The Östergarnsholm field station

1. slow response ('profile') measurements of wind and temperature at 5 levels and humidity, radiation and concentration of carbon dioxide at one level
2. turbulence measurements at three levels with Gill sonic anemometers, giving instantaneous values of the three copmponents of the wind and of temperature 20 times per second and, in addition,rapid humidity fluctuations at one level. Additional instrumentation for measurment of turbulent fluctuations of humidity and carbon dioxide are mounted on at the tower top and at 10m. In 2006, extra sensitive sensors for temperature fluctuations were mounted on the sonics.
   

The meteorological measurements on the tower are supplemented by wave measurements, performed with the aid of a 3D Waverider Buoy (owned and run by the Finnish Institute for Marine Research, Helsinki, Finland) moored about 4 km southeast of the tower site. A second buoy is situated about 1 km to the souteast of the island. Attached to this buoy at 5m depth, an instrument measures the partial pressure of carbon dioxide in the water. This measurement is made once every hour.

The measurements at Östergarnsholm run on a semi-continuous basis, which means that we aim at 100% coverage, but that the combination of high technical complexity and remote location of the site results in certain data coverage reduction. In addition, the wave measurements are usually interrupted during a mid-winter period with imminent risk of ice damaging the instrument buoy.

At several occasions, intensive measuring campaigns have been launched at Östergarnsholm, which means that during periods of a few weeks duration each special measurements have been carried out with the purpose to probe the entire atmospheric boundary layer in addition to the continuous measurements on the tower. These special measurements include balloon-borne measurements of wind, temperature and humidity, measurements with aircrafts equipped with dedicated instrumentation and with sophisticated remote sensing techiques.

The crucial point with land-borne measurements in a marine environment is: do the measurements from the tower actually represent undisturbed oceanic conditions? This question can be reformulated in terms of the ‘flux-footprint concept’: When measurements are performed at a certain height above the surface, it does not ‘see’ the area immediately around it but an area which is displaced some distance upwind. In ref. A6 we showed that theory originally developed for calculating dispersion of air pollutants could be successfully employed to delineate flux-footprint areas for our turbulence measurements on the tower at 10, 16 and 26 m above the water surface. It turned out that these areas are displaced upwind to areas so far removed from the island (of the order of a kilometre or more) that we expect little effects from shoaling (at least for wind not much in excess of 15 ms^-1). These theoretical conclusions received strong support from measurements during conditions with pure wind sea (only waves generated by the local wind), which were successfully compared with corresponding data from several deep sea expeditions, A20.

                      Recently a more direct evaluation of the representatives of the Östergarnsholm measurements was made possible. In the autumn of 2003 an instrumented Air-Sea-Interaction Spar Buoy (ASIS) was deployed by scientists from University of Miami, USA in 45 m deep water about 4 km to the SE of the tower. Turbulence was measured at 2.5 and 5 m above the water on ASIS. Thus, one month of concurrent measurements on the tower and the ASIS enabled comparison of turbulent fluxes of momentum and heat as well as of other relevant parameters. The result of this study, A36, is that in the mean, the instruments on the tower and on the ASIS indeed measure the same thing. This means that the measurements from Östergarnsholm in general can be considered to be as representative for open sea conditions as would a tower placed in 45 m deep water.  

                      The International Evaluation of Swedish Research in Meteorology (2004) rated the Östergarnsholm field station a “world-class facility”. The evaluation had been initiated by the Swedish Research Council, who has contributed substantially to the Östergarnsholm project from its start in 1995 and onwards. The 2004 International Evaluation of Meteorology makes the following statement concerning the position within atmospheric sciences of our research field: “Air-sea interaction is recognised as a critically important component of the climate system, and air-sea interaction provides the science base behind many prediction systems involved in offshore energy research and coastal zone management. Since the existing theoretical base has major limitations in coastal domains, there is tremendous opportunity for scientific growth.”  - The research was rated ‘excellent’ by the evaluation.

                      As already mentioned, the measurements on the Östergarnsholm tower have been running on a semi-continuous basis since May, 1995. In addition to these basic measurements, there have been made many  other measurements by ourselves and by other groups of scientists, including radio soundings (by our group on numerous occasions), airborne measurements (by our group with an instrumented twin jet in 1995 and by UK Met Office with a C-130 aircraft in 1997), remote sensing techniques (by Max- Planck-Institute for Meteorology, Hamburg, Germany in 1998), research ships (German ship R/V Heinke in 1998, Finnish ship R/V Aranda in 2003 and Swedish ship R/V Skagerak in 2006) and the ASIS (Univ. Miami, USA) buoy mentioned earlier. 

