| 1. |
- Donadini, Fabio, et al.
(författare)
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Holocene geomagnetic paleointensities: A blind test of absolute paleointensity techniques and materials
- 2007
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Ingår i: Physics of the Earth and Planetary Interiors. - : Elsevier BV. - 0031-9201. ; 161:1-2, s. 19-35
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Tidskriftsartikel (refereegranskat)abstract
- Through several decades of research, absolute paleointensity estimates have been obtained from a wide range of rocks with varying ages, covering the last 3.45 Ga years. These paleointensity data are crucial to study the past geodynamo and the geological evolution of the Earth's deep interior. However, paleointensity data are often difficult to interpret, and on-going discussions concerning the past geomagnetic field invariably focus on data reliability, and which kind of material, and method, is best suited for absolute paleointensity experiments. In this paper we blind test paleointensity methods and materials using the new GEOMAGIA50 database that contains all available experimental information about published paleointensity estimates covering the last 50,000 years. Our analysis is built on simple comparisons of results obtained from different materials, and methodologies, to investigate their possible influence on the paleointensity estimate. We also study the effect of various numbers of samples, and dating accuracies. We focus on paleointensity estimates from the last 12,000 years, which includes the Holocene epoch. The advantage of this period is that data have been obtained from a wide array of methodological techniques as well as a wide selection of materials, including both natural ones (e.g. lava flows) and various archaeological artifacts. Our main observations are (i) that well-fired archaeological materials (bricks, potteries, clays, and ceramics) show the best correlation with the rest of the dataset, which we interpret to suggest that these data are the most reliable, and (ii) although paleointensity data obtained from lava flows are slightly more scattered, there is no evidence for paleointensity data from lavas to be significantly lower or higher than other data, irrespective of the methodology. (c) 2007 Elsevier B.V. All rights reserved.
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| 2. |
- 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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| 3. |
- Semedo, Alvaro, 1966-, et al.
(författare)
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Wave induced wind in the marine boundary layer
- 2009
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 66:8, s. 2256-2271
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Tidskriftsartikel (refereegranskat)abstract
- Recent field observations and large-eddy simulations have shown that the impact of fast swell on the marine atmospheric boundary layer (MABL) might be stronger than previously assumed. For low to moderate winds blowing in the same direction as the waves, swell propagates faster than the mean wind. The momentum flux above the sea surface will then have two major components: the turbulent shear stress, directed downward, and the swell-induced stress, directed upward. For sufficiently high wave age values, the wave-induced component becomes increasingly dominant, and the total momentum flux will be directed into the atmosphere. Recent field measurements have shown that this upward momentum transfer from the ocean into the atmosphere has a considerable impact on the surface layer flow dynamics and on the turbulence structure of the overall MABL. The vertical wind profile will no longer exhibit a logarithmic shape because an acceleration of the airflow near the surface will take place, generating a low-level wave-driven wind maximum (a wind jet). As waves propagate away from their generation area as swell, some of the wave momentum will be returned to the atmosphere in the form of wave-driven winds. A model that qualitatively reproduces the wave-following atmospheric flow and the wave-generated wind maximum, as seen from measurements, is proposed. The model assumes a stationary momentum and turbulent kinetic energy balance and uses the dampening of the waves at the surface to describe the momentum flux from the waves to the atmosphere. In this study, simultaneous observations of wind profiles, turbulent fluxes, and wave spectra during swell events are presented and compared with the model. In the absence of an established model for the linear damping ratio during swell conditions, the model is combined with observations to estimate the wave damping. For the cases in which the observations showed a pronounced swell signal and almost no wind waves, the agreement between observed and modeled wind profiles is remarkably good. The resulting attenuation length is found to be relatively short, which suggests that the estimated damping ratios are too large. The authors attribute this, at least partly, to processes not accounted for by the model, such as the existence of an atmospheric background wind. In the model, this extra momentum must be supplied by the waves in terms of a larger damping ratio.
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| 4. |
- 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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