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  • 1. Agarwal, Sahil
    et al.
    Wettlaufer, John S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Yale University, United States; University of Oxford, United Kingdom.
    The Statistical Properties of Sea Ice Velocity Fields2017Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, nr 13, s. 4873-4881Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    By arguing that the surface pressure field over the Arctic Ocean can be treated as an isotropic, stationary, homogeneous, Gaussian random field, Thorndike estimated a number of covariance functions from two years of data (1979 and 1980). Given the active interest in changes of general circulation quantities and indices in the polar regions during the recent few decades, the spatial correlations in sea ice velocity fields are of particular interest. It is thus natural to ask, "How persistent are these correlations?'' To this end, a multifractal stochastic treatment is developed to analyze observed Arctic sea ice velocity fields from satellites and buoys for the period 1978-2015. Since it was previously found that the Arctic equivalent ice extent (EIE) has a white noise structure on annual to biannual time scales, the connection between EIE and ice motion is assessed. The long-term stationarity of the spatial correlation structure of the velocity fields and the robustness of their white noise structure on multiple time scales is demonstrated; these factors (i) combine to explain the white noise characteristics of the EIE on annual to biannual time scales and (ii) explain why the fluctuations in the ice velocity are proportional to fluctuations in the geostrophic winds on time scales of days to months. Moreover, it is shown that the statistical structure of these two quantities is commensurate from days to years, which may be related to the increasing prevalence of free drift in the ice pack.

  • 2. Andre, M.
    et al.
    Behlke, R.
    Wahlund, J. E.
    Vaivads, A.
    Eriksson, A. I.
    Tjulin, A.
    Carozzi, T. D.
    Cully, C.
    Gustafsson, G.
    Sundkvist, D.
    Khotyaintsev, Y.
    Cornilleau-Wehrlin, N.
    Rezeau, L.
    Maksimovic, M.
    Lucek, E.
    Balogh, A.
    Dunlop, M.
    Lindqvist, Per-Arne
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Mozer, F.
    Pedersen, A.
    Fazakerley, A.
    Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause2001Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, nr 12-okt, s. 1471-1481Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.

  • 3. Arnqvist, J.
    et al.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Dellwik, E.
    Bergström, H.
    Wind Statistics from a Forested Landscape2015Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, nr 1, s. 53-71Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40 wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.

  • 4.
    Augier, Pierre
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    A New Formulation of the Spectral Energy Budget of the Atmosphere, with Application to Two High-Resolution General Circulation Models2013Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 70, nr 7, s. 2293-2308Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new formulation of the spectral energy budget of kinetic and available potential energies of the atmosphere is derived, with spherical harmonics as base functions. Compared to previous formulations, there are three main improvements: (i) the topography is taken into account, (ii) the exact three-dimensional advection terms are considered, and (iii) the vertical flux is separated from the energy transfer between different spherical harmonics. Using this formulation, results from two different high-resolution GCMs are analyzed: the Atmospheric GCM for the Earth Simulator (AFES) T639L24 and the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) T1279L91. The spectral fluxes show that the AFES, which reproduces quite realistic horizontal spectra with a k(-5/3) inertial range at the mesoscales, simulates a strong downscale energy cascade. In contrast, neither the k(-5/3) vertically integrated spectra nor the downscale energy cascade are produced by the ECMWF IFS.

  • 5.
    Becerra, Marley
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion. ABB Corporate Research, Power Technologies, Electrical Apparatus and Diagnostics group, Sweden .
    Corona discharges and their effect on lightning attachment revisited: Upward leader initiation and downward leader interception2014Inngår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 149, s. 316-323Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Previous studies have suggested the possibility of using glow corona discharges to control the frequency of lightning flashes to grounded objects. In order to revisit the theoretical basis of this proposal, the self-consistent leader inception and propagation model - SLIM - is used together with a two-dimensional glow corona drift model. The analysis is performed to quantify the effect of glow corona generated at the tip of ground-based objects on the initiation and propagation of upward positive connecting leaders under the influence of downward lightning leaders. It is found that the presence of glow corona does not influence the performance of Franklin lightning rods shorter than 15 m, while it slightly reduces the lateral distance of rods up to 60 m tall by a maximum of 10%. Furthermore, the results indicate that it is not possible to suppress the initiation of upward connecting leaders by means of glow corona. It is found instead that unconventional lightning protection systems based on the generation of glow corona attract downward lightning flashes in a similar way as a standard lightning rod with the same height.

  • 6.
    Boman, Eva
    KTH, Tidigare Institutioner, Mark- och vattenteknik.
    The effects of noise and gender on children's episodic and semantic memory2004Inngår i: Scandinavian Journal of Psychology, ISSN 0036-5564, E-ISSN 1467-9450, Vol. 45, nr 5, s. 407-416Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The main objectives in the present study were to examine meaningful irrelevant speech and road traffic noise effects on episodic and semantic memory, and to evaluate whether gender differences in memory performance interact with noise. A total of 96 subjects, aged 13-14 years (n= 16 boys and 16 girls in each of three groups), were randomly assigned to a silent or two noise conditions. Noise effects found were restricted to impairments from meaningful irrelevant speech on recognition and cued recall of a text in episodic memory and of word comprehension in semantic memory. The obtained noise effect suggests that the meaning of the speech were processed semantically by the pupils, which reduced their ability to comprehend a text that also involved processing of meaning. Meaningful irrelevant speech was also assumed to cause a poorer access to the knowledge base in semantic memory. Girls outperformed boys in episodic and semantic memory materials, but these differences did not interact with noise.

  • 7.
    Borgemo, Joakim
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Lärande.
    Roth, Richard
    KTH, Skolan för industriell teknik och management (ITM), Lärande.
    Gymnasiefysikens meteorologiinnehåll: En metodkombinationsstudie om hur klimat och väder implementerats i fysikundervisningen2018Independent thesis Advanced level (professional degree), 20 poäng / 30 hpOppgave
    Abstract [sv]

    Studien undersökte implementeringen och undervisningen av klimat och väder som år 2011 lades till under Fysik 1 i den svenska gymnasieskolan. Undersökningen använde sig av metodkombination som omfattade lärarintervjuer kompletterade med intervju av ett undervisningsråd på Skolverket och med textanalys av kursböcker. Lärarna upplevde en del problem kopplat till vad det nya ämnesområdet skulle innehålla. Oklarheten kring vad området skulle innehålla märktes också i kursböckernas framställning. Intervjupersonerna var överens om att viktiga aspekter inom ämnesområdet var växthuseffekten och global uppvärmning. Växthuseffekten och global uppvärmning hade även en framträdande roll i kursböckernas framställning. Från lärarintervjuerna framkom att lärarna gärna inkluderade Samhällsfrågor med Naturvetenskapligt Innehåll (SNI) i sin undervisning, men i bedömningen av elevernas lärande var det fysikalisk begreppsförståelse som accentuerades. Ämnesöverskridande samarbeten beskrevs av lärarna som önskvärt och förekom ofta.

