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  • 1.
    Bengtsson, Lisa
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Körnich, Heiner
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Källén, Erland
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Large-Scale Dynamical Response to Subgrid-Scale Organization Provided by Cellular Automata2011Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 12, s. 3132-3144Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Due to the limited resolution of numerical weather prediction (NWP) models, sub-grid scale physical processes are parameterized, and represented by grid-box means. However, some physical processes are better represented by a mean and its variance, a typical example being deep convection, with scales varying from individual updraughts to organized meso-scale systems. In this study, we investigate, in an idealized setting, whether a cellular automaton (CA) can be used in order to enhance sub-grid scale organization by forming clusters representative of the convective scales, and yield a stochastic representation of sub-grid scale variability. We study the transfer of energy from the convective to the larger atmospheric scales through nonlinear wave interactions. This is done using a shallow water (SW) model initialized with equatorial wave modes. By letting a CA act on a finer resolution than that of the SW model, it can be expected to mimic the effect of, for instance, gravity wave propagation on convective organization. Employing the CA-scheme allows to reproduce the observed behaviour of slowing down equatorial Kelvin modes in convectively active regions, while random perturbations fail to feed back on the large-scale flow. The analysis of kinetic energy spectra demonstrates that the CA sub-grid scheme introduces energy back-scatter from the smallest model scales to medium scales. However, the amount of energy back-scattered depends almost solely on the memory time scale introduced to the sub-grid scheme, whereas any variation in spatial scales generated does not influence the energy spectra markedly.

  • 2.
    Bengtsson, Lisa
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Steinheimer, M.
    Bechtold, P.
    Geleyn, J-F.
    A stochastic parameterization for deep convection using cellularautomataIngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469Artikel i tidskrift (Refereegranskat)
  • 3.
    Caballero, Rodrigo
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Carlson, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Surface superrotation2018Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 10, s. 3671-3689Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earth-like climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.

  • 4.
    Caballero, Rodrigo
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hanley, John
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Midlatitude Eddies, Storm-Track Diffusivity, and Poleward Moisture Transport in Warm Climates2012Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 69, nr 11, s. 3237-3250Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent work using both simplified and comprehensive GCMs has shown that poleward moisture transport across midlatitudes follows Clausius-Clapeyron scaling at temperatures close to modern, but that it reaches a maximum at sufficiently elevated temperatures and then decreases with further warming. This study explores the reasons for this nonmonotonic behavior using a sequence of NCAR Community Atmosphere Model, version 3 (CAM3) simulations in an aquaplanet configuration spanning a broad range of climates. No significant change is found in the scale, structure, or organization of midlatitude eddies across these simulations. Instead, the high-temperature decrease in poleward moisture transport is attributed to the combined effect of decreasing eddy velocities and contracting mixing lengths. The contraction in mixing length is, in turn, a consequence of the decreasing eddy velocities in combination with constant eddy decorrelation time scales.

  • 5.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Analysis of the Meridional Energy Transport by Atmospheric Overturning Circulations2011Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 8, s. 1806-1820Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The atmospheric meridional overturning circulation is computed using the interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) data. Meridional mass transport streamfunctions are calculated not only using pressure as a vertical coordinate but also using temperature, specific humidity, and geopotential height as generalized vertical coordinates. Moreover, mass transport streamfunctions are calculated using the latent, the dry static, or the moist static energy as generalized vertical coordinates. The total meridional energy transport can be obtained by integrating these streamfunctions vertically over their entire energy range. The time-averaged mass transport streamfunctions are also decomposed into mean-flow and eddy-induced components. The meridional mass transport streamfunctions with temperature and specific humidity as independent variables yield a two-cell structure with a tropical Hadley-like cell and a pronounced extratropical Ferrel-like cell, which carries warm and moist air poleward. These Ferrel-like cells are much stronger than the Eulerian zonal-mean Ferrel cell, a feature that can be understood by considering the residual circulation related to specific humidity or temperature. Regardless of the generalized vertical coordinate, the present meridional mass transport streamfunctions yield essentially a two-layer structure with one poleward and one equatorward branch. The strongest meridional overturning in the midlatitudes is obtained when the specific humidity or the moist static energy is used as the vertical coordinate, indicating that the specific humidity is the variable that best distinguishes between poleward- and equatorward-moving air in the lower troposphere.

