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  • 1.
    Bittenbinder, Daniil
    Göteborgs universitet/Institutionen för geovetenskaper.
    Vinteruppvärmningshändelser i Abisko och Latnjajaure från 1992 till 2017.: Studie om fall av extrema vinteruppvärmningshändelser i nordvästra Lappland2021Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Under de senaste decennier har temperaturer i Arktis och Subarktis ökat dramatiskt jämfört medandra delar av världen. Konsekvensen av detta kan vara en ökad frekvens, intensitet ochvaraktighet av så kallade vinteruppvärmningshändelser. Vinteruppvärmningshändelserdefinieras i denna studie som två eller fler dagar i rad med en lufttemperatur > 0ºC undervinterhalvåret. Dessa vinteruppvärmningshändelser kan orsaka betydande konsekvenser förekosystem samt flora och fauna i Subarktis.Det finns få studier, i vilka man analyserat varaktigheten, frekvensen och intensiteten av dessavinteruppvärmningshändelser i den subarktiska regionen. I den här studien användslufttemperaturmätningar från stationerna Abisko och Latnjajaure för att identifiera ochklassificera vinteruppvärmningshändelser gällande varaktighet, frekvens och intensitet underperioden 1992 till 2017. Vidare undersöks om dessa uppvärmningshändelser kan kopplas tillden Nordatlantiska oscillationen samt under vilka månader dessa händelser förekommer.Resultatet visar på flera uppvärmningshändelser i området. Majoritet av dessa händelser har envaraktighet på mellan 2 till 4 dagar, med den längsta uppvärmningshändelsen på 12 dagarskedde 2011. Under perioden 2000 till 2010 förekom ett stort antal händelser med långvaraktighet. De flesta av uppvärmningshändelserna sker i slutet av april och början avnovember. En korrelation med faserna av Nordatlantiska Oscillationen testades men visadeingen statistisk signifikant korrelation mellan dessa fenomen under den aktuella periodenmellan åren 1992 och 2017.

  • 2.
    Horst, Axel
    et al.
    Stockholms universitet, Institutionen för tillämpad miljövetenskap (ITM).
    Thornton, Brett F.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Holmstrand, Henry
    Stockholms universitet, Institutionen för tillämpad miljövetenskap (ITM).
    Andersson, Per
    Crill, Patrick M.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Gustafsson, Örjan
    Stockholms universitet, Institutionen för tillämpad miljövetenskap (ITM).
    Stable bromine isotopic composition of atmospheric CH3Br2013Ingår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 65, artikel-id 21040Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Tropospheric methyl bromide (CH3Br) is the largest source of bromine to the stratosphere and plays an important role in ozone depletion. Here, the first stable bromine isotope composition (delta Br-81) of atmospheric CH3Br is presented. The delta Br-81 of higher concentration Stockholm samples and free air subarctic Abisko samples suggest a source/background value of -0.04 +/- 0.28 parts per thousand ranging up to +1.75 +/- 0.12 parts per thousand. The Stockholm delta Br-81 versus concentration relationship corresponds to an apparent isotope enrichment factor of -4.7 +/- 3.7 parts per thousand, representing the combined reaction sink. This study demonstrates the scientific potential of atmospheric delta Br-81 measurements, which in the future may be combined with other isotope systems in a top-down inverse approach to further understand key source and sink processes of methyl bromide.

  • 3. Igel, Adele L.
    et al.
    Ekman, Annica M. L.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Savre, Julien
    Sedlar, Joseph
    Stockholms universitet, Meteorologiska institutionen (MISU).
    The free troposphere as a potential source of arctic boundary layer aerosol particles2017Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 13, s. 7053-7060Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study investigates aerosol particle transport from the free troposphere to the boundary layer in the summertime high Arctic. Observations from the Arctic Summer Cloud Ocean Study field campaign show several occurrences of high aerosol particle concentrations above the boundary layer top. Large-eddy simulations suggest that when these enhanced aerosol concentrations are present, they can be an important source of aerosol particles for the boundary layer. Most particles are transported to the boundary layer by entrainment. However, it is found that mixed-phase stratocumulus clouds, which often extend into the inversion layer, also can mediate the transport of particles into the boundary layer by activation at cloud top and evaporation below cloud base. Finally, the simulations also suggest that aerosol properties at the surface sometimes may not be good indicators of aerosol properties in the cloud layer.

