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  • 201.
    Gustafsson, Örjan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Kruså, Martin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Zencak, Zdenek
    Sheesley, Rebecca J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Granat, Lennart
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Erik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Praveen, P.S.
    Rao, P.S.P.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rodhe, Henning
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Brown clouds over South Asia: Biomass or fossil fuel combustion?2009Ingår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 323, nr 23 January, s. 495-498Artikel i tidskrift (Refereegranskat)
  • 202. Hakkinen, S. A. K.
    et al.
    Aijala, M.
    Lehtipalo, K.
    Junninen, H.
    Backman, J.
    Virkkula, A.
    Nieminen, T.
    Vestenius, M.
    Hakola, H.
    Ehn, M.
    Worsnop, D. R.
    Kulmala, M.
    Petaja, T.
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Long-term volatility measurements of submicron atmospheric aerosol in Hyytiala, Finland2012Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 22, s. 10771-10786Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The volatility of submicron atmospheric aerosol particles was investigated at a boreal forest site in Hyytiala, Finland from January 2008 to May 2010. These long-term observations allowed for studying the seasonal behavior of aerosol evaporation with a special focus on compounds that remained in the aerosol phase at 280 degrees C. The temperature-response of evaporation was also studied by heating the aerosol sample step-wise to six temperatures ranging from 80 degrees C to 280 degrees C. The mass fraction remaining after heating (MFR) was determined from the measured particle number size distributions before and after heating assuming a constant particle density (1.6 g cm(-3)). On average 19% of the total aerosol mass remained in the particulate phase at 280 degrees C. The particles evaporated less at low ambient temperatures during winter as compared with the warmer months. Black carbon (BC) fraction of aerosol mass correlated positively with the MFR at 280 degrees C, but could not explain it completely: most of the time a notable fraction of this nonvolatile residual was something other than BC. Using additional information on ambient meteorological conditions and results from an Aerodyne aerosol mass spectrometer (AMS), the chemical composition of MFR at 280 degrees C and its seasonal behavior was further examined. Correlation analysis with ambient temperature and mass fractions of polycyclic aromatic hydrocarbons (PAHs) indicated that MFR at 280 degrees C is probably affected by anthropogenic emissions. On the other hand, results from the AMS analysis suggested that there may be very low-volatile organics, possibly organonitrates, in the non-volatile (at 280 degrees C) fraction of aerosol mass.

  • 203. Hakkinen, S. A. K.
    et al.
    Manninen, H. E.
    Yli-Juuti, T.
    Merikanto, J.
    Kajos, M. K.
    Nieminen, T.
    D'Andrea, S. D.
    Asmi, A.
    Pierce, J. R.
    Kulmala, M.
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Semi-empirical parameterization of size-dependent atmospheric nanoparticle growth in continental environments2013Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 15, s. 7665-7682Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The capability to accurately yet efficiently represent atmospheric nanoparticle growth by biogenic and anthropogenic secondary organics is a challenge for current atmospheric large-scale models. It is, however, crucial to predict nanoparticle growth accurately in order to reliably estimate the atmospheric cloud condensation nuclei (CCN) concentrations. In this work we introduce a simple semi-empirical parameterization for sub-20 nm particle growth that distributes secondary organics to the nanoparticles according to their size and is therefore able to reproduce particle growth observed in the atmosphere. The parameterization includes particle growth by sulfuric acid, secondary organics from monoterpene oxidation (SORG(MT)) and an additional condensable vapor of non-monoterpene organics (background). The performance of the proposed parameterization was investigated using ambient data on particle growth rates in three diameter ranges (1.5-3 nm, 3-7 nm and 7-20 nm). The growth rate data were acquired from particle / air ion number size distribution measurements at six continental sites over Europe. The longest time series of 7 yr (2003-2009) was obtained from a boreal forest site in Hyytiala, Finland, while about one year of data (2008-2009) was used for the other stations. The extensive ambient measurements made it possible to test how well the parameterization captures the seasonal cycle observed in sub-20 nm particle growth and to determine the weighing factors for distributing the SORG(MT) for different sized particles as well as the background mass flux (concentration). Besides the monoterpene oxidation products, background organics with a concentration comparable to SORGMT, around 6x10(7) cm(-3) (consistent with an additional global SOA yield of 100 Tg yr(-1)) was needed to reproduce the observed nanoparticle growth. Simulations with global models suggest that the background could be linked to secondary biogenic organics that are formed in the presence of anthropogenic pollution.

  • 204.
    Hallberg, Anneli
    Stockholms universitet, Naturvetenskapliga fakulteten.
    Aerosol particle properties influencing cloud droplet nucleation1994Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 205.
    Hamacher-Barth, Evelyne
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The high Arctic summer aerosol: Size, chemical composition, morphology and evolution over the pack-ice2017Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Aerosol particles, especially in the high Arctic are still not very well represented in climate models. Particle size and number concentrations are strongly under-predicted and temporal variations of aerosol composition and size are still not very well understood, mainly due to the sparsity of observations.

    The main objective of this thesis is the characterization of the high Arctic summer aerosol by means of electron microscopy in order to extend the existing data set from previous expeditions by size resolved data on aerosol number, morphology and chemical composition and to gain a better understanding of the evolution of the aerosol in the atmosphere.

    Ambient aerosol was collected over the pack ice during the Arctic Summer Cloud and Ocean (ASCOS) campaign to the high Arctic in summer 2008. Aerosol particles were evaluated with scanning electron microscopy and subsequent digital image processing to assess particle size and morphology. More than 3900 aerosol particles from 9 sampling events were imaged with scanning electron microscopy and merged into groups of similar morphology which contributed to different degrees to the total aerosol: single particles (82%), gel particles (11%) and halo particles (7%). Single particles were observed over the whole size range with a maximum at 64 nm in diameter, gel particles appeared > 45 nm with a maximum in number at 174 nm, halo particles appeared > 75 nm with a maximum in number at 161 nm. The majority of particles showed the morphology of marine gels, no sea salt or otherwise crystalline particles were observed. Transmission electron microscopy enabled more subtle insights into particle morphology and allowed further subdivision of gel particles into aggregates, aggregates with film and mucus-like particles. Energy dispersive X-ray spectroscopy of individual particles revealed a gradual transition in the content of Na+/K+ and Ca2+/Mg2+ between particle morphologies. Single particles and aggregate particles preferentially contained Na+/K+ whereas aggregate with film particles and mucus-like particles mainly contained Ca2+/Mg2+ suggesting a connection between particle morphology and ion content. Back-trajectory analysis was used to identify aerosol sources and to understand the evolution of the aerosol as a function of the synoptic weather situation. Particle numbers, size and morphology changed with the days the air mass spent over the pack-ice. A morphological descriptor applied to gel particles showed a clear trend suggesting that the contour of the particles becomes sharper and more distinct with increased time spent over the pack-ice. For a very long time over the pack-ice, however, we observed a morphology comparable to freshly emitted particles suggesting aerosol sources over the inner pack-ice.

    Size resolved aerosol chemical composition measurements were utilized to investigate the inorganic composition of laboratory generated nascent sea spray aerosol particles and ambient aerosol samples collected during ASCOS. A significant enrichment of Ca2+ was observed in submicrometer particles in either case with a tendency for increasing Ca2+ enrichment with decreasing particle size. This has strong implications for the alkalinity of sea spray aerosol particles with consequences for the sulfur chemistry in the marine boundary layer, the hygroscopicity and thus the potential of sea spray aerosol particles to act as cloud condensation nuclei.

