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| 2. |
- Högström, Ulf, et al.
(författare)
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Air–Sea Interaction Features in the Baltic Sea and at a Pacific Trade-Wind Site : An Inter-comparison Study
- 2013
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 147:1, s. 139-163
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Tidskriftsartikel (refereegranskat)abstract
- A systematic comparison of wind profiles and momentum exchange at a trade wind site outside Oahu, Hawaii and corresponding data from the Baltic Sea is presented. The trade wind data are to a very high degree swell dominated, whereas the Baltic Sea data include a more varied assortment of wave conditions, ranging from a pure growing sea to swell. In the trade wind region swell waves travel predominantly in the wind direction, while in the Baltic, significant cross-wind swells are also present. Showing the drag coefficient as a function of the 10-m wind speed demonstrates striking differences for unstable conditions with swell for the wind-speed range 2 m s−1 < U 10 < 7 m s−1, where the trade-wind site drag values are significantly larger than the corresponding Baltic Sea values. In striking contrast to this disagreement, other features studied are surprisingly similar between the two sites. Thus, exactly as found previously in Baltic Sea studies during unstable conditions and swell, the wind profile in light winds (3 m s−1) shows a wind maximum at around 7–8 m above the water, with close to constant wind speed above. Also, for slightly higher wind speeds (4 m s−1 < U 10 < 7 m s−1), the similarity between wind profiles is striking, with a strong wind-speed increase below a height of about 7–8 m followed by a layer of virtually constant wind speed above. A consequence of these wind-profile features is that Monin–Obukhov similarity is no longer valid. At the trade-wind site this was observed to be the case even for wind speeds as high as 10 m s−1. The turbulence kinetic energy budget was evaluated for four cases of 8–16 30- min periods at the trade-wind site, giving results that agree very well with corresponding figures from the Baltic Sea.
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| 3. |
- Högström, Ulf, et al.
(författare)
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Momentum fluxes and wind gradients in the marine boundary layer : a multi platform study
- 2008
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Ingår i: Boreal environment research. - 1239-6095 .- 1797-2469. ; 13:6, s. 475-502
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Tidskriftsartikel (refereegranskat)abstract
- During five autumn weeks, measurements of turbulent fluxes were obtained in the Baltic Sea at three levels on a 30-m tower and two levels on an ASIS buoy 4 km from the tower together with profiles of wind and temperature. Wave data and SST were obtained from ASIS. In the mean, momentum fluxes measured on the tower and on ASIS during onshore winds agree closely. Dimensionless wind gradients phi(m)(z/L) for (i) stable conditions are linear in z/L (L is the Obukhov length); (ii) unstable, growing sea conditions are much smaller than predicted by 'standard' equations, due to an indirect effect of the boundary layer height. Individual wind profiles extrapolated from ASIS to tower by integration of phi(m)(z/L) deviate by about 0.5 m s(-1) from measured values, but corresponding mean profiles agree well for all levels from 1.18 m to 30 m. This random variation in the wind field is shown to be related to inherent dynamics of the atmospheric surface layer.
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| 4. |
- Högström, Ulf, et al.
(författare)
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Surface Stress over the Ocean in Swell-Dominated Conditions during Moderate Winds
- 2015
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 72:12, s. 4777-4795
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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.
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| 5. |
- Högström, Ulf, et al.
(författare)
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The atmospheric boundary layer during swell – a field study and interpretation of the turbulent kinetic energy budget for high wave ages
- 2009
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 66:9, s. 2764-2779
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Tidskriftsartikel (refereegranskat)abstract
- Analysis of the turbulent kinetic energy (TKE) budget for five slightly unstable cases with swell has been performed based on measurements of mechanical production, buoyancy production, turbulent transport, and dissipation at five levels over the sea, from 2.5 to 26 m. The time rate of change and advection of TKE were found to be small, so the TKE residual is interpreted as an estimate of the pressure transport term (T-p). In two cases with high wave age, the T-p term is a gain at all heights. For three cases with smaller wave age, T-p is a loss in the TKE budget below 5-10 m and a gain for greater heights, where the decrease is exponential, thus showing the combined effects of swell waves and a range of waves traveling slower than the wind. The TKE budget for a case with growing sea but similar wind speed and stability as some of the swell cases has T-p close to zero at all heights. It is shown that the observed characteristic wind profile with either a low-level maximum in the 5-10-m range or a distinct "knee'' at that height is an effect of the T-p term.
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| 6. |
- Högström, Ulf, et al.
(författare)
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The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions
- 2018
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 75:8, s. 2579-2588
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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
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| 7. |
- Norman, Maria, 1973-
(författare)
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Air-Sea Fluxes of CO2 : Analysis Methods and Impact on Carbon Budget
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon dioxide (CO2) is an important greenhouse gas, and the atmospheric concentration of CO2 has increased by more than 100 ppm since prior to the industrial revolution. The global oceans are considered an important sink of atmospheric CO2, since approximately one third of the anthropogenic emissions are absorbed by the oceans. To be able to model the global carbon cycle and the future climate, it is important to have knowledge of the processes controlling the air-sea exchange of CO2. In this thesis, measurements as well as a model is used in order to increase the knowledge of the exchange processes.The air-sea flux of CO2 is estimated from high frequency measurements using three methods; one empirical method, and two methods with a solid theoretical foundation. The methods are modified to be applicable for various atmospheric stratifications, and the agreement between methods is good in average.A new parameterization of the transfer velocity (the rate of transfer across the air-sea interface), is implemented in a Baltic Sea model. The new parameterization includes also the mechanism of water-side convection. The impact of including the new parameterization is relatively small due to feedback processes in the model. The new parameterization is however more representative for flux calculations using in-situ measurement or remote sensing products. When removing the feedback to the model, the monthly average flux increases by up to 20% in some months, compared to when water-side convection is not included.The Baltic Sea carbon budget was estimated using the Baltic Sea model, and the Baltic Sea was found to be a net sink of CO2. This is consistent with some previous studies, while contradictory to others. The dissimilarity between studies indicates the difficulty in estimating the carbon budget mainly due to variations of the CO2 uptake/release in time and space. Local variations not captured by the model, such as coastal upwelling, give uncertainties to the model. Coastal upwelling can alter the uptake/release of CO2 in a region by up to 250%. If upwelling would be included in the model, the Baltic Sea might be considered a smaller sink of CO2.