  • 8.
    Bramsäter, Robin
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Topography and morphology analysis of marine nanoparticles and a pedagogical study of representations used for improving a high school experiment2012Independent thesis Advanced level (professional degree), 20 poäng / 30 hpOppgave
    Abstract [en]

    The Arctic Summer Cloud Ocean Study expedition took place during the autumn of 2008 and brought back water and air samples. One theory was that marine particles were shot into the atmosphere by bubble bursting and, while in the atmosphere, acted as cloud condensation nuclei. Particles collected from the subsurface water, surface microlayer and just above the surface had their topography and morphology analyzed using a scanning electron microscope. Due to a lack of EDS analysis it's impossible to say for sure if the particles found were the same found in previous studies, just that it is highly likely that they are. No evidence against the marine particles being able to act as cloud condensation nuclei was found but the data gathered was not sufficient to strengthen the theory either.

    The scanning electron microscope was also used in a pedagogical study, analyzing how operators with different knowledge and prior experience interact with the microscope's images. These results as well as knowledge gained from literature studies were used to improve a high school experiment regarding centripetal acceleration. The main issue with the experiment was that what the students learned performing the experiment was not the same as the theoretical models the teachers wished the students to learn. The reason for this was because the experimental equipment lacked the centripetal model's external representations such as force arrows. If a simulator would be incorporated into the lab centripetal acceleration representations could be visualized and a clearer connection between experiment and theory could be made.

  • 9.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    STELLAR MIXING LENGTH THEORY with ENTROPY RAIN2016Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 832, nr 1, artikkel-id 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effects of a non-gradient flux term originating from the motion of convective elements with entropy perturbations of either sign are investigated and incorporated into a modified version of stellar mixing length theory (MLT). Such a term, first studied by Deardorff in the meteorological context, might represent the effects of cold intense downdrafts caused by the rapid cooling in the granulation layer at the top of the convection zone of late-type stars. These intense downdrafts were first seen in the strongly stratified simulations of Stein & Nordlund in the late 1980s. These downdrafts transport heat nonlocally, a phenomenon referred to as entropy rain. Moreover, the Deardorff term can cause upward enthalpy transport even in a weakly Schwarzschild-stably stratified layer. In that case, no giant cell convection would be excited. This is interesting in view of recent observations, which could be explained if the dominant flow structures were of small scale even at larger depths. To study this possibility, three distinct flow structures are examined: one in which convective structures have similar size and mutual separation at all depths, one in which the separation increases with depth, but their size is still unchanged, and one in which both size and separation increase with depth, which is the standard flow structure. It is concluded that the third possibility with fewer and thicker downdrafts in deeper layers remains the most plausible, but it may be unable to explain the suspected absence of large-scale flows with speeds and scales expected from MLT.

  • 10.
    Campagne, Antoine
    et al.
    LEGI, Université Grenoble Alpes.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Chassagne, Rémi
    LEGI, Université Grenoble Alpes.
    Micard, Diane
    LMFA, École Centrale de Lyon.
    Mordant, Nicolas
    LEGI, Université Grenoble Alpes.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Sommeria, Joel
    LEGI, Université Grenoble Alpes.
    Viboud, Samuel
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Augier, Pierre
    LEGI, Université Grenoble Alpes.
    First report of the MILESTONE experiment: strongly stratified turbulence and mixing efficiency in the Coriolis platform2016Inngår i: VIIIth International Symposium on Stratified Flows (ISSF), 2016, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Strongly stratified turbulence is a possible interpretation of oceanic and atmospheric mea-surements. However, this regime has never been produced in a laboratory experiment be-cause of the two conditions of very small horizontal Froude number Fh and large buoyancy Reynolds number R which require a verily large experimental facility. We present a new attempt to study strongly stratified turbulence experimentally in the Coriolis platform.The flow is forced by a slow periodic movement of an array of six vertical cylinders of 25 cm diameter with a mesh of 75 cm. Five cameras are used for 3D-2C scanned horizontalparticles image velocimetry (PIV) and stereo 2D vertical PIV. Five density-temperatureprobes are used to measure vertical and horizontal profiles and signals at fixed positions.The first preliminary results indicate that we manage to produce strongly stratified tur-bulence at very small Fh and large R in a laboratory experiment.

  • 11. Carlsen, Henrik
    et al.
    Klein, Richard J. T.
    Wikman-Svahn, Per
    Filosofi och historia, KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Filosofi och teknikhistoria, Filosofi.
    Transparent scenario development2017Inngår i: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 7, nr 9, s. 613-613Artikkel i tidsskrift (Fagfellevurdert)
  • 12. Chiacchio, Marc
    et al.
    Pausata, Fransesco S. R.
    Messori, Gabriele
    Hannachi, Abdel
    Chin, Mian
    Önskog, Thomas
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.), Matematisk statistik.
    Ekman, Annica M. L.
    Barrie, Leonard
    On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins2017Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 122, s. 802-822Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.

  • 13. Dangendorf, Soenke
    et al.
    Müller-Navarra, Sylvin
    Jensen, Juergen
    Schenk, Frederik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wahl, Thomas
    Weisse, Ralf
    North Sea Storminess from a Novel Storm Surge Record since AD 18432014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 10, s. 3582-3595Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The detection of potential long-term changes in historical storm statistics and storm surges plays a vitally important role for protecting coastal communities. In the absence of long homogeneous wind records, the authors present a novel, independent, and homogeneous storm surge record based on water level observations in the North Sea since 1843. Storm surges are characterized by considerable interannual-to-decadal variability linked to large-scale atmospheric circulation patterns. Time periods of increased storm surge levels prevailed in the late nineteenth and twentieth centuries without any evidence for significant long-term trends. This contradicts with recent findings based on reanalysis data, which suggest increasing storminess in the region since the late nineteenth century. The authors compare the wind and pressure fields from the Twentieth-Century Reanalysis (20CRv2) with the storm surge record by applying state-of-the-art empirical wind surge formulas. The comparison reveals that the reanalysis is a valuable tool that leads to good results over the past 100 yr; previously the statistical relationship fails, leaving significantly lower values in the upper percentiles of the predicted surge time series. These low values lead to significant upward trends over the entire investigation period, which are in turn supported by neither the storm surge record nor an independent circulation index based on homogeneous pressure readings. The authors therefore suggest that these differences are related to higher uncertainties in the earlier years of the 20CRv2 over the North Sea region.

  • 14. Decremer, Damien
    et al.
    Chung, Chul E.
    Ekman, Annica M. L.
    Brandefelt, Jenny
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Which significance test performs the best in climate simulations?2014Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, nr 1, s. 23139-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student's t-test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student's t-test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375-1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> +/- 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student's t-test by the advanced techniques in most cases.

  • 15.
    Deusebio, Enrico
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical studies in rotating and stratified turbulence2013Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides measurements and experiments, to give some insights into turbulent dynamics. In this thesis, numerical simulations are employed to study homogeneous and wall-bounded turbulence in rotating and stably stratified conditions, as encountered in geophysical flows where the rotation of the Earth as well as the vertical density variation influence the dynamics.

    In the context of homogeneous turbulence, we investigate how the transfer of energy among scales is affected by the presence of strong but finite rotation and stratification. Unlike geostrophic turbulence, we show that there is a forward energy cascade towards small scales which is initiated at the forcing scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where it leads to a shallowing of the energy spectrum, from k-3 to k-5/3. Two-point statistics show a good agreement with measurements in the atmosphere, suggesting that this process is an important mechanism for energy transfer in the atmosphere.

