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541.
  • Wallin, Marcus B., et al. (författare)
  • Spatiotemporal variability of the gas transfer coefficient (KCO2) in boreal streams : Implications for large scale estimates of CO2 evasion
  • 2011
  • Ingår i: Global Biogeochemical Cycles. - : John Wiley & Sons. - 0886-6236 .- 1944-9224. ; 25:3, s. GB3025-
  • Tidskriftsartikel (refereegranskat)abstract
    • Boreal streams represent potentially important conduits for the exchange of carbon dioxide (CO(2)) between terrestrial ecosystems and the atmosphere. The gas transfer coefficient of CO(2) (K(CO2)) is a key variable in estimating this source strength, but the scarcity of measured values in lotic systems creates a risk of incorrect flux estimates even when stream gas concentrations are well known. This study used 114 independent measurements of K(CO2) from 14 stream reaches in a boreal headwater system to determine and predict spatiotemporal variability in K(CO2). The K(CO2) values ranged from 0.001 to 0.207 min(-1) across the 14 sites. Median K(CO2) for a specific site was positively correlated with the slope of the stream reach, with higher gas transfer coefficients occurring in steeper stream sections. Combining slope with a width/depth index of the stream reach explained 83% of the spatial variability in K(CO2). Temporal variability was more difficult to predict and was strongly site specific. Variation in K(CO2), rather than pCO(2), was the main determinant of stream CO(2) evasion. Applying published generalized gas transfer velocities produced an error of up to 100% in median instantaneous evasion rates compared to the use of actual measured K(CO2) values from our field study. Using the significant relationship to local slope, the median K(CO2) was predicted for 300,000 km of watercourses (ranging in stream order 1-4) in the forested landscape of boreal/nemoral Sweden. The range in modeled stream order specific median K(CO2) was 0.017-0.028 min(-1) and there was a clear gradient of increasing K(CO2) with lower stream order. We conclude that accurate regional scale estimates of CO(2) evasion fluxes from running waters are possible, but require a good understanding of gas exchange at the water surface.
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542.
  • Wang, D.Y., et al. (författare)
  • Validation of nitric acid retrieved by the IMK-IAA processor from MIPAS/ENVISAT measurements
  • 2007
  • Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 7, s. 721-738
  • Tidskriftsartikel (refereegranskat)abstract
    • The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the ENVISAT satellite provides profiles of temperature and various trace-gases from limb-viewing mid-infrared emission measurements. The stratospheric nitric acid (HNO(3)) from September 2002 to March 2004 was retrieved from the MIPAS observations using the science-oriented data processor developed at the Institut fur Meteorologie und Klimaforschung (IMK), which is complemented by the component of non-local thermodynamic equilibrium (non-LTE) treatment from the Instituto de Astrofisica de Andalucia (IAA). The IMK-IAA research product, different from the ESA operational product, is validated in this paper by comparison with a number of reference data sets. Individual HNO3 profiles of the IMK-IAA MIPAS show good agreement with those of the balloon-borne version of MIPAS (MIPAS-B) and the infrared spectrometer MkIV, with small differences of less than 0.5 ppbv throughout the entire altitude range up to about 38 km, and below 0.2 ppbv above 30 km. However, the degree of consistency is largely affected by their temporal and spatial coincidence, and differences of 1 to 2 ppbv may be observed between 22 and 26 km at high latitudes near the vortex boundary, due to large horizontal inhomogeneity of HNO(3). Statistical comparisons of MIPAS IMK-IAA HNO(3) VMRs with respect to those of satellite measurements of Odin/SMR, ILAS-II, ACE-FTS, as well as the MIPAS ESA product show good consistency. The mean differences are generally +/- 0.5 ppbv and standard deviations of the differences are of 0.5 to 1.5 ppbv. The maximum differences are 2.0 ppbv around 20 to 25 km. This gives confidence in the general reliability of MIPAS HNO(3) VMR data and the other three satellite data sets.
