| 1. |
- Nilsson, Erik Olof, 1983-
(författare)
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Fluxes and Mixing Processes in the Marine Atmospheric Boundary Layer
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric models are strongly dependent on the turbulent exchange of momentum, sensible heat and moisture (latent heat) at the surface. Oceans cover about 70% of the Earth’s surface and understanding the processes that control air-sea exchange is of great importance in order to predict weather and climate. In the atmosphere, for instance, hurricane development, cyclone intensity and track depend on these processes.Ocean waves constitute an obvious example of air-sea interaction and can cause the air-flow over sea to depend on surface conditions in uniquely different ways compared to boundary layers over land. When waves are generated by wind they are called wind sea or growing sea, and when they leave their generation area or propagate faster than the generating wind they are called swell. The air-sea exchange is mediated by turbulent eddies occurring on many different scales. Field measurements and high-resolution turbulence resolving numerical simulations have here been used to study these processes.The standard method to measure turbulent fluxes is the eddy covariance method. A spatial separation is often used between instruments when measuring scalar flux; this causes an error which was investigated for the first time over sea. The error is typically smaller over ocean than over land, possibly indicating changes in turbulence structure over sea.Established and extended analysis methods to determine the dominant scales of momentum transfer was used to interpret how reduced drag and sometimes net upward momentum flux can persist in the boundary layer indirectly affected by swell. A changed turbulence structure with increased turbulence length scales and more effective mixing was found for swell.A study, using a coupled wave-atmosphere regional climate model, gave a first indication on what impact wave mixing have on atmosphere and wave parameters. Near surface wind speed and wind gradients was affected especially for shallow boundary layers, which typically increased in height from the introduced wave-mixing. A large impact may be expected in regions of the world with predominant swell. The impact of swell waves on air-sea exchange and mixing should be taken into account to develop more reliable coupled Earth system models.
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| 2. |
- Nilsson, Erik, 1983-
(författare)
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Turbulence Structure and Fluxes over Sea
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the processes that control air-sea exchange of momentum, sensible heat and moisture (latent heat) is of great importance for atmospheric and oceanic modelling. In the atmosphere, for instance, cyclone intensity and track depends on these processes. Combined effects of wind drag and surface convection affects the mean wind and turbulence structure near the surface, with additional indirect effects on the upper parts of the atmospheric boundary layer (by for example entrainment). For the oceanic side the atmospheric forcing generates waves, provides mixing and drives ocean currents. The most reliable way to estimate exchanges between the ocean and the atmosphere are through accurate direct measurements. The standard method used for measuring turbulent fluxes is the eddy covariance method. When using this method to measure scalar fluxes, there is often a spatial separation between the instruments measuring the scalar and the wind. This leads to an error in the calculated scalar flux if the separation is not accounted for. This source of error is investigated for the first time over sea, in addition the measured two-point covariance can also be used to indicate a changed turbulence structure over sea in comparison to over flat terrain. We show the impact of a process that changes the turbulence structure over sea using numerical simulations. Long fast swell waves on the ocean surface are shown to induce turbulent wind eddies in the flow at the scale of the dominant wave and thereby alter the turbulence structure and momentum exchange in the marine atmospheric boundary layer. Analysis of turbulence structure and momentum exchange is carried out for a range of conditions that have previously been shown to be prevalent at lower latitudes, where the impact of swell is expected to be higher than at mid and high latitudes.
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| 3. |
- 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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