     The results of the research cover a wide range of subjects, such as

• Stable atmospheric stratification. Such conditions prevail over the Baltic during most of spring and early summer. During such conditions, the air-sea exchange of sensible heat was observed to be very small, A3, A9, A29. A new mechanism which explains this finding, ‘shear-sheltering’, was identified and explained theoretically, A26.

• Unstable (convective) conditions. During late summer, autumn and winter, the atmosphere over the Baltic is more or less unstable. In a previous study over land, A11, we showed that the conditions in the surface layer are not solely defined in terms of z/L, where z is measurement height and L the Obukhov length (defined from the local flux of momentum and heat) as predicted by classical Monin-Obukhov theory, but also depends on z_i/L, where z_i is the depth of the convective boundary layer. Our measurements at Östergarnsholm show results in complete agreement, A27, A36, with the earlier over-land measurements. Climatologically it appears that z_i is often fairly low (only a couple of hundred meters) over the Baltic, which was shown to have the effect that vertical wind gradients are reduced considerably compared to what is expected from traditional expressions, based solely on Monin-Obukhov theory.

• Near-neutral conditions. Strong wind in combination with low heat flux often leads to near-neutral conditions over the Baltic Sea (as well as over other high-latitude seas). A study of the turbulence regime in such atmospheric conditions, A14, showed a ‘detached eddy’ model developed for high-Reynolds-number flow by fluid dynamic scientists from UK to model the observed atmospheric flow much better than traditional theory.

          Whereas the turbulent momentum flux was found to vary continuously from    very slightly unstable to very slightly stable conditions, studies of the observed turbulent flux of heat and scalars (such as moisture) was found to change discontinuously at strict neutrality. To explain this phenomenon, which was observed to occur both over the sea (Östergarnsholm) and under similar conditions over land, a new theory was developed for the ‘Unstable Very Close to Neutral regime’, the ‘UVCN-regime’, A31. The turbulent exchange of sensible heat and of moisture was found to be strongly increased in this regime compared to corresponding predictions from traditional schemes, A30, A34.

          In this context it is worth noticing that the studies relevant for the UVCN-regime were made with a special turbulence instrument developed by our group, the MIUU-instrument, A23, A30. Systematic comparisons with standard sonic instruments, A30, revealed that these instruments, which are widely used, are inadequate for recording the heat flux in a wind above 10 ms^-1.

• Effects of surface waves on the marine boundary layer, including swell. Several studies from Östergarnsholm clearly show the effects of the surface waves on the atmosphere above. In A20, it is shown that observed features are functions not only of one single wave age parameter, but that two parameters are needed for an accurate description. In A20 it was also shown that the wind profile over the sea is not necessarily logarithmic in neutral conditions as traditionally assumed; for a logarithmic profile to occur it is also required that pure wind sea conditions prevail.

          When the local wave field is dominated by long waves (waves travelling faster than the wind), the term swell is applicable. Swell waves are not locally produced but come from distant storms. In the deep sea, there may be swell coming from several directions simultaneously, making the pattern very complicated. The Baltic Sea being limited in size, means that it is unlikely to have more than one storm at a time. This creates unidirectional swell at a measuring site like Östergarnsholm, which is a situation better suited for fundamental studies of  the influence of swell on the atmosphere than corresponding deep sea cases. A very ‘clean’ swell case from September, 1995 was analyzed in detail in A6. It revealed several new features, such as a ‘wave-driven wind’ just a few meters above the water, effects of the waves being clearly observable at our highest measuring level, 30 m as well as effects of ‘inactive’ turbulence brought down from the top of the boundary layer. These and other features related to swell and presented by us were originally met with some scepticism by the international air-sea research community, but later Dr. Peter Sullivan from NCAR, USA, managed to model the ideal swell boundary layer with LES (Large-Eddy-Simulations), which indeed reproduced our experimental results. Significant swell features were also identified in DNS (Direct Numerical Simulations), A41. Recent analysis of swell data from Östergarnsholm, which combine ASIS-data, tower data and radio sonde data, A37, give a very complete picture of the uni-directional swell situation.

        Several other features of the marine atmospheric boundary layer have been studied, such as sea breeze effects and internal boundary layers, A8 and stratified turbulence in the stable atmosphere above the boundary layer, A5.