    Boundary layers subjected to system rotation around the wall-normal axis are usually referred to as Ekman layers and they can be seen as a model of the atmospheric and oceanic boundary layers developing at mid and high latitudes. We study the turbulent dynamics in Ekman layers by means of numerical simulations, focusing on the turbulent structures developing at moderately high Reynolds numbers. For neutrally stratified conditions, we show that there exists a turbulent helicity cascade in the logarithmic region. We focus on the effect of a stable stratification produced by a vertical positive temperature gradient. For moderate stratification, continuously turbulent regimes are produced which are in fair agreement with existing theories and models used in the context of atmospheric boundary layer dynamics. For larger degree of stratification, we show that laminar and turbulent motions coexist and displace along inclined patterns similar to what has been recently observed in other transitional flows.

  • 16.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Helicity in the Ekman boundary layer2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 654-671Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Helicity, which is defined as the scalar product of velocity and vorticity, H = u . omega, is an inviscidly conserved quantity in a barotropic fluid. Mean helicity is zero in flows that are parity invariant. System rotation breaks parity invariance and has therefore the potential of giving rise to non-zero mean helicity. In this paper we study the helicity dynamics in the incompressible Ekman boundary layer. Evolution equations for the mean field helicity and the mean turbulent helicity are derived and it is shown that pressure flux injects helicity at a rate 2 Omega G(2) over the total depth of the Ekman layer, where G is the geostrophic wind far from the wall and Omega = Omega e(y) is the rotation vector and e(y) is the wall-normal unit vector. Thus right-handed/left-handed helicity will be injected if Omega is positive/negative. We also show that in the uppermost part of the boundary layer there is a net helicity injection with opposite sign as compared with the totally integrated injection. Isotropic relations for the helicity dissipation and the helicity spectrum are derived and it is shown that it is sufficient to measure two transverse velocity components and use Taylor's hypothesis in the mean flow direction in order to measure the isotropic helicity spectrum. We compare the theoretical predictions with a direct numerical simulation of an Ekman boundary layer and confirm that there is a preference for right-handed helicity in the lower part of the Ekman layer and left-handed helicity in the uppermost part when Omega > 0. In the logarithmic range, the helicity dissipation conforms to isotropic relations. On the other hand, spectra show significant departures from isotropic conditions, suggesting that the Reynolds number considered in the study is not sufficiently large for isotropy to be valid in a wide range of scales. Our analytical and numerical results strongly suggest that there is a turbulent helicity cascade of right-handed helicity in the logarithmic range of the atmospheric boundary layer when Omega > 0, consistent with recent measurements by Koprov, Koprov, Ponomarev & Chkhetiani (Dokl. Phys., vol. 50, 2005, pp. 419-422). The isotropic relations which are derived may facilitate future measurements of the helicity spectrum in the atmospheric boundary layer as well as in controlled wind tunnel experiments.

  • 17.
    Ezhova, Ekaterina
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. University of Helsinki, Finland.
    Cenedese, C.
    Brandt, L.uca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Dynamics of a turbulent Buoyant Plume in a stratified fluid: An idealized model of subglacial discharge in Greenland Fjords2017Inngår i: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 47, nr 10, s. 2611-2630Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study reports the results of large-eddy simulations of an axisymmetric turbulent buoyant plume in a stratified fluid. The configuration used is an idealized model of the plume generated by a subglacial discharge at the base of a tidewater glacier with an ambient stratification typical of Greenland fjords. The plume is discharged from a round source of various diameters and characteristic stratifications for summer and winter are considered. The classical theory for the integral parameters of a turbulent plume in a homogeneous fluid gives accurate predictions in the weakly stratified lower layer up to the pycnocline, and the plume dynamics are not sensitive to changes in the source diameter. In winter, when the stratification is similar to an idealized two-layer case, turbulent entrainment and generation of internal waves by the plume top are in agreement with the theoretical and numerical results obtained for turbulent jets in a two-layer stratification. In summer, instead, the stratification is more complex and turbulent entrainment by the plume top is significantly reduced. The subsurface layer in summer is characterized by a strong density gradient and the oscillating plume generates internal waves that might serve as an indicator of submerged plumes not penetrating to the surface.

  • 18. Feser, F.
    et al.
    Barcikowska, M.
    Krueger, O.
    Schenk, Frederik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Weisse, R.
    Xia, L.
    Storminess over the North Atlantic and northwestern Europe: A review2015Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, nr 687, s. 350-382Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    This review assesses storm studies over the North Atlantic and northwestern Europe regarding the occurrence of potential long-term trends. Based on a systematic review of available articles, trends are classified according to different geographical regions, datasets, and time periods. Articles that used measurement and proxy data, reanalyses, regional and global climate model data on past and future trends are evaluated for changes in storm climate. The most important result is that trends in storm activity depend critically on the time period analysed. An increase in storm numbers is evident for the reanalyses period for the most recent decades, whereas most long-term studies show merely decadal variability for the last 100-150 years. Storm trends derived from reanalyses data and climate model data for the past are mostly limited to the last four to six decades. The majority of these studies find increasing storm activity north of about 55-60° N over the North Atlantic with a negative tendency southward. This increase from about the 1970s until the mid-1990s is also mirrored by long-term proxies and the North Atlantic Oscillation and constitutes a part of their decadal variability. Studies based on proxy and measurement data or model studies over the North Atlantic for the past which cover more than 100 years show large decadal variations and either no trend or a decrease in storm numbers. Future scenarios until about the year 2100 indicate mostly an increase in winter storm intensity over the North Atlantic and western Europe. However, future trends in total storm numbers are quite heterogeneous and depend on the model generation used.

  • 19.
    Flache, Denis
    et al.
    Federal Armed Forces Research University, Munich, Germany.
    Rakov, Vladimir
    University of Florida, Gainesville, USA.
    Heidler, Fridolin
    Federal Armed Forces Research University, Munich, Germany.
    Zischank, Wolfgang
    Federal Armed Foprces Research University, Munich, Germany.
    Thottappillil, Rajeev
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Leader/Return Stroke versus M-Component Mode of Charge Transfer to Ground in Initial-Stage Pulses of Upward Lightning2009Konferansepaper (Annet vitenskapelig)
  • 20.
    Gustafsson, David
    et al.
    KTH, Tidigare Institutioner, Mark- och vattenteknik.
    Lewan, E.
    Jansson, Per-Erik
    KTH, Tidigare Institutioner, Mark- och vattenteknik.
    Modeling water and heat balance of the boreal landscape - comparison of forest and arable land in Scandinavia2004Inngår i: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 43, nr 11, s. 1750-1767Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The water and heat balances of an arable field and a forest in the boreal zone in Scandinavia were explored using 3 yr of observations and simulations with two different soil - vegetation - atmosphere transfer (SVAT) models over a 30-yr period. Results from a detailed mechanistic model [ coupled heat and mass transfer model ( COUP)] were compared with those obtained with a large-scale type of SVAT model used in the weather prediction model at the European Centre for Medium-Range Weather Forecasts [ECMWF tiled land surface scheme (TESSEL)]. The COUP model simulations agreed well with the observations from a seasonal perspective. The TESSEL model differed significantly from the measurements when standard operational parameter values were used. The introduction of a seasonal variation in leaf-area index values, tuned canopy resistance for forest, and a reduced roughness length over snow-covered open land reduced the discrepancies. Net radiation was 40% higher in the forest when compared with the arable land, based on 30-yr simulations with both models. Furthermore, the forest was a net source of sensible heat flux, whereas the arable land was a net sink. Because of different treatment of winter interception evaporation, forest latent heat flux based on the COUP model considerably exceeded that from the TESSEL model, and suggested that the total annual evaporation was higher from the forest than from arable land. The representation of interception evaporation in winter, as well as seasonal dynamics in vegetation properties are, thus, of considerable importance for adequate simulation of forest and arable land energy fluxes within the boreal zone.

