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  • 701.
    Zagar, Mark
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    A method for determining the small-scale variations of the surface turbulent momentum flux seaward of the coast2003Inngår i: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 42, s. 291-307Artikkel i tidsskrift (Fagfellevurdert)
  • 702.
    Zagar, Mark
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    High spatial and temporal variability of dry deposition in a coastal region2005Inngår i: Environmental Fluid Mechanics, ISSN 1567-7419, E-ISSN 1573-1510, Vol. 5, s. 357-372Artikkel i tidsskrift (Fagfellevurdert)
  • 703. Zemp, D. C.
    et al.
    Schleussner, C. F.
    Barbosa, H. M. J.
    van der Ent, R. J.
    Donges, Jonathan F.
    Stockholms universitet, Naturvetenskapliga fakulteten, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research (PIK), Germany.
    Heinke, J.
    Sampaio, G.
    Rammig, A.
    On the importance of cascading moisture recycling in South America2014Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, nr 23, s. 13337-13359Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Continental moisture recycling is a crucial process of the South American climate system. In particular, evapotranspiration from the Amazon basin contributes substantially to precipitation regionally as well as over other remote regions such as the La Plata basin. Here we present an in-depth analysis of South American moisture recycling mechanisms. In particular, we quantify the importance of cascading moisture recycling (CMR), which describes moisture transport between two locations on the continent that involves re-evaporation cycles along the way. Using an Eulerian atmospheric moisture tracking model forced by a combination of several historical climate data sets, we were able to construct a complex network of moisture recycling for South America. Our results show that CMR contributes about 9-10% to the total precipitation over South America and 17-18% over the La Plata basin. CMR increases the fraction of total precipitation over the La Plata basin that originates from the Amazon basin from 18-23 to 24-29% during the wet season. We also show that the south-western part of the Amazon basin is not only a direct source of rainfall over the La Plata basin, but also a key intermediary region that distributes moisture originating from the entire Amazon basin towards the La Plata basin during the wet season. Our results suggest that land use change in this region might have a stronger impact on downwind rainfall than previously thought. Using complex network analysis techniques, we find the eastern side of the sub-tropical Andes to be a key region where CMR pathways are channeled. This study offers a better understanding of the interactions between the vegetation and the atmosphere on the water cycle, which is needed in a context of land use and climate change in South America.

  • 704.
    Zhang, Qiong
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Körnich, Heiner
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Holmgren, Karin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    How well do reanalyses represent the southern African precipitation?2013Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, nr 3-4, s. 951-962Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Monthly-mean precipitation observations over southern Africa are used to evaluate the performance of eight global reanalyses: ERA-40, ERA-interim, JRA-25, MERRA, CFSR, NCEP-R1, NCEP-R2 and 20CRv2. All eight reanalyses reproduce the regionally averaged seasonal cycle fairly well; a few spatial mismatches with the observations are found in the climate mean for the rainy season. Principal component analyses show a dipole in the leading modes of all reanalyses, however with crucial differences in its spatial position. Possible reasons for the differences between the reanalyses are discussed on the basis of the ERA-interim and 20CRv2 results. A comparison between the moisture transports shows that ERA-interim manifests a very strong moisture convergence over the eastern equatorial Atlantic, resulting in the strong precipitation here. This excessive convergence may be due to the water-vapor assimilation and convection parameterization. Over the Indian Ocean, the ITCZ is shifted northward in ERA-interim compared to its position in 20CRv2. This discrepancy is most likely attributable to the meridional SST gradients in the Indian Ocean which are significantly larger in the ERA-interim than those in the 20CRv2, and the resulting atmospheric response prevents a southward shift of the ITCZ. Overall, the consistent description of the dynamical circulation of the atmosphere and the hydrological cycle appears as a crucial benchmark for reanalysis data. Based on our evaluation, the preferential reanalysis for investigating the climate variability over southern Africa is 20CRv2 that furthermore spans the longest time period, hence permitting the most precise investigations of interannual to decadal variability.

