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1
Perevedentsev, Y. P., N. V. Ismagilov, N. A. Mirsaeva, V. V. Guryanov, A. A. Nikolaev et K. M. Shantalinsky. « Seasonal Variations in Stratospheric Circulation and Interactions between the Troposphere and the Stratosphere ». Известия Российской академии наук. Физика атмосферы и океана 59, no 6 (1 novembre 2023) : 720–30. http://dx.doi.org/10.31857/s000235152306007x.
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Based on the data of the ERA5 reanalysis, the dates of spring and autumn rearrangements of the stratospheric circulation on isobaric surfaces of 30, 20 and 10 gPa in the latitude zone of 30–90° C. in the period 1979–2020 were obtained. Of the 42 cases of spring restructuring, 10 belong to the early, 15 to the middle and 17 to the late. The spread in the dates of spring rearrangements on the surface of 10 hPa is 69 days. Most often, the spring restructuring of the circulation occurs from top to bottom, in some years, the delay of spring restructuring on the surface of 30 gPa relative to the surface of 10 gPa reaches 22–25 days. Autumn perestroika takes place from the bottom up and their terms at the 3 levels under consideration are close to each other. The relationship between the timing of the spring restructuring of the stratospheric circulation with solar activity and large sudden winter stratospheric warming is shown. Analysis of the fields of anomalies of daily temperature values and zonal wind velocity in the 1000-1 hPa layer in the period January-May showed their significant spatio-temporal difference in the case of early and late spring perestroika. Thus, foci of positive anomalies of temperature and wind speed are formed initially in the upper stratosphere, and then shifted from top to bottom. The interrelations between the layers of the atmosphere in different seasons are considered.
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Cohen, Judah, Mathew Barlow, Paul J. Kushner et Kazuyuki Saito. « Stratosphere–Troposphere Coupling and Links with Eurasian Land Surface Variability ». Journal of Climate 20, no 21 (1 novembre 2007) : 5335–43. http://dx.doi.org/10.1175/2007jcli1725.1.
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Abstract A diagnostic of Northern Hemisphere winter extratropical stratosphere–troposphere interactions is presented to facilitate the study of stratosphere–troposphere coupling and to examine what might influence these interactions. The diagnostic is a multivariate EOF combining lower-stratospheric planetary wave activity flux in December with sea level pressure in January. This EOF analysis captures a strong linkage between the vertical component of lower-stratospheric wave activity over Eurasia and the subsequent development of hemisphere-wide surface circulation anomalies, which are strongly related to the Arctic Oscillation. Wintertime stratosphere–troposphere events picked out by this diagnostic often have a precursor in autumn: years with large October snow extent over Eurasia feature strong wintertime upward-propagating planetary wave pulses, a weaker wintertime polar vortex, and high geopotential heights in the wintertime polar troposphere. This provides further evidence for predictability of wintertime circulation based on autumnal snow extent over Eurasia. These results also raise the question of how the atmosphere will respond to a modified snow cover in a changing climate.
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Marcheggiani, Andrea, et Thomas Spengler. « Diabatic effects on the evolution of storm tracks ». Weather and Climate Dynamics 4, no 4 (3 novembre 2023) : 927–42. http://dx.doi.org/10.5194/wcd-4-927-2023.
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Abstract. Despite the crucial role of moist diabatic processes in mid-latitude storm tracks and related model biases, we still lack a more complete theoretical understanding of how diabatic processes affect the evolution of storm tracks. To alleviate this shortcoming, we investigate the role of diabatic processes in the evolution of the northern hemispheric storm tracks using a framework based on the tendency of the slope of isentropic surfaces as a measure of baroclinic development. We identify opposing behaviours in the near-surface and free troposphere for the relationship between the flattening of the slope of isentropic surfaces and its restoration by diabatic processes. Near the surface (900–825 hPa), cold air advection associated with cold air outbreaks initially acts to flatten isentropic surfaces, with air–sea interactions ensuing to restore surface baroclinicity. In the free troposphere (750–350 hPa), on the other hand, the diabatic generation of the slope of isentropic surfaces precedes its depletion due to tilting by eddies, suggesting the primary importance of moist diabatic processes in triggering subsequent baroclinic development. The same phasing between diabatic and tilting tendencies of the slope is observed both in upstream and downstream sectors of the North Atlantic and North Pacific storm tracks. This suggests that the reversed behaviour between near-surface and free troposphere is a general feature of mid-latitude storm tracks. In addition, we find a correspondence between the diabatic generation of the slope of isentropic surfaces and enhanced precipitation as well as moisture availability, further underlining the crucial role of moisture and moist processes in the self-maintenance of storm tracks.