  • 21.
    Hartung, Kerstin
    et al.
    Stockholm Univ, Dept Meteorol, Stockholm, Sweden.;Stockholm Univ, Bolin Ctr Climate Res, Stockholm, Sweden.;Swedish E Sci Res Ctr, Stockholm, Sweden..
    Svensson, Gunilla
    Stockholm Univ, Dept Meteorol, Stockholm, Sweden.;Stockholm Univ, Bolin Ctr Climate Res, Stockholm, Sweden.;Swedish E Sci Res Ctr, Stockholm, Sweden..
    Kjellstrom, Erik
    Stockholm Univ, Dept Meteorol, Stockholm, Sweden.;Stockholm Univ, Bolin Ctr Climate Res, Stockholm, Sweden.;Swedish Meteorol & Hydrol Inst, Rossby Ctr, Norrkoping, Sweden..
    Resolution, physics and atmosphere-ocean interaction - How do they influence climate model representation of Euro-Atlantic atmospheric blocking?2017Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, artikkel-id 1406252Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric blocking events are known to locally explain a large part of climate variability. However, despite their relevance, many current climate models still struggle to represent the observed blocking statistics. In this study, simulations of the global climate model EC-Earth are analysed with respect to atmospheric blocking. Seventeen simulations map the uncertainty space defined by the three-model characteristics: atmospheric resolution, physical parameterization and complexity of atmosphere-ocean interaction, namely an atmosphere coupled to an ocean model or forced by surface data. Representation of the real-world statistics is obtained from reanalyses ERA-20C, JRA-55 and ERA-Interim which agree on Northern Hemisphere blocking characteristics. Blocking events are detected on a central blocking latitude which is individually determined for each simulation. The frequency of blocking events tends to be underestimated relative to ERA-Interim over the Atlantic and western Eurasia in winter and overestimated during spring months. However, only few model setups show statistically significant differences compared to ERA-Interim which can be explained by the large inter-annual variability of blocking. Results indicate slightly larger biases relative to ERA-Interim in coupled than in atmosphere-only models but differences between the two are not statistically significant. Although some resolution dependence is present in spring, the signal is weak and only statistically significant if the physical parameterizations of the model are improved simultaneously. Winter blocking is relatively more sensitive to physical parameterizations, and this signal is robust in both atmosphere-only and coupled simulations, although stronger in the latter. Overall, the model can capture blocking frequency well despite biases in representing the mean state of geopotential height over this area. Blocking signatures of geopotential height are represented more similar to ERA-Interim and only weak sensitivities to model characteristics remain.

  • 22. Hede, T.
    et al.
    Li, Xin
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Leck, C.
    Tu, Yaoquan
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Model HULIS compounds in nanoaerosol clusters: investigations of surface tension and aggregate formation using molecular dynamics simulations2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 13, s. 6549-6557Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cloud condensation nuclei act as cores for water vapour condensation, and their composition and chemical properties may enhance or depress the ability for droplet growth. In this study we use molecular dynamics simulations to show that model humic-like substances (HULIS) in systems containing 10 000 water molecules mimic experimental data well referring to reduction of surface tension. The model HULIS compounds investigated in this study are cis-pinonic acid (CPA), pinic acid (PAD) and pinonaldehyde (PAL). The structural properties examined show the ability for the model HULIS compounds to aggregate inside the nanoaerosol clusters.

  • 23. Hede, Thomas
    et al.
    Leck, Caroline
    Sun, Lu
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Tu, Yaoquan
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    A theoretical study revealing the promotion of light-absorbing carbon particles solubilization by natural surfactants in nanosized water droplets2013Inngår i: Atmospheric Science Letters, ISSN 1530-261X, E-ISSN 1530-261X, Vol. 14, nr 2, s. 86-90Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many identified effects of atmospheric aerosol particles on climate come from pollutants. The effects of light-absorbing carbon particles (soot) are amongst the most uncertain and they are also considered to cause climate warming on the same order of magnitude as anthropogenic carbon dioxide. This study contributes to the understanding of the potential for transformation of the surface character of soot from hydrophobic to hydrophilic, which in clouds promotes a build-up of water-soluble material. We use molecular dynamics simulations to show how natural surfactants facilitate solubilization of fluoranthene, which we use as a model compound for soot in nanoaerosol water clusters.

  • 24.
    Howells, Mark
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Energisystemanalys.
    Hermann, Sebastian
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Energisystemanalys.
    Welsch, Manuel
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Energisystemanalys.
    Bazilian, Morgan
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Energisystemanalys.
    Segerström, Rebecka
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Energisystemanalys.
    Alfstad, Thomas
    Gielen, Dolf
    Rogner, Holger
    Fischer, Guenther
    van Velthuizen, Harrij
    Wiberg, David
    Young, Charles
    Roehrl, R. Alexander
    Mueller, Alexander
    Steduto, Pasquale
    Ramma, Indoomatee
    Integrated analysis of climate change, land-use, energy and water strategies2013Inngår i: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 3, nr 7, s. 621-626Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Land, energy and water are our most precious resources, but the manner and extent to which they are exploited contributes to climate change. Meanwhile, the systems that provide these resources are themselves highly vulnerable to changes in climate. Efficient resource management is therefore of great importance, both for mitigation and for adaptation purposes. We postulate that the lack of integration in resource assessments and policy-making leads to inconsistent strategies and inefficient use of resources. We present CLEWs (climate, land-use, energy and water strategies), a new paradigm for resource assessments that we believe can help to remedy some of these shortcomings.

  • 25.
    Ivchenko, Nickolay V.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Facciolo, Luca
    KTH, Tidigare Institutioner, Alfvénlaboratoriet. Politecnico di Torino, Italy.
    Lindqvist, Per-Arne
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Kekkonen, P.
    Holback, B.
    Disturbance of plasma environment in the vicinity of the Astrid-2 microsatellite2001Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, nr 6, s. 655-666Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The presence of a satellite disturbs the ambient plasma. The charging of the spacecraft creates a sheath around it, and the motion of the satellite creates a wake disturbance. This modification of the plasma environment introduces difficulties in measuring electric fields and plasma densities using the probe technique. We present a study of the structure of the sheath and wake around the Astrid-2 microsatellite, as observed by the probes of the EMMA and LINDA instruments, Measurements with biased LINDA probes, as well as current sweeps on the EMMA probes, show a density enhancement upstream of the satellite and a plasma depletion behind the satellite. The electric field probes detect disturbances in the plasma potential on magnetic field lines connected to the satellite.