  • 206.
    Hamacher-Barth, Evelyne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jansson, Kjell
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    A method for sizing submicrometer particles in air collected on Formvar films and imaged by scanning electron microscopy2013Ingår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 6, nr 12, s. 3459-3475Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A method was developed to systematically investigate individual aerosol particles collected onto a polyvinyl formal (Formvar)-coated copper grid with scanning electron microscopy. At very mild conditions with a low accelerating voltage of 2 kV and Gentle Beam mode aerosol particles down to 20 nm in diameter can be observed. Subsequent processing of the images with digital image analysis provides size resolved and morphological information (elongation, circularity) on the aerosol particle population. Polystyrene nanospheres in the expected size range of the ambient aerosol particles (20–900 nm in diameter) were used to confirm the accuracy of sizing and determination of morphological parameters. The relative standard deviation of the diameters of the spheres was better than ±10% for sizes larger than 40 nm and ±18% for 21 nm particles compared to the manufacturer's certificate. Atmospheric particles were collected during an icebreaker expedition to the high Arctic (north of 80°) in the summer of 2008. Two samples collected during two different meteorological regimes were analyzed. Their size distributions were compared with simultaneously collected size distributions from a Twin Differential Mobility Particle Sizer, which confirmed that a representative fraction of the aerosol particles was imaged under the electron microscope. The size distributions obtained by scanning electron microscopy showed good agreement with the Twin Differential Mobility Sizer in the Aitken mode, whereas in the accumulation mode the size determination was critically dependent on the contrast of the aerosol with the Formvar-coated copper grid. The morphological properties (elongation, circularity) changed with the number of days the air masses spent over the pack-ice area north of 80° before the aerosol particles were collected at the position of the icebreaker and are thus an appropriate measure to characterize transformation processes of ambient aerosol particles.

  • 207.
    Hamacher-Barth, Evelyne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The evolution of the high Arctic summer aerosol over the pack iceManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Ambient aerosol samples were collected over the pack ice north of 80° N during the summer of 2008 during the course of the ASCOS campaign. Aerosol particles were collected during nine sampling events and were subsequently sorted into five groups according to the number of days the air spent over the pack ice since last contact with the ice edge. As a tracer for marine sources, the molar ratio of methane sulfonic acid/non-sea-salt-sulfate was used. Scanning electron microscopy allowed size resolved identification of three types of aerosol particles, single particles, gel particles and halo particles within each sample group. Between the five groups we found significant differences in aerosol morphology, largely dependent on the time of advection over the pack ice (days over ice, DOI) and the synoptic weather encountered. The most obvious differences were observed for marine gel particles. The fraction of these particles in the lower accumulation mode, ≤ 100 nm, increased from 15% (DOI = 1) over 20% (DOI = 3.2) and 27% (DOI= 6.7) to 30% (DOI = 8.9). In parallel the gel particle morphology changed with increasing DOI value, from a widely outspread and weakly contrasting morphology (DOI = 1) over a more distinct and better contrasting appearance (DOI = 3.2) to sharply and well contoured particles (DOI = 6.7). The gel particles with the highest DOI value (DOI = 8.9), however, showed a branched and widely outspread morphology that indicated a relatively recent emission of these particles into the submicrometer size range, either from sources over the pack ice or through fragmentation of supermicrometer particles.

  • 208. Hamburger, T.
    et al.
    McMeeking, G.
    Minikin, A.
    Birmili, W.
    Dall'Osto, M.
    O'Dowd, C.
    Flentje, H.
    Henzing, B.
    Junninen, H.
    Kristensson, A.
    de Leeuw, G.
    Stohl, A.
    Burkhart, J. F.
    Coe, H.
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Petzold, A.
    Overview of the synoptic and pollution situation over Europe during the EUCAARI-LONGREX field campaign2011Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 3, s. 1065-1082Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In May 2008 the EUCAARI-LONGREX aircraft field campaign was conducted within the EUCAARI intensive observational period. The campaign aimed at studying the distribution and evolution of air mass properties on a continental scale. Airborne aerosol and trace gas measurements were performed aboard the German DLR Falcon 20 and the British FAAM BAe-146 aircraft. This paper outlines the meteorological situation over Europe during May 2008 and the temporal and spatial evolution of predominantly anthropogenic particulate pollution inside the boundary layer and the free troposphere. Time series data of six selected ground stations are used to discuss continuous measurements besides the single flights. The observations encompass total and accumulation mode particle number concentration (0.1–0.8 μm) and black carbon mass concentration as well as several meteorological parameters. Vertical profiles of total aerosol number concentration up to 10 km are compared to vertical profiles probed during previous studies.

    During the first half of May 2008 an anticyclonic blocking event dominated the weather over Central Europe. It led to increased pollutant concentrations within the centre of the high pressure inside the boundary layer. Due to long-range transport the accumulated pollution was partly advected towards Western and Northern Europe. The measured aerosol number concentrations over Central Europe showed in the boundary layer high values up to 14 000 cm−3 for particles in diameter larger 10 nm and 2300 cm−3 for accumulation mode particles during the high pressure period, whereas the middle free troposphere showed rather low concentrations of particulates. Thus a strong negative gradient of aerosol concentrations between the well mixed boundary layer and the clean middle troposphere occurred.

  • 209.
    Hamburger, Thomas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Matisans, Modris
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Tunved, Peter
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Calderon, S.
    Hoffmann, P.
    Hochschild, G.
    Gross, J.
    Schmeissner, T.
    Wiedensohler, A.
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Long-term in situ observations of biomass burning aerosol at a high altitude station in Venezuela - sources, impacts and interannual variability2013Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 19, s. 9837-9853Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron particle volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night-time and observations of boundary layer air masses during daytime and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron particle volume reached 1.4 +/- 1.3 mu m(3) cm(-3), refractory particle number concentrations (at 300 degrees C) 510+/-420 cm(-3) and the absorption coefficient 0.91+/-1.2 Mm(-1). The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19+/-0.25 mu m(3) cm-3, 150+/-94 cm(-3) and 0.15+/-0.26 Mm(-1). A decrease of particle concentrations during the dry seasons from 2007-2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Nino-Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Nina conditions, high biomass burning activity followed El Nino conditions.

  • 210.
    Hamburger, Thomas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    McMeeking, G.
    Minikin, A.
    Petzold, A.
    Coe, H.
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Airborne observations of aerosol microphysical properties and particle ageing processes in the troposphere above Europe2012Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 23, s. 11533-11554Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In-situ measurements of aerosol microphysical properties were performed in May 2008 during the EUCAARI-LONGREX campaign. Two aircraft, the FAAM BAe-146 and DLR Falcon 20, operated from Oberpfaffenhofen, Germany. A comprehensive data set was obtained comprising the wider region of Europe north of the Alps throughout the whole tropospheric column. Prevailing stable synoptic conditions enabled measurements of accumulating emissions inside the continental boundary layer reaching a maximum total number concentration of 19 000 particles cm(-3) stp. Ultra-fine particles as indicators for nucleation events were observed within the boundary layer during high pressure conditions and after updraft of emissions induced by frontal passages above 8 km altitude in the upper free troposphere. Aerosol ageing processes during air mass transport are analysed using trajectory analysis. The ratio of particles containing a non-volatile core (250 degrees C) to the total aerosol number concentration was observed to increase within the first 12 to 48 h from the particle source from 50 to 85% due to coagulation. Aged aerosol also features an increased fraction of accumulation mode particles of approximately 40% of the total number concentration. The presented analysis provides an extensive data set of tropospheric aerosol microphysical properties on a continental scale which can be used for atmospheric aerosol models and comparisons of satellite retrievals.