  • 4.
    Jantunen, Liisa
    et al.
    Environment Canada, Canada.
    Wong, Fiona
    Stockholm University.
    Gawor, Anya
    Environment Canada.
    Kylin, Henrik
    Linköpings universitet, Tema Miljöförändring.
    Helm, Paul
    Ontario Ministry of the Environment, Canada.
    Stern, Gary
    University of Manitoba, Canada.
    Strachan, William
    Environment Canada, Canada.
    Burniston, Deborah
    Environment Canada, Canada.
    Bidleman, Terry
    Umeå University.
    20 Years of Air-Water Gas Exchange Observations for Pesticides in the Western Arctic Ocean2015Ingår i: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 49, nr 23, s. 13844-13852Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Arctic has been contaminated by legacy organochlorine pesticides (OCPs) and currently used pesticides (CUPs) through atmospheric transport and oceanic currents. Here we report the time trends and air−water exchange of OCPs and CUPs from research expeditions conducted between 1993 and 2013. Compounds determined in both air and water were trans- and cis-chlordanes (TC, CC), trans- and cis-nonachlors (TN, CN), heptachlor exo-epoxide (HEPX), dieldrin (DIEL), chlorobornanes (ΣCHBs and toxaphene), dacthal (DAC), endosulfans and metabolite endosulfan sulfate (ENDO-I, ENDO-II, and ENDO SUL), chlorothalonil (CHT), chlorpyrifos (CPF), and trifluralin (TFN). Pentachloronitrobenzene (PCNB and quintozene) and its soil metabolite pentachlorothianisole (PCTA) were also found in air. Concentrations of most OCPs declined in surface water, whereas some CUPs increased (ENDO-I, CHT, and TFN) or showed no significant change (CPF and DAC), and most compounds declined in air. Chlordane compound fractions TC/(TC + CC) and TC/(TC + CC + TN) decreased in water and air, while CC/(TC + CC + TN) increased. TN/(TC + CC + TN) also increased in air and slightly, but not significantly, in water. These changes suggest selective removal of more labile TC and/or a shift in chlordane sources. Water−air fugacity ratios indicated net volatilization (FR > 1.0) or near equilibrium (FR not significantly different from 1.0) for most OCPs but net deposition (FR < 1.0) for ΣCHBs. Net deposition was shown for ENDO-I on all expeditions, while the net exchange direction of other CUPs varied. Understanding the processes and current state of air−surface exchange helps to interpret environmental exposure and evaluate the effectiveness of international protocols and provides insights for the environmental fate of new and emerging chemicals.

  • 5. Kozlovsky, A.
    et al.
    Myllymaa, M.
    Lukianova, R.
    Raita, T.
    Lester, M.
    Influence of Atmospheric Circulation on Orientation of Auroral Arcs2023Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, nr 7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigated statistically the orientation of about 10,000 auroral arcs observed during 2016?2021 in Abisko (68.36°N, 18.81°E, Sweden) in the equatorward part of the nightside auroral oval. The observations were made between 19 and 06 magnetic local time (MLT). On average, the orientation of the arcs, that is, the angle between an arc and the West-East direction, linearly changed with MLT at the rate 2.2°/hr. In most cases the mean orientation of the auroral arcs near midnight was parallel to the geomagnetic latitude of Abisko, except a few late winter or spring months. These anomalies cannot be explained by geomagnetic disturbances or interplanetary conditions. They do, however, coincide with spring transitions in the atmospheric circulations from winter-to summer-type. The spring transitions were manifested in the data of a meteor radar in the region of observations as reversals of the zonal wind at altitudes 90?100 km from eastward to westward. We propose that these spring transition changes in the global atmospheric circulation couple also the overlying thermosphere at 100?150 km where neutral winds may affect the ionosphere and subsequently the whole ionosphere-magnetosphere system. Then, changes in the coupled ionosphere-magnetosphere system are manifested in changes of the orientation of auroral arcs.

  • 6. Loewe, Katharina
    et al.
    Ekman, Annica M. L.
    Paukert, Marco
    Sedlar, Joseph
    SMHI, Atmosfärisk fjärranalys.
    Tjernstrom, Michael
    Hoose, Corinna
    Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)2017Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, nr 11, s. 6693-6704Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Arctic climate is changing; temperature changes in the Arctic are greater than at midlatitudes, and changing atmospheric conditions influence Arctic mixed-phase clouds, which are important for the Arctic surface energy budget. These low-level clouds are frequently observed across the Arctic. They impact the turbulent and radiative heating of the open water, snow, and sea-ice-covered surfaces and influence the boundary layer structure. Therefore the processes that affect mixed-phase cloud life cycles are extremely important, yet relatively poorly understood. In this study, we present sensitivity studies using semi-idealized large eddy simulations (LESs) to identify processes contributing to the dissipation of Arctic mixed-phase clouds. We found that one potential main contributor to the dissipation of an observed Arctic mixed-phase cloud, during the Arctic Summer Cloud Ocean Study (ASCOS) field campaign, was a low cloud droplet number concentration (CDNC) of about 2 cm-3. Introducing a high ice crystal concentration of 10 L-1 also resulted in cloud dissipation, but such high ice crystal concentrations were deemed unlikely for the present case. Sensitivity studies simulating the advection of dry air above the boundary layer inversion, as well as a modest increase in ice crystal concentration of 1 L-1, did not lead to cloud dissipation. As a requirement for small droplet numbers, pristine aerosol conditions in the Arctic environment are therefore considered an important factor determining the lifetime of Arctic mixed-phase clouds.