  • 26.
    Jansson, Christer
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    The urban-park climate system: Testing the dynamic behaviour of a small-scale numerical modelManuskript (Annet vitenskapelig)
  • 27.
    Jansson, Christer
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Jansson, Per-Erik
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Gustafsson, David
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Near surface climate in an urban vegetated park and its surroundings2007Inngår i: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 89, nr 34, s. 185-193Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Near surface climate was observed through temperature profiling from the surface to 2.47 m height in an urban vegetated park and its surroundings in central Stockholm, Sweden. Measurements were conducted during three summer days by mobile traverses. Air temperature differences between the built-up area and the park were in the range of 0.5-0.8°C during the day and reached a maximum of 2°C at sunset. The thermal stratification of the air was mainly stable in the park and unstable in the built-up area. Inverse air temperature profiles in the park were less stable in open than in shady areas, and close to neutral at midday. The most unstable air was found in the north-south orientated canyons in the early afternoon. Possible heat advection from the surroundings, and thus uncoupling between the surface and the air, was identified through temperature gradients pointing at different directions within the 2.47 m profile. Examples at midday indicated that warm air advected as far as 150 m into the park.

  • 28.
    Jourdier, Bénédicte
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Kraft- och värmeteknologi.
    Study and implementation of mesoscale weather forecasting models in the wind industry2012Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    As the wind industry is developing, it is asking for more reliable short-term wind forecasts to better manage the wind farms’ operations and electricity production. Developing new wind farms also requires correct assessments of the long-term wind potentials to decide whether to install a wind farm at a specific location. This thesis is studying a new generation of numerical weather forecasting models, named mesoscale models, to see how they could answer those needs. It is held at the company Maïa Eolis which operates several wind farms in France. A mesoscale model, the Weather Research and Forecasting model (WRF), was chosen and used to generate high resolution forecasts based on lower resolution forecasts from NCEP’s Global Forecasting System.

    The stages for implementation of daily forecasts for the company’s wind farms were: explore and configure the model, automate the runs, develop post-processing tools and forecasts visualization software which was intended to be used by the management team. WRF was also used to downscale wind archives of NCEP’s Final Analysis and determine the possibility to use these in assessing wind potentials. Finally the precision of the model in both cases and for each wind farm was assessed by comparing attained data from the model with real power production.

  • 29. Khotyaintsev, Y. V.
    et al.
    Lindqvist, Per-Arne
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Cully, C. M.
    Eriksson, A. I.
    Andre, M.
    In-flight calibration of double-probe DC electric field measurements on Cluster2014Inngår i: Geoscientific Instrumentation, Methods and Data Systems, ISSN 2193-0856, E-ISSN 2193-0864, Vol. 4, nr 1, s. 85-107Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Double-probe electric field instrument with long wire booms is one of the most popular techniques for in situ measurement of DC and AC electric fields in plasmas on spinning spacecraft platforms, which have been employed on a large number of space missions. Here we present an overview of the calibration procedure used for the EFW instrument on Cluster, which involves spin fits of the data and correction of several offsets. We also describe the procedure for the offset determination and present results for the long-term evolution of the offsets.

  • 30. Lapenta, Giovanni
    et al.
    Pierrard, Viviane
    Keppens, Rony
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Poedts, Stefaan
    Sebek, Ondrej
    Travnicek, Pavel M.
    Henri, Pierre
    Califano, Francesco
    Pegoraro, Francesco
    Faganello, Matteo
    Olshevsky, Vyacheslav
    Restante, Anna Lisa
    Nordlund, Åke
    Frederiksen, Jacob Trier
    Mackay, Duncan H.
    Parnell, Clare E.
    Bemporad, Alessandro
    Susino, Roberto
    Borremans, Kris
    SWIFF: Space weather integrated forecasting framework2013Inngår i: Journal of Space Weather and Space Climate, ISSN 2115-7251, E-ISSN 2115-7251, Vol. 3, s. A05-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    SWIFF is a project funded by the Seventh Framework Programme of the European Commission to study the mathematical-physics models that form the basis for space weather forecasting. The phenomena of space weather span a tremendous scale of densities and temperature with scales ranging 10 orders of magnitude in space and time. Additionally even in local regions there are concurrent processes developing at the electron, ion and global scales strongly interacting with each other. The fundamental challenge in modelling space weather is the need to address multiple physics and multiple scales. Here we present our approach to take existing expertise in fluid and kinetic models to produce an integrated mathematical approach and software infrastructure that allows fluid and kinetic processes to be modelled together. SWIFF aims also at using this new infrastructure to model specific coupled processes at the Solar Corona, in the interplanetary space and in the interaction at the Earth magnetosphere.

  • 31.
    Lazeroms, Werner
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulence modelling applied to the atmospheric boundary layer2015Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required. 

  • 32.
    Lazeroms, Werner M. J.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Svensson, G.
    Bazile, E.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models2016Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 161, nr 1, s. 19-47Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We test a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the atmospheric boundary layer. First of all, we consider a stable boundary layer used as the well-known first test case from the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS1). The model is shown to agree well with data from large-eddy simulations (LES), and this agreement is significantly better than for a standard operational scheme with a prognostic equation for turbulent kinetic energy. Furthermore, we apply the model to a case with a (idealized) diurnal cycle and make a qualitative comparison with a simpler first-order model. Some interesting features of the model are highlighted, pertaining to its stronger foundation on physical principles. In particular, the use of more prognostic equations in the model is shown to give a more realistic dynamical behaviour. This qualitative study is the first step towards a more detailed comparison, for which additional LES data are needed.

  • 33.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Svensson, Gunilla
    Stockholm University, Department of Meteorology.
    Bazile, Eric
    Météo-France, CNRM-GAME.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Study of transitions in the atmospheric boundary layer using explicit algebraic turbulence models.Manuskript (preprint) (Annet vitenskapelig)
  • 34.
    Li, Xiang-Yung
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Svensson, G.
    Haugen, N. E. L.
    Mehlig, B.
    Rogachevskii, lgor
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Effect of turbulence on collisional growth of cloud droplets2018Inngår i: Journal of the Atmospheric Sciences, Vol. 75, nr 10, s. 3469-3487Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Weinvestigate the effect of turbulence on the collisional growth of micrometer-sized droplets through highresolution numerical simulations with well-resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate ε, but only weakly on the Reynolds number. This can be explained through the « dependence of the mean collision rate described by the Saffman-Turner collision model. Consistent with the Saffman-Turner collision model and its extensions, the collision rate increases as ε1/2 even when coalescence is invoked. The size distribution exhibits power-law behavior with a slope of 23.7 from a maximum at approximately 10 up to about 40 mm. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is because turbulence enhances the collisional growth between similar-sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth. 