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Daibova, Elena B., Tamara S. Minakova, Valeriy S. Zakharenko, Natalia I. Kosova, Irina A. Kurzina et Alla B. Zotova. « Acid-Base and Photoinduced Processes on Magnesium-Containing Minerals and their Influence on the Troposphere Cleaning ». Advanced Materials Research 1085 (février 2015) : 119–23. http://dx.doi.org/10.4028/www.scientific.net/amr.1085.119.
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Acid-base, adsorption and photosorption properties of microparticles surface of magnesium-containing compounds produced by dispersion of natural minerals after exposure to the air were studied. It was revealed that the reduction in predominant basicity of minerals surface in a range of periclase (MgO), brucite (Mg (OH)2) and magnesite (MgCO3) can be considered as a result of the presence of iron, silicon and magnesium oxide compounds. Quantum-chemical calculations made during the investigation, the decrease of pressure in the process of long-term contact of Freon 22 with the surface of MgO, irreversible character of adsorption and the absence of fluorine and chlorine-containing products in a gaseous phase give evidence of CFC destructive adsorption on the surface of magnesium oxide. The interaction of chlorine and fluorine derivatives of methane with the surface of aerosol particles from minerals in the darkness and under the influence of sunlight tropospheric radiation was studied. The evaluation of such interactions influence on the process of the Earth troposphere purification from carbon dioxide and dichlorofluoromethane during their adsorption and photosorption on the surface of precipitated aerosol was made.
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Ait-Chaalal, Farid, et Tapio Schneider. « Why Eddy Momentum Fluxes are Concentrated in the Upper Troposphere ». Journal of the Atmospheric Sciences 72, no 4 (31 mars 2015) : 1585–604. http://dx.doi.org/10.1175/jas-d-14-0243.1.
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Abstract The extratropical eddy momentum flux (EMF) is controlled by generation, propagation, and dissipation of large-scale eddies and is concentrated in Earth’s upper troposphere. An idealized GCM is used to investigate how this EMF structure arises. In simulations in which the poles are heated more strongly than the equator, EMF is concentrated near the surface, demonstrating that surface drag generally is not responsible for the upper-tropospheric EMF concentration. Although Earth’s upper troposphere favors linear wave propagation, quasi-linear simulations in which nonlinear eddy–eddy interactions are suppressed demonstrate that this is likewise not primarily responsible for the upper-tropospheric EMF concentration. The quasi-linear simulations reveal the essential role of nonlinear eddy–eddy interactions in the surf zone in the upper troposphere, where wave activity absorption away from the baroclinic generation regions occurs through the nonlinear generation of small scales. In Earth-like atmospheres, wave activity that is generated in the lower troposphere propagates upward and then turns meridionally, eventually being absorbed nonlinearly in the upper troposphere. The level at which the wave activity begins to propagate meridionally appears to be set by the typical height reached by baroclinic eddies. This can coincide with the tropopause height but also can lie below it if convection controls the tropopause height. In the latter case, EMF is maximal well below the tropopause. The simulations suggest that EMF is concentrated in Earth’s upper troposphere because typical baroclinic eddies reach the tropopause.
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Kim, So-Young, Song-You Hong, Young Cheol Kwon, Yong Hee Lee et Da-Eun Kim. « Effects of Modified Surface Roughness Length over Shallow Waters in a Regional Model Simulation ». Atmosphere 10, no 12 (16 décembre 2019) : 818. http://dx.doi.org/10.3390/atmos10120818.