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543.
  • Wang, Jianfeng, 1984-, et al. (författare)
  • A Hybrid Statistical-Dynamical Downscaling of Air Temperature over Scandinavia Using the WRF Model
  • 2020
  • Ingår i: Advances in Atmospheric Sciences. - : Springer. - 0256-1530 .- 1861-9533. ; 37:1, s. 57-74
  • Tidskriftsartikel (refereegranskat)abstract
    • An accurate simulation of air temperature at local scales is crucial for the vast majority of weather and climate applications. In this work, a hybrid statistical–dynamical downscaling method and a high-resolution dynamical-only downscaling method are applied to daily mean, minimum and maximum air temperatures to investigate the quality of localscale estimates produced by downscaling. These two downscaling approaches are evaluated using station observation data obtained from the Finnish Meteorological Institute over a near-coastal region of western Finland. The dynamical downscaling is performed with the Weather Research and Forecasting (WRF) model, and the statistical downscaling method implemented is the Cumulative Distribution Function-transform (CDF-t). The CDF-t is trained using 20 years of WRF-downscaled Climate Forecast System Reanalysis data over the region at a 3-km spatial resolution for the central month of each season. The performance of the two methods is assessed qualitatively, by inspection of quantile-quantile plots, and quantitatively, through the Cramer-von Mises, mean absolute error, and root-mean-square error diagnostics. The hybrid approach is found to provide significantly more skillful forecasts of the observed daily mean and maximum air temperatures than those of the dynamical-only downscaling (for all seasons). The hybrid method proves to be less computationally expensive, and also to give more skillful temperature forecasts (at least for the Finnish near-coastal region).
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544.
  • Wetterhall, Fredrik, et al. (författare)
  • Statistical downscaling of daily precipitation over Sweden using GCM output
  • 2009
  • Ingår i: Journal of Theoretical and Applied Climatology. - : Springer Science and Business Media LLC. - 0177-798X .- 1434-4483. ; 96:1-2, s. 95-103
  • Tidskriftsartikel (refereegranskat)abstract
    • A classification of Swedish weather patterns (SWP) was developed by applying a multi-objective fuzzy-rule-based classification method (MOFRBC) to large-scale-circulation predictors in the context of statistical downscaling of daily precipitation at the station level. The predictor data was mean sea level pressure (MSLP) and geopotential heights at 850 (H850) and 700 hPa (H700) from the NCEP/NCAR reanalysis and from the HadAM3 GCM. The MOFRBC was used to evaluate effects of two future climate scenarios (A2 and B2) on precipitation patterns on two regions in south-central and northern Sweden. The precipitation series were generated with a stochastic, autoregressive model conditioned on SWP. H850 was found to be the optimum predictor for SWP, and SWP could be used instead of local classifications with little information lost. The results in the climate projection indicated an increase in maximum 5-day precipitation and precipitation amount on a wet day for the scenarios A2 and B2 for the period 2070-2100 compared to 1961-1990. The relative increase was largest in the northern region and could be attributed to an increase in the specific humidity rather than to changes in the circulation patterns.
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545.
