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- Guo Larsén, Xiaoli, 1974-
(författare)
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Air-Sea Exchange of Momentum and Sensible Heat Over the Baltic Sea
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Long term measurements are used to explore the wave impact on the momentum and sensible heat transfer in the marine atmospheric surface layer. This is important in determining the boundary conditions of atmosphere-ocean coupled models of many scales. In those models classical similarity theories developed over land are commonly used, but they have shown to be questionable over sea. It is found that during pure wind sea conditions, the wind law, which is valid over land, is also valid over the sea. The sea surface roughness resembles that of a solid rigid surface. It scales with the surface elevation and decreases with wave age. When the sea surface is dominated by swell, wave influence is felt through the whole boundary layer. The wind profile is shown to be a function of wave state, and the classical wind law is modified when swell is present. Measurements and a wind over wave coupled model reveal that the momentum exchange depends on a combination of wind and wave condition. The exchange of sensible heat does not show the similar dependence on the wave state as the momentum. However at strong winds when waves break, the effect of spray becomes significant, increasing the exchange coefficient considerably.
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| 5. |
- Hodge, Bri-Mathias, et al.
(författare)
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Wind Power Forecasting Error Distributions : An International Comparison
- 2012
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Konferensbidrag (refereegranskat)abstract
- Wind power forecasting is essential for greater penetration of wind power into electricity systems. Because no wind forecasting system is perfect, a thorough understanding of the errors that may occur is a critical factor for system operation functions, such as the setting of operating reserve levels. This paper provides an international comparison of the distribution of wind power forecasting errors from operational systems, based on real forecast data. The paper concludes with an assessment of similarities and differences between the errors observed in different locations.
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| 6. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Natural hazards and extreme events in the Baltic Sea region
- 2022
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Ingår i: Earth System Dynamics. - : Copernicus Publications. - 2190-4979 .- 2190-4987. ; 13:1, s. 251-301
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Tidskriftsartikel (refereegranskat)abstract
- A natural hazard is a naturally occurring extreme event that has a negative effect on people and society or the environment. Natural hazards may have severe implications for human life and can potentially generate economic losses and damage ecosystems. A better understanding of their major causes, probability of occurrence, and consequences enables society to be better prepared to save human lives as well as to invest in adaptation options. Natural hazards related to climate change are identified as one of the Grand Challenges in the Baltic Sea region. Here, we summarize existing knowledge about extreme events in the Baltic Sea region with a focus on the past 200 years as well as on future climate scenarios. The events considered here are the major hydro-meteorological events in the region and include wind storms, extreme waves, high and low sea levels, ice ridging, heavy precipitation, sea-effect snowfall, river floods, heat waves, ice seasons, and drought. We also address some ecological extremes and the implications of extreme events for society (phytoplankton blooms, forest fires, coastal flooding, offshore infrastructure, and shipping). Significant knowledge gaps are identified, including the response of large-scale atmospheric circulation to climate change and also concerning specific events, for example, the occurrence of marine heat waves and small-scale variability in precipitation. Suggestions for future research include the further development of high-resolution Earth system models and the potential use of methodologies for data analysis (statistical methods and machine learning). With respect to the expected impacts of climate change, changes are expected for sea level, extreme precipitation, heat waves and phytoplankton blooms (increase), and cold spells and severe ice winters (decrease). For some extremes (drying, river flooding, and extreme waves), the change depends on the area and time period studied.
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| 7. |
- Sahlée, Erik, et al.