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The effects of modified sea-surface roughness length over shallow waters are examined in a regional climate simulation over East Asia centered on the Korean Peninsula, using the Advanced Research Weather Research and Forecasting model (WRF-ARW). The control experiment calculates the sea-surface roughness length as a function of friction velocity based on the Charnock relationship. The experiment considering water depth in the sea-surface roughness length over shallow waters is compared with the control experiment. In the experiment considering water depth, the excessive near-surface wind speed over shallow waters is reduced compared to that of the control experiment. Wind speed is reduced also in the lower troposphere. The effects of modified surface roughness over shallow waters are not localized to the lower troposphere but extended into the upper troposphere. Through the vertical interaction between the lower and upper levels, upper tropospheric wind—which is underestimated in the control experiment—is enhanced in the experiment with modified sea-surface roughness length, not only over the shallow waters, but also over the entire domain. As a result, the vertical shear of zonal wind increases, leading to the enhancement of the negative meridional temperature gradient in the mid troposphere.
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Haase, Sabine, et Katja Matthes. « The importance of interactive chemistry for stratosphere–troposphere coupling ». Atmospheric Chemistry and Physics 19, no 5 (18 mars 2019) : 3417–32. http://dx.doi.org/10.5194/acp-19-3417-2019.
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Abstract. Recent observational and modeling studies suggest that stratospheric ozone depletion not only influences the surface climate in the Southern Hemisphere (SH), but also impacts Northern Hemisphere (NH) spring, which implies a strong interaction between dynamics and chemistry. Here, we systematically analyze the importance of interactive chemistry with respect to the representation of stratosphere–troposphere coupling and in particular the effects on NH surface climate during the recent past. We use the interactive and specified chemistry version of NCAR's Whole Atmosphere Community Climate Model coupled to an ocean model to investigate differences in the mean state of the NH stratosphere as well as in stratospheric extreme events, namely sudden stratospheric warmings (SSWs), and their surface impacts. To be able to focus on differences that arise from two-way interactions between chemistry and dynamics in the model, the specified chemistry model version uses a time-evolving, model-consistent ozone field generated by the interactive chemistry model version. We also test the effects of zonally symmetric versus asymmetric prescribed ozone, evaluating the importance of ozone waves in the representation of stratospheric mean state and variability. The interactive chemistry simulation is characterized by a significantly stronger and colder polar night jet (PNJ) during spring when ozone depletion becomes important. We identify a negative feedback between lower stratospheric ozone and atmospheric dynamics during the breakdown of the stratospheric polar vortex in the NH, which contributes to the different characteristics of the PNJ between the simulations. Not only the mean state, but also stratospheric variability is better represented in the interactive chemistry simulation, which shows a more realistic distribution of SSWs as well as a more persistent surface impact afterwards compared with the simulation where the feedback between chemistry and dynamics is switched off. We hypothesize that this is also related to the feedback between ozone and dynamics via the intrusion of ozone-rich air into polar latitudes during SSWs. The results from the zonally asymmetric ozone simulation are closer to the interactive chemistry simulations, implying that under a model-consistent ozone forcing, a three-dimensional (3-D) representation of the prescribed ozone field is desirable. This suggests that a 3-D ozone forcing, as recommended for the upcoming CMIP6 simulations, has the potential to improve the representation of stratospheric dynamics and chemistry. Our findings underline the importance of the representation of interactive chemistry and its feedback on the stratospheric mean state and variability not only in the SH but also in the NH during the recent past.
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Abbatt, Jonathan P. D. « Interaction of HNO3with water-ice surfaces at temperatures of the free troposphere ». Geophysical Research Letters 24, no 12 (15 juin 1997) : 1479–82. http://dx.doi.org/10.1029/97gl01403.
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Daibova, Elena B., Tamara S. Minakova, Valeriy S. Zakharenko, Natalia I. Kosova, Irina A. Kurzina, Магina E. Kirillova et Ludmila Yu Minakova. « Physicochemical and Photosorption Properties of Oxygen-Containing Calcium Compounds – Components of Troposferic Aerosol ». Advanced Materials Research 1085 (février 2015) : 124–29. http://dx.doi.org/10.4028/www.scientific.net/amr.1085.124.