  • 705. Zibordi, G.
    et al.
    Ruddick, K.
    Ansko, I.
    Moore, G.
    Kratzer, Susanne
    Stockholms universitet, Naturvetenskapliga fakulteten, Systemekologiska institutionen.
    Icely, J.
    Reinart, A.
    In situ determination of the remote sensing reflectance: an inter comparison2012Inngår i: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 8, nr 4, s. 567-586Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Inter-comparison of data products from simultaneous measurements performed with independent systems and methods is a viable approach to assess the consistency of data and additionally to investigate uncertainties. Within such a context the inter-comparison called Assessment of In Situ Radiometric Capabilities for Coastal Water Remote Sensing Applications (ARC) was carried out at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea to explore the accuracy of in situ data products from various in- and above-water optical systems and methods. Measurements were performed under almost ideal conditions, including a stable deployment platform, clear sky, relatively low sun zenith angles and moderately low sea state. Additionally, all optical sensors involved in the experiment were inter-calibrated through absolute radiometric calibration performed with the same standards and methods. Inter-compared data products include spectral waterleaving radiance L-w(lambda), above-water downward irradiance E-d(0(+),lambda) and remote sensing reflectance R-rs(lambda). Data products from the various measurement systems/methods were directly compared to those from a single reference system/method. Results for R-rs(lambda) indicate spectrally averaged values of relative differences comprised between - 1 and +6 %, while spectrally averaged values of absolute differences vary from approximately 6% for the above-water systems/methods to 9 % for buoy-based systems/methods. The agreement between R-rs(lambda) spectral relative differences and estimates of combined uncertainties of the inter-compared systems/methods is noteworthy.

  • 706.
    Zieger, Paul
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM). Paul Scherrer Institute.
    Fierz-Schmidhauser, R.
    Weingartner, E.
    Baltensperger, U.
    Effects of relative humidity on aerosol light scattering: results from different European sites2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 21, s. 10609-10631Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of aerosol water uptake on the aerosol particle light scattering coefficient (sigma(sp)) is described in this study by comparing measurements from five European sites: the Jungfraujoch, located in the Swiss Alps at 3580 m a.s.l.; Ny-angstrom lesund, located on Spitsbergen in the Arctic; Mace Head, a coastal site in Ireland; Cabauw, a rural site in the Netherlands; and Melpitz, a regional background site in Eastern Germany. These sites were selected according to the aerosol type usually encountered at that location. The scattering enhancement factor f(RH, lambda) is the key parameter to describe the effect of water uptake on the particle light scattering. It is defined as the sigma(sp)(RH) at a certain relative humidity (RH) and wavelength lambda divided by its dry value. f(RH) at the five sites varied widely, starting at very low values of f(RH = 85%, lambda = 550 nm) around 1.28 for mineral dust, and reaching up to 3.41 for Arctic aerosol. Hysteresis behavior was observed at all sites except at the Jungfraujoch (due to the absence of sea salt). Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f(RH). Both parameters are also needed to successfully predict f(RH). Finally, the measurement results were compared to the widely used aerosol model, OPAC (Hess et al., 1998). Significant discrepancies were seen, especially at intermediate RH ranges; these were mainly attributed to inappropriate implementation of hygroscopic growth in the OPAC model. Replacement of the hygroscopic growth with values from the recent literature resulted in a clear improvement of the OPAC model.