  • 211. Hammarstrand, Ulla
    Convection in large scale numerical models: parameterization and forecasts of precipitation probability1980Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 212.
    Hamrud, Mats
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Statistical properties of the atmosphere influencing long-range transport of air pollutants1985Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This thesis deals with some statistical properties of the atmosphere of importance for the transport and removal of air pollutants. Clouds and precipitation can strongly influence the behaviour of atmospheric pollutants. Thus, we need to know the frequency of occurrence of cloud and rain events in order to estimate e.g. removal times of pollutants. The frequency of cloud and rain passages should be estimated in a Lagrangian frame of reference, i.e. moving with the wind. As these estimates are difficult to obtain, estimates made at fixed points in space are often used instead. When comparing estimates of frequency of rain events obtained in a Lagrangian frame of reference with estimates obtained at fixed points no systematic difference was found.

    To estimate the residence time of atmospheric gases one normally needs to know the sources and sinks. An alternative approach is to use the information contained in the atmospheric variability of these gases. The estimate of the residence time obtained from the variability will have an uncertainty of at least a factor of 10 if the source and sink distribution is unknown. If we know the location of the sources the estimate will be much improved bringing the uncertainty down to perhaps a factor of two.

  • 213.
    Hanley, John
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Extreme Storms in the North Atlantic and Europe2013Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    A study of the most extreme cyclones affecting the North Atlantic and Europe is presented with particular focus on extreme windstorms over the densely populated area of Western Europe, whose associated high surface wind speeds are capable of causing extensive structural damage and occasionally a loss of life.

    A novel cyclone identification and tracking algorithm is presented which explicitly recognises ‘multi-centre cyclones’ (MCCs), defined as cyclonic systems which contain two or three sea-level pressure minima. The method also recognises cyclone merging and splitting events and reduces the number of tracks which would have been spuriously split at some point in their life-cycle. MCC frequency is shown to increase with storm intensity, with approximately 60% of the top 30% of cyclones constituting MCCs at some point in their life-cycle.

    The first findings of the IMILAST (Inter-comparison of MId-LAtitudeSTorm diagnostics) project, an intercomparison study of 15 cyclone identification and tracking algorithms, are presented. Each method was applied to a 20 year period of the ERA-Interim dataset and results for cyclone frequency, intensity, life-cycle and track location were compared across the methods.

    The relationship between the evolution of the most intense wind storms affecting Western Europe (Britain and Ireland, Scandinavia, and Western Continental Europe) and the large-scale atmospheric flow is investigated using an automated cyclone tracking algorithm and an objective measure of cyclone destructiveness applied to ERA40 and ERA-Interim reanalysis data as well as EC-Earth model output data at two different spatial resolutions. Composite analyses reveal a clear connection between the precise location of upper-level anti-cyclonic wave breaking and cold air intrusion from the north and the position and orientation of an intense jet; this, in turn, plays a crucial role in determining into which region a developing extreme storm will be steered.

  • 214.
    Hanley, John
    et al.
    University College Dublin, Ireland.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Objective identification and tracking of multi-centre cyclones in the ERA-Interim reanalysis dataset2012Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, nr 664, s. 612-625Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a novel cyclone identification and tracking method that explicitly recognizes multicentre cyclones (MCCs), defined as a cyclonic system with two or three sea-level pressure minima within its outermost contour. The method allows for the recognition of cyclone merger and splitting events in a natural way, and provides a consistent measure of the cyclone extent. Using the ERA-Interim reanalysis dataset, we compute a climatology using this method and show that MCCs occur in about 32 of all cyclone tracks and are much more prevalent in more intense storms. We also show that the method permits reconnection of tracks that would have been spuriously split using a conventional method. We present spatial maps of cyclone mergers, splitting, genesis and lysis using the method and also compute statistics of precipitation falling within cyclones, showing that it is strongly concentrated in the most intense cyclones.

  • 215.
    Hanley, John
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rossby wave breaking and extreme windstorms over Western EuropeManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We investigate the role of upper-level Rossby wave breaking in the evolution ofthemost extreme windstorms affecting three regions inWestern Europe: Britainand Ireland, Scandinavia and Western Continental Europe. Using ERA40and ERA-Interim reanalysis data along with EC-Earth model output at twodifferent spatial resolutions, we first construct an extreme wind climatologyover the selected regions and inter-compare the model climatology with thatcomputed from the reanalysis data. Using a storm destructiveness measure, wethen select the top 25 most destructive storms in each region from a multidecadalclimatology in each of our four datasets; track-by-track analysis revealsa good agreement in the trajectories and evolution of these storms in bothmodel resolutions compared to the reanalysis data. Temporal MSLP reanalysiscomposites demonstrate that in each region, there exists a set of large-scaleconditions conducive to the development of these storms; similar composites ofmodel output data show that these surface conditions are broadly well capturedby both model resolutions. Temporal composites of potential temperature onthe 2-PVU surface using reanalysis data reveal that these regional large-scalesurface patterns can be associated with exceptional cyclonic and anti-cyclonicwave breaking occurring contemporaneously in the North Atlantic; the preciselocation of these wave breaking events controls the position and orientation ofan intense upper-level jet which in turn determines into which region the stormsare steered. Similar composites using model output data show qualitativelythe same picture, but with an overall positive bias most likely due to a lowertropopause height in the model.

  • 216.
    Hannachi, Abdel
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Regularised empirical orthogonal functions2016Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 68, artikel-id 31723Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Empirical orthogonal functions, extensively used in weather/climate research, suffer serious geometric drawbacks such as orthogonality in space and time and mixing. The present paper presents a different version, the regularised (or smooth) empirical orthogonal function (EOF) method, by including a regularisation constraint, which originates from the field of regression/correlation of continuous variables. The method includes an extra unknown, the smoothing parameter, and solves a generalised eigenvalue problem and can overcome, therefore, some shortcomings of EOFs. For example, the geometrical constraints satisfied by conventional EOFs are relaxed. In addition, the method can help alleviate the mixing drawback. It can also be used in combination with other methods, which are based on downscaling or dimensionality reduction. The method has been applied to sea level pressure and sea surface temperature and yields an optimal value of the smoothing parameter. The method shows, in particular, that the leading sea level pressure pattern, with substantially larger explained variance compared to its EOF counterpart, has a pronounced Arctic Oscillation compared to the mixed North Atlantic Oscillation/Arctic Oscillation pattern of the leading EOF. The analysis of the remaining leading patterns and the application to sea surface temperature field and trend EOFs are also discussed.

  • 217.
    Hannachi, Abdel
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Iqbal, Waheed
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability2019Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 76, nr 1, s. 333-356Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nonlinearity in the Northern Hemisphere’s winter time atmospheric flow is investigated from both an intermediate complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasi-geostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel PC space in consistency with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 reanalyses using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20-hPa) and troposphere (500-hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced and undisturbed states of the winter polar vortex. In the troposphere the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with signature of negative North Atlantic Oscillation (NAO). Over the North Atlantic/European sector a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or –NAO) positions of the eddy-driven jet strteam. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (decrease) in the frequency of the polar high (low) consistent with an increase of zonal flow frequency.