Texte intégralRésumé :
The Importance of this Paper is Determined by the Research of Conditions of Photosorption Processes Occurance on the Surface of Aerosol Particles Resulting in the Removal of Toxic Substances from the Atmosphere. Acid-Base Properties of Oxygen-Containing Calcium Compounds being Components of Troposphere Aerosol Particles were Studied by Methods of Ph-Metry and Hammet’s Indicators. the Basic Properties of the Investigated Compounds Surfaces are Predominant Ones: рНiis of Calcium Oxide and Hydroxide has a Value of 9.3 – 9.5, and that for Carbonate and Calcite Equals to 12.3-12.4. Indicator Method Distinguishes Three Areas of Spectrum Corresponding to Lewis Base, and Brensted Neutral and Basic Centers. the Intencity of Peaks is much Higher for Ca(OH)2 and CaO. the Interaction Process of Halogen-Containing Organic Compounds (Freons: 134a, 22 and 12) with Calcium Carbonate Surface under Illumination in Conditions close to Tropospheric Conditions was Studied. it is Shown that the Interaction is the Destructive Photosorption of Freons (134a or 22). the Spectral Dependence of Effective Quantum Yield of Destructive Photosorption is Determined. as a Result of the Interaction Calcium Fluoride and Calcium Chloride are Formed at the Surface.
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Qin, Huiling, et Hiroshi Kawamura. « Air-sea interaction throughout the troposphere over a very high sea surface temperature phenomenon ». Geophysical Research Letters 37, no 1 (janvier 2010) : n/a. http://dx.doi.org/10.1029/2009gl041685.
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Taguchi, Masakazu. « Predictability of stratospheric sudden warming and vortex intensification and their effects on the troposphere ». Impact 2020, no 3 (13 mai 2020) : 14–16. http://dx.doi.org/10.21820/23987073.2020.3.14.
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The atmosphere of the Earth is composed of several different layers that extend out into space. The layer that occupies between the first 9 and 17 km from the Earth's surface is the troposphere, where most of our weather occurs. The stratosphere extends above it up to about 50 km altitude. Associate Professor Masakazu Taguchi at the Department of Earth Science, Aichi University of Education in Japan, is currently focusing his research on understanding the dynamical interaction between the extratropical stratosphere and troposphere and the role of stratospheric variations in the weather and climate.
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Butchart, Neal. « The stratosphere : a review of the dynamics and variability ». Weather and Climate Dynamics 3, no 4 (7 novembre 2022) : 1237–72. http://dx.doi.org/10.5194/wcd-3-1237-2022.
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Abstract. Large-scale, intra-seasonal to inter-annual variability of the stratosphere is reviewed. Much of the variability is dynamical and induced by waves emanating from the troposphere. It is largely characterized by fluctuations in the strength of the polar vortex in winter and a quasi-biennial oscillation in the equatorial winds. Existing theories for the variability are generally formulated in terms of wave–mean-flow interactions, with refinements due, in part, to teleconnections between the tropics and extratropics. Climate and seasonal forecast models are able to reproduce much of the observed polar stratospheric variability and are increasingly successful in the tropics too. Compared to the troposphere the models display longer predictability timescales for variations within the stratosphere. Despite containing just ∼17 % of the atmosphere's mass, the stratosphere's variability exerts a powerful downward influence on the troposphere that can affect surface extremes. The stratosphere is therefore a useful source of additional skill for surface predictions. However, a complete dynamical explanation for the downward coupling is yet to be established.
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Yang, Xiao-Yi, Rui Xin Huang et Dong Xiao Wang. « Decadal Changes of Wind Stress over the Southern Ocean Associated with Antarctic Ozone Depletion ». Journal of Climate 20, no 14 (15 juillet 2007) : 3395–410. http://dx.doi.org/10.1175/jcli4195.1.