  • 707. Zilitinkevich, S. S.
    et al.
    Elperin, T.
    Kleeorin, N.
    Rogachevskii, I.
    Esau, I.
    Mauritsen, Thorsten
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Miles, M. W.
    Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimes2008Inngår i: The Quarterly Journal of the Royal Meteorological Society. Part B, Vol. 134, nr 633, s. 792-799Artikkel i tidsskrift (Fagfellevurdert)
  • 708.
    Zurovac-Jevtic, Dance
    Stockholms universitet.
    Dynamic modeling of cirrus cloud characteristics1999Doktoravhandling, med artikler (Annet vitenskapelig)
  • 709. Zveryaev, Igor I.
    et al.
    Hannachi, Abdel A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Interannual variability of Mediterranean evaporation and its relation to regional climate2012Inngår i: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 38, nr 3-4, s. 495-512Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Gridded monthly evaporation data for 1958-2006 from the Woods Hole Oceanographic Institution data set are used to investigate interannual variability of Mediterranean evaporation during cold and hot seasons and its relation to regional atmospheric dynamics, sea surface temperature and atmospheric elements of the hydrological cycle. The first EOF mode of Mediterranean evaporation, explaining more than 50% of its total variance, is characterized by the monopole pattern both in winter and summer. However, despite structural similarity, the EOF-1 of Mediterranean evaporation is affected by different climate signals in cold and hot seasons. During winter the EOF-1 is associated with the East Atlantic teleconnection pattern. In summer, there is indication of tropical influence on the EOF-1 of Mediterranean evaporation (presumably from Asian monsoon). Both in winter and summer, principal components of EOF-1 demonstrate clear interdecadal signals (with a stronger signature in summer) associated with large sea surface temperature anomalies. The results of a sensitivity analysis suggest that in winter both the meridional wind and the vertical gradient of saturation specific humidity (GSSH) near the sea surface contribute to the interdecadal evaporation signal. In summer, however, it is likely that the signal is more related to GSSH. Our analysis did not reveal significant links between the Mediterranean evaporation and the North Atlantic Oscillation in any season. The EOF-2 of evaporation accounts for 20% (11%) of its total variance in winter (in summer). Both in winter and summer the EOF-2 is characterized by a zonal dipole with opposite variations of evaporation in western and eastern parts of the Mediterranean Sea. This mode is associated presumably with smaller scale (i.e., local) effects of atmospheric dynamics. Seasonality of the leading modes of the Mediterranean evaporation is also clearly seen in the character of their links to atmospheric elements of the regional hydrological cycle. In particular, significant links to precipitation in some regions have been found in winter, but not in summer.

  • 710. Zveryaev, Igor I.
    et al.
    Hannachi, Abdel B. A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Interdecadal changes in the links between Mediterranean evaporation and regional atmospheric dynamics during extended cold season2017Inngår i: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 37, nr 3, s. 1322-1340Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Monthly evaporation data for 1958-2010 from the Woods Hole Oceanographic Institution dataset are used to investigate interdecadal changes in interannual variability of Mediterranean evaporation and its links to regional climate. Analysis performed for the two climate periods characterized by the downward (1958-1978) and upward (1979-2010) trends of evaporation revealed significant season-dependent interdecadal changes in its interannual variability. The largest changes in interannual variability have been revealed during autumn when the leading empirical orthogonal functions (EOFs) are characterized by a zonal dipole pattern (except in November 1979-2010). During winter and spring, the EOF-1 and EOF-2 are characterized, respectively, by a monopole pattern and a zonal dipole and are associated with the East Atlantic (EA) and the East Atlantic-West Russia (EAWR) teleconnections. It is shown that interdecadal changes in interannual variability of Mediterranean evaporation during cold season were determined by the EA transition from the strongly negative to more neutral phase that occurred in late 1970s. Seasonally dependent changes in the structure of the leading evaporation EOFs reflect changing roles of the EA and EAWR which impact near surface air temperature, specific humidity and wind.

  • 711.
    Zábori, Julia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mårtensson, E. Monica
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    de Leeuw, Gerrit
    University of Helsinki, Department of Physics.
    Nilsson, E. Douglas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Wintertime Arctic Ocean sea water properties and primary marine aerosol concentrations2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 21, s. 10405-10421Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sea spray aerosols are an important part of the climate system through their direct and indirect effects. Due to the diminishing sea ice, the Arctic Ocean is one of the most rapidly changing sea spray aerosol source areas. However, the influence of these changes on primary particle production is not known.

    In laboratory experiments we examined the influence of Arctic Ocean water temperature, salinity, and oxygen saturation on primary particle concentration characteristics. Sea water temperature was identified as the most important of these parameters. A strong decrease in sea spray aerosol production with increasing water temperature was observed for water temperatures between −1°C and 9°C. Aerosol number concentrations decreased from at least 1400 cm−3 to 350 cm−3. In general, the aerosol number size distribution exhibited a robust shape with one mode close to dry diameter Dp 0.2 μm with approximately 45% of particles at smaller sizes. Changes in sea water temperature did not result in pronounced change of the shape of the aerosol size distribution, only in the magnitude of the concentrations. Our experiments indicate that changes in aerosol emissions are most likely linked to changes of the physical properties of sea water at low temperatures. The observed strong dependence of sea spray aerosol concentrations on sea water temperature, with a large fraction of the emitted particles in the typical cloud condensation nuclei size range, provide strong arguments for a more careful consideration of this effect in climate models