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  • 7. Marshall, Gareth J.
    et al.
    Jylhä, Kirsti
    Kivinen, Sonja
    Laapas, Mikko
    Dyrrdal, Anita Verpe
    The role of atmospheric circulation patterns in driving recent changes in indices of extreme seasonal precipitation across Arctic Fennoscandia2020Ingår i: Climatic Change, ISSN 0165-0009, E-ISSN 1573-1480, Vol. 162, nr 2, s. 741-759Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Extreme precipitation events (EPEs) have a major impact across Arctic Fennoscandia (AF). Here we examine the spatial variability of seasonal 50-year trends in three EPEs across AF for 1968–2017, using daily precipitation data from 46 meteorological stations, and analyse how these are related to contemporaneous changes in the principal atmospheric circulation patterns that impact AF climate. Positive trends in seasonal wet-day precipitation (PRCPTOT) are widespread across AF in all seasons except autumn. Spring (autumn) has the most widespread negative (positive) trends in consecutive dry days (CDD). There is less seasonal dependence for trends in consecutive wet days (CWDs), but the majority of the stations show an increase. Clear seasonal differences in the circulation pattern that exerted most influence on these AF EPE trends exist. In spring, PRCPTOT and CDD are most affected by the Scandinavian pattern at more than half the stations while it also has a marked influence on CWD. The East Atlantic/Western Russia pattern generally has the greatest influence on the most station EPE trends in summer and autumn, yet has no effect during either spring or winter. In winter, the dominant circulation pattern across AF varies more between the different EPEs, with the North Atlantic Oscillation, Polar/Eurasia and East Atlantic patterns all exerting a major influence. There are distinct geographical distributions to the dominant pattern affecting particular EPEs in some seasons, especially winter, while in others there is no discernible spatial relationship.

  • 8. Marshall, Gareth J.
    et al.
    Kivinen, Sonja
    Jylhä, Kirsti
    Vignols, Rebecca M.
    Rees, W. G.
    The accuracy of climate variability and trends across Arctic Fennoscandia in four reanalyses2018Ingår i: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 38, nr 10, s. 3878-3895Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations reveal a statistically significant warming across Arctic Fennoscandia: three reanalyses show a similar regional warming of smaller magnitude while the fourth is anomalous, even having a slight cooling in some areas. Spatial patterns of precipitation trends across the region differ markedly between the reanalyses, which have varying success at matching observations but generally fail to replicate sites with significant observed trends. There is a clear spatial mismatch between reanalyses and observations in regions of steep and complex orography.

  • 9.
    Mashayekhy Rad, Farshid
    Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi.
    Analytical methods for biomolecules involved in atmospheric aerosol formation in the Arctic2018Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In the Arctic, increasing ice-free conditions and nutrients freed from the melting ice must strongly influence the marine life. Aerosol emissions from microbiological marine processes may affect the low clouds and fogs over the summer Arctic, which in turn have effects on the melting of sea ice. The radiative properties of the high Arctic low clouds are strongly dependent on the number concentration of airborne water-soluble particles, known as cloud condensation nuclei (CCN). If the effects of CCN on cloud optical properties is to be fully understood it is important to be able to specify the source and concentrations of the Arctic aerosol particles.

    Previous studies in the Arctic have indicated that organic material formed in the uppermost ocean surface is transferred to the atmosphere and plays a potentially very important role in the aerosol-fog/cloud cycle. However, many aspects of this process remain unverified and chemical characterisation of targeted groups of biomolecules is still notably fragmentary or non-existing. Investigation of biomolecules, particularly amino acids, peptides and proteins together with mono- and polysac­charides and fatty acids in the airborne aerosol, and their relative contributions to fog/cloud water, requires development of an array of “cutting edge” analytical techniques and methods.

    In this thesis, electrospray ionization mass spectrometry was used for all applications and target biomolecules. The measurements in the Arctic turned out to be challenging due to the highly complex, salty matrices, combined with very low concentration and high diversity of the target biomolecules, and each step of the analytical chain needed careful consideration. To increase the detectability of the very low levels of polysaccharides and proteins in aerosols, these compounds were hydrolyzed to their subunits, monosaccharides and amino acids. Monosaccharides were separated using hydrophilic interaction chromatography, which was beneficial for their detection in electrospray ionization mass spectrometry. Amino acids were derivatized, yielding improvement in reversed-phase chromatographic separation, ionization efficiency as well as selectivity. For fatty acids in a sea surface sample, a novel fast screening method was developed, utilizing travelling-wave ion mobility separation as an orthogonal technique connected to mass spectrometry. In addition, a method for the detection of wood burning as an anthropogenic source of aerosols was developed, utilizing anhydrous monosaccharides as markers. This method can be used in the upcoming expeditions for source apportionment studies.