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Abstract Using 40-yr ECMWF Re-Analysis (ERA-40) data and in situ observations, the positive trend of Southern Ocean surface wind stress during two recent decades is detected, and its close linkage with spring Antarctic ozone depletion is established. The spring Antarctic ozone depletion affects the Southern Hemisphere lower-stratospheric circulation in late spring/early summer. The positive feedback involves the strengthening and cooling of the polar vortex, the enhancement of meridional temperature gradients and the meridional and vertical potential vorticity gradients, the acceleration of the circumpolar westerlies, and the reduction of the upward wave flux. This feedback loop, together with the ozone-related photochemical interaction, leads to the upward tendency of lower-stratospheric zonal wind in austral summer. In addition, the stratosphere–troposphere coupling, facilitated by ozone-related dynamics and the Southern Annular Mode, cooperates to relay the zonal wind anomalies to the upper troposphere. The wave–mean flow interaction and the meridional circulation work together in the form of the Southern Annular Mode, which transfers anomalous wind signals downward to the surface, triggering a striking strengthening of surface wind stress over the Southern Ocean.
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Raveh-Rubin, Shira. « Dry Intrusions : Lagrangian Climatology and Dynamical Impact on the Planetary Boundary Layer ». Journal of Climate 30, no 17 (septembre 2017) : 6661–82. http://dx.doi.org/10.1175/jcli-d-16-0782.1.
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Dry-air intrusions (DIs) are dry, deeply descending airstreams from the upper troposphere toward the planetary boundary layer (PBL). The significance of DIs spans a variety of aspects, including the interaction with convection, extratropical cyclones and fronts, the PBL, and extreme surface weather. Here, a Lagrangian definition for DI trajectories is used and applied to ECMWF interim reanalysis (ERA-Interim) data. Based on the criterion of a minimum descent of 400 hPa during 48 h, a first global Lagrangian climatology of DI trajectories is compiled for the years 1979–2014, allowing quantitative understanding of the occurrence and variability of DIs, as well as the dynamical and thermodynamical interactions that determine their impact. DIs occur mainly in winter. While traveling equatorward from 40°–50° latitude, DIs typically reach the lower troposphere (with maximum frequencies of ~10% in winter) in the storm-track regions, as well as over the Mediterranean Sea, Arabian Sea, and eastern North Pacific, off the western coast of South America, South Africa, and Australia, and across the Antarctic coast. The DI descent is nearly adiabatic, with a mean potential temperature decrease of 3 K in two days. Relative humidity drops strongly during the first descent day and increases in the second day, because of mixing into the moist PBL. Significant destabilization of the lower levels occurs beneath DIs, accompanied by increased 10-m wind gusts, intense surface heat and moisture fluxes, and elevated PBL heights. Interestingly, only 1.2% of all DIs are found to originate from the stratosphere.
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Raupp, Carlos F. M., Pedro L. Silva Dias, Esteban G. Tabak et Paul Milewski. « Resonant Wave Interactions in the Equatorial Waveguide ». Journal of the Atmospheric Sciences 65, no 11 (1 novembre 2008) : 3398–418. http://dx.doi.org/10.1175/2008jas2387.1.
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Abstract Weakly nonlinear interactions among equatorial waves have been explored in this paper using the adiabatic version of the equatorial β-plane primitive equations in isobaric coordinates. Assuming rigid lid vertical boundary conditions, the conditions imposed at the surface and at the top of the troposphere were expanded in a Taylor series around two isobaric surfaces in an approach similar to that used in the theory of surface–gravity waves in deep water and capillary–gravity waves. By adopting the asymptotic method of multiple time scales, the equatorial Rossby, mixed Rossby–gravity, inertio-gravity, and Kelvin waves, as well as their vertical structures, were obtained as leading-order solutions. These waves were shown to interact resonantly in a triad configuration at the O(ɛ) approximation. The resonant triads whose wave components satisfy a resonance condition for their vertical structures were found to have the most significant interactions, although this condition is not excluding, unlike the resonant conditions for the zonal wavenumbers and meridional modes. Thus, the analysis has focused on such resonant triads. In general, it was found that for these resonant triads satisfying the resonance condition in the vertical direction, the wave with the highest absolute frequency always acts as an energy source (or sink) for the remaining triad components, as usually occurs in several other physical problems in fluid dynamics. In addition, the zonally symmetric geostrophic modes act as catalyst modes for the energy exchanges between two dispersive waves in a resonant triad. The integration of the reduced asymptotic equations for a single resonant triad shows that, for the initial mode amplitudes characterizing realistic magnitudes of atmospheric flow perturbations, the modes in general exchange energy on low-frequency (intraseasonal and/or even longer) time scales, with the interaction period being dependent upon the initial mode amplitudes. Potential future applications of the present theory to the real atmosphere with the inclusion of diabatic forcing, dissipation, and a more realistic background state are also discussed.