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| 8. |
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| 9. |
- Sahlée, Erik, et al.
(författare)
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Measurements of the damping of temperature fluctuations in a tube
- 2009
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 132:2, s. 339-348
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Tidskriftsartikel (refereegranskat)abstract
- Using field measurements we evaluate a previously deduced semi-empirical model for the dampening of temperature fluctuations in a circular tube. The measurements show that the model is a reasonably good approximation, and the previous recommendation of using a tube length of the order of 1,000 times the inner diameter of the tube to remove most of the temperature fluctuations seems to hold. Part of the difference between the empirical results and the model is possibly due to deviations from the idealized conditions assumed in the model such as no inclusion of the influence from the ambient atmospheric stability. Limitations of the measurements are also likely to affect the evaluation.
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| 10. |
- Sahlée, Erik, et al.
(författare)
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Waves and air-sea fluxes from a drifting ASIS buoy during the Southern Ocean Gas Exchange experiment
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. C08003-
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Ocean, while widely acknowledged as playing a major role within the Earth's climate system, remains the most poorly sampled and understood of the world's ocean basins. The High Latitude Surface Flux Working Group of U.S. CLIVAR (Climate Variability and Predictability, part of the World Climate Research Programme) has accordingly identified as a key priority the need for further measurements in the Southern Ocean. During the 2008 Southern Ocean Gas Exchange experiment, an Air-Sea Interaction Spar (ASIS) buoy was deployed to measure air-sea fluxes, surface waves, and mean properties of the upper ocean and lower atmosphere. During its eight-day deployment in the Atlantic sector of the Southern Ocean, the drifting buoy captured two storm events, with winds reaching 20 m s−1 and waves approaching 6 m significant height. The wavefield was observed to be dominated by swell waves except for the storm periods. In a combined analysis using data from two other ASIS deployments, existing relations for fetch limited wave growth were evaluated. Measured moisture flux showed good comparison with previous findings indicating a near-constant Dalton number. The drag coefficient was found to be significantly higher than previous parameterization predictions, due to an effect of swell wave interaction with the atmospheric turbulence. This enhanced momentum flux in the swell dominated seas of the Southern Ocean must be accounted for in regional bulk flux relations.
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| 11. |
- Smedman, Ann-Sofi, et al.
(författare)
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Observational study of the marine atmospheric boundary layer characteristics during swell
- 2009
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 66:9, s. 2747-2763
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Tidskriftsartikel (refereegranskat)abstract
- By combining simultaneous data from an instrumented Air-Sea Interaction Spar (ASIS) buoy and a 30-m tower, profiles of wind and turbulence characteristics have been obtained at several heights from about 1 to 30 m above the water surface during swell conditions. Five cases formed as averages over time periods ranging from 2.5 to 9.5 h, representing quasi-steady conditions, have been selected. They represent a range of typical wave age and include wind-following swell cases and cross-swell cases. For relatively large wave age, the wind profile exhibits a well-defined maximum in the height range 5-10 m; for more modest wave age, this maximum turns into a sharp "knee'' in the wind profile. Below the maximum ( or knee), the wind increases rapidly with height; above that point the wind is very nearly constant up to the highest measuring level on the tower, 30 m. Analysis of balloon data from one day with swell indicates that the layer with constant wind in fact extends to the top of the boundary layer, in this case similar to 200 m. Analysis of the complete swell dataset from the 45 days of the 2003 Baltic Swell experiment shows that the results concerning wind profile shape obtained from the selected cases are generally valid in this experiment. Analysis of the nondimensional wind profile phi(m) shows that Monin-Obukhov scaling is not valid during swell. Wind and turbulence characteristics are found not to vary to a significant degree with the wind/swell angle within the range of angles encountered, +/- 90 degrees.
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| 12. |
- Tamura, Hitoshi, et al.
(författare)
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Spectral form and source term balance of short gravity wind waves
- 2014
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Ingår i: Journal Of Geophysical Research - Oceans. - 2169-9275. ; 119:11, s. 7406-7419
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the spectral structure and source term balance of short gravity waves, based on in situ observations of wave number spectra retrieved by air-sea interaction spar (ASIS) buoys. The behaviors of wave number spectra up to 10 rad/m (the gravity wave regime) were analyzed for a wide range of wind and wave conditions. The observed wave number spectra showed the spectral power laws described by Toba (1973) and Phillips (1958) in addition to the characteristic nodal point at approximate to 10 rad/m where spectral energy becomes constant over the entire wind speed range. We also improved the third-generation wave model using the nonlinear dissipation term. The wave model reproduced the spectral form in the higher wave number domain. In the equilibrium range, nonlinear transfer played a major role in maintaining equilibrium conditions. On the other hand, in the saturation range, which starts at the upper limit of the equilibrium range, the nonlinear transfer tended to be out of balance with other source terms, and the dissipation term was in balance with wind input.Key Points We investigated the characteristics of short gravity wind waves The observed wave number spectra showed the spectral power laws The nonlinear dissipation term significantly improved the model results.
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