  • Yao, Ruzhen, et al. (författare)
  • Runoff Response to Climate in Two River Basins Supplied by Small Glacier Meltwater in Southern and Northern Tibetan Plateau
  • 2023
  • Ingår i: Atmosphere. - 2073-4433. ; 14:4
  • Tidskriftsartikel (refereegranskat)abstract
    • The Tibetan Plateau (TP) has experienced amplified warming in recent decades, causing glaciers to melt and affecting river runoff. It is well established that the southern and northern areas of the TP have responded to climate changes differently, with the north dominated by a westerly climate and the south by the Indian monsoon. While there are more glaciers in the TP than in any other region outside the polar areas, most of these glaciers are tiny, and only a limited number of them have been monitored to study mass balance and downward runoff. This study used the mass balance measured at two glaciers along with in situ and satellite data to drive a hydrological model called the Alpine Runoff Predictor that includes glacier melt to simulate glacial melting and the accompanying hydrological processes of the two glacierized basins, analyze their contributions to the river runoffs, and investigate their responses to local climate changes. The results show that the glacier meltwater in both river basins showed an increasing trend, with values of 0.001 x 10(8) m(3) a(-1) in the Kyanjing River basin and 0.0095 x 10(8) m(3) a(-1) in the Tuole River basin. However, their multi-year average contributions to the runoff were 12.5% and 5.6%, respectively. In contrast to the Tuole River basin, where runoff is increasing (0.0617 x 10(8) m(3) a(-1)), the Kyanjing River basin has decreasing runoff (-0.0216 x 10(8) m(3) a(-1)) as a result of decreasing precipitation. This result highlights the dominant role played by precipitation changes in the two basins under study, which are characterized by small glacier meltwater contributions.
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546.
  • Ye, Kunhui, et al. (författare)
  • Inter-model spread in the wintertime Arctic amplification in the CMIP6 models and the important role of internal climate variability
  • 2021
  • Ingår i: Global and Planetary Change. - : Elsevier BV. - 0921-8181 .- 1872-6364. ; 204
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the characteristics of wintertime Arctic amplification (AA) and the inter-model spread in model output from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the Community Earth System Model Large Ensemble Project (CESM-LE). The CMIP6 models exhibit a large spread in AA intensity (as measured by an AA index, ranging from about −0.2 to about 1.1 °C/decade in the Historical simulations) and in its spatial/vertical structure (no AA to AA extending to upper-troposphere). A large model spread in Arctic sea-ice trends (from about −3.1 to about 0.3 %/decade) is also found. Intra-model spread in the ensemble simulations of six selected CMIP6 models, which we interpret as a measure of internal climate variability, is similarly large. We further find that CMIP6 model integrations where AA extends to the upper-troposphere, show weaker warming in the midlatitudes. The observed recent midlatitude cooling trends are generally not seen in the one-realization CMIP6 models but are seen in some ensemble members in the CMIP6 ensemble simulations; They are likely not a response to Arctic sea ice decline according to our analysis and are more likely caused by internal climate variability. By comparing the CMIP6 inter-model spread to the intra-model spread in the six selected CMIP6 ensembles and CESM-LE simulations, we argue that a large part of the CMIP6 inter-model spread in AA index, surface air temperature (SAT) trends and Arctic sea ice trends can be attributed to internal climate variability: about 67% for AA index, 58% for Arctic SAT and 22% for Arctic sea-ice. However, the inferred internal climate variability has been relatively stable in the recent several decades, while the inter-model spread in Arctic temperature trends and particularly Arctic sea-ice trends has been increasing in the CMIP6 models. This highlights the importance of constraining CMIP6 model discrepancies in simulating recent climate variations, which will potentially benefit the future multimodel climate projections.
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547.
  •  
548.
