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  • 1. Bocquet, Florence
    et al.
    Balsley, Ben
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer2011In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 138, no 1, p. 43-60Article in journal (Refereed)
    Abstract [en]

    Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (epsilon) are reasonably well correlated with concurrent measurements of vertical velocity variance (sigma(2)(w)) obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e can exhibit values as low as 10(-7) m(2) s(-3). We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e to provide a proxy measurement for sigma(2)(w) at altitudes higher than tower heights.

  • 2. Bosveld, Fred C.
    et al.
    Baas, Peter
    Steeneveld, Gert-Jan
    Holtslag, Albert A. M.
    Angevine, Wayne M.
    Bazile, Eric
    de Bruijn, Evert I. F.
    Deacu, Daniel
    Edwards, John M.
    Ek, Michael
    Larson, Vincent E.
    Pleim, Jonathan E.
    Raschendorfer, Matthias
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding2014In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 152, no 2, p. 157-187Article in journal (Refereed)
    Abstract [en]

    We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.

  • 3. Grisogono, Branko
    et al.
    Ström, Linda
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Small-scale variability in the coastal atmospheric boundary layer1998In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 88, p. 23-46Article in journal (Refereed)
  • 4. Ketterer, C.
    et al.
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Paul Scherrer Institute, Switzerland .
    Bukowiecki, N.
    Coen, M. Collaud
    Maier, O.
    Ruffieux, D.
    Weingartner, E.
    Investigation of the Planetary Boundary Layer in the Swiss Alps Using Remote Sensing and In Situ Measurements2014In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 151, no 2, p. 317-334Article in journal (Refereed)
    Abstract [en]

    The development of the planetary boundary layer (PBL) has been studied in a complex terrain using various remote sensing and in situ techniques. The high-altitude research station at Jungfraujoch (3,580 m a.s.l.) in the Swiss Alps lies for most of the time in the free troposphere except when it is influenced by the PBL reaching the station, especially during the summer season. A ceilometer and a wind profiler were installed at Kleine Scheidegg, a mountain pass close to Jungfraujoch, located at an altitude of 2,061 m a.s.l. Data from the ceilometer were analyzed using two different algorithms, while the signal-to-noise ratio of the wind profiler was studied to compare the retrieved PBL heights. The retrieved values from the ceilometer and wind profiler agreed well during daytime and cloud-free conditions. The results were additionally compared with the PBL height estimated by the numerical weather prediction model COSMO-2, which showed a clear underestimation of the PBL height for most of the cases but occasionally also a slight overestimation especially around noon, when the PBL showed its maximum extent. Air parcels were transported upwards by slope winds towards Jungfraujoch when the PBL was higher than 2,800 m a.s.l. during cloud-free cases. This was confirmed by the in situ aerosol measurements at Jungfraujoch with a significant increase in particle number concentration, particle light absorption and scattering coefficients when PBL-influenced air masses reached the station in the afternoon hours. The continuous aerosol in situ measurements at Jungfraujoch were clearly influenced by the local PBL development but also by long-range transport phenomena such as Saharan dust or pollution from the south.

  • 5. Lazeroms, W. M. J.
    et al.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bazile, E.
    Brethouwer, G.
    Wallin, S.
    Johansson, A. V.
    Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models2016In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 161, no 1, p. 19-47Article in journal (Refereed)
    Abstract [en]

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

  • 6.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Grisogono, Branko
    Department of Geophysics, Faculty of Science, Zagreb, Croatia.
    Wave flow simulations over Arctic leads2005In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 117, no 2, p. 259-273Article in journal (Refereed)
    Abstract [en]

    We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.

