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  • 251.
    Gidhagen, Lars
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kahelin, Hanna
    Schmidt-Thomé, Philip
    Johansson, Christer
    Anthropogenic and natural levels of arsenic in PM10 in Central and Northern Chile2002Inngår i: Atmospheric Environment, ISSN 1352-2310, Vol. 36, nr 23, s. 3803-3817Artikkel i tidsskrift (Fagfellevurdert)
  • 252.
    Glantz, Paul
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Bourassa, Adam
    Herber, Andreas
    Iversen, Trond
    Karlsson, Johannes
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Kirkevag, Alf
    Maturilli, Marion
    Seland, Oyvind
    Stebel, Kerstin
    Struthers, Hamish
    Tesche, Matthias
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Thomason, Larry
    Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations2014Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 119, nr 13, s. 8169-8188Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (Delta AOT = +/- 0.03 +/- 0.05 . AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer.

  • 253. Goldner, A.
    et al.
    Huber, M.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Does Antarctic glaciation cool the world?2013Inngår i: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, nr 1, s. 173-189Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, we compare the simulated climatic impact of adding an Antarctic ice sheet (AIS) to the greenhouse world of the Eocene and removing the AIS from the modern world. The modern global mean surface temperature anomaly (Delta T) induced by Antarctic Glaciation depends on the background CO2 levels and ranges from -1.22 to -0.18 K. The Eocene Delta T is nearly constant at similar to-0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as Delta T divided by the change in effective radiative forcing (Delta Q(Antarctica)) which includes some fast feedbacks imposed by prescribing the glacial properties of Antarctica. The main difference between the modern and Eocene responses is that a negative cloud feedback warms much of the Earth's surface as a large AIS is introduced in the Eocene, whereas this cloud feedback is weakly positive and acts in combination with positive sea-ice feedbacks to enhance cooling introduced by adding an ice sheet in the modern. Because of the importance of cloud feedbacks in determining the final temperature sensitivity of the AIS, our results are likely to be model dependent. Nevertheless, these model results suggest that the effective radiative forcing and feedbacks induced by the AIS did not significantly decrease global mean surface temperature across the Eocene-Oligocene transition (EOT -34.1 to 33.6 Ma) and that other factors like declining atmospheric CO2 are more important for cooling across the EOT. The results illustrate that the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. This implies that using EOT paleoclimate proxy data by itself to estimate climate sensitivity for future climate prediction requires climate models and consequently these estimates will have large uncertainty, largely due to uncertainties in modelling low clouds.

  • 254. Goosse, Hugues
    et al.
    Kay, Jennifer E.
    Armour, Kyle C.
    Bodas-Salcedo, Alejandro
    Chepfer, Helene
    Docquier, David
    Jonko, Alexandra
    Kushner, Paul J.
    Lecomte, Olivier
    Massonnet, Francois
    Park, Hyo-Seok
    Pithan, Felix
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Vancoppenolle, Martin
    Quantifying climate feedbacks in polar regions2018Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, artikkel-id 1919Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The concept of feedback is key in assessing whether a perturbation to a system is amplified or damped by mechanisms internal to the system. In polar regions, climate dynamics are controlled by both radiative and non-radiative interactions between the atmosphere, ocean, sea ice, ice sheets and land surfaces. Precisely quantifying polar feedbacks is required for a process-oriented evaluation of climate models, a clear understanding of the processes responsible for polar climate changes, and a reduction in uncertainty associated with model projections. This quantification can be performed using a simple and consistent approach that is valid for a wide range of feedbacks, offering the opportunity for more systematic feedback analyses and a better understanding of polar climate changes.

  • 255.
    Gordon, Line J.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för systemekologi.
    Steffen, Will
    Jönsson, Bror F.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Folke, Carl
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för systemekologi.
    Falkenmark, Malin
    Johannessen, Åse
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för systemekologi.
    Human modification of global water vapor flows from the land surface2005Inngår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 102, nr 21, s. 7612-7617Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    It is well documented that human modification of the hydrological cycle has profoundly affected the flow of liquid water across the Earth’s land surface. Alteration of water vapor flows through land-use changes has received comparatively less attention, despite compelling evidence that such alteration can influence the functioning of the Earth System. We show that deforestation is as large a driving force as irrigation in terms of changes in the hydrological cycle. Deforestation has decreased global vapor flows from land by 4% (3,000 km3/yr), a decrease that is quantitatively as large as the increased vapor flow caused by irrigation (2,600 km3/yr). Although the net change in global vapor flows is close to zero, the spatial distributions of deforestation and irrigation are different, leading to major regional transformations of vapor-flow patterns. We analyze these changes in the light of future land-use-change projections that suggest widespread deforestation in sub-Saharan Africa and intensification of agricultural production in the Asian monsoon region. Furthermore, significant modification of vapor flows in the lands around the Indian Ocean basin will increase the risk for changes in the behavior of the Asian monsoon system. This analysis suggests that the need to increase food production in one region may affect the capability to increase food production in another. At the scale of the Earth as a whole, our results emphasize the need for climate models to take land-use change, in both land cover and irrigation, into account.