  • Zanatta, M., et al. (författare)
  • A European aerosol phenomenology-5 : Climatology of black carbon optical properties at 9 regional background sites across Europe
  • 2016
  • Ingår i: Atmospheric Environment. - : Elsevier BV. - 1352-2310 .- 1873-2844. ; 145, s. 346-364
  • Tidskriftsartikel (refereegranskat)abstract
    • A reliable assessment of the optical properties of atmospheric black carbon is of crucial importance for an accurate estimation of radiative forcing. In this study we investigated the spatio-temporal variability of the mass absorption cross-section (MAC) of atmospheric black carbon, defined as light absorption coefficient (sigma(ap)) divided by elemental carbon mass concentration (m(EC)). sigma(ap) and m(EC) have been monitored at supersites of the ACTRIS network for a minimum period of one year. The 9 rural background sites considered in this study cover southern Scandinavia, central Europe and the Mediterranean. sigma(ap) was determined using filter based absorption photometers and m(EC) using a thermal-optical technique. Homogeneity of the data-set was ensured by harmonization of all involved methods and instruments during extensive intercomparison exercises at the European Center for Aerosol Calibration (ECAC). Annual mean values of sigma(ap) at a wavelength of 637 nm vary between 0.66 and 1.3 Mm(-1) in southern Scandinavia, 3.7-11 Mm(-1) in Central Europe and the British Isles, and 2.3-2.8 Mm(-1) in the Mediterranean. Annual mean values of mEC vary between 0.084 and 0.23 mu g m(-3) in southern Scandinavia, 0.28 -1.1 in Central Europe and the British Isles, and 0.22-0.26 in the Mediterranean. Both sigma(ap) and mEC in southern Scandinavia and Central Europe have a distinct seasonality with maxima during the cold season and minima during summer, whereas at the Mediterranean sites an opposite trend was observed. Annual mean MAC values were quite similar across all sites and the seasonal variability was small at most sites. Consequently, a MAC value of 10.0 m(2) g(-1) (geometric standard deviation = 133) at a wavelength of 637 nm can be considered to be representative of the mixed boundary layer at European background sites, where BC is expected to be internally mixed to a large extent. The observed spatial variability is rather small compared to the variability of values in previous literature, indicating that the harmonization efforts resulted in substantially increased precision of the reported MAC. However, absolute uncertainties of the reported MAC values remain as high as +/- 30-70% due to the lack of appropriate reference methods and calibration materials. The mass ratio between elemental carbon and non-light-absorbing matter was used as a proxy for the thickness of coatings around the BC cores, in order to assess the influence of the mixing state on the MAC of BC. Indeed, the MAC was found to increase with increasing values of the coating thickness proxy. This provides evidence that coatings do increase the MAC of atmospheric BC to some extent, which is commonly referred to as lensing effect.
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549.
  • Önskog, Thomas, et al. (författare)
  • Predictability and non-Gaussian Characteristics of the North Atlantic Oscillation
  • 2018
  • Ingår i: Journal of Climate. - : American Meteorological Society. - 0894-8755 .- 1520-0442. ; 31, s. 537-554
  • Tidskriftsartikel (refereegranskat)abstract
    • The North Atlantic Oscillation (NAO) is the dominant mode of climate variability over the North Atlantic basin and has a significant impact on seasonal climate and surface weather conditions. It is the result of complex and nonlinear interactions between many spatiotemporal scales. Here, the authors study the statistical properties of two time series of the daily NAO index. Previous NAO modeling attempts only considered Gaussian noise, which can be inconsistent with the system complexity. Here, it is found that an autoregressive model with non-Gaussian noise provides a better fit to the time series. This result holds also when considering time series for the four seasons separately. The usefulness of the proposed model is evaluated by means of an investigation of its forecast skill.
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550.
  • Abbasi, Saeed, et al. (författare)
  • A pin-on-disc study of the rate of airborne wear particle emissions from railway braking materials
  • 2012
  • Ingår i: Wear. - UK : Elsevier. - 0043-1648 .- 1873-2577. ; 284, s. 18-29
  • Tidskriftsartikel (refereegranskat)abstract
    • The current study investigates the characteristics of particles generated from the wear of braking materials, and provides an applicable index for measuring and comparing wear particle emissions. A pin-on-disc tribometer equipped with particle measurement instruments was used. The number concentration, size, morphology, and mass concentration of generated particles were investigated and reported for particles 10 nm-32 mu m in diameter. The particles were also collected on filters and investigated using EDS and SEM. The effects of wear mechanisms on particle morphology and changes in particle concentration are discussed. A new index, the airborne wear particle emission rate (AWPER), is suggested that could be used in legislation to control non-exhaust emissions from transport modes, particularly rail transport.
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