(författare)
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New Findings Concerning the Exchange of Sensible Heat And Water Vapor Over the Ocean
- 2004
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Ingår i: 16th Symposium on Boundary Layers and Turbulence.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The exchange of sensible and latent heat at the sea surface has been studied with the aid of a comprehensive data set from the marine site Östergarnsholm in the Baltic Sea, with additional data from another site in the Baltic Sea, Nässkär. The measurements include turbulent flux at about 10 m above the water surface, profile data of temperature and wind at several levels on towers, sea surface temperature and wave data. The Stanton number, CH was found to follow predictions from surface-renewal theory quite well for unstable conditions up to a wind speed of about 10 ms-1. For higher wind speed, the experimental data deviate to an increasing degree from the prediction based on surface-renewal theory, giving about 30% higher values at 14 ms-1and 50% higher values at 16 ms-1. For wind speed less than 10 ms-1, co-spectra of heat flux have the same shape as found over land with one peak at a fairly low frequency (~10-2 Hz). For increasing wind speed, a secondary maximum around 0.5 Hz is forming. The ratio between the spectral values of the high frequency peak and the low-frequency peak is found to be a strong function of significant wave height, Hs, being as high as 2 for Hs > 2.5 m. The interpretation is made that spray is the cause of the rapid increase of heat exchange with wind speed above c. 10 ms-1. In a plot of CH against stability, z/L, the data show a jump at neutral stratification from about 2 on the unstable side to 0.5 in stable air. For stable conditions, CH-data are widely scattered, It appears though as if the data have an approximate upper bound given by surface-renewal theory. It is demonstrated that at least some of the strong suppression of the flux of sensible heat during stable conditions and low wind speed can be explained as a shear sheltering effect caused by the presence of a wind maximum at a low height. For wind speed above 10 ms-1, the spray-mediated sensible heat flux is lowering the negative heat flux, thus giving a lower CH. The combination of those two mechanisms gives a low mean value of CH for the entire stable range and explains the ‘jump’ at neutral stratification. The exchange coefficient for humidity, CE behaves in much the same way as the Stanton number. Because of the failure of Monin-Obukhov theory over the sea, the conventional way to reduce CH and CE to their neutral values cannot be used.
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| 10. |
- Svensson, Nina, 1988-
(författare)
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Wind and atmospheric stability characteristics over the Baltic Sea
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years there has been an increase in offshore wind energy, which poses the need for accurate wind speed estimates in the marine environment, especially in coastal areas where most wind turbines will be placed. This thesis is focused on the Baltic Sea, which is a small, semi-enclosed sea where land-sea interaction play an important role in explaining the wind patterns.Mesoscale model simulations can be used to study the marine environment, where observations are often scarce. In this thesis the Weather Research and Forecasting (WRF) model is used. In the first study simulations show that stable stratification over sea is very common in spring and summer and is associated with an increase in low-level jet occurrence and increased wind shear below 200 m, at heights where wind turbines are erected. The model performance in stable conditions is evaluated against aircraft measurements using several boundary layer parametrization schemes, and it is shown that the low-level jet height and strength is not accurately captured with any of the parametrizations.In the second study the advection of land features is investigated. From simulations, aircraft observations and satellite images it is shown that boundary layer rolls are created in the convective boundary layer over land, and advected several tens of kilometres out over sea surface, despite the stable stratification, where convective turbulence dissipates quickly. The occurrence of boundary layer rolls gives rise to horizontal wind speed variations of several meters per second over distances of kilometres, which can increase the uncertainty of short term wind speed forecasts in coastal areas with offshore flow.It is shown that mesoscale processes in and above the marine boundary layer are important in modifying the wind field in distances of at least 100 km from the coast and that models still need to be improved in order to capture these conditions.
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| 11. |
- Wu, Lichuan, et al.
(författare)
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The impact of waves and sea spray on modelling storm track and development
- 2015
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 67
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Tidskriftsartikel (refereegranskat)abstract
- In high wind speed conditions, sea spray generated by intensely breaking waves greatly influences the wind stress and heat fluxes. Measurements indicate that the drag coefficient decreases at high wind speeds. The sea spray generation function (SSGF), an important term of wind stress parameterisation at high wind speeds, is usually treated as a function of wind speed/friction velocity. In this study, we introduce a wave-state-dependent SSGF and wave-age-dependent Charnock number into a high wind speed–wind stress parameterisation. The newly proposed wind stress parameterisation and sea spray heat flux parameterisation were applied to an atmosphere–wave coupled model to study the mid-latitude storm development of six storm cases. Compared with measurements from the FINO1 platform in the North Sea, the new wind stress parameterisation can reduce wind speed simulation errors in the high wind speed range. Considering only sea spray impact on wind stress (and not on heat fluxes) will intensify the storms (in terms of minimum sea level pressure and maximum wind speed), but has little effect on the storm tracks. Considering the impact of sea spray on heat fluxes only (not on wind stress) can improve the model performance regarding air temperature, but it has little effect on the storm intensity and storm track performance. If the impact of sea spray on both the wind stress and heat fluxes is taken into account, the model performs best in all experiments for minimum sea level pressure, maximum wind speed and air temperature.
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