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- Smedman, Ann-Sofi, et al.
(författare)
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Precipitation and evaporation budgets over the Baltic Proper: observations and modelling
- 2005
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Ingår i: Journal of Atmospheric and Ocean Science. - : Informa UK Limited. - 1741-7538 .- 1741-7546. ; 10:3, s. 163-191
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation and evaporation budgets over the Baltic Sea were studied in a concerted project called PEP in BALTEX, combining extensive field measurements and modelling efforts. Eddy-correlation-measurements of turbulent heat flux were made on a semi-continuous basis for a 12 months period at four well-exposed coastal sites in the Baltic Proper. Precipitation was measured at land-based sites with standard gauges and on four merchant ships travelling between Germany and Finland with the aid of specially designed ship rain gauges (SRG).The evaporation and precipitation regime of the Baltic Sea was modelled for a 12 months period by applying a wide range of numerical models: The operational atmospheric High Resolution Limited Area Model HIRLAM (Swedish and Finnish versions), the German atmospheric REgional-scale MOdel, REMO, the operational German Europe-Model (only precipitation), the oceanographic model PROBE-Baltic, and two models that use interpolation of ground-based data, the Swedish MESAN model of SMHI and a German model of IFM-GEOMAR Kiel.Modelled precipitation was compared with ship rain gauge measurements on board the ships. A reasonable correlation was obtained, but the regional scale models and MESAN give some 20 percent higher precipitation over the sea than measured.Bulk parameterisation schemes for evaporation were evaluated against measurements. A constant value of CHN and CEN with wind speed underestimated large fluxes of both sensible and latent heat flux. The limited area models do not resolve the influence of the height of the marine boundary layer in coastal zones and the entrainment processes, which may explain the observed low correlations between modelled and measured latent heat fluxes.Estimates of evaporation, E, and precipitation, P, for the entire Baltic Proper were made with several models for a 12 months period. While the annual variation was well represented by all predictions, there are still important differences in the total mean. Evaporation ranges from 509 to 625 mm/year and precipitation between 624 and 805 mm/year for the particular 12 months period. Taking the results of model verification from the present study into account, the best estimate of P - E is about 100 50 mm for the particular 12 months period. But the annual mean of P - E varies considerably from year to year. This is reflected in simulations with the PROBE-Baltic model for an 18 years period, which gave 95 mm/year for the 12 months period studied here and 32 mm/year as an average for 18 years.
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| 2. |
- Högström, Ulf, et al.
(författare)
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Air–Sea Interaction Features in the Baltic Sea and at a Pacific Trade-Wind Site : An Inter-comparison Study
- 2013
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 147:1, s. 139-163
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Tidskriftsartikel (refereegranskat)abstract
- A systematic comparison of wind profiles and momentum exchange at a trade wind site outside Oahu, Hawaii and corresponding data from the Baltic Sea is presented. The trade wind data are to a very high degree swell dominated, whereas the Baltic Sea data include a more varied assortment of wave conditions, ranging from a pure growing sea to swell. In the trade wind region swell waves travel predominantly in the wind direction, while in the Baltic, significant cross-wind swells are also present. Showing the drag coefficient as a function of the 10-m wind speed demonstrates striking differences for unstable conditions with swell for the wind-speed range 2 m s−1 < U 10 < 7 m s−1, where the trade-wind site drag values are significantly larger than the corresponding Baltic Sea values. In striking contrast to this disagreement, other features studied are surprisingly similar between the two sites. Thus, exactly as found previously in Baltic Sea studies during unstable conditions and swell, the wind profile in light winds (3 m s−1) shows a wind maximum at around 7–8 m above the water, with close to constant wind speed above. Also, for slightly higher wind speeds (4 m s−1 < U 10 < 7 m s−1), the similarity between wind profiles is striking, with a strong wind-speed increase below a height of about 7–8 m followed by a layer of virtually constant wind speed above. A consequence of these wind-profile features is that Monin–Obukhov similarity is no longer valid. At the trade-wind site this was observed to be the case even for wind speeds as high as 10 m s−1. The turbulence kinetic energy budget was evaluated for four cases of 8–16 30- min periods at the trade-wind site, giving results that agree very well with corresponding figures from the Baltic Sea.
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| 6. |
- Högström, Ulf, et al.
(författare)
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Surface Stress over the Ocean in Swell-Dominated Conditions during Moderate Winds
- 2015
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 72:12, s. 4777-4795
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Tidskriftsartikel (refereegranskat)abstract
- Atmospheric and surface wave data from several oceanic experiments carried out on the Floating Instrument Platform(FLIP) and the Air–Sea Interaction Spar (ASIS) have been analyzed with the purpose of identifying swell-related effects on the surface momentum exchange during near-neutral atmospheric conditions and wind-following or crosswind seas. All data have a pronounced negative maximum in uw cospectra centered at the frequency of the dominant swell np, meaning a positive contribution to the stress. A similar contribution at this frequency is also obtained for the corresponding crosswind cospectrum. The magnitude of the cospectral maximum is shown to be linearly related to the square of the orbital motion, being equal to , where Hsd is the swell-significant wave height, the effect tentatively being due to strong correlation between the surface component of the orbital motion and the pattern of capillary waves over long swell waves.A model for prediction of the friction velocity from measurements of Hsd, np, and the 10-m wind speed U10 is formulated and tested against an independent dataset of ~400 half-hour measurements during swell, giving good result.The model predicts that the drag coefficient CD, which is traditionally modeled as a function of U10 alone (e.g., the COARE algorithm), becomes strongly dependent on the magnitude of the swell factor and that CD can attain values several times larger than predicted by wind speed–only models. According to maps of the global wave climate, conditions leading to large effects are likely to be widespread over the World Ocean.
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| 7. |
- Högström, Ulf, et al.
(författare)
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The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions
- 2018
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 75:8, s. 2579-2588
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Tidskriftsartikel (refereegranskat)abstract
- Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U 5 4ms21 . For the lowest observed wind speeds U , 2.4 m s21 , the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height Hsd in the range 0.6 , Hsd , 1.4 m. For Hsd , 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed
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