  • 256.
    Gouirand, Isabelle
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Linderholm, Hans W.
    Moberg, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi. Meteorologiska institutionen (MISU).
    Wohlfarth, Barbara
    Institutionen för geologi och geokemi.
    On the spatio-temporal characteristics of Fennoscandian tree-ring based summer temperatures reconstructions2008Inngår i: Theoretical and Applied Climatology, Vol. 91, s. 1-25Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We analyse the spatial representation of five previously published multi-century to millennial length dendroclimatological reconstructions of Fennoscandian summer temperatures. The reconstructions, ranging from local to regional scale, were based on either tree-ring width (TRW) or maximum latewood density (MXD) data or on a combination of the two. TRW chronologies are shown to provide reasonably good spatial information mainly for July temperatures, but a combination of TRW and MXD yields a better spatial representation for the whole summer season (June–August). A multiple-site reconstruction does not necessarily provide better spatial representation than a single site reconstruction, depending on the criterion for selecting data and also on the strength of the climate signal in the tree-ring data. In a new approach to analyse the potential for further developing Fennoscandian temperature reconstructions, we selected from a network of TRW and MXD chronologies those having the strongest temperature information a priori, to obtain a strong common climate signal suitable for a regional-scale reconstruction. Seven separate, but not independent, reconstructions based on progressively decreasing numbers of chronologies were created. We show that it is possible to improve the spatial representation of available reconstructions back to around AD 1700, giving high correlations (>0.7) with observed summer temperatures for nearly the whole of Fennoscandia, and even higher correlations (>0.85) over much of central-northern Fennoscandia. Further sampling of older trees (e.g. dry-dead and subfossil wood) would be needed to achieve the same high correlations prior to AD 1700. Our analysis suggests that it should be possible to select a few key sites for improving the reconstructions before AD 1700. Since tree-ring data from northern Fennoscandia are used in all available hemispheric-scale temperature reconstructions for the last millennium, there is also a potential for slightly improving the quality of the hemispheric-scale reconstructions, by including an improved reconstruction for Fennoscandia. However, adding new chronologies from previously unsampled regions would potentially improve hemispheric-scale temperature reconstructions more substantially.

  • 257.
    Gouirand, Isabelle
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Moberg, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi. Meteorologiska institutionen (MISU).
    Zorita, Eduardo
    Climate variability in Scandinavia for the past millennium simulated by an atmosphere-ocean general circulation model2007Inngår i: Tellus: Tellus, Vol. 59A, s. 30-49Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atmosphere-ocean model ECHO-G, run with solar, volcanic and greenhouse gas forcing for the past millennium, is used to analyse winter and summer temperature variability in Scandinavia. Relationships with atmospheric circulation, North Atlantic SSTs and Northern Hemisphere (NH) temperatures are investigated at timescales longer and shorter than 10 yr. The simulated response to volcanic forcing is also analysed. Realistic relationships with the atmospheric circulation, with some deficiencies in summer, are found. High-frequency co-variations with SSTs and NH temperatures are too weak, but low-frequency co-variations with NH temperatures in winter are apparently too strong. The summer cooling response to volcanic forcing is realistic, but the expected winter warming is absent. The simulated long-term temperature evolution agrees broadly with proxy data. Combinations of several forcing factors can lead to decadal and multidecadal anomalies from the centennial trends. Decreased solar forcing can account for cold intervals in both summer and winter. A systematic negative North Atlantic Oscillation (NAO) phase can explain the coldest winter temperatures during 1590–1650. Several strong volcanic forcing events can have contributed to a simultaneous summer cooling. Proxy data also indicate cold summers and winters, and a negative NAO in winter, in the same period.