        During the last few years, research related to the exchange of carbon dioxide has been initiated, A42. Thus mean CO₂-levels are measured both on the tower and in the water. CO₂ in the water is measured using a SAMI sensor at about 1 km SE of the tower. The turbulent flux of CO₂ is measured at two levels on the tower, 8 and 26 m above the water, in order to detect flux divergence. Analysis of the first year of data showed that the fluxes are very sensitive to the variability of partial pressure of CO₂ in the surface water as well as of the transfer coefficient, A42.

One major problem in measuring the flux of constituents like CO₂ is the correction of density differences between upward and downward moving air (the so called Webb correction). In A35 a new method for including this correction directly on the measured signal enables the analysis of spectra and cospectra without any disturbance of heat and humidity fluctuations.

        As exemplified above, the Östergarnsholm project has given considerable new knowledge concerning the exchange of momentum, sensible heat and moisture for a wide range of environmental conditions. Many findings are such that they are likely to influence the outcome of weather prediction and climate models. In A33 it was, for example, shown that the fluxes of sensible and latent heat were enhanced by 10% for wind speeds above 9 m/s when including the UVCN-regime in a regional climate model (the Swedish RCA model). Further work is now underway to implement the new findings concering swell in the RCA model. Test runs will be made to systematically evaluate the role of these changes. Measurements of turbulent fluxes from the Östergarnsholm station have also been used to verify the heat fluxes in numericla models, as well as in gridded data bases, A9, A38, A40. Showing for example the problem of describing sensible heat fluxes correctly using satellite data for stable stratification, A40.

Active researchers:

• Prof. Ann-Sofi Smedman (project leader)
• Prof. emeritus Ulf Högström
• Dr Hans Bergström
• Dr Anna Rutgersson ( responsible for CO₂ measurements and modelling activities)
• Dr. Cecilia Johansson
• Dr. Kimmo Kahma, FIMR, Helsinki (responsible for wave buoy measurements)
• Dr. Heidi Pettersson, FIMR, Helsinki (responsible for wave buoy analysis)

Active PhD-students: Björn Carlsson, Maria Norman and Alvaro Semedo

The project has up to now generated 6 PhD-thesis and several master thesis. About 40 papers have been published in international journals with ‘peer review procedure’. The Swedish Natural Research Council (VR / NFR) together with The Swedish Energy Agency have sponsored the project since 1995.

International experiments at Östergarnsholm: The Östergarnsholm site has become something like a nucleus for process studies over the sea in  the BALTEX  project (A12)  and several EU-funded experiments have been partly performed at and around Östergarnsholm (PEP in BALTEX (A29), AUTOFLUX , SFINCS, BASYS, BASIS and Marie-Curie PhD programme).

Paper published in journals with peer review

A1. A. Smedman, H. Bergström and B. Grisogono, 1996: Evolution of Stable Internal Boundary  Layers over a Cold Sea. J. Geophys. Res., 102 (C9), 21049-21059.

A2. A. Smedman, U. Högström and H. Bergström, 1996: Low-Level Jets - a Decisive Factor for Off-Shore Wind Energy Siting in the Baltic Sea. Wind Engineering 20(3),137-147.

A3. A. Smedman, U. Högström and H. Bergström, 1997: The turbulence regime of a very stable marine airflow with quasi-frictional decoupling. J. Geophys. Res., 102, 21049-21059.

A4. B. Källstrand and A. Smedman,1997: A case study of the near-neutral coastal internal boundary-layer growth: Aircraft measurements compared with different model estimates. Boundary-Layer Meteorol., 85, 1-33.

A5. U. Högström A. Smedman and H. Bergström, 1999: A case study of two-dimensional stratified turbulence. J. Atm. Sci., 56, 959-976.

A6. A. Smedman, U. Högström, H. Bergström, A. Rutgersson, K. K. Kahma and H. Pettersson, 1999: A case-study of air-sea interaction during swell conditions.  J. Geophys. Res., 104(C11), 25833-25851.

A7.  H. Bergström and  A. Smedman, 1999: Wind climatology at a well-exposed site in the Baltic Sea. Journal of Wind Engineering, 23(2), 133-142.

A8.  B. Källstrand, H. Bergström, J. Höjstrup and A. Smedman, 2000: Meso-scale wind field modifications over the Baltic Sea. Bound. -Layer Meteorol., 95, 161-188.