  • 6.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Söderberg, Anders
    Impact of Two-Way Aerosol-Cloud Interaction and Changes in Aerosol Size Distribution on Simulated Aerosol-Induced Deep Convective Cloud Sensitivity2011Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 4, s. 685-698Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent cloud-resolving model studies of single (isolated) deep convective clouds have shown contradicting results regarding the response of the deep convection to changes in the aerosol concentration. In the present study, a cloud-resolving model including explicit aerosol physics and chemistry is used to examine how the complexity of the aerosol model, the size of the aerosols, and the aerosol activation parameterization influence the aerosol-induced deep convective cloud sensitivity. Six sensitivity series are conducted. A significant difference in the aerosol-induced deep convective cloud sensitivity is found when using different complexities of the aerosol model and different aerosol activation parameterizations. In particular, graupel impaction scavenging of aerosols appears to be a crucial process because it efficiently may limit the number of cloud condensation nuclei (CCN) at a critical stage of cloud development and thereby dampen the convection. For the simulated case, a 100% increase in aerosol concentration results in a difference in average updraft between the various sensitivity series that is as large as the average updraft increase itself. The change in graupel and rain formation also differs significantly. The sign of the change in precipitation is not always directly proportional to the change in updraft velocity and several of the sensitivity series display a decrease of the rain amount with increasing updraft velocity. This result illustrates the need to account for changes in evaporation processes and subsequent cooling when assessing aerosol effects on deep convective strength. The model simulations also show that an increased number of aerosols in the Aitken mode (here defined by 23 <= d <= 100.0 nm) results in a larger impact on the convective strength compared to an increased number of aerosols in the accumulation mode (here defined by 100 <= d <= 900.0 nm). When accumulation mode aerosols are activated and grow at the beginning of the cloud cycle, the supersaturation near the cloud base is lowered, which to some extent limits further aerosol activation. The simulations indicate a need to better understand and represent the two-way interaction between aerosols and clouds when studying aerosol-induced deep convective cloud sensitivity.

  • 7.
    Fitch, Anna C.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Pacific Northwest National Laboratory, Washington, USA.
    An Improved Double-Gaussian Closure for the Subgrid Vertical Velocity Probability Distribution Function2019Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 76, s. 285-304Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The vertical velocity probability distribution function (PDF) is analyzed throughout the depth of the lower atmosphere, including the subcloud and cloud layers, in four large-eddy simulation (LES) cases of shallow cumulus and stratocumulus. Double-Gaussian PDF closures are examined to test their ability to represent a wide range of turbulence statistics, from stratocumulus cloud layers characterized by Gaussian turbulence to shallow cumulus cloud layers displaying strongly non-Gaussian turbulence statistics. While the majority of the model closures are found to perform well in the former case, the latter presents a considerable challenge. A new model closure is suggested that accounts for high skewness and kurtosis seen in shallow cumulus cloud layers. The well-established parabolic relationship between skewness and kurtosis is examined, with results in agreement with previous studies for the subcloud layer. In cumulus cloud layers, however, a modified relationship is necessary to improve performance. The new closure significantly improves the estimation of the vertical velocity PDF for shallow cumulus cloud layers, in addition to performing well for stratocumulus. In particular, the long updraft tail representing the bulk of cloudy points is much better represented and higher-order moments diagnosed from the PDF are also greatly improved. However, some deficiencies remain owing to fundamental limitations of representing highly non-Gaussian turbulence statistics with a double-Gaussian PDF.

  • 8.
    Garcia-Carreras, Luis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Parker, D. J.
    Marsham, J. H.
    Rosenberg, P. D.
    Brooks, I. M.
    Lock, A. P.
    Marenco, F.
    McQuaid, J. B.
    Hobby, M.
    The Turbulent Structure and Diurnal Growth of the Saharan Atmospheric Boundary Layer2015Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 72, nr 2, s. 693-713Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The turbulent structure and growth of the remote Saharan atmospheric boundary layer (ABL) is described with in situ radiosonde and aircraft measurements and a large-eddy simulation model. A month of radiosonde data from June 2011 provides a mean profile of the midday Saharan ABL, which is characterized by a well-mixed convective boundary layer, capped by a small temperature inversion (<1K) and a deep, near-neutral residual layer. The boundary layer depth varies by up to 100% over horizontal distances of a few kilometers due to turbulent processes alone. The distinctive vertical structure also leads to unique boundary layer processes, such as detrainment of the warmest plumes across the weak temperature inversion, which slows down the warming and growth of the convective boundary layer. As the boundary layer grows, overshooting plumes can also entrain free-tropospheric air into the residual layer, forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer, thus slowing down boundary layer growth further. A single-column model is unable to accurately reproduce the evolution of the Saharan boundary layer, highlighting the difficulty of representing such processes in large-scale models. These boundary layer processes are special to the Sahara, and possibly hot, dry, desert environments in general, and have implications for the large-scale structure of the Saharan heat low. The growth of the boundary layer influences the vertical redistribution of moisture and dust, and the spatial coverage and duration of clouds, with large-scale dynamical and radiative implications.