  • 218.
    Hannachi, Abdel
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Iqbal, Waheed
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Signature of tropospheric nonlinear regime behavior in northern hemisphere winter via flow tendencies and kernel empirical orthogonal functions2018Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870Artikel i tidskrift (Refereegranskat)
  • 219.
    Hannachi, Abdelwaheb
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Intermittency, autoregression and censoring: a first-order AR model for daily precipitation2014Ingår i: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 21, nr 2, s. 384-397Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Daily precipitation is investigated in this study in terms of simple first order autoregressive models. The methodology is based on combining theory from censored processes with continuous autoregressive models to model intermittent phenomena. The choice of short-memory autoregressive models is corroborated further by recent findings on scaling properties of daily precipitation records. The theory and application to synthetic models are presented. The methodology is then applied to Northern Ireland Armagh Observatory daily precipitation for the period 1950-2001 for each month. Both zero- and non zero-mean processes are considered. The analysis indicates that the model parameters do capture seasonality where, for example, the autocorrelation co-efficient is larger in winter, compared to in the summer. This is arguably a reflection of the stronger effect of large-scale processes on rainfall in winter compared to summer. Interestingly, the parameters of the zero- and non zero-mean processes are found to be quite similar, reflecting the symmetric nature of the truncated processes in the midlatitude and extratropics. It is suggested, in particular, that the process mean can be used as a measure to quantify dryness or wetness of a given region. Ways of model improvement, including power transformation, based on the square root, to represent extremes using exploratory quantile-quantile plots better are also discussed. Copyright (c) 2012 Royal Meteorological Society

  • 220.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Barnes, Elizabeth A.
    Woollings, Tim
    Behaviour of the winter North Atlantic eddy-driven jet stream in the CMIP3 integrations2013Ingår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, nr 3-4, s. 995-1007Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A systematic analysis of the winter North Atlantic eddy-driven jet stream latitude and wind speed from 52 model integrations, taken from the coupled model intercomparison project phase 3, is carried out and compared to results obtained from the ERA-40 reanalyses. We consider here a control simulation, twentieth century simulation, and two time periods (2046-2065 and 2081-2100) from a twenty-first century, high-emission A2 forced simulation. The jet wind speed seasonality is found to be similar between the twentieth century simulations and the ERA-40 reanalyses and also between the control and forced simulations although nearly half of the models overestimate the amplitude of the seasonal cycle. A systematic equatorward bias of the models jet latitude seasonality, by up to 7A degrees, is observed, and models additionally overestimate the seasonal cycle of jet latitude about the mean, with the majority of the models showing equatorward and poleward biases during the cold and warm seasons respectively. A main finding of this work is that no GCM under any forcing scenario considered here is able to simulate the trimodal behaviour of the observed jet latitude distribution. The models suffer from serious problems in the structure of jet variability, rather than just quantitiative errors in the statistical moments.

  • 221.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Iqbal, Waheed
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Finish Meteorological Institute, Finland.
    Bimodality of hemispheric winter atmospheric variability via average flow tendencies and kernel EOFs2019Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, nr 1, artikel-id 1633847Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The topic of the existence of planetary winter circulation regimes has gone through a long debate. This article contributes to this debate by investigating nonlinearity in a 3-level quasi-geostrophic model and the Japanese JRA-55 reanalysis. The method uses averaged flow tendencies and kernel principal component (PC) analysis. Within two-dimensional (2D) kernel PCs the model reveals two fixed (or stationary) points. The probability density function (PDF) within this space is strongly bimodal where the modes match the regions of low tendencies in consistency with low-order conceptual models. The circulation regimes represent respectively zonal and blocked flows. Application to daily winter northern hemisphere sea level pressure and 500-hPa geopotential height yields strong bimodal PDFs. The modes represent respectively polar highs and lows with signatures of North Atlantic Oscillation. A clear climate change signal is observed showing a clear reduction (increase) of occurrence probability of polar high (low), translating into an increase of probability of zonal flow. Relation of the climate change signal to the polar amplification hypothesis is discussed.

  • 222.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mitchell, D.
    Gray, L.
    Charlton-Perez, A.
    On the Use of Geometric Moments to Examine the Continuum of Sudden Stratospheric Warmings2011Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 3, s. 657-674Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The polar winter stratospheric vortex is a coherent structure that undergoes different types of deformation that can be revealed by the geometric invariant moments. Three moments are used the aspect ratio, the centroid latitude, and the area of the vortex based on stratospheric data from the 40-yr ECMWF Re-Analysis (ERA-40) project-to study sudden stratospheric warmings. Hierarchical clustering combined with data image visualization techniques is used as well. Using the gap statistic, three optimal clusters are obtained based on the three geometric moments considered here. The 850-K potential vorticity field, as well as the vertical profiles of polar temperature and zonal wind, provides evidence that the clusters represent, respectively, the undisturbed (U), displaced (D), and split (S) states of the polar vortex. This systematic method for identifying and characterizing the state of the polar vortex using objective methods is useful as a tool for analyzing observations and as a test for climate models to simulate the observations. The method correctly identifies all previously identified major warmings and also identifies significant minor warmings where the atmosphere is substantially disturbed but does not quite meet the criteria to qualify as a major stratospheric warming.

  • 223.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Turner, A. G.
    20th century intraseasonal Asian monsoon dynamics viewed from Isomap2013Ingår i: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 20, nr 5, s. 725-741Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Asian summer monsoon is a high-dimensional and highly nonlinear phenomenon involving considerable moisture transport towards land from the ocean, and is critical for the whole region. We have used daily ECMWF reanalysis (ERA-40) sea-level pressure (SLP) anomalies on the seasonal cycle, over the region 50-145 degrees E, 20 degrees S-35 degrees N, to study the nonlinearity of the Asian monsoon using Isomap. We have focused on the two-dimensional embedding of the SLP anomalies for ease of interpretation. Unlike the unimodality obtained from tests performed in empirical orthogonal function space, the probability density function, within the two-dimensional Isomap space, turns out to be bimodal. But a clustering procedure applied to the SLP data reveals support for three clusters, which are identified using a three-component bivariate Gaussian mixture model. The modes are found to appear similar to active and break phases of the monsoon over South Asia in addition to a third phase, which shows active conditions over the western North Pacific. Using the low-level wind field anomalies, the active phase over South Asia is found to be characterised by a strengthening and an eastward extension of the Somali jet. However during the break phase, the Somali jet is weakened near southern India, while the monsoon trough in northern India also weakens. Interpretation is aided using the APHRODITE gridded land precipitation product for monsoon Asia. The effect of large-scale seasonal mean monsoon and lower boundary forcing, in the form of ENSO, is also investigated and discussed. The outcome here is that ENSO is shown to perturb the intraseasonal regimes, in agreement with conceptual ideas.

  • 224.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Woollings, T.
    Fraedrich, K.
    The North Atlantic jet stream: a look at preferred positions, paths and transitions2012Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, nr 665, s. 862-877Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Preferred jet stream positions and their link to regional circulation patterns over the winter North Atlantic/European sector are investigated to corroborate findings of multimodal behaviour of the jet positions and to analyse patterns of preferred paths and transition probabilities between jet regimes using ERA-40 data. Besides the multivariate Gaussian mixture model, hierarchical clustering and data image techniques are used for this purpose. The different approaches all yield circulation patterns that correspond to the preferred jet regimes, namely the southern, central and the northern positions associated respectively with the Greenland anticyclone or blocking, and two opposite phases of an East Atlantic-like flow pattern. Growth and decay patterns as well as preferred paths of the system trajectory are studied using the mixture model within the delay space. The analysis shows that the most preferred paths are associated with central to north and north to south jet stream transitions with a typical time-scale of about 5 days, and with life cycles of 12 weeks. The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking.