A9. A. Rutgersson, A. Smedman and A. Omstedt, 2001: Measured and simulated sensible and latent heat fluxes at two marine sites in the Baltic Sea.  Bound.-Layer Meteorol., 99, 53-84.

A10. A. Rutgersson, A. Smedman and U. Högström, 2001: The use of conventional stability parameters during swell. J. Geophys. Res., 106, C11, 27.117-27.134.

A11. C. Johansson, A. Smedman, U. Högström, J. Brasseur and S. Kanna, 2001: A critical test of the validity of Monin-Obukhov similarity during convective conditions. J. Atm. Sci., 58, 1549-1566.

A12. E. Raschke, J. Meywerk, K. Warrach, U. Andrae, S. Bergström, F. Beyrich, F. Bosveld, C. Fortelius, L.P. Graham, S.-E. Gryning, S. Halldin, L. Hasse, M. Heikinheimo, H.-J. Isemer, D. Jacob, I. Jauja, K.-G. Karlsson, S. Keevallik, J. Koistinen, A. van Lammeren, U. Lass, J. Launiainen, A. Lehmann, B. Liljebladh, M. Lobmeyr, W. Matthäus, T. Mengelkamp, D. B. Michelson, J. Napiórkowski, A. Omstedt, J. Piechura, B. Rockel, F. Rubel, E. Ruprecht, A.-S. Smedman, A. Stigebrandt, 2001: BALTEX (Baltic Sea Experiment):   A European Contribution to Investigate the Energy and Water Cycle over a Large Drainage Basin. Bull. Am. Meteor. Soc., 82, 2389-2413.

A13. A. Sjöblom and A. Smedman, 2002: The turbulent kinetic budget over the sea. J. Geophys. Res. 107 (C10) 3142, doi:1029/2001JC001016.

A14. Högström U., J. Hunt and A. Smedman, 2002: Theory and measurements for turbulence spectra and variances  in the near neutral surface layer. Bound.-Layer Meteorol., 103, 101-124.

A15. C. Johansson, A. Smedman, U. Högström and J. Brasseur, 2001: Replay to ‘: A critical test of the validity of Monin-Obukhov similarity during convective conditions.’ By Edgar L. Andreas and Bruce B. Hicks. J. Atm. Sci., 59, 2608-2614.

A16. A. Sjöblom and A. Smedman, 2003: Vertical structure in the marine atmospheric boundary layer and its implication for the inertial dissipation method.  Bound.-Layer Meteorol., 109, 1-25.

A17. A. Smedman, U. Högström and A. Sjöblom, 2003: A note on velocity spectra in the marine boundary layer. Bound.- Layer Meteorol., 109, 27-28.

A18. B. Hennemuth-Oberle, A. Rutgersson, A. Omstedt,  K. Bumke and M. Clemens, D. Jacob and A. Smedman, 2003: Net precipitation over the Baltic Sea for one year using several methods. Tellus, 55A, 352-367.

A19. L. Mahrt, D. Vickers, P. Fredrickson and A. Smedman, 2003: Sea-surface aerodynamic roughness. J. Geophys. Res., 108 (C6), 3171 doi:10.1029/2002JC001383,2003.

A20. A. Smedman, X. Guo-Larsén, U. Högström, K. Kahma and H. Pettersson, 2003: The effect of sea state on the monmentum exchange over the sea during neutral conditions. J. Geophys. Res., 108(C11), 3367, doi: 10.1029/2002JC001526, 2003.

A21. X. Guo Larsén, V. Makin and A. Smedman, 2003: Comparison of modelled and measured shearing stress over the Baltic Sea. Global Atmos. Ocean System, 9(3).

A22. A. Smedman, U. Högström, H. Bergström, C. Johansson, A. Sjöblom and X. Guo Larsén, 2003: New findings concerning the structure of the marine atmospheric boundary layer over the Baltic Sea – possible implications for wind energy installations. Journal of Wind Engineering, 27 (6), 431-447.

A23. U. Högström and A. Smedman, 2004: Accuracy of sonic anemometers: Laminar wind-tunnel calibrations compared to atmospheric in situ calibrations against a reference instrument. Bound.-Layer Meteorol. 111(1), 33-54.

A24. X. Guo-Larsén, A. Smedman, 2004: Air-sea exchange of sensible heat over the Baltic Sea. Quart. J. Roy. Meteorol. Soc., 130, 1-25.