  • 9.
    Hannachi, Abdel
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Iqbal, Waheed
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability2019Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 76, nr 1, s. 333-356Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 10.
    Hannachi, Abdelwaheb
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Mitchell, D.
    Gray, L.
    Charlton-Perez, A.
    On the Use of Geometric Moments to Examine the Continuum of Sudden Stratospheric Warmings2011Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 3, s. 657-674Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 11. Herbert, Corentin
    et al.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bouchet, Freddy
    Atmospheric Bistability and Abrupt Transitions to Superrotation: Wave-Jet Resonance and Hadley Cell Feedbacks2020Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 77, nr 1, s. 31-49Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Strong eastward jets at the equator have been observed in many planetary atmospheres and simulated in numerical models of varying complexity. However, the nature of the transition from a conventional state of the general circulation, with easterlies or weak westerlies in the tropics, to such a superrotating state remains unclear. Is it abrupt or continuous? This question may have far-reaching consequences, as it may provide a mechanism for abrupt climate change in a planetary atmosphere, both through the loss of stability of the conventional circulation and through potential noise-induced transitions in the bistability range. We study two previously suggested feedbacks that may lead to bistability between a conventional and a superrotating state: the Hadley cell feedback and a wave-jet resonance feedback. We delineate the regime of applicability of these two mechanisms in a simple model of zonal acceleration budget at the equator. Then we show using numerical simulations of the axisymmetric primitive equations that the wave-jet resonance feedback indeed leads to robust bistability, while the bistability governed by the Hadley cell feedback, although observed in our numerical simulations, is much more fragile in a multilevel model.

  • 12.
    Kjellsson, Joakim
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Döös, Kristofer
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Laliberté, Frédéric B.
    Zika, Jan
    The Atmospheric General Circulation in Thermodynamical Coordinates2014Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 71, nr 3, s. 916-928Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic stream function in dry static energy versus latent heat coordinates. Diabatic motions in the tropical circulations and fluxes driven by midlatitude eddies are found to form a single, global thermodynamic cycle. Calculations based on the ERA-Interim reanalysis dataset indicate that the cycle has a peak transport of 428 Sv (Sv = 109 kg s−1). The thermodynamic cycle encapsulates a globally interconnected heat and water cycle comprising ascent of moist air where latent heat is converted into dry static energy, radiative cooling where dry air loses dry static energy, and a moistening branch where air is warmed and moistened. It approximately follows a tropical moist adiabat and is bounded by the Clausius-Clapeyron relationship for near-surface air. The variability of the atmospheric general circulation is related to ENSO events using reanalysis data from recent years (1979-2009) and historical simulations from the EC-Earth coupled climate model (1850-2005). The thermodynamic cycle in both EC-Earth and ERA-Interim widens and weakens with positive ENSO phases and narrows and strengthens during negative ENSO phases with a high correlation coefficient. Weakening in amplitude suggests a reduction in moist convection in the tropics, while widening suggests an increase in mean tropical near-surface moist static energy.

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  • 13. Lachmy, Orli
    et al.
    Harnik, Nili
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Tel Aviv University, Israel.
    Wave and Jet Maintenance in Different Flow Regimes2016Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 73, nr 6, s. 2465-2484Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The wave spectrum and zonal-mean-flow maintenance in different flow regimes of the jet stream are studied using a two-layer modified quasigeostrophic (QG) model. As the wave energy is increased by varying the model parameters, the flow transitions from a subtropical jet regime to a merged jet regime and then to an eddy-driven jet regime. The subtropical jet is maintained at the Hadley cell edge by zonal-mean advection of momentum, while eddy heat flux and eddy momentum flux convergence (EMFC) are weak and concentrated far poleward. The merged jet is narrow and persistent and is maintained by EMFC from a narrow wave spectrum, dominated by zonal wavenumber 5. The eddy-driven jet is wide and fluctuating and is maintained by EMFC from a wide wave spectrum. The wave-mean flow feedback mechanisms that maintain each regime are explained qualitatively.The regime transitions are related to transitions in the wave spectrum. An analysis of the wave energy spectrum budget and a comparison with a quasi-linear version of the model show that the balance maintaining the spectrum in the merged and subtropical jet regimes is mainly a quasi-linear balance, whereas in the eddy-driven jet regime nonlinear inverse energy cascade takes place. The amplitude and wavenumber of the dominant wave mode in the merged and subtropical jet regimes are determined by the constraint that this mode would produce the wave fluxes necessary for maintaining a mean flow that is close to neutrality. In contrast, the equilibrated mean flow in the eddy-driven jet regime is weakly unstable.