  • 258.
    Granat, Lennart
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Erik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Praveen, Siva
    Rodhe, Henning
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Light Absorbing Material ("Soot") in Rainwater and in aerosol particles in the Maldives2010Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, s. D16307-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simultaneous measurements of soot (absorbing material at 528 nm) and inorganic ions in aerosol and precipitation at the Maldives Climate Observatory Hanimaadhoo during the period May 2005 to February 2007 have made it possible to calculate the washout ratio (WR) of these components as a measure of how efficiently they are scavenged by precipitation. Based on air trajectories the data have been separated into days with polluted air arriving from the Indian subcontinent in a north-easterly sector during winter and clean monsoon days with southerly flow from the Indian Ocean. The average soot concentration was a factor of ten higher in the former situations.

    Despite considerable scatter for individual days a systematic pattern emerged when the WR for the different components were compared with each other. During the monsoon season the WR for soot was similar to that of sulphate and other fine mode aerosol components, indicating that soot containing particles in these situations were efficient as cloud condensation nuclei. The origin of the light absorbing material during the monsoon season is unclear. We speculate that light absorbing material from the tropical ocean surface could contribute to the concentration of "soot" during the monsoon season.

    During the polluted winter days, on the other hand, the WR for soot was 3 times smaller than that of sulphate. This indicates that, even after a travel time of several days, the soot containing particles from India have retained much of their hydrophobic property and that the soot must be mainly externally mixed. The low WR and the infrequent rain during this season probably contribute to extending the atmospheric lifetime of soot well beyond several days.

    Surprisingly high concentrations of non sea salt calcium were measured during the monsoon season, substantially higher than during the winter season. The origin of these high values might be long-range transport from the Australian or African continents. Another possibility might be exopolymer gels derived from the ocean surface micro-layer.

  • 259.
    Grand Graversen, Rune
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Wang, Minghuai
    Polar amplification in a coupled climate model with locked albedo2009Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 33, nr 5, s. 629-643Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling of the atmospheric CO2 content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature amplification.

  • 260.
    Grantinger, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ensemble Kalman filters in mesoscale limited-area numerical weather prediction2007Licentiatavhandling, monografi (Annet vitenskapelig)
  • 261.
    Graversen, Rune
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Drijfhout, S.
    Hazeleger, W.
    van de Wal, R.
    Bintanja, R.
    Helsen, M.
    Greenland’s contribution to global sea-level rise by the end of the 21st century2011Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, s. 1427-1442Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Greenland ice sheet holds enough water to raise the global sea level with *7 m. Over the last few decades, observations manifest a substantial increase of the mass loss of this ice sheet. Both enhanced melting and increase of the dynamical discharge, associated with calving at the outlet-glacier fronts, are contributing to the mass imbalance. Using a dynamical and thermodynamical ice-sheet model, and taking into account speed up of outlet glaciers, we estimate Greenland’s contribution to the 21stcentury global sea-level rise and the uncertainty of this estimate. Boundary fields of temperature and precipitation extracted from coupled climate-model projections used for the IPCC Fourth Assessment Report, are applied to the icesheet model. We implement a simple parameterization for increased flow of outlet glaciers, which decreases the bias of the modeled present-day surface height. It also allows for taking into account the observed recent increase in dynamical discharge, and it can be used for future projections associated with outlet-glacier speed up. Greenland contributes 0–17 cm to global sea-level rise by the end of the 21st century. This range includes the uncertainties in climate-model projections, the uncertainty associated with scenarios of greenhouse-gas emissions, as well as the uncertainties in future outlet-glacier discharge. In addition, the range takes into account the uncertainty of the ice-sheet model and its boundary fields.

  • 262.
    Graversen, Rune G.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Do Changes in the Midlatitude Circulation Have Any Impact on the Arctic Surface Air Temperature Trend?2006Inngår i: Journal of Climate, ISSN 0894-8755, Vol. 19, nr 20, s. 5422-5437Artikkel i tidsskrift (Fagfellevurdert)
  • 263. Graversen, Rune G.
    et al.
    Burtu, Mattias
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Arctic amplification enhanced by latent energy transport of atmospheric planetary waves2016Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, nr 698, s. 2046-2054Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atmospheric northward energy transport plays a crucial role for the Arctic climate; this transport brings to the Arctic an amount of energy comparable to that provided directly by the sun. The transport is accomplished by atmospheric waves-for instance large-scale planetary waves and meso-scale cyclones-and the zonal-mean circulation. These different components of the energy transport impact the Arctic climate differently. A split of the transport into stationary and transient waves constitutes a traditional way of decomposing the transport. However this procedure does not take into account the transport accomplished separately by the planetary and synoptic-scale waves. Here a Fourier decomposition is applied, which decomposes the transport with respect to zonal wave numbers. Reanalysis and model data reveal that the planetary waves impact Arctic temperatures much more than do synoptic-scale waves. In addition the latent transport by these waves affects the Arctic climate more than does the dry-static part. Finally, the EC-Earth model suggests that changes of the energy transport over the twentyfirst century will contribute to Arctic warming, despite the fact that in this model the total energy transport to the Arctic will decrease. This apparent contradictory result is due to the cooling induced by a decrease of the dry-static transport by planetary waves being more than compensated for by a warming caused by the latent counterpart.

