| 1. |
- Strandberg, Gustav, 1977-, et al.
(författare)
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High-resolution regional simulation of last glacial maximum climate in Europe
- 2011
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 63:1, s. 107-125
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Tidskriftsartikel (refereegranskat)abstract
- A fully coupled atmosphere-ocean general circulation model is used to simulate climate conditions during the last glacial maximum (LGM). Forcing conditions include astronomical parameters, greenhouse gases, ice sheets and vegetation. A 50-yr period of the global simulation is dynamically downscaled to 50 km horizontal resolution over Europe with a regional climate model (RCM). A dynamic vegetation model is used to produce vegetation that is consistent with the climate simulated by the RCM. This vegetation is used in a final simulation with the RCM. The resulting climate is 5-10 degrees C colder than the recent past climate (representative of year 1990) over ice-free parts of Europe as an annual average; over the ice-sheet up to 40 degrees C colder in winter." The average model-proxy error is about the same for summer and winter, for pollen-based proxies. The RCM results are within (outside) the uncertainty limits for winter (summer). Sensitivity studies performed with the RCM indicate that the simulated climate is sensitive to changes in vegetation, whereas the location of the ice sheet only affects the climate around the ice sheet. The RCM-simulated interannual variability in near surface temperature is significantly larger at LGM than in the recent past climate.
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| 2. |
- Saetra, Øyvind, 1959-, et al.
(författare)
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Can polar lows lead to a warming of the ocean surface?
- 2008
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Wiley-Blackwell. - 0280-6495 .- 1600-0870. ; 60:1, s. 141-153
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Tidskriftsartikel (refereegranskat)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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| 3. |
- Andersson, Lotta, 1958-, et al.
(författare)
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Assessment of climate change impact on water resources in the Pungwe river basin.
- 2011
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 63:1, s. 138-157
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Tidskriftsartikel (refereegranskat)abstract
- The Rossby Centre Regional Climate Model (RCA3) and the hydrological model HBV were linked to assess climate change impacts on water resources in the Pungwe basin until 2050. RCA3 was capable of simulating the most important aspects of the climate for a control period at the regional scale. At the subbasin scale, additional scalingwas needed. Three climate change experiments using ECHAM4-A2, B2 and CCSM3-B2 as input to RCA3 were carried out. According to the simulations annual rainfall in 2050 would be reduced by approximately 10% with increasing interannual variability of rainfall and dry season river flow and later onset of the rainy season. The ECHAM4-A2 driven experiment did also indicate a slight increase of high flows. If the results indeed reflect the future, they will worsen the already critical situation for water resources, regarding both floods and droughts. Uncertainties, however in the downscaled scenarios make it difficult to prioritize adaptation options. This calls for inclusion of more climate change experiments, in an ensemble of climate scenarios possibly by using a combination of dynamical and statistical downscaling of general circulation models, as well as extending the simulations to 2100 to further ensure robustness of the signal.
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| 4. |
- Bojarova, Jelena, et al.
(författare)
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The EKTF rescaling scheme in HIRLAM
- 2011
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 63:3, s. 685-401
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The ETKF rescaling scheme has been implemented into the HIRLAM forecasting system in order to estimate the uncertainty of the model state. The main purpose is to utilize this uncertainty information for modelling of flow-dependent background error covariances within the framework of a hybrid variational ensemble data assimilation scheme. The effects of rank-deficiency in the ETKF formulation is explained and the need for variance inflation as a way to compensate for these effects is justified. A filter spin-up algorithm is proposed as a refinement of the variance inflation. The proposed spin-up algorithm will also act to prevent ensemble collapse since the ensemble will receive ‘fresh blood’ in the form of additional perturbation components, generated on the basis of a static background error covariance matrix. The resulting ETKF-based ensemble perturbations are compared with ensemble perturbations based on targeted singular vectors and are shown to have more realistic spectral characteristics.
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| 5. |
- Lindborg, Erik, et al.
(författare)
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A condition on the average Richardson number for weak non-linearity of internal gravity waves
- 2007
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 59:5, s. 781-784
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Tidskriftsartikel (refereegranskat)abstract
- A condition on the average Richardson number, Ri, for weak non-linearity of an internal gravity wavefield is derived using a quasi-normal assumption. For weak non-linearity to be satisfied it is required that Ri(-1) << 0.5. This condition is very rarely satisfied in the ocean at vertical scales up to the order of 100 m, for which it is often found that Ri(-1) similar to 1. The analysis suggests that non-linear effects are of no less importance than linear effects in the dynamics of the interior of the ocean at these scales.
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| 6. |
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| 7. |
- Zarroug, Moundheur, 1978-, et al.
(författare)
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Energetics of tidally generated internal waves
- 2010
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 62:1, s. 71-79
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Tidskriftsartikel (refereegranskat)abstract
- The generation of internal waves from barotropic tides can be quantified in terms of the conversion rates. These have often been obtained by applying the WKB approximation, which yields an expression for the conversion rates which is proportional to the seabed buoyancy frequency NB . For small values of NB or strong variations of the buoyancy profile N (z), this gives unreliable results. Using homogenization theory it is here shown that the conversion rate instead depends on the value of N (z) averaged over a vertical region at the bottom of the same magnitude as the vertical length-scale of the internal wave, which for the lowest modes is of the same order as the entire ocean depth. This gives a substantially larger conversion rate.
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| 9. |
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| 10. |
- 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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