  • 14.
    Li, Qiang
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi.
    Lindborg, Erik
    Weakly or Strongly Nonlinear Mesoscale Dynamics Close to the Tropopause?2018Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 4, s. 1215-1229Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recently, it has been discussed whether the mesoscale energy spectra in the upper troposphere and lower stratosphere are generated by weakly or strongly nonlinear dynamics. A necessary condition for weak non- linearity is that the Rossby number Ro vertical bar zeta(z)vertical bar/f << 1, where zeta(z) is the vertical vorticity and f is the Coriolis parameter. First, it is shown that Ro can be estimated by integration of the rotational wavenumber energy spectrum E-r. Then divergence and rotational energy spectra and their ratio, R E-d/E-r, are calculated from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) dataset, and it is shown that at least 1000 flight segments are needed to obtain converged results. It is found that R < 1 in the upper troposphere, ruling out the hypothesis that the spectra are produced by inertia-gravity waves with frequencies larger than f. In the lower stratosphere R is slightly larger than unity. An analysis separating between land and ocean data shows that E-d and temperature spectra have somewhat larger magnitude over land compared to ocean in the upper troposphere-a signature of orographically or convectively forced gravity waves. No such effect is seen in the lower stratosphere. At midlatitudes the Rossby number is on the order of unity and at low latitudes it is larger than unity, indicating that strong nonlinearities are prevalent. Also the temperature spectra, when converted into potential energy spectra, have larger magnitude than predicted by the weakly nonlinear wave hypothesis.

  • 15.
    Li, Xiang-Yu
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado Boulder, USA.
    Brandenburg, Axel
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. University of Colorado Boulder, USA.
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden.
    Haugen, Nils E. L.
    Mehlig, Bernhard
    Rogachevskii, Igor
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Ben-Gurion University of the Negev, Israel.
    Condensational and Collisional Growth of Cloud Droplets in a Turbulent Environment2020Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 77, nr 1, s. 337-353Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high-resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water vapor mixing ratio, and the resulting supersaturation fields together with the Navier-Stokes equation. We find that the droplet size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation-fluctuation-induced broadening of droplet size distributions enhances the collisional growth. This is contrary to classical (nonturbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.

  • 16.
    Li, Xiang-Yu
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado, USA.
    Brandenburg, Axel
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. University of Colorado, USA.
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Centre, Sweden; Global & Climate Dynamics, National Center for Atmospheric Research, USA.
    Haugen, Nils E. L.
    Mehlig, Bernhard
    Rogachevskii, Igor
    Effect of turbulence on collisional growth of cloud droplets2018Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, s. 3469-3487Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the effect of turbulence on the collisional growth of um-sized droplets through high- resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. This can be explained through the energy dissipation rate dependence of the mean collision rate described by the Saffman-Turner collision model. Consistent with the Saffman-Turner collision model and its extensions, the collision rate increases as the square root of the energy dissipation rate even when coalescence is invoked. The size distribution exhibits power law behavior with a slope of -3.7 between a maximum at approximately 10 um up to about 40 um. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is due to the fact that turbulence enhances the collisional growth between similar sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth.

  • 17. Lofverstrom, Marcus
    et al.
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Messori, Gabriele
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Stationary Wave Reflection as a Mechanism for Zonalizing the Atlantic Winter Jet at the LGM2016Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 73, nr 8, s. 3329-3342Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Current estimates of the height of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) range from around 3000 to 4500 m. Modeling studies of the LGM, using low-end estimates of the LIS height, show a relatively weak and northeastward-tilted winter jet in the North Atlantic, similar to the modern jet, while simulations with high-end LIS elevations show a much more intense and zonally oriented jet. Here, an explanation for this response of the Atlantic circulation is sought using a sequence of LGM simulations spanning a broad range of LIS elevations. It is found that increasing LIS height favors planetary wave breaking and nonlinear reflection in the subtropical North Atlantic. For high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. This entails a zonalization of the stationary wave phase lines and thus of the midlatitude jet.

