| 1. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Using land-based stations for air–sea interaction studies
- 2020
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Informa UK Limited. - 0280-6495 .- 1600-0870. ; 72:1, s. 1-23
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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.
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| 2. |
- Sahlée, Erik, et al.
(författare)
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Influence from Surrounding Land on the Turbulence Measurements Above a Lake
- 2014
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 150:2, s. 235-258
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane, TeX , observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of TeX to the surface methane flux is due to the shallow internal boundary layer that develops above the lake, allowing methane to accumulate in a relatively small volume.
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| 3. |
- Svensson, Nina, 1988-, et al.
(författare)
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A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface
- 2017
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Ingår i: Advances in Meteorology. - : Hindawi Limited. - 1687-9309 .- 1687-9317. ; 2017
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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.
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| 4. |
- Svensson, Nina, 1988-, et al.
(författare)
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Measurements and Modelling of Offshore Wind Profiles in a Semi-Enclosed Sea
- 2019
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Ingår i: Atmosphere. - : MDPI AG. - 2073-4433 .- 2073-4433. ; 10:4
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Tidskriftsartikel (refereegranskat)abstract
- A conically scanning, continuous-wave LIDAR is placed on an island in the central Baltic Sea with large open-water fetch, providing wind and turbulence profiles up to 300 m height. LIDAR and Weather Research and Forecasting (WRF) profiles from one year are used to characterize the marine boundary layer, at the same time performing an evaluation of the WRF model against LIDAR measurements with a focus on low-level jet representation. A good agreement is found between the average wind speed profile in WRF and LIDAR, with the largest bias occurring during stable conditions. The LLJ frequency is highest in May with frequency of occurrence ranging between 18% and 27% depending on the method of detection. Most of the LLJs occur during nighttime, indicating that most of them do not have local origin. For cases with simultaneous LLJs in both data sets the WRF agrees well with the LIDAR. In many cases, however, the LLJ is misplaced in time or space in the WRF simulations compared to the LIDAR. This shows that models still must be improved to capture mesoscale effects in the coastal zone.
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| 5. |
- Svensson, Nina, 1988-
(författare)
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Mesoscale Processes over the Baltic Sea
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The objective of this thesis is to study mesoscale processes above the Baltic Sea, which is a small, semi-enclosed sea where land-sea interaction may have a large impact on the offshore conditions. It is only the last tens of years that offshore research has become more popular, and one reason for this is the increasing offshore wind energy, which poses the need for accurate estimates of wind speed and turbulence conditions in the marine environment. In this thesis a range of mesoscale processes over the Baltic Sea are studied using the mesoscale model WRF (Weather Research and Forecasting) and different types of measurements.It is found that mesoscale effects are largest during spring and summer, when stable conditions dominate. The whole Baltic sea surface is affected by warm-air advection and low-level jets. There is very little spatial variation in seasonally averaged fields, which shows that the extent of mesoscale effects is several hundred kilometres. Wind speed and temperature profiles can thus not be described solely by the local conditions even far out over sea surface. Sea breezes are relatively unimportant for modyfing the seasonally averaged wind fields, but other types of low-level jets have a large influence. Results show that most of the low-level jets are likely created by inertial oscillations initiated when air flows across a coastline from the convective land surface to stable sea surface.Evaluation of the model shows that the discrepancies are largest during very stable conditions, but are also quite large during very unstable. The reasons for this are discussed.Several cases of boundary layer rolls are investigated using measurements and simulations and it is found that the rolls are likely created over the convective mainland and advected out over the stable sea surface, which may be a new finding that has not been reported before.
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| 6. |
- Svensson, Nina, 1988-, et al.
(författare)
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Modification of the Baltic Sea wind field by land-sea interaction
- 2019
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 22:6, s. 764-779
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Tidskriftsartikel (refereegranskat)abstract
- The wind and turbulence fields over a small, high‐latitude sea are investigated. These fields are highly influenced by the proximity to the coast, which is never more than 200 km away. Simulations with the WRF model over the Baltic Sea are compared with a simplified, stationary wind model driven by the synoptic forcing. The difference between the models is therefore representative of the mesoscale influence. The results show that the largest wind‐field modifications compared with a neutral atmosphere occur during spring and summer, with a mean monthly increase of up to approximately 1 ms−1 at typical hub heights and upper rotor area (120‐170 m height) in the WRF model. The main reason for this is large‐scale low‐level jets caused by the land‐sea temperature differences, likely increasing in strength due to inertial oscillations. These kind of events can be persistent for approximately 12 hours and cover almost the entire basin, causing wind speed and wind shear to increase considerably. The strongest effect is around 2000 to 2300 local time. Sea breezes and coastal low‐level jets are of less importance, but while sea breezes are mostly detected near the coastline, other types of coastal jets can extend large distances off the coast. During autumn and winter, there are fewer low‐level jet occurrences, but the wind profile cannot be explained by the classical theory of the one‐dimensional model. This indicates that the coastal environment is complex and may be affected by advection from land surfaces to a large degree even when unstable conditions dominate.
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| 7. |
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| 8. |
- Svensson, Nina, et al.
(författare)
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Stable conditions over the Baltic sea: model evaluation and climatology
- 2016
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Ingår i: Boreal environment research. - 1239-6095 .- 1797-2469. ; 21, s. 387-404
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Tidskriftsartikel (refereegranskat)abstract
- The WRF (Weather Research and Forecasting model) was evaluated against flight measurements over the Baltic Sea during stable conditions, focusing on vertical profiles of temperature and wind speed. Six different boundary layer parameterization schemes were used. It is shown that there are generally small differences between the boundary layer schemes, and that all schemes have problems in capturing the strength and height of low-level jets. Climatological simulations over the Baltic Sea show that there is a strong sea-sonality in the stability over the sea with up to 80% stable conditions in spring as compared with 10% in winter. Low-level jets are common, and occur up to 45% of the time in spring. The entire Baltic Sea, not only its coastal areas, is affected by stable stratification.
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| 9. |
- 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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