  • 264.
    Graversen, Rune G.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Källén, Erland
    Tjernström, Michael
    Körnich, Heiner
    Atmospheric mass-transport inconsistencies in the ERA-40 reanalysis2007Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, Vol. 133, nr 624, s. 673-680Artikkel i tidsskrift (Fagfellevurdert)
  • 265.
    Graversen, Rune G.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Langen, Peter L.
    Mauritsen, Thorsten
    Polar Amplification in CCSM4: Contributions from the Lapse Rate and Surface Albedo Feedbacks2014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 12, s. 4433-4450Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A vertically nonuniform warming of the troposphere yields a lapse rate feedback by altering the infrared irradiance to space relative to that of a vertically uniform tropospheric warming. The lapse rate feedback is negative at low latitudes, as a result of moist convective processes, and positive at high latitudes, due to stable stratification conditions that effectively trap warming near the surface. It is shown that this feedback pattern leads to polar amplification of the temperature response induced by a radiative forcing. The results are obtained by suppressing the lapse rate feedback in the Community Climate System Model, version 4 (CCSM4). The lapse rate feedback accounts for 15% of the Arctic amplification and 20% of the amplification in the Antarctic region. The fraction of the amplification that can be attributed to the surface albedo feedback, associated with melting of snow and ice, is 40% in the Arctic and 65% in Antarctica. It is further found that the surface albedo and lapse rate feedbacks interact considerably at high latitudes to the extent that they cannot be considered independent feedback mechanisms at the global scale.

  • 266. Graversen, Rune G.
    et al.
    Mauritsen, Thorsten
    Drijfhout, Sybren
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mårtensson, Sebastian
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 20072011Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, nr 11-12, s. 2103-2112Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989-2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.

  • 267.
    Graversen, Rune G.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mauritsen, Thorsten
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Källén, Erland
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Vertical structure of recent Arctic warming2008Inngår i: Nature, ISSN 0028-0836, Vol. 451, nr 7174, s. 53-56Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Near-surface warming in the Arctic has been almost twice as large as the global average over recent decades1, 2, 3, 4, 5—a phenomenon that is known as the 'Arctic amplification'. The underlying causes of this temperature amplification remain uncertain. The reduction in snow and ice cover that has occurred over recent decades6, 7 may have played a role5, 8. Climate model experiments indicate that when global temperature rises, Arctic snow and ice cover retreats, causing excessive polar warming9, 10, 11. Reduction of the snow and ice cover causes albedo changes, and increased refreezing of sea ice during the cold season and decreases in sea-ice thickness both increase heat flux from the ocean to the atmosphere. Changes in oceanic and atmospheric circulation, as well as cloud cover, have also been proposed to cause Arctic temperature amplification12, 13, 14, 15, 16, 17. Here we examine the vertical structure of temperature change in the Arctic during the late twentieth century using reanalysis data. We find evidence for temperature amplification well above the surface. Snow and ice feedbacks cannot be the main cause of the warming aloft during the greater part of the year, because these feedbacks are expected to primarily affect temperatures in the lowermost part of the atmosphere, resulting in a pattern of warming that we only observe in spring. A significant proportion of the observed temperature amplification must therefore be explained by mechanisms that induce warming above the lowermost part of the atmosphere. We regress the Arctic temperature field on the atmospheric energy transport into the Arctic and find that, in the summer half-year, a significant proportion of the vertical structure of warming can be explained by changes in this variable. We conclude that changes in atmospheric heat transport may be an important cause of the recent Arctic temperature amplification.

  • 268.
    Graversen, Rune Grand
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    On the recent Arctic Warming2008Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The Arctic region attracts considerable scientific interest in these years. Some of the Earth's most pronounced signs of the recent climate change are found here. The summer sea-ice cover is shrinking at an alarming rate. At the same time the region warms faster than the rest of the globe.