  • 18. Lubis, Sandro W.
    et al.
    Matthes, Katja
    Omrani, Nour-Eddine
    Harnik, Nili
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Tel Aviv University, Israel.
    Wahl, Sebastian
    Influence of the Quasi-Biennial Oscillation and Sea Surface Temperature Variability on Downward Wave Coupling in the Northern Hemisphere2016Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 73, nr 5, s. 1943-1965Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Downward wave coupling occurs when an upward-propagating planetary wave from the troposphere decelerates the flow in the upper stratosphere and forms a downward reflecting surface that redirects waves back to the troposphere. To test this mechanism and potential factors influencing the downward wave coupling, three 145-yr sensitivity simulations with NCAR's Community Earth System Model [CESM1(WACCM)], a state-of-the-art high-top chemistry-climate model, are analyzed. The results show that the quasi-biennial oscillation (QBO) and SST variability significantly impact downward wave coupling. Without the QBO, the occurrence of downward wave coupling is significantly suppressed. In contrast, stronger and more persistent downward wave coupling occurs when SST variability is excluded. The above influence on the occurrence of downward wave coupling is mostly due to a direct influence of the QBO and SST variability on stratospheric planetary wave source and propagation. The strengths of the tropospheric circulation and surface responses to a given downward wave coupling event, however, behave differently. The surface anomaly is significantly weaker (stronger) in the experiment with fixed SSTs (without QBO), even though the statistical signal of downward wave coupling is strongest (weakest) in this experiment. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy-mean flow feedback and the possible contribution of SST anomalies in the North Atlantic during the downward wave coupling event. The weaker synoptic-scale eddy-mean flow feedback and the absence of the positive NAO-related SST-tripole pattern in the fixed SST experiment are consistent with a weaker tropospheric response to downward wave coupling. The results highlight the importance of synoptic-scale eddies in setting the tropospheric response to downward wave coupling.

  • 19.
    Messori, Gabriele
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Imperial College London, United Kingdom.
    Geen, R.
    Czaja, A.
    On the Spatial and Temporal Variability of Atmospheric Heat Transport in a Hierarchy of Models2017Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 74, nr 7, s. 2163-2189Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The present study analyzes the spatial and temporal variability of zonally integrated meridional atmospheric heat transport due to transient eddies in a hierarchy of datasets. These include a highly idealized two-layer model seeded with point geostrophic vortices, an intermediate complexity GCM, and the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) data. The domain of interest is the extratropics. Both the two-layer model and the GCMdisplay a pronounced temporal variability in the zonally integrated meridional transport, with the largest values (or pulses) of zonally integrated transport being associated with extended regions of anomalously strong local heat transport. In the two-layer model these largescale coherent transport regions, termed heat transport bands,'' are linked to densely packed baroclinic vortex pairs and can be diagnosed as low-wavenumber streamfunction anomalies. In the GCM they are associated with both the warm and cold sectors of midlatitude weather systems. Both these features are also found in ERA-Interim: the heat transport bands match weather systems and occur primarily in the storm-track regions, which in turn correspond to planetary-scale climatological streamfunction anomalies. The authors hypothesize that the temporal variability of the zonally integrated heat transport is partly linked to oscillatory variations in the stormtrack activity but also contains a background red noise component. The existence of a pronounced variability in the zonally integrated meridional heat transport can have important consequences for the interplay between midlatitude dynamics and the energy balance of the high latitudes.

  • 20. Rogers, David
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Turbulence structure in decoupled marine stratocumulus: A case study from the ASTEX field experiment1996Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 53, s. 598-619Artikel i tidskrift (Refereegranskat)
  • 21.
    Sardina, Gaetano
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Center (SeRC), Sweden; Chalmers University of Technology, Sweden.
    Poulain, Stephane
    Brandt, Luca
    Caballero, Rodrigo
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Swedish e-Science Research Center (SeRC), Sweden.
    Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation2018Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, nr 2, s. 451-467Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a large-eddy simulation (LES) approach. The authors investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed onto a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending the authors' previous results regarding stochastic condensation, the authors introduce a new theoretical estimation of the droplet size spectrum broadening that accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.