  • 225. Hansen, A. M. K.
    et al.
    Kristensen, K.
    Nguyen, Q. T.
    Zare, A.
    Cozzi, F.
    Nøjgaard, J. K.
    Skov, H.
    Brandt, J.
    Christensen, J. H.
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Tunved, Peter
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM). University of Helsinki, Finland.
    Glasius, M.
    Organosulfates and organic acids in Arctic aerosols: speciation, annual variation and concentration levels2014Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 15, s. 7807-7823Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sources, composition and occurrence of secondary organic aerosols in the Arctic were investigated at Zeppelin Mountain, Svalbard, and Station Nord, northeastern Greenland, during the full annual cycle of 2008 and 2010, respectively. Speciation of organic acids, organosulfates and nitrooxy organosulfates - from both anthropogenic and biogenic precursors were in focus. A total of 11 organic acids (terpenylic acid, benzoic acid, phthalic acid, pinic acid, suberic acid, azelaic acid, adipic acid, pimelic acid, pinonic acid, diaterpenylic acid acetate and 3-methyl-1,2,3-butanetricarboxylic acid), 12 organosulfates and 1 nitrooxy organosulfate were identified in aerosol samples from the two sites using a high-performance liquid chromatograph (HPLC) coupled to a quadrupole Time-of-Flight mass spectrometer. At Station Nord, compound concentrations followed a distinct annual pattern, where high mean concentrations of organosulfates (47 +/- 14 ng m(-3)) and organic acids (11.5 +/- 4 ng m(-3)) were observed in January, February and March, contrary to considerably lower mean concentrations of organosulfates (2 +/- 3 ng m(3-)) and organic acids (2.2 +/- 1 ng m(-3)) observed during the rest of the year. At Zeppelin Mountain, organosulfate and organic acid concentrations remained relatively constant during most of the year at a mean concentration of 15 +/- 4 ng m(-3) and 3.9 +/- 1 ng m(-3), respectively. However during four weeks of spring, remarkably higher concentrations of total organosulfates (23-36 ng m(-3)) and total organic acids (7-10 ngm(-3)) were observed. Elevated organosulfate and organic acid concentrations coincided with the Arctic haze period at both stations, where northern Eurasia was identified as the main source region. Air mass transport from northern Eurasia to Zeppelin Mountain was associated with a 100% increase in the number of detected organosulfate species compared with periods of air mass transport from the Arctic Ocean, Scandinavia and Greenland. The results from this study suggested that the presence of organic acids and organosulfates at Station Nord was mainly due to long-range transport, whereas indications of local sources were found for some compounds at Zeppelin Mountain. Furthermore, organosulfates contributed significantly to organic matter throughout the year at Zeppelin Mountain (annual mean of 13 +/- 8 %) and during Arctic haze at Station Nord (7 +/- 2 %), suggesting organosulfates to be important compounds in Arctic aerosols.

  • 226.
    Hansson, Hans-Christen
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Bhend, Jonas
    Causes of Regional Change - Aerosols2015Ingår i: Second Assessment of Climate Change for the Baltic Sea Basin / [ed] The BACC II Author Team, Springer, 2015, s. 441-452Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    This chapter starts by introducing the complex nature of atmospheric aerosols, their sources, formation and properties and describes how they interact with clouds. This is important background information for discussing how aerosols affect climate, both directly and indirectly by affecting the radiative properties of clouds. The complexity of the aerosol-cloud-climate interaction causes large uncertainty in the projections of future climate. Results from different modelling studies on the European region are presented, and these show that the large spatial and temporal variations in atmospheric aerosol concentrations and properties have large regional differences in their effect on climate. This chapter concludes with an example of a co-beneficial global air quality and climate change mitigation scenario.

  • 227.
    Hansson, Margareta
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Detecting changes in climate and atmospheric composition with tracers in Arctic ice caps1993Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Changes in climate and atmospheric composition over decades to a hundred thousand years have been studied by extracting records of various tracers from two Arctic ice cores. A seven metre long ice core of superimposed ice was recovered from the Storöya ice cap in north-eastern Svalbard. The annual increments of superimposed ice were identified by combining records of ice texture, ice structure, insoluble particle content and solid electrical conductivity. A net balance record over the time period A.D. 1954 to 1979 was established and the obtained information about past local climate is found to agree with observations. The chemical composition of the insoluble particles was examined but only limited information is gained about atmospheric aerosol composition due to the disturbed deposition record as is found in superimposed ice.

    The records from a 325 metre long surface-to-bedrock ice core from the Renland ice cap, East Greenland, cover a full glacial cycle into the previous interglacial Eem 130 000 years B.P. Changes in climate are revealed by the record of stable oxygen isotopes. Fast and large variations in temperature between glacial and interglacial periods, but also in mid and late parts of the last glaciation, are found in agreement with other Greenland deep ice cores. The precipitation rate is found to vary significantly between different climatic regimes. Comparison with records from Antarctica suggests a roughly parallel climatic development in the two hemispheres. Changes in the composition of the atmospheric aerosol have been studied by extracting the first Northern Hemisphere record of major anions, major cations, insoluble dust and acidity over a full climatic cycle. The concentration in the ice of most impurities is higher in cold glacial stages. However, for impurities with a strong biogenic component (nitrate, ammonium and methanesulphonate) the concentration is lower in cold glacial stages which is opposite to Southern Hemisphere records. The ammonium record has been utilised to discuss source areas of the greenhouse gas methane. High ammonium concentrations in mild glacial stages are found to coincide with high global concentrations of methane, which points to the terrestrial biosphere at high northern latitudes as a possible source. A simple model simulation indicates that concentration changes in the ice of one order of magnitude between different climatic regimes can be explained entirely by changes in the hydrological cycle and the general circulation of the atmosphere. These physical changes of the atmosphere affects the radiation balance of the atmosphere, just as changes in source emissions, by strongly changing the airborne concentration and composition of the atmospheric aerosol. 

  • 228. Harnik, Nili
    et al.
    Dritschel, David G.
    Heifetz, Eyal
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Tel Aviv University, Israel.
    On the equilibration of asymmetric barotropic instability2014Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 140, nr 685, s. 2444-2464Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The conjunction of turbulence, waves and zonal jets in geophysical flows gives rise to the formation of potential vorticity staircases and to the sharpening of jets by eddies. The effect of eddies on jet structure, however, is fundamentally different if the eddies arise from barotropic rather than from baroclinic instability. As is well known, barotropic instability may occur on zonal jets when there is a reversal of potential vorticity gradients at the jet flanks. In this article we focus on the nonlinear stages of this instability and its eventual saturation. We consider an idealized initial state consisting of an anticyclonic potential vorticity strip sitting in the flanks of an eastward jet. This asymmetric configuration, a generalization of the Rayleigh problem, is one of the simplest barotropic jet configurations which incorporates many fundamental aspects of real flows, including linear instability and its equilibration, nonlinear interactions, scale cascades, vortex dynamics, and jet sharpening. We make use of the simplicity of the problem to conduct an extensive parameter sweep, and develop a theory relating the properties of the equilibrated flow to the initial flow state by considering the marginal stability limit, together with conservation of circulation and wave activity.

  • 229. Harrison, S. P.
    et al.
    Bartlein, P. J.
    Brewer, S.
    Prentice, I. C.
    Boyd, Meighan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hessler, I.
    Holmgren, Karin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Izumi, K.
    Willis, K.
    Climate model benchmarking with glacial and mid-Holocene climates2014Ingår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 43, nr 3-4, s. 671-688Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Past climates provide a test of models' ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.