    The sea-ice reduction implies an increase of solar-radiation absorption at the surface leading to warming which is expected to be larger at higher than at lower latitudes. It is therefore often assumed that the sea-ice reduction is a major cause of the observed Arctic temperature amplification. However, results presented in this thesis suggest that the snow and ice-albedo feedbacks are a contributing but not dominating mechanism behind the Arctic amplification. A coupled climate-model experiment with a doubling of the atmospheric CO2 concentration reveals a considerable Arctic surface-air-temperature amplification in a world without surface-albedo feedback. The amplification is only 8 % larger when this feedback is included. Instead the greenhouse effect associated with an increase of humidity and cloud cover over the Arctic seems to play a major role for the amplification.

    Reanalysis data, which are partly based on observations, show Arctic temperature amplification well above the surface in the troposphere. In the summer season, the amplification has its maximum at ~ 2 km height. These trends cannot be explained by the snow- and ice-albedo feedbacks which are expected to induce the largest amplification near the surface. Instead, a considerable part of the trends aloft can be linked to an increase of the atmospheric energy transport into the Arctic.

    A major topic of this thesis is the linkage between the mid-latitude circulation and the Arctic warming. It is suggested that the atmospheric meridional energy transport is an efficient indicator of this linkage.

  • 269.
    Graversen, Rune Grand
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Källén, Erland
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Körnich, Heiner
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Atmospheric mass-budget inconsistency in the ERA-40 reanalysis2007Inngår i: Quarterly Journal of the Royal Meteorological society, ISSN 0035-9009, Vol. 133, s. 673-680Artikkel i tidsskrift (Fagfellevurdert)
  • 270.
    Graversen, Rune Grand
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mauritsen, Thorsten
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Källén, Erland
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Reply: Communications arising2008Inngår i: Nature, Vol. 455, s. E4-E5Artikkel i tidsskrift (Fagfellevurdert)
  • 271. Green, J. A. Mattias
    et al.
    Nycander, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    A comparison of tidal conversion parameterizations for tidal models2013Inngår i: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 43, nr 1, s. 104-119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The conversion of barotropic to baroclinic tidal energy in the global abyssal ocean is calculated using three different formulations. The calculations are done both offline, that is, using externally given tidal currents to estimate the energy conversion, and online, that is, by using the formulations to parameterize linear wave drag in a prognostic tidal model. All three schemes produce globally integrated offline dissipation rates beneath 500-m depth of similar to 0.6-0.8TW for the M-2 constituent, but the spatial structures vary significantly between the parameterizations. Detailed investigations of the energy transfer in local areas confirm the global results: there are large differences between the schemes, although the horizontally integrated conversion rates are similar. The online simulations are evaluated by comparing the sea surface elevation with data from the TOPEX/Poseidon database, and the error is then significantly lower when using the parameterization provided by Nycander than with the other two parameterizations examined.

  • 272. Grisogono, Branko
    et al.
    Ström, Linda
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Small-scale variability in the coastal atmospheric boundary layer1998Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 88, s. 23-46Artikkel i tidsskrift (Fagfellevurdert)
  • 273. Grisogono, Branko
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Thermal mesoscale circulations on the Baltic coast 2. Perturbation of surface parameters1996Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 101, s. 18999-19012Artikkel i tidsskrift (Fagfellevurdert)
  • 274. Gualtieri, P.
    et al.
    Picano, F.
    Sardina, Gaetano
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Casciola, C. M.
    Exact regularized point particle method for multiphase fows in the two-way coupling regime2015Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 773, s. 520-561Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Particulate flows have mainly been studied under the simplifying assumption of a one-way coupling regime where the disperse phase does not modify the carrier fluid. A more complete view of multiphase flows can be gained calling into play two-way coupling effects, i.e. by accounting for the inter-phase momentum exchange, which is certainly relevant at increasing mass loading. In this paper we present a new methodology rigorously designed to capture the inter-phase momentum exchange for particles smaller than the smallest hydrodynamical scale, e.g. the Kolmogorov scale in a turbulent flow. The momentum coupling mechanism exploits the unsteady Stokes flow around a small rigid sphere, where the transient disturbance produced by each particle is evaluated in a closed form. The particles are described as lumped point masses, which would lead to the appearance of singularities. A rigorous regularization procedure is conceived to extract the physically relevant interactions between the particles and the fluid which avoids any 'ad hoc' assumption. The approach is suited for high-efficiency implementation on massively parallel machines since the transient disturbance produced by the particles is strongly localized in space. We will show that hundreds of thousands of particles can be handled at an affordable computational cost, as demonstrated by a preliminary application to a particle-laden turbulent shear flow.