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Posselt, Derek J., Susan van den Heever, Graeme Stephens et Matthew R. Igel. « Changes in the Interaction between Tropical Convection, Radiation, and the Large-Scale Circulation in a Warming Environment ». Journal of Climate 25, no 2 (15 janvier 2012) : 557–71. http://dx.doi.org/10.1175/2011jcli4167.1.
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Abstract This paper explores the response of the tropical hydrologic cycle to surface warming through the lens of large-domain cloud-system-resolving model experiments run in a radiative–convective equilibrium framework. Simulations are run for 55 days and are driven with fixed insolation and constant sea surface temparatures (SSTs) of 298 K, 300 K, and 302 K. In each experiment, convection organizes into coherent regions of large-scale ascent separated by areas with relatively clear air and troposphere-deep descent. Aspects of the simulations correspond to observed features of the tropical climate system, including the transition to large precipitation rates above a critical value of total column water vapor, and an increase in convective intensity with SST amidst weakening of the large-scale overturning circulation. However, the authors also find notable changes to the interaction between convection and the environment as the surface warms. In particular, organized convection in simulations with SSTs of 298 and 300 K is inhibited by the presence of a strong midtropospheric stable layer and dry upper troposphere. As a result, there is a decrease in the vigor of deep convection and an increase in stratiform precipitation fraction with an increase in SST from 298 to 300 K. With an increase in SST to 302 K, moistening of the middletroposphere and increase in lower-tropospheric buoyancy serve to overcome these limitations, leading to an overall increase in convective intensity and larger increase in upper-tropospheric relative humidity. The authors conclude that, while convective intensity increases with SST, the aggregate nature of deep convection is strongly affected by the details of the thermodynamic environment in which it develops. In particular, the positive feedback between increasing SST and a moistening upper troposphere found in the simulations, operates as a nonmonotonic function of SST and is modulated by a complex interaction between deep convection and the environmental relative humidity and static stability profile. The results suggest that projected changes in convection that assume a monotonic dependence on SST may constitute an oversimplification.
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Palmer, Paul I. « Quantifying sources and sinks of trace gases using space-borne measurements : current and future science ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 366, no 1885 (octobre 2008) : 4509–28. http://dx.doi.org/10.1098/rsta.2008.0176.
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We have been observing the Earth's upper atmosphere from space for several decades, but only over the past decade has the necessary technology begun to match our desire to observe surface air pollutants and climate-relevant trace gases in the lower troposphere, where we live and breathe. A new generation of Earth-observing satellites, capable of probing the lower troposphere, are already orbiting hundreds of kilometres above the Earth's surface with several more ready for launch or in the planning stages. Consequently, this is one of the most exciting times for the Earth system scientists who study the countless current-day physical, chemical and biological interactions between the Earth's land, ocean and atmosphere. First, I briefly review the theory behind measuring the atmosphere from space, and how these data can be used to infer surface sources and sinks of trace gases. I then present some of the science highlights associated with these data and how they can be used to improve fundamental understanding of the Earth's climate system. I conclude the paper by discussing the future role of satellite measurements of tropospheric trace gases in mitigating surface air pollution and carbon trading.