  • 230.
    Hartung, Kerstin
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Paths to improving atmospheric models across scales: The importance of the unresolved scales2018Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Correct representation of physical processes, the parametrizations, and their interaction with the resolved circulation is crucial for the performance of numerical models. Here, focus is put on understanding model biases and developing tools to alleviate existing biases. Atmospheric blocking can divert the typical atmospheric flow for several days up to weeks and thereby impacts the mean climate of the region experiencing blocking. Models typically underestimate the frequency of atmospheric blocking. Based on results from the global climate model EC-Earth, it is found that the atmospheric model resolution is not strongly influencing the representation of atmospheric blocking once the grid reaches about 80 km grid length in the horizontal. Updating several physical parametrizations, and thereby the model version, is the largest contributor to advancements in simulating atmospheric blocking. The importance of the topography for the large-scale atmospheric flow is further investigated with the reanalysis ERA-Interim by applying a simplified theoretical analysis. It is found that the idealized topographic forcing theory can explain some part of the observed large-scale properties of the flow, though the method does mainly produce relative results. The explained part of the large-scale structure is increased during periods of northwesterly flow and when the flow impinges the mountain ridge almost orthogonally.

    Small-scale processes acting in air masses transported from midlatitudes to the Arctic are also discussed. Numerical models often struggle with representing the stable conditions in the Arctic and tend to underestimate the downward longwave impact during cloudy conditions. A comparison of single-column models (SCMs) indicates that most models can capture the bimodal longwave distribution which develops from alternating cloudy and clear-sky conditions. SCMs are often used for model development as they allow to decouple the parametrized physical processes from the large-scale environment and enable many parameter sensitivity tests. A new tool is presented which can be used for the development of physical parametrizations in marine and polar conditions. It combines one-dimensional models of the atmosphere and ocean, including sea-ice, into a coupled atmosphere-ocean SCM (AOSCM). The presented setup constitutes an advantage compared to SCMs of one component because the coupling is directly modelled and the interaction between the respective boundary layers does not dependent on prescribed boundary conditions.

  • 231.
    Hartung, Kerstin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-sciene Research Centre, Sweden.
    Shepherd, Theodore
    Hoskins, Brian
    Methven, John
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-sciene Research Centre, Sweden.
    Diagnosing topographic forcing in an atmospheric dataset: the case of the North American CordilleraManuskript (preprint) (Övrigt vetenskapligt)
  • 232.
    Hartung, Kerstin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden.
    Kjellström, Erik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish Meteorological and Hydrological Institute, Sweden.
    Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?2017Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, nr 1, artikel-id 1406252Artikel i tidskrift (Refereegranskat)
    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.

  • 233.
    Hartung, Kerstin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden.
    Struthers, Hamish
    Deppenmeier, Anna-Lena
    Hazeleger, Wilco
    An EC-Earth coupled atmosphere-ocean single-column model (AOSCM) for studying coupled marine and polar processesManuskript (preprint) (Övrigt vetenskapligt)
  • 234. Havnes, Ove
    et al.
    Antonsen, Tarjei
    Baumgarten, Gerd
    Hartquist, Thomas W.
    Biebricher, Alexander
    Fredriksen, Ashild
    Friedrich, Martin
    Hedin, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    A new method of inferring the size, number density, and charge of mesospheric dust from its in situ collection by the DUSTY probe2019Ingår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 12, nr 3, s. 1673-1683Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a new method of analyzing measurements of mesospheric dust made with DUSTY rocket-borne Faraday cup probes. It can yield the variation in fundamental dust parameters through a mesospheric cloud with an altitude resolution down to 10 cm or less if plasma probes give the plasma density variations with similar height resolution. A DUSTY probe was the first probe that unambiguously detected charged dust and aerosol particles in the Earth's mesosphere. DUSTY excluded the ambient plasma by various biased grids, which however allowed dust particles with radii above a few nanometers to enter, and it measured the flux of charged dust particles. The flux measurements directly yielded the total ambient dust charge density. We extend the analysis of DUSTY data by using the impact currents on its main grid and the bottom plate as before, together with a dust charging model and a secondary charge production model, to allow the determination of fundamental parameters, such as dust radius, charge number, and total dust density. We demonstrate the utility of the new analysis technique by considering observations made with the DUSTY probes during the MAXIDUSTY rocket campaign in June-July 2016 and comparing the results with those of other instruments (lidar and photometer) also used in the campaign. In the present version we have used monodisperse dust size distributions.

  • 235.
    Hede, Thomas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Beyond Köhler theory: Molecular dynamics simulations as a tool for atmospheric science2013Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this thesis, the results from molecular dynamics (MD) simulations of nanoaerosol clusters are discussed. The connecting link of these studies is the Köhler theory, which is the theory of condensational growth and activation of cloud droplets to form clouds. By investigating parameters such as the surface tension, state of mixture and morphology of nanoaerosol particles, conclusions can be drawn to improve the Köhler theory to include the effects of organic compounds previously unaccounted for.

    For the terrestrial environment, the simulations show that the natural surfactant cis-pinonic acid, an oxidation product evaporated from boreal trees, spontaneously accumulates at the surface of nanoaerosol clusters and thereby reduces the surface tension. The surface tension depression is related to the concentration of the surfactant and the size of the clusters. Surface tension is an important parameter of the Köhler theory. A decrease of the surface tension can lower the critical water vapour supersaturation needed for cloud droplet activation, giving rise to more, but smaller cloud droplets (Twomey effect) which in turn could change the optical properties of the cloud. It was also shown that the three organic surfactants, being model compounds for so called Humic-like substances (HULIS) have the ability to form aggregates inside the nanoaerosol clusters. These HULIS aggregates can also promote the solubilization of hydrophobic organic carbon in the form of fluoranthene, enabling soot taking part in cloud drop formation.

    Dissolved intermediately surface-active free amino acids were shown to be of some relevance for cloud formation over remote marine areas. The MD simulations showed differences between the interacting forces for spherical and planar interfaces of amino acids solutions.

    This thesis has emphasized the surface-active properties of organic compounds, including model HULIS and amino acids and their effect on surface tension and molecular orientation including aggregate formation in nanoaerosol clusters and their activation to form droplets. This thesis shows that the Köhler equation does not fully satisfactory describe the condensational growth of nano-sized droplets containing organic surfactants. Different approaches are suggested as revisions of the Köhler theory.

  • 236.
    Hede, Thomas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Sun, Lu
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    Tu, Yaoquan
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    Ågren, Hans
    Royal Inst Technol, Sch Biotechnol, Dept Theoret Chem, S-10691 Stockholm, Sweden.
    A theoretical study revealing the promotion of light absorbing carbon particles solubilization by natural surfactants in nanosized water droplets2013Ingår i: Atmospheric Science Letters, ISSN 1530-261X, E-ISSN 1530-261X, Vol. 14, nr 2, s. 86-90Artikel i tidskrift (Refereegranskat)
    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 buil-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.

  • 237.
    Hede, Thomas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Li, Xin
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tu, Yaoquan
    Ågren, Hans
    Model HULIS compounds in nanoaerosol clusters - investigations of surface tension and aggregate formation using molecular dynamics simulations2011Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 13, s. 6549-6557Artikel i tidskrift (Refereegranskat)
    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.