  • 275. Guineva, V
    et al.
    Witt, Georg
    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).
    Werner, R
    Hedin, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Neichev, S
    Kirov, B
    Bankov, L
    Gramatikov, P
    Tashev, V
    Popov, M
    Hauglund, K
    Hansen, G
    Ilstad, J
    Wold, H
    Lyman-alpha Detector Designed for Rocket Measurements of the Direct Solar Radiation at 121.5 nm2007Inngår i: Bulgarian Journal of Physics, Vol. 3, s. 116-127Artikkel i tidsskrift (Fagfellevurdert)
  • 276. Guineva, V.
    et al.
    Witt, Georg
    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).
    Werner, R.
    Hedin, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Neichev, S.
    Kirov, B.
    Bankov, L.
    Gramatikov, P.
    Tashev, V.
    Popov, M.
    Hauglund, K.
    Hansen, G.
    Ilstad, J.
    Wold, H.
    O-2 density and temperature profiles retrieving from direct solar Lyman-alpha radiation measurements2009Inngår i: Geomagnetism and Aeronomy, ISSN 0016-7932, E-ISSN 1555-645X, Vol. 49, nr 8, s. 1292-1295Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The resonance transition P-2-S-2 of the atomic hydrogen (Lyman-alpha emission) is the strongest and most conspicuous feature in the solar EUV spectrum. The Lyman-alpha radiation transfer depends on the resonance scattering from the hydrogen atoms in the atmosphere and on the O-2 absorption. Since the Lyman-alpha extinction in the atmosphere is a measure for the column density of the oxygen molecules, the atmospheric O-2 density and temperature profiles can be calculated thereof. A detector of solar Lyman-alpha radiation was manufactured in the Stara Zagora Department of the Solar-Terrestrial Influences Laboratory (STIL). Its basic part is an ionization camera, filled in with NO. A 60 V power supply is applied to the chamber. The produced photoelectric current from the sensor is fed to a two-channel amplifier, providing analog signal. The characteristics of the Lyman-alpha detector were studied. It passed successfully all tests and the results showed that the so-designed instrument could be used in rocket experiments to measure the Lymanalpha flux. From the measurements of the detector, the Lyman-alpha vertical profile can be obtained. Programs are created to compute the O-2 density, atmospheric power and temperature profiles based on Lymanalpha data. The detector design appertained to ASLAF project (Attenuation of the Solar Lyman-Alpha Flux), a scientific cooperation between STIL-Bul.Acad.Sci., Stara Zagora Department and the Atmospheric Physics Group at the Department of Meteorology (MISU), Stockholm University, Sweden. The joint project was part of the rocket experiment HotPay I, in the ALOMAR eARI Project, EU's 6th Framework Programme, Andoya Rocket Range, Andenes, Norway. The project is partly financed by the Bulgarian Ministry of Science and Education.

  • 277. Gumbel, J.
    et al.
    Megner, Linda
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The importance of charging processes for mesospheric ice nucleationInngår i: Atmospheric chemistry and physicsArtikkel i tidsskrift (Fagfellevurdert)
  • 278.
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rocket-borne optical measurements of minor constituents in the middle atmosphere1997Doktoravhandling, monografi (Annet vitenskapelig)
  • 279.
    Gumbel, Jörg
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Fan, Z. Y.
    Wademarsson, Tomas
    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).
    Llewellyn, E. J.
    She, C.-Y.
    Plane, J. M. C.
    Retrieval of the global mesopsheric sodium density from the Odin satellite2007Inngår i: Geophysical Research Letters, Vol. 34, nr L04813Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Satellite observations of the Na D dayglow at 589 nm provide a global database for the climatology of the mesospheric sodium layer. More than five years of Na D limb observations are available from the Optical Spectrograph and InfraRed Imager System onboard the Odin satellite. We describe a robust retrieval method that provides individual sodium density profiles with a typical accuracy of 20% and altitude resolution of 2 km. Retrieved column abundances and density profiles are validated against sodium resonance lidar measurements at mid-latitudes. Examples of the seasonal and latitudinal variation of the sodium layer illustrate Odin's potential for climatological studies of mesospheric metals.

  • 280.
    Gumbel, Jörg
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Fan, Z. Y.
    Waldemarsson, T.
    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).
    Llewellyn, E. J.
    She, C. -Y
    Plane, J. M. C.
    Retrieval of global mesospheric sodium densities from the Odin satellite2007Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, nr 4, s. L04813-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Satellite observations of the Na D dayglow at 589 nm provide a global database for the climatology of the mesospheric sodium layer. More than five years of Na D limb observations are available from the Optical Spectrograph and InfraRed Imager System onboard the Odin satellite. We describe a robust retrieval method that provides individual sodium density profiles with a typical accuracy of 20% and altitude resolution of 2 km. Retrieved column abundances and density profiles are validated against sodium resonance lidar measurements at mid- latitudes. Examples of the seasonal and latitudinal variation of the sodium layer illustrate Odin's potential for climatological studies of mesospheric metals.

