| 1. |
- Hallgren, Christoffer, et al.
(författare)
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Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
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Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
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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.
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| 2. |
- Janzon, Erik, et al.
(författare)
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Impacts of land cover heterogeneity on wind turbine icing
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric icing affecting wind turbines is a challenge for energy companies operating in cold climate regions. In addition, low level super cooled clouds that cause rime ice in these areas are often subject to surface-atmosphere interactions. In numerical weather prediction (NWP) models, the detailed characteristics of the surface boundary--including the presence of snow cover and the patchiness, or heterogeneity, of land cover--are unknown and must be parameterized. This study examines the sensitivity of ice accretion to the land surface, with respect to changing the heterogeneity length scale ($L_{f}$) of roughness elements and the presence of snow. This is tested using large eddy simulation (LES) integrated over a semi-idealized partial diurnal cycle in combination with an offline ice accretion model for both an early- and late-season case. The ice accretion is found to be most sensitive to the heterogeneity length scale when snow is present in the bare patches, although an opposite icing response with respect to $L_{f}$ is found for different solar insolation. The ice accretion response to $L_{f}$ is shown to be mostly due to changes in the content of liquid water and cloud ice when snow is at the surface.
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| 3. |
- Janzon, Erik
(författare)
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Local Effects On Icing Forecasts for Wind Power In Cold Climate
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 4. |
- Janzon, Erik
(författare)
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Local Effects On Icing Forecasts for Wind Power In Cold Climate
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the risk of atmospheric icing on wind turbines is crucial for the operation of wind farms in cold climate regions. Processes leading to atmospheric icing in the planetary boundary layer are subject to interactions with the land surface--the details of which are largely unknown in current numerical weather prediction models and must be parameterized. This thesis examines the impact of the representation of land surface cover in these models on meteorological variables related to wind turbine icing. First, a sensitivity analysis is conducted using a single-column model to test the relative impacts of the vegetation fraction on ice accretion at the elevation of a modern commercial wind turbine. The impact of the representation of surface roughness due to the vegetation fraction and parameterization of processes related to the forest canopy is found to have the largest impact on icing in the simulations. This effect is combined with an important secondary role due to the albedo of the surface, which impacts the evolution of the icing event under early season solar insolation. Next, large eddy simulations (LES) are conducted to test the impact of the land cover heterogeneity length scale on wind profiles during a semi-idealized diurnal cycle in dry, subarctic conditions. The effective surface roughness is found to decrease as a function of the land cover heterogeneity length scale and the blending height is found to be limited by the height of the atmospheric boundary layer. Using the findings of this study in combination with previous work in the literature, a dynamic blending height model is presented as a possible coupling strategy for surface parameterizations. Following the results of the aforementioned study, LES simulations of different heterogeneity length scales are used to validate an analytical model that is used to simulate mean wind profiles and wind stress over arbitrary patterns of surface roughness. Lastly, semi-idealized LES simulations of supercooled low-level clouds are used to test the sensitivity of meteorological variables related to wind turbine icing to land cover heterogeneity length scales. Here, the presence of snow cover in bare patches has a significant effect on the icing prediction. The results of this thesis show that the representation of the surface can have a significant impact on wind turbine icing forecasts and that these findings will be helpful in identifying uncertainties in these predictions.
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| 5. |
- Janzon, Erik, et al.
(författare)
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Modelling the effects of surface heterogeneity on the internal boundary layer during a diurnal cycle
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Annan publikation (övrigt vetenskapligt/konstnärligt)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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| 6. |
- Janzon, Erik, et al.
(författare)
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Modelling the flow response to surface heterogeneity during a semi-idealized diurnal cycle
- 2023
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Ingår i: Journal of Applied Meteorology and Climatology. - : American Meteorological Society. - 1558-8424 .- 1558-8432. ; 62:4, s. 511-527
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Tidskriftsartikel (refereegranskat)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.
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| 7. |
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