  • 238.
    Hede, Thomas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Murugan, N. Arul
    Kongsted, Jacob
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ågren, Hans
    Simulations of Light Absorption of Carbon Particles in Nanoaerosol Clusters2014Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 118, nr 10, s. 1879-1886Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Black carbon soot (BS) is considered to be the second most contributing organic matter next to carbon dioxide for the global warming effect. There is, however, so far no consensus on the quantitative warming effect due to the increased distribution of black carbon in the atmosphere. A recent report (Science 2012, 337, 1078) suggests that due to BS there is only a few percentage enhancement in absorption of BS-immersed aerosols. To get proper interpretation of the available experimental data, it becomes essential to obtain details of the microscopic origin of the absorption and scattering processes of the aerosol clusters due to the presence of soot. However, so far, due to the large spatial scale and the need for a quantum mechanical description of the particles involved in the absorption and scattering, this quest has posed an insurmountable challenge. In the present work we propose the use of a multiscale integrated approach based on molecular dynamics and a quantum mechanical molecular mechanical method to model the optical property of molecules immersed in nanosized aerosol particles. We choose fluoranthene (FA) with varying cis-pinonic acid (CPA) impurity concentration as an illustrative example of application. We observe that normally FA tends to be on the surface of the nanoaerosols but in the presence of CPA impurities its spatial location changes to a core aggregate to some extent. We find that the absorption maximum is only slightly red-shifted in the presence of increased CPA concentrations and that the oscillator strengths are not altered significantly. The comparable values for the oscillator strengths of all the low energy excitations suggest that the absorption enhancement of the aerosol due to BS will not be substantial, which is in line with the recent experimental report in Science.

  • 239. Hedfors, Jim
    et al.
    Aldahan, Ala
    Kulan, Abdulhadi
    Karlsson, Karl-Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish Meteorological and Hydrological Institute, Norrköping.
    Vintersved, Ingemar
    Clouds and Be-7: Perusing connections between cosmic rays and climate2006Ingår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, nr D2, s. D02208-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    [1] Time series data on Be-7, precipitation, temperature, and satellite imagery of cloud cover over Scandinavia, together with cosmic ray and sunspot activity, were used to elucidate the relationship between cosmic rays and clouds, and ultimately climate change. The results indicate a coherent negative correlation between total cloud cover and Be-7 on intraseasonal, seasonal, and decadal scales. Although the reasons behind this correlation are unclear, a full-scale implication of this feature is in the possible use of Be-7 and Be-10 records for proxy paleo-reconstruction of total cloud cover. This is a strongly needed, but generally difficult to quantify parameter in climate models.

  • 240.
    Hedin, Jonas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Khaplanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Witt, Georg
    Stegman, Jacek
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Optical studies of noctilucent clouds in the extreme ultraviolet2008Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, nr 5, s. 1109-1119Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In order to better understand noctilucent clouds (NLC) and their sensitivity to the variable environment of the polar mesosphere, more needs to be learned about the actual cloud particle population. Optical measurements are today the only means of obtaining information about the size of mesospheric ice particles. In order to efficiently access particle sizes, scattering experiments need to be performed in the Mie scattering regime, thus requiring wavelengths of the order of the particle size. Previous studies of NLC have been performed at wavelengths down to 355 nm from the ground and down to about 200 nm from rockets and satellites. However, from these measurements it is not possible to access the smaller particles in the mesospheric ice population. This current lack of knowledge is a major limitation when studying important questions about the nucleation and growth processes governing NLC and related particle phenomena in the mesosphere. We show that NLC measurements in the extreme ultraviolet, in particular using solar Lyman-α radiation at 121.57 nm, are an efficient way to further promote our understanding of NLC particle size distributions. This applies both to global measurements from satellites and to detailed in situ studies from sounding rockets. Here, we present examples from recent rocket-borne studies that demonstrate how ambiguities in the size retrieval at longer wavelengths can be removed by invoking Lyman-α. We discuss basic requirements and instrument concepts for future rocket-borne NLC missions. In order for Lyman-α radiation to reach NLC altitudes, high solar elevation and, hence, daytime conditions are needed. Considering the effects of Lyman-α on NLC in general, we argue that the traditional focus of rocket-borne NLC missions on twilight conditions has limited our ability to study the full complexity of the summer mesopause environment.

  • 241.
    Hedin, Jonas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rapp, Markus
    On the efficiency of rocket-borne particle detection in the mesosphere2007Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 7, nr 14, s. 3701-3711Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Meteoric smoke particles have been proposed as a key player in the formation and evolution of mesospheric phenomena. Despite their apparent importance still very little is known about these particles. Important questions concern the smoke number density and size distribution as a function of altitude as well as the fraction of charged particles. Sounding rockets are used to measure smoke in situ, but aerodynamics has remained a major challenge. Basically, the small smoke particles tend to follow the gas flow around the payload rather than reaching the detector if aerodynamics is not considered carefully in the detector design. So far only indirect evidence for the existence of meteoric smoke has been available from measurements of heavy charge carriers. Quantitative ways are needed that relate these measured particle population to the atmospheric particle population. This requires in particular knowledge about the size-dependent, altitude-dependent and charge-dependent detection efficiency for a given instrument. In this paper, we investigate the aerodynamics for a typical electrostatic detector design. We first quantify the flow field of the background gas, then introduce particles in the flow field and determine their trajectories around the payload structure. We use two different models to trace particles in the flow field, a Continuous motion model and a Brownian motion model. Brownian motion is shown to be of basic importance for the smallest particles. Detection efficiencies are determined for three detector designs, including two with ventilation holes to allow airflow through the detector. Results from this investigation show that rocket-borne smoke detection with conventional detectors is largely limited to altitudes above 75 km. The flow through a ventilated detector has to be relatively large in order to significantly improve the detection efficiency.

  • 242.
    Hedin, Jonas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Stegman, Jacek
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Witt, Georg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Use of O2 airglow for calibrating direct atomic oxygen measurements from sounding rockets2009Ingår i: Atmospheric Measurement Techniques, ISSN 1867-1381, Vol. 2, s. 801-812Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Accurate knowledge about the distribution of atomic oxygen is crucial for many studies of the mesosphere and lower thermosphere. Direct measurements of atomic oxygen by the resonance fluorescence technique at 130 nm have been made from many sounding rocket payloads in the past. This measurement technique yields atomic oxygen profiles with good sensitivity and altitude resolution. However, accuracy is a problem as calibration and aerodynamics make the quantitative analysis challenging. Most often, accuracies better than a factor 2 are not to be expected from direct atomic oxygen measurements. As an example, we present results from the NLTE (Non Local Thermodynamic Equilibrium) sounding rocket campaign at Esrange, Sweden, in 1998, with simultaneous O2 airglow and O resonance fluorescence measurements. O number densities are found to be consistent with the nightglow analysis, but only within the uncertainty limits of the resonance fluorescence technique. Based on these results, we here describe how better atomic oxygen number densities can be obtained by calibrating direct techniques with complementary airglow photometer measurements and detailed aerodynamic analysis. Night-time direct O measurements can be complemented by photometric detection of the O2 (b1g+X3g-) Atmospheric Band at 762 nm, while during daytime the O2 (a1ΔgX3g-) Infrared Atmospheric Band at 1.27 μm can be used. The combination of a photometer and a rather simple resonance fluorescence probe can provide atomic oxygen profiles with both good accuracy and good height resolution.

  • 243.
    Hedin, Jonas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Waldemarsson, Tomas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Giovane, Frank
    Naval Research Laboratory, Washington, DC.
    The aerodynamics of the MAGIC meteoric smoke sampler2007Ingår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 40, nr 6, s. 818-824Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The detection of nanometre-sized meteoric smoke particles in the Earth’s mesosphere and lower thermosphere is difficult. The particles are too small for optical detection and so far only the charged fraction of the particles has been probed by rocket-borne instruments. One way to obtain maximum information about the smoke particles is direct sampling with rocket-borne particle samplers. The MAGIC project (Mesospheric Aerosol – Genesis, Interaction and Composition) aims to quantitatively answer fundamental questions about the properties of smoke in the atmosphere. The first launch of such particle samplers was performed during the MAGIC rocket campaign from Esrange, Sweden, in January 2005. In order to characterise the sampling process, we have performed simulations of the trajectories of nanometre-sized dust particles towards the MAGIC detectors with a new particle motion model. An important feature of this model is the Brownian motion of the particles due to thermal collisions of the gas molecules. As a result, we obtain the detection efficiency for the MAGIC detectors as a function of altitude and particle size. Our simulations confirm that particles of radii down to 0.75 nm impact on the sampling surface with an efficiency exceeding 80% over the entire mesospheric altitude range of interest.

