy and momentum exchange have been found to be wave field dependent and this increases the variability of air-sea exchange, especially in low wind conditions when swell waves can propagate faster than the overlaying wind. Other processes such as wind shear and surface heat flux is however also well-known to influence turbulence in the atmospheric boundary layer and surface exchange processes. Direct measurements are unarguably the most reliable way to estimate the surface exchange. Measurements can, however, often be difficult to interpret and therefore numerical simulations can be used as a tool for investigating the complex interaction between various processes that occurs within the marine boundary layer. A better understanding of how different air-sea processes can act by themselves or interact with each other to enhance or reduce surface exchange could be of interest from a global climatological point of view because 70% of earth's surface is covered by ocean. In this introductory essay the governing equations for atmospheric motions is reviewed and Large Eddy Simulation is introduced as a research tool for high-Reynolds number turbulent flow. Some theories about turbulence are briefly mentioned. The main focus is kept on influences from surface waves as reported from recent work.
|
|
| 49. |
|
|
| 50. |
- Nilsson, Erik O., 1983-, et al.
(författare)
-
Characterization of Wave Energy Potential for the Baltic Sea with Focus on the Swedish Exclusive Economic Zone
- 2019
-
Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 12:5
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, a third-generation wave model is used to examine the wave power resource for the Baltic Sea region at an unprecedented one-kilometer-scale resolution for the years 1998 to 2013. Special focus is given to the evaluation and description of wave field characteristics for the Swedish Exclusive Economic Zone (SEEZ). It is carried out to provide a more detailed assessment of the potential of waves as a renewable energy resource for the region. The wave energy potential is largely controlled by the distance from the coast and the fetch associated with the prevailing dominant wave direction. The ice cover is also shown to significantly influence the wave power resource, especially in the most northern basins of the SEEZ. For the areas in focus here, the potential annual average wave energy flux reaches 45 MWh/m/year in the two sub-basins with the highest wave energies, but local variations are up to 65 MWh/m/year. The assessment provides the basis for a further detailed identification of potential sites for wave energy converters. An outlook is given for additional aspects studied within a broad multi-disciplinary project to assess the conditions for offshore wave energy conversion within the SEEZ.
|
|
