| 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. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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Atmospheric CO2 variation over the Baltic Sea and the impact on air–sea exchange
- 2009
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Ingår i: Boreal environment research. - 1239-6095 .- 1797-2469. ; 14:1, s. 238-249
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Tidskriftsartikel (refereegranskat)abstract
- The variability in time and space of the atmospheric molar fraction of CO2 over the Baltic Sea was investigated using data from seven stations from the World Data Center for Greenhouse Gases. The variation on a monthly timescale of CO2 was divided into a global trend, a regional anthropogenic contribution and a natural seasonal cycle. For the Baltic Sea stations the anthropogenic and terrestrial contributions were largest at the coastal sites in the southern Baltic Sea (an offset of 9 ppm), decreasing towards the north over the Baltic Sea (to about 2 ppm). When calculating the air–sea flux of CO2 using the difference in partial pressure between air and sea, uncertainties in the atmospheric molar fraction of CO2 were shown to be of secondary importance as compared with uncertainties in other parameters (< 10%). Realistic uncertainties in the sea surface partial pressure, wind speed or transfer velocity resulted in significantly larger uncertainties in a calculated air–sea flux.
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| 4. |
- Rutgersson, Anna, 1971-, et al.
(författare)
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The annual cycle of carbon dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper
- 2008
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Ingår i: Journal of Marine Systems. - : Elsevier BV. - 0924-7963 .- 1879-1573. ; 74:1-2, s. 381-394
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Tidskriftsartikel (refereegranskat)abstract
- A land-based field station, two moored buoys and data from the Finnpartner ship were used to investigate the variability of the air-sea CO2-flux and parameters controlling the flux during one year in the Baltic Sea region.The agreement between the sea surface partial pressure of CO2 measured near the tower and from the ship in the central parts of the Baltic Proper was relatively good during most of the period.But, during periods with intense biological activity or strong upwelling there were significant differences.The flux of CO2 was measured with the eddy-correlation method.The transfer velocity was calculated from the flux measurements and the instrumental uncertainty in calculations of the hourly values of transfer velocity was of the order of 20%.The calculated value of the transfer velocity increased with increasing the wind speed.The relation showed, however, great scatter and no clear wind-dependent relation could be determined.It was shown that for the measured flux and for transfer velocities estimated from measurements it is important to know the variability of pCO2w in the footprint area.This is of particular importance when investigating the processes influencing the flux.When calculating the air-sea flux of CO2 the greatest uncertainty is in the determination of the transfer velocity, but it was shown that also the partial pressure of CO2 in the surface water is crucial to determine with good accuracy.
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| 5. |
- Sahlée, Erik, et al.
(författare)
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Re-evaluation of the bulk exchange coefficient for humidity at sea during unstable and neutral conditions
- 2008
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Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 38:1, s. 257-272
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Tidskriftsartikel (refereegranskat)abstract
- Processes influencing the air-sea exchange of humidity during unstable and neutral stratification were studied using tower measurements from the island of Ostergarnsholm in the Baltic Sea. For small air-sea temperature differences, the neutral exchange coefficient for humidity C-EN was found to increase with increasing wind speed, attaining a value of approximately 1.8 x 10(-3) at 13 m s(-1). The high C-EN values were observed during situations when the characteristics of the turbulence structure differed from what would be expected from traditional theory. Results from spectral analysis point to a situation in which the vertical transport of humidity is dominated by smaller-scale eddies. Quadrant analysis showed that these eddies enhance the humidity flux by bringing down drier air from layers aloft. These findings are consistent with recent analyses of the neutral boundary layer in which a change of turbulence regime has been observed. The conclusion is made that this dynamic effect accounts for the observed increase in C-EN. Here, C-EN was calculated using a wave-dependent normalized wind gradient, which had the effect of reducing the value by about 10% during swell relative to calculations using a non-wave-dependent normalized wind gradient.
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