  • 22. Smedman, Ann-Sofi
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Högström, Ulf
    The near-neutral marine atmospheric boundary layer with no surface shearing shress: a case study1994Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 51, s. 3399-3411Artikel i tidskrift (Refereegranskat)
  • 23.
    Söderberg, Stefan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Diurnal cycle of supercritical along-coast flows2002Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 59, s. 2615-2624Artikel i tidskrift (Refereegranskat)
  • 24.
    Tjernström, Michael
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Balsley, Ben B.
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nappo, Carmen J.
    The Effects of Critical Layers on Residual Layer Turbulence2009Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 66, nr 2, s. 468-480Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The authors report results of a study of finescale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The study covers two nights during the Cooperative Atmosphere-Surface Exchange Study 1999 (CASES-99) field experiment that exhibit significant differences in turbulence, as indicated by the observed turbulence dissipation rates in the RL. The RL turbulence sometimes reaches intensities comparable to those in the underlying stable boundary layer. The commonly accepted concept of turbulence generation below critical values of the gradient Richardson number (Ri(g)) is scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical Ri(g) values (approximate to 0.25) occur at vertical scales of only a few tens of meters. The very small scale for the occurrence of subcritical Ri poses problems for incorporating experimentally determined Ri(g)-based methods in model closures in models with poor resolution. There appear to be two distinct turbulence ""regimes"" in the RL: a very weak but ever-present background turbulence level with minimal temporal and spatial structure and a more intense intermittent regime during which turbulent intensity can approach near-surface nighttime turbulent intensities. It is hypothesized that the locally produced RL turbulence can be related to upward-propagating atmospheric gravity waves generated by flow over the low-relief terrain. The presence of critical layers in the RL, caused by wind turning with height, results in the generation of intermittent turbulence.

  • 25.
    Tjernström, Michael
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Balsley, Ben
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nappo, Carmen
    The effects of critical layers on residual layer turbulence2009Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 66, s. 468-480Artikel i tidskrift (Refereegranskat)
  • 26.
    Tjernström, Michael
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Koracin, Darko
    Modeling the impact of marine stratocumulus on boundary layer structure1995Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 52, s. 863-878Artikel i tidskrift (Refereegranskat)
  • 27.
    Zhang, Qiong
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi.
    Holmgren, Karin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi.
    Sundqvist, Hanna
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi.
    Decadal Rainfall Dipole Oscillation over Southern Africa Modulated by Variation of Austral Summer Land-Sea Contrast along the East Coast of Africa2015Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 72, nr 5, s. 1827-1836Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern. It is found that the southern African rainfall dipole index is highly correlated to the land-sea contrast along the east coast of Africa. When the land-sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south. On a decadal time scale, an ENSO-like SST pattern acts to modulate this land-sea contrast and the consequent rainfall dipole. During a wet in the south and dry in the north dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land-sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud-SST coupling acts to further strengthen the land-sea contrast.

  • 28.
    Žagar, Nedjeljka
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Assimilation of equatorial waves by line of sight wind observations2004Ingår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 61, nr 15, s. 1877-1893Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper investigates the potential of line-of-sight (LOS) wind information from a spaceborne Doppler wind lidar to reduce uncertainties in the analysis fields of equatorial waves. The benefit of LOS winds is assessed by comparing their impact to that of a single wind component, full wind field information, and mass field data in three- and four-dimensional variational data assimilation.

    The dynamical framework consists of nonlinear shallow-water equations solved in spectral space and a background error term based on eigenmodes derived from linear equatorial wave theory. Based on observational evidence, simulated wave motion fields contain equatorial Kelvin, Rossby, mixed Rossby–gravity, and the lowest two modes of the westward-propagating inertio–gravity waves. The same dynamical structures are included, entirely or partially, into the background error covariance matrix for the multivariate analysis. The relative usefulness of LOS data is evaluated by carrying out “identical twin” observing system simulation experiments and assuming a perfect model.

    Results from the experiments involving a single observation or an imperfect background error covariance matrix illustrate that the assimilation increments due to LOS wind information rely more on the background error term specification than the full wind field information. This sensitivity is furthermore transferred to the balanced height field increments.

    However, all assimilation experiments suggest that LOS wind observations have a capability of being valuable and need supplemental information to the existing satellite mass field measurements in the Tropics. Although the new wind information is incomplete, it has a potential to provide reliable analysis of tropical wave motions when it is used together with the height data.

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