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Nair, Vijayakumar Sivadasan, Filippo Giorgi et Usha Keshav Hasyagar. « Amplification of South Asian haze by water vapour–aerosol interactions ». Atmospheric Chemistry and Physics 20, no 22 (28 novembre 2020) : 14457–71. http://dx.doi.org/10.5194/acp-20-14457-2020.
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Abstract. Air pollution and wintertime fog over South Asia is a major concern due to its significant implications for air quality, visibility and health. Using a regional climate model coupled with chemistry, we assess the contribution of the hygroscopic growth of aerosols (ambient–dry) to the total aerosol optical depth and demonstrate that the increased surface cooling due to the hygroscopic effects of aerosols further increases the humidity in the boundary layer and thus enhances the confinement of pollutants through aerosol–boundary layer interactions. This positive feedback mechanism plays an important role in the prevalence of wintertime fog and poor air quality conditions over South Asia, where water vapour contributes more than half of the aerosol optical depth. The aerosol–boundary layer interactions lead to moistening of the boundary layer and drying of the free troposphere, which amplifies the long-term trend in relative humidity over the Indo-Gangetic Plain during winter. Hence, the aerosol–water vapour interaction plays a decisive role in the formation and maintenance of the wintertime fog conditions over South Asia, which needs to be considered for planning mitigation strategies.
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Eshel, Gidon, Daniel P. Schrag et Brian F. Farrell. « Troposphere–Planetary Boundary Layer Interactions and the Evolution of Ocean Surface Density : Lessons from Red Sea Corals ». Journal of Climate 13, no 2 (janvier 2000) : 339–51. http://dx.doi.org/10.1175/1520-0442(2000)013<0339:tpblia>2.0.co;2.
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Yang, K., Y. Y. Chen et J. Qin. « Some practical notes on the land surface modeling in the Tibetan Plateau ». Hydrology and Earth System Sciences Discussions 6, no 1 (27 février 2009) : 1291–320. http://dx.doi.org/10.5194/hessd-6-1291-2009.
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Abstract. The Tibetan Plateau is a key region of land-atmosphere interactions, as it provides an elevated heat source to the middle-troposphere. The Plateau surfaces are typically characterized by alpine meadows and grasslands in the central and eastern part while by alpine deserts in the western part. This study evaluates performance of three state-of-the-art land surface models (LSMs) for the Plateau typical land surfaces. The LSMs of interest are SiB2 (the Simple Biosphere), CoLM (Common Land Model), and Noah. They are run with default parameters at typical alpine meadow sites in the central Plateau and typical alpine desert sites in the western Plateau. The recognized key processes and modeling issues are as follows. First, soil stratification is a typical phenomenon beneath the alpine meadows, with dense roots and soil organic matters within the topsoil, and it controls the profile of soil moisture in the central and eastern Plateau; all models significantly under-estimate the soil moisture within the topsoil. Second, a soil surface resistance controls the surface evaporation from the alpine deserts but it has not been reasonably modeled in LSMs; a new scheme is proposed to determine this resistance from soil water content. Third, an excess resistance controls sensible heat fluxes from dry bare-soil or sparsely vegetated surfaces, and all LSMs significantly under-predict the ground-air temperature difference in the daytime. A parameterization scheme for this resistance has been shown effective to remove this bias.
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Garfinkel, Chaim I., et Dennis L. Hartmann. « The Influence of the Quasi-Biennial Oscillation on the Troposphere in Winter in a Hierarchy of Models. Part II : Perpetual Winter WACCM Runs ». Journal of the Atmospheric Sciences 68, no 9 (1 septembre 2011) : 2026–41. http://dx.doi.org/10.1175/2011jas3702.1.
