| 1. |
- Carlsson, Björn, 1980-, et al.
(författare)
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Impact of swell on simulations using a regional atmospheric climate model
- 2009
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 61:4, s. 527-538
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Tidskriftsartikel (refereegranskat)abstract
- When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared to under wind sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A three-year simulation and two case studies using the wave field from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and of heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2°C annually on average) while the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg1). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.
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| 2. |
- Carlsson, Björn, 1980-, et al.
(författare)
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Investigating the effect of a wave-dependent momentum flux in a process oriented ocean model
- 2009
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Ingår i: Boreal environment research. - 1239-6095 .- 1797-2469. ; 14:1, s. 3-17
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Tidskriftsartikel (refereegranskat)abstract
- New expressions of the drag coefficient were developed using measurements from the Östergarnsholm site in the Baltic Sea. The drag coefficient was significantly lower in the presence of waves travelling faster than the wind (swell). The expressions were implemented in an oceanographic process-oriented model in a 45-year simulation. Since no wave information was included we did an analysis of the potential impact of swell on an ocean model. Current velocity and surface stress were significantly altered during periods with low wind speed but the temperature and the mixing depth in the ocean were not significantly changed. The implementation of the swell effect in a process oriented ocean model is thus of limited importance. There is, however, an indication that for studies of current velocity it is crucial to have a correct description of the drag coefficient.
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| 3. |
- 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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| 4. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Enhancement of CO2 transfer velocity due to water-side convection
- 2010
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Ingår i: Journal of Marine Systems. - : Elsevier BV. - 0924-7963 .- 1879-1573. ; 80:1-2, s. 125-134
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Tidskriftsartikel (refereegranskat)abstract
- The exchange of CO2 between ocean and atmosphere is controlled by the air-sea difference in partial pressure of CO2 and by the efficiency of the transfer processes, the efficiency can be represented by a resistance of the surface and expressed by transfer velocity, k.Measurements made at the Östergarnsholm field station in the Baltic Sea indicate that the CO2 transfer velocity is also a function not only of wind speed, but also of the mixed-layer depth of the water and, to a lesser degree, of the stratification of the atmosphere. The transfer velocity is significantly enhanced by a large mixed-layer depth, the enhancement increasing as the surface cooling increases. The impact of mixed layer depth is expressed by the convective velocity scale of the water (analogous to atmospheric convective scaling).Enhancement due to convection is an important factor affecting the diurnal cycle of air-sea fluxes. Large air-sea temperature differences also occur due to air mass advection. Here water-side convection at wind speeds between 2.5 and 6.5 m s-1 is investigated.The enhancement due to convection can be added to the traditional transfer velocity.
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