| 1. |
- Carlsson, Björn, 1980-
(author)
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Implementation and Analysis of Air-Sea Exchange Processes in Atmosphere and Ocean Modelling
- 2008
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Doctoral thesis (other academic/artistic)abstract
- To understand and to predict the weather and climate, numerical models are important tools and it is crucial that the controlling processes are described correctly. Since 70% of the global surface is covered with water the description how the ocean and atmosphere communicates has a considerable impact. The ocean–atmosphere exchange occurs through transport of momentum (friction) and heat, governed by turbulent eddies. The sea surface is also an important source of turbulence in both directions. The scales of the turbulent eddies cannot be resolved in ocean and climate models. Therefore, the turbulent exchanges have to be related to mean variables, such as wind speed and temperature differences. By using measurements, new methods to describe the air–sea exchange during two specific processes were developed. These processes are the so-called UVCN-regime (Unstable Very Close to Neutral stratification) and swell, i.e. waves which are not produced by the local wind. These processes were included in an ocean model and in a regional atmospheric climate model and the impact was investigated.The UVCN-regime enhances the heat transport significantly during the autumn and winter months in the ocean model. This results in a shallower well-mixed surface layer in the ocean. Wind-following swell reduces the surface friction, which is very important for the atmosphere. Some secondary effects in the climate model are reduced low-level cloud cover and reduced precipitation by more than 10% over sea areas. Locally and for short periods the impact is large. It is important to include the UVCN-regime and the swell impact in models, to make simulations more reliable.
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| 2. |
- Christensen, Kai H., et al.
(author)
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Surface wave measurements using a ship-mounted ultrasonic altimeter
- 2013
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In: Methods in Oceanography. - : Elsevier BV. - 2211-1220. ; 6, s. 1-15
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Journal article (peer-reviewed)abstract
- We present a method for measuring one-dimensional surface wave spectra using a ship-mounted ultrasonic altimeter in combination with a motion correction device. The instruments are mounted at the bow of the ship and provide high-resolution, local, wave information. We present results from three recent field studies. The results are compared with data from a conventional waverider buoy and, when in-situ observations are not available, with wave model analyses and satellite altimetry. We find good agreement with regard to integrated parameters such as significant wave height and mean period. Comparison with a waverider demonstrates fair agreement with regard to spectral shape, but the representation of the low frequency part depends on the quality of the motion correction data.
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| 3. |
- Linders, Torsten, 1971, et al.
(author)
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Can CAPE maintain polar lows?
- 2010
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In: Journal of the Atmospheric Sciences. - 0022-4928. ; 67:8, s. 2559-2571
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Journal article (peer-reviewed)abstract
- A unique dataset of atmospheric observations over the Nordic Seas has been analyzed to investigate the role of convective available potential energy (CAPE) for the energetics of polar lows. The observations were made during the flight campaign of the Norwegian International Polar Year (IPY) and The Observing System Research and Predictability Experiment (THORPEX) in February and March 2008, which specifically targeted polar lows. The data reveal virtually no conditional instability and very limited CAPE. It is suggested that the significance of CAPE values should be assessed by calculating the time scale tCAPE that is necessary for the heat fluxes from the ocean to transfer the corresponding amount of energy. Even the largest CAPE values have a tCAPE of less than 1 h. These CAPE values are associated with unconditional instability. It is concluded that the observed CAPE should be seen as a temporary stage in an energy flux rather than as an energy reservoir. Based on the findings in this investigation, it is proposed that significant reservoirs of CAPE over the marine Arctic atmosphere are impossible since CAPE production will automatically trigger convection and CAPE is consumed as it is produced.
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| 4. |
- Linders, Torsten, 1971-, et al.
(author)
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Can CAPE maintain polar lows?
- 2010
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In: Journal of the Atmospheric Sciences. - : American Meteorological Society. - 0022-4928 .- 1520-0469. ; 67:8, s. 2559-2571
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Journal article (peer-reviewed)
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| 5. |
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| 6. |
- Linders, Torsten, 1971, et al.
(author)
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Limited polar low sensitivity to SST
- 2011
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In: Quarterly Journal of the Royal Meteorological Society. - 1477-870X. ; 137:654, s. 58-69
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Journal article (peer-reviewed)abstract
- The sensitivity of the intensity of polar lows to changes in the sea-surface temperature (SST) is investigated. This is done by using an axisymmetric non-hydrostatic numerical model. It is found that the intensity of the simulated polar lows responds linearly to SST perturbations, with positive perturbations leading to deeper sea-level pressure (SLP) depressions and higher azimuthal winds. The numerical simulations give an SLP depression sensitivity of only about −0.6 hPa K−1 and a sensitivity of the maximum azimuthal wind of only about +0.6 m s−1 K−1. This is about one order of magnitude less than the sensitivity of the theoretical maximum intensity. It is suggested that the very limited sensitivity can be explained within the wind-induced surface heat exchange (WISHE) theory. The limited sensitivity can thus be attributed, in roughly equal parts, to the limited efficiency with which the polar low extracts heat from the ocean and the limited mean height of the convection. Copyright © 2011 Royal Meteorological Society
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| 7. |
- Saetra, Øyvind, 1959-, et al.
(author)
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Can polar lows lead to a warming of the ocean surface?
- 2008
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In: Tellus. Series A, Dynamic meteorology and oceanography. - : Wiley-Blackwell. - 0280-6495 .- 1600-0870. ; 60:1, s. 141-153
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Journal article (peer-reviewed)abstract
- Possible surface warming by strong wind-forcing from polar lows in the North-Atlantic has been investigated using a numerical model for vertical entrainment of waters from a subsurface warm core, and microwave satellite images of sea-surface temperature during polar low events. The hypothesis is based on the frequently observed subsurface warm core in oceans influenced by the North-Atlantic current (NAC) or by outflowing surface water from the Arctic Ocean. CTD-soundings from the Nordic Seas reveal that the waters from the NAC are located under colder and less saline surface waters in winter. For sufficiently strong wind events, turbulent entrainment of this subsurface warm core may lead to a rapid surface warming. Our main findings is that the surface warming of more than 1 °C may take place within a few hours. The result is based on model runs with initial temperature and salinity profiles from CTD-observations. Observational evidence of surface temperatures that support the hypothesis are found in microwave satellite observations from a polar low event. In the case presented here, increased sea-surface temperatures between 1 and 2 °C were observed. We believe that rapid surface warming of this magnitude may be a potential positive feedback mechanism for the cyclone intensity.
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| 8. |
- Semedo, Alvaro, 1966-, et al.
(author)
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Wave induced wind in the marine boundary layer
- 2009
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In: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 66:8, s. 2256-2271
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Journal article (peer-reviewed)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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