    The results from the analyses of the aerosol and fog water samples, collected over the summer pack ice north of 80 °N, show that both total polysaccharides and total proteinaceous compounds (sum of proteins, peptides and amino acids) occurred at the pmol m-3 to nmol m-3 level. Interestingly, the levels were found higher between different years, suggested to be coupled to less ice coverage and thus to a higher biological activity in the ocean surface. The highest concentrations of polysaccharides, as an indication of marine polymer gels, were found during the summer over the pack ice area. In addition, a pilot source apportionment study was carried out combining the measurement of different molecular tracers, used as source markers. This study indicates the seasonality and abundance of marine polymer gels as an important feature of the Arctic Ocean connected to the melting and freezing of sea ice. It should be further studied how the abundance of these gels, which have a high potential for cloud droplet activation, affect the melting and freezing of the perennial sea ice.

    Given the successful development of analytical methods for targeted groups of biomolecules, this thesis has supported the importance of biomolecules as CCN and for cloud formation in the Arctic. Less ice coverage may further increase the number of biomolecular CCN which could change the radiative balance, by the formation of more low-level clouds. Overall, more studies are required to further unravel the complex relationship of biogenic sources, atmospheric chemistry and meteorology to assess the impact of climate change on the Arctic.

  • 10. Rannik, Üllar
    et al.
    Vesala, Timo
    Peltola, Olli
    Novick, Kimberly A.
    Aurela, Mika
    Järvi, Leena
    Montagnani, Leonardo
    Mölder, Meelis
    Peichl, Matthias
    Pilegaard, Kim
    Mammarella, Ivan
    Impact of coordinate rotation on eddy covariance fluxes at complex sites2020Ingår i: Agricultural and Forest Meteorology, ISSN 0168-1923, E-ISSN 1873-2240, nr 287, artikel-id 07940Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The choice of coordinate system to calculate eddy covariance fluxes becomes particularly relevant at complex measurement sites. The traditional way is to perform double rotation (DR) of the coordinate system i.e., to calculate turbulent fluxes in a coordinate system that is aligned with the flow streamlines within the flux averaging period (e.g., Kaimal and Finnigan, 1994). The second approach, the so-called planar-fitted (PF) coordinate system, averages the flow over a longer period of time, in practice a month or more. The PF method allows to derive an intercept coefficient of the vertical wind speed which can be attributed to the offset of the sonic anemometer or the average vertical flow related to meteorological conditions. We evaluated the variants of the PF methods using data from a variety of sites ranging from complex urban and forest sites to nearly ideal forest and peatland sites. At complex sites, we found that the intercept of the vertical wind speed derived from the PF method is a function of wind direction, time of day and/or stability. The sector-wise PF (SPF) method frequently led to insignificant statistical relationships. We tested a continuous PF (CPF) method where the relationship establishing the coordinate frame was represented as the continuous function in the form of Fourier series. The method enabled to obtain the PF with lower uncertainty as compared to the SPF method, by selecting necessary number of harmonics for each site based on confidence intervals of estimated parameters. Therefore, we recommend to use the CPF method in cases when the number of observations in some wind direction interval is low or the obtained SPF is insignificant due to large variance in measurements. We also showed that significant systematic difference can exist in cumulative turbulent fluxes between the DR and PF methods over a longer period of time. Derived vertical advection of carbon dioxide exhibited large variability with wind direction due to topography at complex sites and therefore, without considering horizontal advection, cannot be used to improve the net ecosystem exchange estimation during nocturnal, low turbulence conditions.

  • 11. Sedlar, Joseph
    et al.
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Rinke, Annette
    Orr, Andrew
    Cassano, John
    Fettweis, Xavier
    Heinemann, Guenther
    Seefeldt, Mark
    Solomon, Amy
    Matthes, Heidrun
    Phillips, Tony
    Webster, Stuart
    Confronting Arctic Troposphere, Clouds, and Surface Energy Budget Representations in Regional Climate Models With Observations2020Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 125, nr 6, artikel-id e2019JD031783Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A coordinated regional climate model (RCM) evaluation and intercomparison project based on observations from a July-October 2014 trans-Arctic Ocean field experiment (ACSE-Arctic Clouds during Summer Experiment) is presented. Six state-of-the-art RCMs were constrained with common reanalysis lateral boundary forcing and upper troposphere nudging techniques to explore how the RCMs represented the evolution of the surface energy budget (SEB) components and their relation to cloud properties. We find that the main reasons for the modeled differences in the SEB components are a direct consequence of the RCM treatment of cloud and cloud-radiative interactions. The RCMs could be separated into groups by their overestimation or underestimation of cloud liquid. While radiative and turbulent heat flux errors were relatively large, they often invoke compensating errors. In addition, having the surface sea-ice concentrations constrained by the reanalysis or satellite observations limited how errors in the modeled radiative fluxes could affect the SEB and ultimately the surface evolution and its coupling with lower tropospheric mixing and cloud properties. Many of these results are consistent with RCM biases reported in studies over a decade ago. One of the six models was a fully coupled ocean-ice-atmosphere model. Despite the biases in overestimating cloud liquid, and associated SEB errors due to too optically thick clouds, its simulations were useful in understanding how the fully coupled system is forced by, and responds to, the SEB evolution. Moving forward, we suggest that development of RCM studies need to consider the fully coupled climate system.