  • 7.
    Sedlar, Joseph
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Stratiform Cloud—Inversion Characterization During the Arctic Melt Season2009In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 132, no 3, p. 455-474Article in journal (Refereed)
    Abstract [en]

    Data collected during July and August from the Arctic Ocean Experiment 2001illustrated a common occurrence of specific-humidity (q) inversions, where moistureincreases with height, coinciding with temperature inversions in the central Arctic boundarylayer and lower troposphere. Low-level stratiform clouds and their relationship to temperatureinversions are examined using radiosonde data and data from a suite of remote sensinginstrumentation. Two low-level cloud regimes are identified: the canonical case of stratiformclouds, where the cloud tops are capped by the temperature inversion base (CCI—CloudsCapped by Inversion) and clouds where the cloud tops were found well inside the inversion(CII—Clouds Inside Inversion). The latter case was found to occur more than twiceas frequently than the former. The characteristic of the temperature inversion is shown tohave an influence on the cloud regime that was supported. Statistical analyses of the cloudregimes using remote sensing instruments suggest that CCI cases tend to be dominated bysingle-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extentof such clouds to the inversion base height. The CII cases, on the other hand, display characteristicsthat can be divided into two situations—(1) clouds that only slightly penetrate thetemperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) cloudsthat extend higher into the inversion and show evidence of a mixed-phase cloud structure.An important interplay between the mixed-phase structure and an increased potential for turbulentmixing across the inversion base appears to support the lifetime of CII cases existingwithin the inversion layer.

  • 8. Smedman, Ann-Sofi
    et al.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Högström, Ulf
    Analysis of the turbulence structure of a marine low-level jet1993In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 66, p. 105-126Article in journal (Refereed)
  • 9.
    Svensson, Gunilla
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holtslag, A. A. M.
    Kumar, V.
    Mauritsen, T.
    Steeneveld, G. J.
    Angevine, W. M.
    Bazile, E.
    Beljaars, A.
    de Bruijn, E. I. F.
    Cheng, A.
    Conangla, L.
    Cuxart, J.
    Ek, M.
    Falk, M. J.
    Freedman, F.
    Kitagawa, H.
    Larson, V. E.
    Lock, A.
    Mailhot, J.
    Masson, V.
    Park, S.
    Pleim, J.
    Söderberg, S.
    Weng, W.
    Zampieri, M.
    Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment2011In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 140, no 2, p. 177-206Article in journal (Refereed)
    Abstract [en]

    We present the main results from the second model intercomparison within the GEWEX (Global Energy and Water cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today's numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled near-surface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identified.

  • 10.
    Svensson, Gunilla
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holtslag, Albert
    Meteorology and Air Quality Section, Wageningen University, Wageningen, The Netherlands.
    Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer2009In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 132, no 2, p. 261-277Article in journal (Refereed)
    Abstract [en]

    The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer.

  • 11.
    Svensson, Gunilla
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Koracin, Darko
    The sensitivity of a stratocumulus transition: Model simulations of the ASTEX first Lagrangian2000In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 95, p. 57-90Article in journal (Refereed)
  • 12.
    Söderberg, Stefan
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Parmhed, Oskar
    Stockholm University, Faculty of Science, Department of Meteorology .
    Numerical simulations and analytical estimates of katabatic flow over a melting outflow glacier2006In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 120, no 3, p. 509-534Article in journal (Refereed)
    Abstract [en]

    A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.

  • 13.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Numerical simulations of stratiform boundary-layer clouds on the meso-gamma-scale. Part I: The influence of terrain height differences1988In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 44, p. 207-230Article in journal (Refereed)
  • 14.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Numerical simulations of stratiform boundary-layer clouds on the meso-gamma-scale. Part II: The influence of a step change in surface roughness and surface temperature1988In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 44, p. 207-230Article in journal (Refereed)
  • 15.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Some tests with a surface energy balance scheme, including a bulk parameterisation for vegetation, in a mesoscale model1989In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 48, p. 33-68Article in journal (Refereed)
  • 16.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    The summer Arctic boundary layer during the Arctic Ocean Experiment 2001 (AOE-2001)2005In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 117, p. 5--36Article in journal (Refereed)
  • 17.
    Tjernström, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mesoscale variability in the summer Arctic boundary layer2009In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 130, no 3, p. 1573-1472Article in journal (Refereed)
    Abstract [en]

    Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.

  • 18.
    Tjernström, Michael
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zagar, Mark
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Cassano, John
    Pfeifer, S
    Rinke, Annette
    Wyser, Klaus
    Dethloff, Klaus
    Jones, Colin
    Semmler, Tido
    Shaw, M
    Modelling the Arctic boundary layer: An evaluation of six ARCMIP regional-scale models using data from the SHEBA project2005In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 117, p. 337-381Article in journal (Refereed)
1 - 18 of 18
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