  • 35.
    Li, Xin
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Hede, T.
    Tu, Y.
    Leck, C.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi (stängd 20110512).
    Glycine in aerosol water droplets: a critical assessment of Köhler theory by predicting surface tension from molecular dynamics simulations2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 2, s. 519-527Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aerosol particles in the atmosphere are important participants in the formation of cloud droplets and have significant impact on cloud albedo and global climate. According to the Kohler theory which describes the nucleation and the equilibrium growth of cloud droplets, the surface tension of an aerosol droplet is one of the most important factors that determine the critical supersaturation of droplet activation. In this paper, with specific interest to remote marine aerosol, we predict the surface tension of aerosol droplets by performing molecular dynamics simulations on two model systems, the pure water droplets and glycine in water droplets. The curvature dependence of the surface tension is interpolated by a quadratic polynomial over the nano-sized droplets and the limiting case of a planar interface, so that the so-called Aitken mode particles which are critical for droplet formation could be covered and the Kohler equation could be improved by incorporating surface tension corrections.

  • 36.
    Li, Xin
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Hede, Thomas
    Tu, Yaoquan
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Leck, Caroline
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Cloud droplet activation mechanisms of amino acid aerosol particles: insight from molecular dynamics simulations2013Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 65, s. 65-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric amino acids constitute a large fraction of water-soluble organic nitrogen compounds in aerosol particles, and have been confirmed as effective cloud condensation nuclei (CCN) materials in laboratory experiments. We present a molecular dynamics (MD) study of six amino acids with different structures and chemical properties that are relevant to the remote marine atmospheric aerosol-cloud system, with the aim of investigating the detailed mechanism of their induced changes in surface activity and surface tension, which are important properties for cloud drop activation. Distributions and orientations of the amino acid molecules are studied; these L-amino acids are serine (SER), glycine (GLY), alanine (ALA), valine (VAL), methionine (MET) and phenylalanine (PHE) and are categorised as hydrophilic and amphiphilic according to their affinities to water. The results suggest that the presence of surface-concentrated amphiphilic amino acid molecules give rise to enhanced Lennard-Jones repulsion, which in turn results in decreased surface tension of a planar interface and an increased surface tension of the spherical interface of droplets with diameters below 10 nm. The observed surface tension perturbation for the different amino acids under study not only serves as benchmark for future studies of more complex systems, but also shows that amphiphilic amino acids are surface active. The MD simulations used in this study reproduce experimental results of surface tension measurements for planar interfaces and the method is therefore applicable for spherical interfaces of nano-size for which experimental measurements are not possible to conduct.

  • 37. Lin, C.
    et al.
    Kao, M. -J
    Yang, James
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Resurser, energi och infrastruktur.
    Teng, P. -H
    Raikar, R. V.
    Study on probabilistic mean features of lower and upper free-surface profiles and velocity fields of a highly fluctuating free jet over a chute2018Inngår i: Journal of Marine Science and Technology (Taiwan), ISSN 1023-2796, Vol. 26, nr 3, s. 309-326Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An optic-based method that utilizes the particle-laden images captured during high-speed particle image velocimetry (HSPIV) measurements is presented, aiming to explore the probabilistic mean characteristics of the free surface profiles and velocity fields of a free jet with high-frequency random fluctuations over a chute. The technique based on the gray-level gradients in the smoothed gray level distribution of the contrast-enhanced images is used to determine the probabilistic mean features of the free jet, right beneath and above which the water-air interfaces have I%/(100 I)% intermittent appearance of air/water phase and (100 I)% /I% fitful show-up of water/air phase. Further, the cross-correlation calculation for HSPIV measurements is employed to obtain the instantaneous and probabilistic mean velocity fields of the free jet. A target experiment of the free jet having a mean water-depth of 2.76 cm and a Froude number of 3.92 over a 17 chute model is performed in a re-circulating water channel to demonstrate the application of this method. The entire process for obtaining the probabilistic mean positions of the free surface profiles is elucidated step-by step. The lower/upper part of the free surface changing from the height at which the possibility of intermittent appearance of water phase is 3%/97%, via the counterpart for 50%/50%, to that for 97%/ 3% is identified precisely. In addition, the probabilistic mean velocity field is further categorized into the conditionally and overall time-averaged ones. Each streamwise velocity profile in the conditionally time-averaged velocity field is fairly uniform. However, the counterpart in the overall time-averaged velocity field evidently shows the non-uniform feature with prominent velocity gradient in the lower/upper part between the height at which the possibility of intermittent appearance of water phase is 3%/97% and the counterpart is 97%/3%.

  • 38.
    Lindborg, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mohanan, Ashwin Vishnu
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A two-dimensional toy model for geophysical turbulence2017Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, nr 11, artikkel-id 111114Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A toy model for large scale geophysical turbulence is constructed by making two modifications of the shallow water model. Unlike the shallow water model, the toy model has a quadratic expression for total energy, which is the sum of Available Potential Energy (APE) and Kinetic Energy (KE). More importantly, in contrast to the shallow water model, the toy model does not produce any shocks. Three numerical simulations with different forcing are presented and compared with the simulation of a full General Circulation Model (GCM). The energy which is injected cascades in a similar way as in the GCM. First, some of the energy is converted from APE to KE at large scales. The wave field then undergoes a forward energy cascade displaying shallow spectra, close to k−5/3, for both APE and KE, while the vortical field either displays a k−3-spectrum or a more shallow spectrum, close to k−5/3, depending on the forcing. In a simulation with medium forcing wave number, some of the energy which is converted from APE to KE undergoes an inverse energy cascade which is produced by nonlinear interactions only involving the rotational component of the velocity field. The inverse energy cascade builds up a vortical field at larger scales than the forcing scale. At these scales, coherent vortices emerge with a strong dominance of anticyclonic vortices. The relevance of the simulation results to the dynamics of the atmosphere is discussed as in possible continuations of the investigation.

  • 39.
    Lindvall, Jenny
    et al.
    KTH, Centra, SeRC - Swedish e-Science Research Centre. Stockholm Univ, Bolin Ctr Climate Res, Dept Meteorol, Stockholm, Sweden..
    Svensson, Gunilla
    KTH, Centra, SeRC - Swedish e-Science Research Centre. Stockholm Univ, Bolin Ctr Climate Res, Dept Meteorol, Stockholm, Sweden.
    Caballero, Rodrigo
    KTH, Centra, SeRC - Swedish e-Science Research Centre. Stockholm Univ, Bolin Ctr Climate Res, Dept Meteorol, Stockholm, Sweden.
    The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)2017Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 48, nr 11, s. 3741-3758Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simulations with the Community Atmosphere Model version 5 (CAM5) are used to analyze the sensitivity of the large-scale circulation to changes in parameterizations of orographic surface drag and vertical diffusion. Many GCMs and NWP models use enhanced turbulent mixing in stable conditions to improve simulations, while CAM5 cuts off all turbulence at high stabilities and instead employs a strong orographic surface stress parameterization, known as turbulent mountain stress (TMS). TMS completely dominates the surface stress over land and reduces the near-surface wind speeds compared to simulations without TMS. It is found that TMS is generally beneficial for the large-scale circulation as it improves zonal wind speeds, Arctic sea level pressure and zonal anomalies of the 500-hPa stream function, compared to ERA-Interim. It also alleviates atmospheric blocking frequency biases in the Northern Hemisphere. Using a scheme that instead allows for a modest increase of turbulent diffusion at higher stabilities only in the planetary boundary layer (PBL) appears to in some aspects have a similar, although much smaller, beneficial effect as TMS. Enhanced mixing throughout the atmospheric column, however, degrades the CAM5 simulation. Evaluating the simulations in comparison with detailed measurements at two locations reveals that TMS is detrimental for the PBL at the flat grassland ARM Southern Great Plains site, giving too strong wind turning and too deep PBLs. At the Sodankyla forest site, the effect of TMS is smaller due to the larger local vegetation roughness. At both sites, all simulations substantially overestimate the boundary layer ageostrophic flow.