  • 12.
    Siegel, Karolina
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Chemical composition of summertime High Arctic aerosols2020Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Denna avhandling ämnar att presentera nya kunskaper om den kemiska sammansättningen av halvflyktiga föreningar i aerosolprover som samlades in i Norra ishavet nära Nordpolen i september 2018. Den innersta delen av Norra ishavet är svårtillgängligt på grund av bristen på fasta landområden ihop med kompakt packis. Därför kvarstår stora kunskapsluckor i förståelsen av klimatsystemet i Arktis, och i synnerhet aerosolpartiklars roll i dess orörda atmosfär.

    Den kemiska sammansättningen av aerosolproverna analyserades på molekylnivå med högupplöst kemisk joniseringsmasspektrometri kopplad till ett filterinsläpp för gaser och aerosoler (FIGAERO-HRToF-CIMS). Analysen visade på en tydlig signal från föreningar som sannolikt har marina källor. En viktig kemisk föregångare till marina aerosoler är dimetylsulfid (DMS), en gas som frigörs av fytoplankton och isalger i Ishavet. DMS oxiderar i atmosfären till oxidationsprodukter som kan bidra till en storleksökning av aerosoler. Genom analys av provluftens ursprung med trajektorieanalys visades att de högsta DMS-koncentrationerna kom från havsområdena runt packisen. Ingen korrelation kunde emellertid visas inom packisområdet mellan DMS i gasfas och dess oxidationsprodukt metansulfonsyra (MSA) i partikelfas.

    Eftersom FIGAERO-HRToF-CIMS ofta används i områden med högre partikelkoncentrationer och aldrig har använts i de inre delarna av Arktis tidigare, visar denna avhandling även att tekniken är lämplig för att mäta den kemiska sammansättningen av aerosoler i detta avlägsna polarområde.

  • 13.
    Sofokleous, Ioannis
    Uppsala universitet, Luft-, vatten och landskapslära.
    Correction of Inhomogeneous Data in the Precipitation Time Series of Sweden Due to the Wind Shield Introduction2016Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Kontinuerliga samt felfria nederbördsmätningar är av stor betydelse för geovetenskaper som klimatologi och hydrologi därför att nederbördsdata är en av de primära meteorologiska parametrarna för forskning om klimatförändringen. Att säkerställa felfria (homogena) nederbörds tidsserier betyder i stort sett att säkerställa homogenitet genom att identifiera och korrigera inhomogena data. Icke homogena data uppkommer på grund av förändringar i mätmetoder och mätförhållanden under observationstiden, sedan 1860-talet tills idag alltså. Denna studies syfte är att beräkna en korrektion som ska användas för att korrigera nederbördsmätningar som utfördes sedan 1860 utan vinskydd. Vindskyddet eller vindskärmen, en speciell utrustning som användas på nederbördsinsamlare, infördes gradvis under perioden 1900-1960 vid de svenska nederbördstationerna. Vindskyddet introducerades med avsikt att minska vindens påverka vid nederbördsinsamling. Men trotts den positiva effekten som vindskyddet ledde till, genom den ökade nederbördsmängden som samlades in, skapade denna förändring av mätarutrustningen inhomogena data.

    Bearbetningen skedde för månadsnederbördsdata från 70 stationer från SMHIs meteorologiska nätverk genom att jämföra nederbördsobservationer som genomfördes under perioderna tio år före och tio år efter införandet av vindskydd. Dessutom användes temperaturdata från samma stationer för att uppskatta nederbördslag (snö/regn). Skälet till detta är att vinskyddseffekten är olika mellan snö och regn. Beräkningarna och bestämningen av nederbördslag ledde till en 5 % respektive 27 % nederbörds ökning för regn och snö för de mätningarna som utfördes utan vindskydd. I genomsnitt har de korrigerade värdena, under perioden som vinskyddet saknades, ökat med omkring 50 mm.