  • 281.
    Gumbel, Jörg
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hultgren, Kristoffer
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mesospheric Airglow/Aerosol Tomography and Spectroscopy (MATS) - a satellite mission on mesospheric wavesManuskript (preprint) (Annet vitenskapelig)
  • 282.
    Gumbel, Jörg
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Körnich, Heiner
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bailey, S. M.
    Lubken, F. -J
    Morris, R.
    Special issue on layered phenomena in the mesopause region Foreword2011Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 73, nr 14-15, s. 2045-2048Artikkel i tidsskrift (Fagfellevurdert)
  • 283.
    Gumbel, Jörg
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Megner, Linda
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Charged meteoric smoke as ice nuclei in the mesosphere. Part 1: A review of basic concepts2009Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 71, nr 12, s. 1225-1235Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The role of meteoric smoke as condensation nuclei for mesospheric ice has recently been challenged by model simulations on the global transport of meteoric material. At the same time a considerable fraction of smoke particles is charged in the mesosphere. This has significant effects on nucleation processes as it can remove the Kelvin barrier. We suggest that in particular nucleation on negatively charged smoke is likely to be a dominant mechanism for mesospheric ice formation. This is in contrast to nucleation on positive ion clusters as the latter is largely hampered by efficient ion/electron recombination. Surprisingly, the large potential of nucleation on charged smoke has so far not been considered in mesospheric ice models. A challenging question concerns the fraction of mesospheric smoke that is actually charged. An improved understanding of mesospheric charging and nucleation will require laboratory experiments on nuclei in the transition regime between molecular and particulate sizes.

  • 284.
    Gustafsson, Nils
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Data assimilation for numerical weather prediction systems: a 25 year perspective1997Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis deals with the assimilation of meteorological observations in numerical weather prediction models. A selection of research and development contributions by the author during 25 years is included and discussed in the light of international development efforts up to the present time.

    The central meteorological data assimilation problem is that the degrees of freedom associated with the state of the atmosphere, as represented in a weather forecast model, is at least one order of magnitude larger than the degrees of freedom of available observational data. To compensate for this lack of observations, various constraints to reduce the degrees of freedom of the model state have to be introduced.

    Development efforts during the early 1970's were devoted to basic spatial interpolation techniques, used to merge observed information from one point in time with earlier observed information, represented in the state variables of a forecast model. The author contributed to the development of statistical interpolation, a technique that has become a standard data assimilation tool for operational numerical weather prediction. The development was started with simple 2-dimensional schemes and continued with a more complete 3-dimensional scheme including simplified dynamical constraints.

    Observations from meteorological satellites have become important for operational numerical weather prediction, and the data assimilation techniques developed by the author have proven to be able to handle also these observations efficiently.

    The Swedish Meteorolgical and Hydrological Institute joined the international High Resolution Limited Area Modelling (HIRLAM) project in 1985 and the author has contributed to the development of the HIRLAM data assimilation. A study on the importance of proper lateral boundary conditions for limited area data assimilation is included in this thesis. More recent development efforts by the author have been devoted to the developement of a new data assimilation system for HIRLAM based on variational techniques. It is shown in a paper, included in this thesis, how high frequency gravity-inertia oscillations can be controlled in 4-dimensional variational data assimilation by means of a weak digital filter constraint. The building of more physical and dynamical knowledge into future data assimilation systems is finally suggested in this thesis. The results of an idealized study in support of this suggestion are presented.

  • 285. Gustafsson, Nils
    et al.
    Berre, Loik
    Hörnquist, Sara
    Huang, Xiang-Yu
    Lindskog, Magnus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Navascues, Beatriz
    Mogensen, Kristian
    Thorsteinsson, Sigurdur
    Three-dimensional variational data assimilation for a limited area model: Part I: General formulation and the background error constraint2001Inngår i: Tellus A, ISSN 0280-6495, Vol. 53, nr 4, s. 425-446Artikkel i tidsskrift (Fagfellevurdert)
  • 286.
    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?2009Inngår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 323, nr 23 January, s. 495-498Artikkel i tidsskrift (Fagfellevurdert)
  • 287.
    Hamacher-Barth, Evelyne
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The high Arctic summer aerosol: Size, chemical composition, morphology and evolution over the pack-ice2017Doktoravhandling, med artikler (Annet vitenskapelig)
    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.