A25. A. Sjöblom and A. Smedman, 2004: Comparison between eddy-correlation and inertial dissipation methods in the marine atmospheric boundary layer.  Bound.- Layer Meteorol., 110, 141-164.

A26.  A. Smedman, J. Hunt and U. Högström, 2004: Effects of shear sheltering in a slightly  stable atmospheric boundary layer with strong shear. Quart. J. Roy. Meteorol. Soc., 130, 31-50

A27. C. Johansson, A. Smedman, H. Bergström and S-E. Gryning, 2005: Influence from the boundary layer height on the turbulence structure in the marine atmospheric surface layer over the Baltic Sea. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 792, ACTA UNIVERSITATIS UPSALIENSIS, and submitted to Bound.-Layer Meteorol.

A28. C. Johansson, B. Hennemuth, B. Bösenberg, H. Linné and A. Smedman, 2005:  Double-layer structure over the Baltic Sea; Climatology, Case Study and Simulation.  Bound.- Layer Meteorol., 114, 389-412

A29. A. Smedman, K. Bumke, U. Högström, A. Rutgersson, S-E. Gryning, E. Batchvarova, G. Peters, B. Hennemuth, B. Tammelin, R. Hyvönen, A. Omstedt, D. Michelson, T. Andersson and M. Clemens,2006: Precipitation and Evaporation Budgets over the Baltic Proper: Observations and Modelling.  J.  of  Atmospheric and Ocean Science,10, 163-191.

 
A30. A. Smedman, U. Högström, E. Sahlée and C. Johansson, 2007: Critical re-evaluation of the bulk transfer coefficient for heat over the ocean. Quart. J. Roy. Meteorol. Soc..In press.

A31.  A. Smedman, U. Högström,  J.C.R. Hunt and E. Sahlée, 2007: Heat/mass transfer in the slightly unstable atmospheric surface layer. Quart. J. Roy. Meteorol. Soc. In press..

A32. U. Högström, A. Smedman and H. Bergström, 2006: Calculation of wind speed variation with height over the sea. Wind Engineering, 30(4), 269-286.

A33. A. Rutgersson, B. Karlsson and A. Smedman, 2007: Modelling sensible and latent heat fluxes over sea during unstable, very close to neutral conditions. Accepted for publication in  Boundary –Layer Meteorol.

A34.  E. Sahlée, A. Smedman, U. Högström and A. Rutgersson, 2007:  Re-evolution of  bulk exchange coefficient for humidity at sea during unstable and neutral conditions. Accepted in J. Phys. Oceanogr.

A35. E. Sahlée, A. Smedman, A. Rutgersson, U. Högström, 2007: Spectra of CO2 and humidity in the marine atmospheric surface layer. Accepted in Boundary- Layer Meteorol.

A36. U.Högström, A. Smedman, E. Sahlée, A. Rutgersson, C. Johansson, K.K. Kahma, H. Pettersson, L.Toumi, W.M. Drennan, 2007: To what extent can we believe measurements on a land-based tower to represent upwind open sea conditions? Manuscript.

A37. A. Smedman, U. Högström, E. Sahlée, A. Rutgersson, W. M. Drennan, K. Kahma and H. Pettersson, 2007: Vertical variation of swell impact on the marine atmospheric boundary layer. Manuscript.

A38. Rutgersson, A. 2000. A comparison between long term measured and modeled sensible heat and momentum flux using a High Resolution Limited Area Model (HIRLAM). Meteorol. Z., 9, 29-37.

A39. Rutgersson, A., Bumke, K., Clemens, M., Foltescu, V., Lindau, R., Michelson, D.,
Omstedt, A., 2001: Precipitation Estimates over the Baltic Sea: Present State of the Art, Nordic Hydrol., 32(4), 285-314.

A40. Rutgersson, A., Anders Omstedt and Youmin Chen, 2005: Evaluation of the heat balance components over the Baltic Sea using four gridded meteorological databases and direct observations. Nordic Hydrol., 36, 381-396.

A41. Rutgersson, A. and P. P. Sullivan, 2005: Investigating the effects of water waves on the turbulence structure in the atmosphere using direct numerical simulations. Dyn. Atmos. Oceans., 38, 147-171.

A42. Rutgersson, A., Schneider, B., Pettersson, H., Smedman, A.-S. 2007: The annual cycle of carbon-dioxide and parameters influencing the air-sea carbon exchange in the Baltic Proper. Manuscript.