  • 14.
    Sotiropoulou, Georgia
    Stockholms universitet, Meteorologiska institutionen (MISU).
    The Arctic Atmosphere: Interactions between clouds, boundary-layer turbulence and large-scale circulation2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Arctic climate is changing fast, but weather forecast and climate models have serious deficiencies in representing the Arctic atmosphere, because of the special conditions that occur in this region. The cold ice surface and the advection of warm air aloft from the south result in a semi-continuous presence of a temperature inversion, known as the “Arctic inversion”, which is governed by interacting large-scale and local processes, such as surface fluxes and cloud formation. In this thesis these poorly understood interactions are investigated using observations from field campaigns on the Swedish icebreaker Oden: The Arctic Summer Cloud Ocean Study (ASCOS) in 2008 and the Arctic Clouds in Summer Experiment (ACSE) in 2014. Two numerical models are also used to explore these data: the IFS global weather forecast model from the European Center for Medium-range Weather Forecasts and the MIMICA LES from Stockholm University.

    Arctic clouds can persist for a long time, days to weeks, and are usually mixed-phase; a difficult to model mixture of super-cooled cloud droplets and ice crystals. Their persistence has been attributed to several mechanisms, such as large-scale advection, surface evaporation and microphysical processes. ASCOS observations indicate that these clouds are most frequently decoupled from the surface; hence, surface evaporation plays a minor role. The determining factor for cloud-surface decoupling is the altitude of the clouds. Turbulent mixing is generated in the cloud layer, forced by cloud-top radiative cooling, but with a high cloud this cannot penetrate down to the surface mixed layer, which is forced primarily by mechanical turbulence. A special category of clouds is also found: optically thin liquid-only clouds with stable stratification, hence insignificant in-cloud mixing, which occur in low-aerosol conditions. IFS model fails to reproduce the cloud-surface decoupling observed during ASCOS. A new prognostic cloud physics scheme in IFS improves simulation of mixed-phase clouds, but does not improve the warm bias in the model, mostly because IFS fails to disperse low surface-warming clouds when observations indicate cloud-free conditions.

    With increasing summer open-water areas in a warming Arctic, there is a growing interest in processes related to the ice marginal zones and the summer-to-autumn seasonal transition. ACSE included measurements over both open-water and sea-ice surfaces, during melt and early freeze. The seasonal transition was abrupt, not gradual as would have been expected if it was primarily driven by the gradual changes in net solar radiation. After the transition, the ocean surface remained warmer than the atmosphere, enhancing surface cooling and facilitating sea-ice formation. Observations in melt season showed distinct differences in atmospheric structure between the two surface types; during freeze-up these largely disappear. In summer, large-scale advection of warm and moist air over melting sea ice had large impacts on atmospheric stability and the surface. This is explored with an LES; results indicate that while vertical structure of the lowest atmosphere is primarily sensitive to heat advection, cloud formation, which is of great importance to the surface energy budget, is primarily sensitive to moisture advection.

  • 15.
    Sotiropoulou, Georgia
    et al.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Sedlar, Joseph
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Forbes, Richard
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Late Summer Arctic clouds in the ECMWF forecast model: an evaluation of cloud parameterization scheme2016Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, nr 694, s. 387-400Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mixed-phase clouds are an integral part of the Arctic climate system, for precipitation and for their interactions with radiation and thermodynamics. Mixed-phase processes are often poorly represented in global models and many use an empirically based diagnostic partition between the liquid and ice phases that is dependent solely on temperature. However, increasingly more complex microphysical parametrizations are being implemented allowing a more physical representation of mixed-phase clouds.

    This study uses in situ observations from the Arctic Summer Cloud Ocean Study (ASCOS) field campaign in the central Arctic to assess the impact of a change from a diagnostic to a prognostic parametrization of mixed-phase clouds and increased vertical resolution in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS). The newer cloud scheme improves the representation of the vertical structure of mixed-phase clouds, with supercooled liquid water at cloud top and ice precipitating below, improved further with higher vertical resolution. Increased supercooled liquid water and decreased ice content are both in closer agreement with observations. However, these changes do not result in any substantial improvement in surface radiation, and a warm and moist bias in the lowest part of the atmosphere remains. Both schemes also fail to capture the transitions from overcast to cloud-free conditions. Moreover, whereas the observed cloud layer is frequently decoupled from the surface, the modelled clouds remain coupled to the surface most of the time. The changes implemented to the cloud scheme are an important step forward in improving the representation of Arctic clouds, but improvements in other aspects such as boundary-layer turbulence, cloud radiative properties, sensitivity to low aerosol concentrations and representation of the sea-ice surface may also need to be addressed.