  • 244.
    Hedin, Jonas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rapp, Markus
    Deutsches Zentrum für Luft und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany.
    Khaplanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Stegman, Jacek
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Witt, Georg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Observations of NO in the upper mesosphere and lower thermosphere during ECOMA 20102012Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 30, s. 1611-1621Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In December 2010 the last campaign of the German-Norwegian sounding rocket project ECOMA (Existence and Charge state Of Meteoric smoke particles in the middle Atmosphere) was conducted from Andøya Rocket Range in northern Norway (69° N, 16° E) in connection with the Geminid meteor shower. The main instrument on board the rocket payloads was the ECOMA detector for studying meteoric smoke particles (MSPs) by active photoionization and subsequent detection of the produced charges (particles and photoelectrons). In addition to photoionizing MSPs, the energy of the emitted photons from the ECOMA flash-lamp is high enough to also photoionize nitric oxide (NO). Thus, around the peak of the NO layer, at and above the main MSP layer, photoelectrons produced by the photoionization of NO are expected to contribute to, or even dominate above the main MSP-layer, the total measured photoelectron current. Among the other instruments on board was a set of two photometers to study the O2(b1Σg+X3Σg-) Atmospheric band and NO2 continuum nightglow emissions. In the absence of auroral emissions, these two nightglow features can be used together to infer NO number densities. This will provide a way to quantify the contribution of NO photoelectrons to the photoelectron current measured by the ECOMA instrument and, above the MSP layer, a simultaneous measurement of NO with two different and independent techniques. This work is still on-going due to the uncertainties, especially in the effort to quantitatively infer NO densities from the ECOMA photoelectron current, and the lack of simultaneous measurements of temperature and density for the photometric study. In this paper we describe these two techniques to infer NO densities and discuss the uncertainties. The peak NO number density inferred from the two photometers on ascent was 3.9 × 108 cm−3 at an altitude of about 99 km, while the concentration inferred from the ECOMA photoelectron measurement at this altitude was a factor of 5 smaller.

  • 245.
    Heifetz, Eyal
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Tel Aviv University, Israel.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    An alternative view on the role of the beta-effect in the Rossby wave propagation mechanism2014Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, s. 22672-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The role of the beta-effect in the Rossby wave propagation mechanism is examined in the linearised shallow water equations directly in momentum-height variables, without recourse to potential vorticity (PV). Rigorous asymptotic expansion of the equations, with respect to the small non-dimensionalised beta parameter, reveals in detail how the Coriolis force acting on the small ageostrophic terms translates the geostrophic leading-order solution to propagate westward in concert. This information cannot be obtained directly from the conventional PV perspective on the propagation mechanism. Furthermore, a comparison between the beta-effect in planetary Rossby waves and the sloping-bottom effect in promoting topographic Rossby waves shows that the ageostrophic terms play different roles in the two cases. This is despite the fact that from the PV viewpoint whether the advection of mean PV gradient is set up by changes in planetary vorticity or by mean depth is inconsequential.

  • 246. Heintzenberg, J.
    et al.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The summer aerosol in the Central Arctic 1991 2008: did it change or not?2012Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 9, s. 3969-3983Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the course of global warming dramatic changes are taking place in the Arctic and boreal environments. However, physical aerosol data in from the central summer Arctic taken over the course of 18 yr from 1991 to 2008 do not show systematic year-to-year changes, albeit substantial interannual variations. Besides the limited extent of the data several causes may be responsible for these findings. The processes controlling concentrations and particle size distribution of the aerosol over the central Arctic perennial pack ice area, north of 80A degrees, may not have changed substantially during this time. Environmental changes are still mainly effective in the marginal ice zone, the ice-free waters and continental rims and have not propagated significantly into the central Arctic yet where they could affect the local aerosol and its sources. The analysis of meteorological conditions of the four expedition summers reveal substantial variations which we see as main causes of the measured variations in aerosol parameters. With combined lognormal fits of the hourly number size distributions of the four expeditions representative mode parameters for the summer aerosol in the central Arctic have been calculated. The combined aerosol statistics discussed in the present paper provide comprehensive physical data on the summer aerosol in the central Arctic. These data are the only surface aerosol information from this region.

  • 247. Heintzenberg, Jost
    et al.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Birmili, W
    Wehner, B
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Wiedensohler, A
    Aerosol number-size distributions during clear and fog periods in the summer high Arctic: 1991, 1996 and 20012006Ingår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 58, s. 41-50Artikel i tidskrift (Refereegranskat)
  • 248.
    Held, A.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Brooks, I. M.
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    On the potential contribution of open lead particle emissions to the = ntral Arctic aerosol concentration2011Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 7, s. 3093-3105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present direct eddy covariance measurements of aerosol number fluxes, dominated by sub-50 nm particles, at the edge of an ice floe drifting in the central Arctic Ocean. The measurements were made during the ice-breaker borne ASCOS (Arctic Summer Cloud Ocean Study) expedition in August 2008 between 2 degrees-10 degrees W longitude and 87 degrees-87.5 degrees N latitude. The median aerosol transfer velocities over different surface types (open water leads, ice ridges, snow and ice surfaces) ranged from 0.27 to 0.68 mm s(-1) during deposition-dominated episodes. Emission periods were observed more frequently over the open lead, while the snow behaved primarily as a deposition surface. Directly measured aerosol fluxes were compared with particle deposition parameterizations in order to estimate the emission flux from the observed net aerosol flux. Finally, the contribution of the open lead particle source to atmospheric variations in particle number concentration was evaluated and compared with the observed temporal evolution of particle number. The direct emission of aerosol particles from the open lead can explain only 5-10% of the observed particle number variation in the mixing layer close to the surface.

  • 249. Held, A.
    et al.
    Orsini, D. A.
    Vaattovaara, P.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Near-surface profiles of aerosol number concentration and temperature over the Arctic Ocean2011Ingår i: Atmospheric Measurement Techniques, ISSN 1867-1381, Vol. 4, nr 8, s. 1603-1616Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Temperature and particle number concentration profiles were measured at small height intervals above open and frozen leads and snow surfaces in the central Arctic. The device used was a gradient pole designed to investigate potential particle sources over the central Arctic Ocean. The collected data were fitted according to basic logarithmic flux-profile relationships to calculate the sensible heat flux and particle deposition velocity. Independent measurements by the eddy covariance technique were conducted at the same location. General agreement was observed between the two methods when logarithmic profiles could be fitted to the gradient pole data. In general, snow surfaces behaved as weak particle sinks with a maximum deposition velocity upsilon(d) = 1.3 mm s(-1) measured with the gradient pole. The lead surface behaved as a weak particle source before freeze-up with an upward flux F(c) = 5.7 x 10(4) particles m(-2) s(-1), and as a relatively strong heat source after freeze-up, with an upward maximum sensible heat flux H = 13.1 W m(-2). Over the frozen lead, however, we were unable to resolve any significant aerosol profiles.

  • 250. Held, Andreas
    et al.
    Brooks, Ian
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    On the potential contribution of open lead particle emissions to the central Arctic aerosol concentration2011Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, s. 3093-3105Artikel i tidskrift (Refereegranskat)
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