  • 40.
    Miki, M.
    et al.
    Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa, Japan.
    Shindo, T.
    Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa, Japan.
    Rakov, Vladimir
    University of Florida, Gainesville, USA.
    Uman, M. A.
    University of Florida, Gainesville, USA.
    Diendorfer, Gerhard
    Austrian Electrotechnical Association (ÖVE), Vienna.
    Mair, M.
    Institute of Power Systems and Energy Economics, Technical University of Vienna, Vienna, Austria.
    Heidler, Fridolin
    Federal Armed Forces Research University, Munich, Germany.
    Zischank, Wolfgang
    Federal Armed Foprces Research University, Munich, Germany.
    Thottappillil, Rajeev
    Uppsala University.
    Wang, D.
    Department of Human and Information Systems, Gifu University, Gifu, Japan..
    Characterisation of current pulses superimposed on the continuous current in upward lightning initiated from tall objects and in rocket triggered lightning2006Inngår i: Proceedings of the 28th International Conference on Lightning Protection, 2006, s. 83-88Konferansepaper (Annet vitenskapelig)
  • 41. Nykvist, Bjorn
    et al.
    Nilsson, Måns
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Hållbar utveckling, miljövetenskap och teknik, Miljöstrategisk analys (fms).
    Rapidly falling costs of battery packs for electric vehicles2015Inngår i: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 5, nr 4, s. 329-332Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    To properly evaluate the prospects for commercially competitive battery electric vehicles (BEV) one must have accurate information on current and predicted cost of battery packs. The literature reveals that costs are coming down, but with large uncertainties on past, current and future costs of the dominating Li-ion technology1-3. This paper presents an original systematic review, analysing over 80 different estimates reported 2007-2014 to systematically trace the costs of Li-ion battery packs for BEV manufacturers. We show that industry-wide cost estimates declined by approximately 14% annually between 2007 and 2014, from above US$1,000 per kWh to around US$410 per kWh, and that the cost of battery packs used by market-leading BEV manufacturers are even lower, at US$300 per kWh, and has declined by 8% annually. Learning rate, the cost reduction following a cumulative doubling of production, is found to be between 6 and 9%, in line with earlier studies on vehicle battery technology2. We reveal that the costs of Li-ion battery packs continue to decline and that the costs among market leaders are much lower than previously reported. This has significant implications for the assumptions used when modelling future energy and transport systems and permits an optimistic outlook for BEVs contributing to low-carbon transport.

  • 42.
    Orrling, Diana
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialens processvetenskap.
    Fitzgerald, E.
    Department of Chemistry and Biochemistry, University of California.
    Ivanov, A.
    Department of Chemistry and Biochemistry, University of California.
    Molina, M.
    Department of Chemistry and Biochemistry, University of California.
    Enhanced Sulfate Formation on Ozone-Exposed Soot2011Inngår i: Journal of Aerosol Science, ISSN 0021-8502, E-ISSN 1879-1964, Vol. 42, nr 9, s. 615-620Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Gas phase experiments were conducted to examine the heterogeneous oxidation of sulfur dioxide on methane soot in the presence of ozone and water vapor. The enhanced formation of sulfate at atmospheric pressure was confirmed by the use of a particle-into-liquid sampler (PILS) coupled with ion chromatography (IC). Due to the ozone oxidation, multilayer adsorption of sulfur dioxide on soot, as well as sulfate formation and physisorption on secondary surface layer sites were observed. The exposure also caused the soot to become hydrophilic, due to the formation of sulfuric acid and also likely the formation of carboxyl groups on the surface. The sulfate yield increased with ozone levels, but no increase was observed at ozone concentrations above 1000 ppm.

  • 43. Pedersen, A.
    et al.
    Decreau, P.
    Escoubet, C. P.
    Gustafsson, G.
    Laakso, H.
    Lindqvist, Per-Arne
    KTH, Tidigare Institutioner (före 2005), Alfvénlaboratoriet.
    Lybekk, B.
    Masson, A.
    Mozer, F.
    Vaivads, Andris
    Four-point high time resolution information on electron densities by the electric field experiments (EFW) on cluster2001Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, nr 12-okt, s. 1483-1489Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For accurate measurements of electric fields, spherical double probes are electronically controlled to be at a positive potential of approximately 1 V relative to the ambient magnetospheric plasma. The spacecraft will acquire a potential which balances the photoelectrons escaping to the plasma and the electron flux collected from the plasma. The probe-to-plasma potential difference can be measured with a time resolution of a fraction of a second, and provides information on the electron density over a wide range of electron densities from the lobes (similar to0.01 cm(-3)) to the magnetosheath (> 10 cm(-3)) and the plasmasphere (> 100 cm(-3)). This technique has been perfected and calibrated against other density measurements on GEOS, ISEE-1, CRRES, GEOTAIL and POLAR. The Cluster spacecraft potential measurements opens the way for new approaches, particularly near boundaries and gradients where four-point measurements will provide information never obtained before. Another interesting point is that onboard data storage of this simple parameter can be done for complete orbits and thereby will provide background information for the shorter full data collection periods on Cluster. Preliminary calibrations against other density measurements on Cluster will be reported.