  • 712.
    Zábori, Julia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Vaattovaara, Petri
    University of Eastern Finland, Department of Applied Physics.
    Ekman, Annica
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mårtensson, Monica
    Uppsala University, Department of Earth Sciences.
    Nilsson, Douglas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Comparison between summertime and wintertime Arctic Ocean primary marine aerosol propertiesInngår i: Atmospheric Chemistry and Physics Discussions, ISSN 1680-7367, E-ISSN 1680-7375Artikkel i tidsskrift (Fagfellevurdert)
  • 713.
    Zábori, Julia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Matisans, Modris
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Nilsson, Douglas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Artificial primary marine aerosol production: a laboratory study with varying water temperature, salinity, and succinic acid concentration2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 22, s. 10709-10724Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Primary marine aerosols are an important component of the climate system, especially in the remote marine environment. With diminishing sea-ice cover, better understanding of the role of sea spray aerosol on climate in the polar regions is required. As for Arctic Ocean water, laboratory experiments with NaCl water confirm that a few degrees change in the water temperature (Tw) gives a large change in the number of primary particles. Small particles with a dry diameter between 0.01 μm and 0.25 μm dominate the aerosol number density, but their relative dominance decreases with increasing water temperature from 0 °C where they represent 85–90% of the total aerosol number to 10 °C, where they represent 60–70% of the total aerosol number. This effect is most likely related to a change in physical properties and not to modification of sea water chemistry. A change of salinity between 15 g kg−1 and 35 g kg−1 did not influence the shape of a particle number size distribution. Although the magnitude of the size distribution for a water temperature change between 0 °C and 16 °C changed, the shape did not. An experiment where succinic acid was added to a NaCl water solution showed, that the number concentration of particles with 0.010 μm < Dp < 4.5 μm decreased on average by 10% when the succinic acid concentration in NaCl water at a water temperature of 0 °C was increased from 0 μmol L−1 to 94 μmol L−1. A shift to larger sizes in the particle number size distribution is observed from pure NaCl water to Arctic Ocean water. This is likely a consequence of organics and different inorganic salts present in Arctic Ocean water in addition to the NaCl.

  • 714.
    Árnason, Geirmundur
    Stockholms universitet.
    A study of nongeostrophic perturbations in a zonal flow with emphasis on criteria for baroclinic stability1963Doktoravhandling, med artikler (Annet vitenskapelig)
  • 715.
    Žagar, Nedjeljka
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Dynamical aspects of atmospheric data assimilation in the tropics2004Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    A faithful depiction of the tropical atmosphere requires three-dimensional sets of observations. Despite the increasing amount of observations presently available, these will hardly ever encompass the entire atmosphere and, in addition, observations have errors. Additional (background) information will always be required to complete the picture. Valuable added information comes from the physical laws governing the flow, usually mediated via a numerical weather prediction (NWP) model. These models are, however, never going to be error-free, why a reliable estimate of their errors poses a real challenge since the whole truth will never be within our grasp.

    The present thesis addresses the question of improving the analysis procedures for NWP in the tropics. Improvements are sought by addressing the following issues:

    - the efficiency of the internal model adjustment,

    - the potential of the reliable background-error information, as compared to observations,

    - the impact of a new, space-borne line-of-sight wind measurements, and

    - the usefulness of multivariate relationships for data assimilation in the tropics.

    Most NWP assimilation schemes are effectively univariate near the equator. In this thesis, a multivariate formulation of the variational data assimilation in the tropics has been developed. The proposed background-error model supports the mass-wind coupling based on convectively-coupled equatorial waves. The resulting assimilation model produces balanced analysis increments and hereby increases the efficiency of all types of observations.

    Idealized adjustment and multivariate analysis experiments highlight the importance of direct wind measurements in the tropics. In particular, the presented results confirm the superiority of wind observations compared to mass data, in spite of the exact multivariate relationships available from the background information. The internal model adjustment is also more efficient for wind observations than for mass data.

    In accordance with these findings, new satellite wind observations are expected to contribute towards the improvement of NWP and climate modeling in the tropics. Although incomplete, the new wind-field information has the potential to reduce uncertainties in the tropical dynamical fields, if used together with the existing satellite mass-field measurements.

    The results obtained by applying the new background-error representation to the tropical short-range forecast errors of a state-of-art NWP model suggest that achieving useful tropical multivariate relationships may be feasible within an operational NWP environment.

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