  • 16. Stevens, Robin G.
    et al.
    Loewe, Katharina
    Dearden, Christopher
    Dimitrelos, Antonios
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Possner, Anna
    Eirund, Gesa K.
    Raatikainen, Tomi
    Hill, Adrian A.
    Shipway, Benjamin J.
    Wilkinson, Jonathan
    Romakkaniemi, Sami
    Tonttila, Juha
    Laaksonen, Ari
    Korhonen, Hannele
    Connolly, Paul
    Lohmann, Ulrike
    Hoose, Corinna
    Ekman, Annica M. L.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Carslaw, Ken S.
    Field, Paul R.
    A model intercomparison of CCN-limited tenuous clouds in the high Arctic2018Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 18, nr 15, s. 11041-11071Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform a model intercomparison of summertime high Arctic ( > 80 degrees N) clouds observed during the 2008 Arctic Summer Cloud Ocean Study (ASCOS) campaign, when observed cloud condensation nuclei (CCN) concentrations fell below 1 cm(-3). Previous analyses have suggested that at these low CCN concentrations the liquid water content (LWC) and radiative properties of the clouds are determined primarily by the CCN concentrations, conditions that have previously been referred to as the tenuous cloud regime. The intercomparison includes results from three large eddy simulation models (UCLALES-SALSA, COSMO-LES, and MIMICA) and three numerical weather prediction models (COSMO-NWP, WRF, and UM-CASIM). We test the sensitivities of the model results to different treatments of cloud droplet activation, including prescribed cloud droplet number concentrations (CDNCs) and diagnostic CCN activation based on either fixed aerosol concentrations or prognostic aerosol with in-cloud processing. There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ice crystal number concentrations (ICNC). The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC depends on the representation of the cloud droplet size distribution within each model, which impacts autoconversion rates. Our results therefore suggest that properly estimating aerosol-cloud interactions requires an appropriate treatment of the cloud droplet size distribution within models, as well as in situ observations of hydrometeor size distributions to constrain them. The results strongly support the hypothesis that the liquid water content of these clouds is CCN limited. For the observed meteorological conditions, the cloud generally did not collapse when the CCN concentration was held constant at the relatively high CCN concentrations measured during the cloudy period, but the cloud thins or collapses as the CCN concentration is reduced. The CCN concentration at which collapse occurs varies substantially between models. Only one model predicts complete dissipation of the cloud due to glaciation, and this occurs only for the largest prescribed ICNC tested in this study. Global and regional models with either prescribed CDNCs or prescribed aerosol concentrations would not reproduce these dissipation events. Additionally, future increases in Arctic aerosol concentrations would be expected to decrease the frequency of occurrence of such cloud dissipation events, with implications for the radiative balance at the surface. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. Therefore, this suggests that linkages between aerosol and clouds, as well as linkages between clouds, surface temperatures, and atmospheric stability need to be considered for weather and climate predictions in this region.

  • 17. Thomson, Neil R.
    et al.
    Clilverd, Mark A.
    Brundell, James B.
    Rodger, Craig J.
    Quiet Night Arctic Ionospheric D Region Characteristics2021Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 126, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    VLF radio propagation recordings are used to determine the characteristics of the nighttime polar lower D region of the ionosphere. Recordings of both VLF phase and amplitude in the Arctic on days within ∼1–2 weeks of the equinoxes enable their day-to-night changes to be determined. These changes are then combined with previously measured daytime polar D region characteristics to find the nighttime characteristics. The previously determined daytime characteristics were measured in the Arctic summer; the NRLMSISE atmosphere model is used to help determine the height change from daytime summer to daytime equinox (∼5 km lower). The principal path used was from the 16.4 kHz Norwegian transmitter JXN (67°N, 14°E) 1,334 km northwards across the Arctic Ocean to Ny-Ålesund (79°N, 12°E), Svalbard. Also used were the 2,014-km path from NRK (37.5 kHz, Grindavik, 64°N, Iceland) to Ny-Ålesund, the 1,655-km path from JXN to Reykjavik (64°N, Iceland), and the 5,302-km path from JXN across the Arctic Ocean to Fairbanks (65°N) in Alaska. The night values of (the Wait parameters) H′ and β were found to average from ∼79 km at equinox down to 77 km near winter solstice (lower than the 85 km at low and midlatitudes by ∼7 km) and 0.6 km−1, respectively. This lower height and its variability are shown to be consistent with the principal source of ionization being energetic electron precipitation.

  • 18.
    Winiger, Patrik
    Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi.
    Isotope-based source apportionment of black carbon aerosols in the Eurasian Arctic2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Aerosols change the Earth's energy balance. Black carbon (BC) aerosols are a product of incomplete combustion of fossil fuels and biomass burning and cause a net warming through aerosol radiation interactions (ari) and aerosol cloud interactions (aci). BC aerosols have potentially strong implications on the Arctic climate, yet the net global climate effect of BC is very uncertain. Best estimates assume a net warming effect, roughly half to that of CO2. However, the time scales during which CO2 emissions affect the global climate are on the order of hundreds of years, while BC is a short-lived climate pollutant (SLCP) with atmospheric life times of days to weeks.

    Climate models or atmospheric transport models struggle to emulate the seasonality and amplitude of BC concentrations in the Arctic, which are low in summer and high in winter/spring during the so called Arctic haze season. The high uncertainties regarding BC's climate impact are not only related to ari and aci, but also due to model parameterizations of BC lifetime and transport, and the highly uncertain estimates of global and regional BC emissions. Given the high uncertainties in technology-based emission inventories (EI), there is a need for an observation-based assessment of sources of BC in the atmosphere.