  • 288.
    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 microscopy2013Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 6, nr 12, s. 3459-3475Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 289.
    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) (Annet vitenskapelig)
    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.

  • 290.
    Hamacher-Barth, Evelyne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jansson, Kjell
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Size-resolved morphological properties of the high Arctic summer aerosol during ASCOS-20082016Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, nr 10, s. 6577-6593Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The representation of aerosol properties and processes in climate models is fraught with large uncertainties. Especially at high northern latitudes a strong underprediction of aerosol concentrations and nucleation events is observed and can only be constrained by in situ observations based on the analysis of individual aerosol particles. To further reduce the uncertainties surrounding aerosol properties and their potential role as cloud condensation nuclei this study provides observational data resolved over size on morphological and chemical properties of aerosol particles collected in the summer high Arctic, north of 80A degrees aEuro-N. Aerosol particles were imaged with scanning and transmission electron microscopy and further evaluated with digital image analysis. In total, 3909 aerosol particles were imaged and categorized according to morphological similarities into three gross morphological groups: single particles, gel particles, and halo particles. Single particles were observed between 15 and 800aEuro-nm in diameter and represent the dominating type of particles (82aEuro-%). The majority of particles appeared to be marine gels with a broad Aitken mode peaking at 70aEuro-nm and accompanied by a minor fraction of ammonium (bi)sulfate with a maximum at 170aEuro-nm in number concentration. Gel particles (11aEuro-% of all particles) were observed between 45 and 800aEuro-nm with a maximum at 154aEuro-nm in diameter. Imaging with transmission electron microscopy allowed further morphological discrimination of gel particles in 'aggregate' particles, 'aggregate with film' particles, and 'mucus-like' particles. Halo particles were observed above 75aEuro-nm and appeared to be ammonium (bi)sulfate (59aEuro-% of halo particles), gel matter (19aEuro-%), or decomposed gel matter (22aEuro-%), which were internally mixed with sulfuric acid, methane sulfonic acid, or ammonium (bi)sulfate with a maximum at 161aEuro-nm in diameter. Elemental dispersive X-ray spectroscopy analysis of individual particles revealed a prevalence of the monovalent ions Na+/K+ for single particles and aggregate particles and of the divalent ions Ca2+/Mg2+ for aggregate with film particles and mucus-like particles. According to these results and other model studies, we propose a relationship between the availability of Na+/K+ and Ca2+/Mg2+ and the length of the biopolymer molecules participating in the formation of the three-dimensional gel networks.

  • 291.
    Hanley, John
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Extreme Storms in the North Atlantic and Europe2013Doktoravhandling, med artikler (Annet vitenskapelig)
    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.

  • 292.
    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 dataset2012Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, nr 664, s. 612-625Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 293.
    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 overWestern EuropeManuskript (preprint) (Annet vitenskapelig)
    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.

  • 294.
    Hanley, John
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The role of large-scale atmospheric flow and Rossby wave breaking in the evolution of extreme windstorms over Europe2012Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, s. L21708-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate the relationship between large-scale atmospheric flow and the evolution of the most extreme windstorms affecting Western Continental Europe. The 25 most destructive Western Continental European wind storms are selected from a 43-year climatology. 22 of these storms are grouped as having a similar trajectory and evolution. We show that these storms typically occur during particularly strong and persistent positive NAO anomalies which peak approximately 2 days before the storms' peak intensity; the NAO pattern then shifts eastward to a position over the European continent when the storms strike Europe. A temporal composite of potential temperature on the 2-PVU surface suggests that this NAO shift is the result of simultaneous cyclonic and anticyclonic wave breaking penetrating further to the east than during a typical high-NAO event. This creates an extremely intense, zonally-orientated jet over the North Atlantic whose baroclinicity favours explosive intensification of storms while steering them into Western Continental Europe.

  • 295.
    Hannachi, Abdel
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Regularised empirical orthogonal functions2016Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 68, artikkel-id 31723Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 296.
    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 variability2018Inngår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 297.
    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 functions2018Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870Artikkel i tidsskrift (Fagfellevurdert)
  • 298.
    Hannachi, Abdelwaheb
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Intermittency, autoregression and censoring: a first-order AR model for daily precipitation2014Inngår i: Meteorological Applications, ISSN 1350-4827, E-ISSN 1469-8080, Vol. 21, nr 2, s. 384-397Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 299.
    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 integrations2013Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, nr 3-4, s. 995-1007Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 300.
    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 Warmings2011Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 3, s. 657-674Artikkel i tidsskrift (Fagfellevurdert)
    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.

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