  • 44.
    Read, Peter L.
    et al.
    Univ Oxford, Dept Phys, Clarendon Lab, Atmospher Ocean & Planetary Phys, Oxford, England..
    Tabataba-Vakili, Fachreddin
    Univ Oxford, Dept Phys, Clarendon Lab, Atmospher Ocean & Planetary Phys, Oxford, England.;Jet Prop Lab, Pasadena, CA USA..
    Wang, Yixiong
    Univ Oxford, Dept Phys, Clarendon Lab, Atmospher Ocean & Planetary Phys, Oxford, England..
    Augier, Pierre
    LEGI, BP 53, F-38041 Grenoble 9, France..
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Valeanu, Alexandru
    Univ Oxford, Dept Phys, Clarendon Lab, Atmospher Ocean & Planetary Phys, Oxford, England..
    Young, Roland M. B.
    Univ Oxford, Dept Phys, Clarendon Lab, Atmospher Ocean & Planetary Phys, Oxford, England.;Sorbonne Univ, LMD, IPSL, Paris, France..
    Comparative terrestrial atmospheric circulation regimes in simplified global circulation models. Part II: Energy budgets and spectral transfers2018Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, nr 717, s. 2558-2576Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The energetics of possible global atmospheric circulation patterns in an Earth-like atmosphere are explored using a simplified global General Circulation Model (GCM) based on the University of Hamburg's Portable University Model for the Atmosphere (designated here as PUMA-S), forced by linear relaxation towards a prescribed temperature field and subject to Rayleigh surface drag and hyperdiffusive dissipation. Results from a series of simulations, obtained by varying planetary rotation rate with an imposed equator-to-pole temperature difference, were analysed to determine the structure and magnitude of the heat transport and other contributions to the energy budget for the time-averaged, equilibrated flow. These show clear trends with rotation rate, with the most intense Lorenz energy cycle for an Earth-sized planet occurring with a rotation rate around half that of the present-day Earth (i.e., =/(E)=1/2, where (E) is the rotation rate of the Earth). Kinetic energy (KE) and available potential energy (APE) spectra, E-K(n) and E-A(n) (where n is total spherical wavenumber), also show clear trends with rotation rate, with n(-3) enstrophy-dominated spectra around =1 and steeper (approximate to n(-5)) slopes in the zonal mean flow with little evidence for the n(-5/3) spectrum anticipated for an inverse KE cascade. Instead, both KE and APE spectra become almost flat at scales larger than the internal Rossby radius, L-d, and exhibit near-equipartition at high wavenumbers. At <<1, the spectrum becomes dominated by KE with E-K(n)approximate to(2-3)E-A(n) at most wavenumbers and a slope that tends towards n(-5/3) across most of the spectrum. Spectral flux calculations show that enstrophy and APE are almost always cascaded downscale, regardless of rotation rate. KE cascades are more complicated, however, with downscale transfers across almost all wavenumbers, dominated by horizontally divergent modes, for less than or similar to 1 / 4 . At higher rotation rates, transfers of KE become increasingly dominated by rotational (horizontally nondivergent) components with strong upscale transfers (dominated by eddy-zonal flow interactions) for scales larger than L-d and weaker downscale transfers for scales smaller than L-d.

  • 45.
    Riehm, Mats
    et al.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Mark- och vattenteknik.
    Nordin, Lina
    Optimization of winter road maintenance energy costs in Sweden: a critique of site specific frost warning techniques2012Inngår i: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 19, nr 4, s. 443-453Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Frost formation on roads may cause slippery conditions and thereby increase the risk of accident occurrence. Roads are often treated with preventive operations using de-icing agents (e.g. road salt), which are ideally planned and performed prior to frost formation. The decisions on when and where to treat different road stretches with salt are based on meteorological measurements and forecasts. This paper investigates how uncertainties in meteorological measurements for frost prediction at road weather stations affect the efficiency of winter road maintenance. Different types of uncertainties and errors are discussed, together with potential solutions. The effects on winter road maintenance efficiency are discussed in terms of energy and cost. It was found that improvements in frost warning accuracy and reliability can lead to considerable savings and more efficient winter road maintenance.

  • 46. Roth, Lorenz
    et al.
    Saur, Joachim
    Retherford, Kurt D.
    Strobel, Darrell F.
    Feldman, Paul D.
    McGrath, Melissa A.
    Nimmo, Francis
    Transient Water Vapor at Europa’s South Pole2014Inngår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 343, nr 6167, s. 171-174Artikkel i tidsskrift (Fagfellevurdert)
  • 47. Roth, Lorenz
    et al.
    Saur, Joachim
    Retherford, Kurt D.
    Strobel, Darrell F.
    Spencer, John R.
    Simulation of Io’s auroral emission: Constraints on the atmosphere in eclipse2011Inngår i: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 214, nr 2, s. 495-509Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the morphology of Io's aurora by comparing simulation results of a three-dimensional (3D) two-fluid plasma model to observations by the high-resolution Long-Range Reconnaissance Imager (LORRI) on-board the New Horizons spacecraft and by the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). In 2007, Io's auroral emission in eclipse has been observed simultaneously by LORRI and ACS and the observations revealed detailed features of the aurora, such as a huge glowing plume at the Tvashtar paterae close to the North pole. The auroral radiation is generated in Io's atmosphere by collisions between impinging magnetospheric electrons and various neutral gas components. We calculate the interaction of the magnetospheric plasma with Io's atmosphere-ionosphere and simulate the auroral emission. Our aurora model takes into account not only the direct influence of the atmospheric distribution on the morphology and intensity of the emission, but also the indirect influence of the atmosphere on the plasma environment and thus on the exciting electrons. We find that the observed morphology in eclipse can be explained by a smooth (non-patchy) equatorial atmosphere with a vertical column density that corresponds to ∼10% of the column density of the sunlit atmosphere. The atmosphere is asymmetric with two times higher density and extension on the downstream hemisphere. The auroral emission from the Tvashtar volcano enables us to constrain the plume gas content for the first time. According to our model, the observed intensity of the Tvashtar plume implies a mean column density of ∼5×1015cm-2 for the plume region.

  • 48.
    Saeidi, Davood
    et al.
    Isfahan Univ Technol, Dept Mech Engn, Esfahan, Iran..
    Saghafian, Mohsen
    Isfahan Univ Technol, Dept Mech Engn, Esfahan, Iran..
    Javanmard, Shaghayegh Haghjooy
    Isfahan Univ Med Sci, Dept Physiol, Cardiovasc Res Inst, Appl Physiol Res Ctr, Esfahan, Iran..
    Hammarström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biomedicinsk fysik och röntgenfysik.
    Wiklund, Martin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Biomedicinsk fysik och röntgenfysik.
    Acoustic dipole and monopole effects in solid particle interaction dynamics during acoustophoresis2019Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 145, nr 6, s. 3311-3319Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A method is presented for measurements of secondary acoustic radiation forces acting on solid particles in a plain ultrasonic standing wave. The method allows for measurements of acoustic interaction forces between particles located in arbitrary positions such as in between a pressure node and a pressure antinode. By utilizing a model that considers both density- and compressibility-dependent effects, the observed particle-particle interaction dynamics can be well understood. Two differently sized polystyrene micro-particles (4.8 and 25 mu m, respectively) were used in order to achieve pronounced interaction effects. The particulate was subjected to a 2-MHz ultrasonic standing wave in a microfluidic channel, such as commonly used for acoustophoresis. Observation of deflections in the particle pathways shows that the particle interaction force is not negligible under this circumstance and has to be considered in accurate particle manipulation applications. The effect is primarily pronounced when the distance between two particles is small, the sizes of the particles are different, and the acoustic properties of the particles are different relative to the media. As predicted by theory, the authors also observe that the interaction forces are affected by the angle between the inter-particle centerline and the axis of the standing wave propagation direction.

  • 49. Sardina, G.
    et al.
    Picano, F.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Caballero, R.
    Direct and large eddy simulations of droplet condensation in turbulent warm clouds2018Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A cloud is a complex multiphase system constituted by a huge number of different substances such as water droplets, ice droplets, water vapor, organic vapors, air.

  • 50. Sardina, Gaetano
    et al.
    Poulain, Stephane
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Caballero, Rodrigo
    Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation2018Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 2, s. 451-467Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a large-eddy simulation (LES) approach. The authors investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed onto a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending the authors' previous results regarding stochastic condensation, the authors introduce a new theoretical estimation of the droplet size spectrum broadening that accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.

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