    We study short-term and long-term observations of elemental carbon (EC), the mass-based analog of optically-defined BC. EC aerosol concentrations and carbon-isotope-based (δ13C and ∆14C) sources were constrained (top-down) for three Arctic receptor sites in Abisko (northern Sweden), Tiksi (East Siberian Russia), and Zeppelin (on Svalbard, Norway). The radiocarbon (∆14C) signature allows to draw conclusion on the EC sources (fossil fuels vs. biomass burning) with high accuracy (<5% variation). Stable carbon isotopic fingerprints (δ13C) give qualitative information of the consumed fuel type, i.e. coal, C3-plants (wood), liquid fossil fuels (diesel) or gas flaring (methane and non-methane hydrocarbons). These fingerprints can be used in conjunction with Bayesian statistics, to estimate quantitative source contributions of the sources. Finally, our observations were compared to predictions from a state of the art atmospheric transport model (coupled to BC emissions), conducted by our collaborators at NILU (Norwegian Institute for Air Research).

    Observed BC concentrations showed a high seasonality throughout the year, with elevated concentrations in the winter, at all sites. The highest concentrations were measured on Svalbard during a short campaign (Jan-Mar 2009) focusing on BC pollution events. Long-term observations showed that Svalbard (2013) had overall the lowest annual BC concentrations, followed by Abisko (2012) and Tiksi (2013). Isotope constraints on BC combustion sources exhibited a high seasonality and big amplitude all across the Eurasian Arctic. Uniform seasonal trends were observed in all three year-round studies, showing fractions of biomass burning of 60-70% in summer and 10-40% in winter. Europe was the major source region (>80%) for BC emissions arriving at Abisko and the main sources were liquid fossil fuels and biomass burning (wood). The model agreed very well with the Abisko observations, showing good model skill and relatively well constrained sources in the European regions of the EI.

  • 19. You, Cheng
    et al.
    Tjernstrom, Michael
    Devasthale, Abhay
    SMHI, Atmosfärisk fjärranalys.
    Warm-Air Advection Over Melting Sea-Ice: A Lagrangian Case Study2020Ingår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations from the 2014 Arctic Clouds in Summer Experiment indicate that, in summer, warm-air advection over melting sea-ice results in a strong surface melting feedback forced by a very strong surface-based temperature inversion and fog formation exerting additional heat flux on the surface. Here, we analyze this case further using a combination of reanalysis dataset and satellite products in a Lagrangian framework, thereby extending the view spatially from the local icebreaker observations into a Langrangian perspective. The results confirm that warm-air advection induces a positive net surface-energy-budget anomaly, exerting positive longwave radiation and turbulent heat flux on the surface. Additionally, as warm and moist air penetrates farther into the Arctic, cloud-top cooling and surface mixing eventually erode the surface inversion downstream. The initial surface inversion splits into two elevated inversions while the air columns below the elevated inversions transform into well-mixed layers.

  • 20. You, Cheng
    et al.
    Tjernström, Michael
    Devasthale, Abhay
    Eulerian and Lagrangian views of warm and moist air intrusions into summer Arctic2021Ingår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 256Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, warm and moist air intrusions (WaMAI) over the sea sectors of Kara, Laptev, East Siberian and Beaufort from 1979 to 2018 are identified in ERA5 reanalysis and their air-mass transformation is analysed using interpolation in ERA5 and satellite products along trajectories. The analysis shows that WaMAIs, driven by blocking high-pressure systems over the respective ocean sectors, induce surface warming (11–18 W m−2) and sea ice melt from positive anomalies of net longwave radiation (5–8 W m−2) and turbulent flux (8–13 W m−2) to the surface, although the anomaly of net shortwave radiation (−9 ~ +1 W m−2) is negative. From a Lagrangian perspective, the surface energy-budget anomaly decreases linearly, while total column cloud liquid water (TCLW) increases linearly with the downstream distance from the sea-ice edge. However, the cloud radiative effects of both longwave and shortwave radiation reach an equilibrium as TCLW increases in a much lower rate beyond 7 degrees north of the sea ice edge. The boundary-layer energy-budget pattern can be categorized into two classes: radiation-dominated and turbulence-dominated, comprised of 26% and 62% WaMAIs respectively. Statistically, turbulence-dominated cases occur with 3 times stronger large-scale subsidence, and also feature a larger anomaly in net shortwave radiation. In radiation-dominated WaMAIs, stratocumulus develops more strongly and hence exerts larger longwave and shortwave forcing to the surface. In both categories, a well-mixed boundary layer deepens by 500 m along the trajectories, from the continuous turbulent mixing.

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