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Abstract Experiments with the Whole Atmosphere Community Climate Model (WACCM) are used to understand the influence of the stratospheric tropical quasi-biennial oscillation (QBO) in the troposphere. The zonally symmetric circulation in thermal wind balance with the QBO affects high-frequency eddies throughout the extratropical troposphere. The influence of the QBO is strongest and most robust in the North Pacific near the jet exit region, in agreement with observations. Variability of the stratospheric polar vortex does not appear to explain the effect of the QBO in the troposphere in the model, although it does contribute to the response in the North Atlantic. Anomalies in tropical deep convection associated with the QBO appear to damp, rather than drive, the effect of the QBO in the extratropical troposphere. Rather, the crucial mechanism whereby the QBO modulates the extratropical troposphere appears to be the interaction of tropospheric transient waves with the axisymmetric circulation in thermal wind balance with the QBO. The response to QBO winds of realistic amplitude is stronger for perpetual February radiative conditions and sea surface temperatures than perpetual January conditions, consistent with the observed response in reanalysis data, in a coupled seasonal WACCM integration, and in dry model experiments described in Part I.
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Zhang, Jingjing, Lanqiang Bai, Zhaoming Li, Yu Du et Shushi Zhang. « High-Frequency Microbarograph-Observed Pressure Variations Associated with Gust Fronts during an Extreme Rainfall Event ». Remote Sensing 16, no 1 (26 décembre 2023) : 101. http://dx.doi.org/10.3390/rs16010101.
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This study aims to explore the roles of multiple gust fronts (i.e., outflow boundaries) during a short-lived extreme rainfall that occurred in the Greater Bay Area of South China in the afternoon of 1 August 2021. Through the use of microbarographs and Doppler weather radars, the research highlights how the interactions of five gust fronts, approaching the region from different directions, have contributed to the high precipitation efficiency and damaging surface winds during the event. The close convergence of these gust fronts funneled unstable air masses into the region of interest, priming the mesoscale convective environment. Some isolated convection initiated before the gust fronts’ arrival. Preceding the arrival of these gust fronts, subtle wave-like pressure jumps were identified from the high-frequency (1 Hz) microbarograph observations. The amplitude of the pressure jump is approximately 40 Pa with minimal changes in air temperature. During the early stage of the gust front passages, very high-frequency oscillations in surface pressure are recognized, indicating interaction between the density currents and the low-level troposphere. As suggested through numerical simulations, the subtle pressure jumps are associated with upward displacements of isentropic surfaces aloft, deepening the moist layer and enhancing the lapse rate that are conducive to convective development. The simulated vertical profiles show no evident capping inversion above the dry neutral boundary layer, suggesting that the pressure jumps are likely to be dynamically induced through the collision of the outflows and environmental air masses. The findings of this study suggest the potential application of microbarographs in the nowcasting of the convective development associated with gust fronts.
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Tromeur, Eric, et William B. Rossow. « Interaction of Tropical Deep Convection with the Large-Scale Circulation in the MJO ». Journal of Climate 23, no 7 (1 avril 2010) : 1837–53. http://dx.doi.org/10.1175/2009jcli3240.1.
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Abstract To better understand the interaction between tropical deep convection and the Madden–Julian oscillation (MJO), tropical cloud regimes are defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud-top pressure—optical thickness joint distributions from the D1 dataset covering 21.5 yr. An MJO index based solely on upper-level wind anomalies is used to study variations of the tropical cloud regimes. The MJO index shows that MJO events are present almost all the time; instead of the MJO event being associated with “on or off” deep convection, it is associated with weaker or stronger mesoscale organization of deep convection. Atmospheric winds and humidity from NCEP–NCAR reanalysis 1 are used to characterize the large-scale dynamics of the MJO; the results show that the large-scale motions initiate an MJO event by moistening the lower troposphere by horizontal advection. Increasingly strong convection transports moisture into the upper troposphere, suggesting a reinforcement of the convection itself. The change of convection organization shown by the cloud regimes indicates a strong interaction between the large-scale circulation and deep convection. The analysis is extended to the complete atmospheric diabatic heating by precipitation, radiation, and surface fluxes. The wave organizes stronger convective heating of the tropical atmosphere, which results in stronger winds, while there is only a passive response of the surface, directly linked to cloud radiative effects. Overall, the results suggest that an MJO event is an amplification of large-scale wave motions by stronger convective heating, which results from a dynamic reorganization of scattered deep convection into more intense mesoscale systems.
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Dirmeyer, Paul A., Yan Jin, Bohar Singh et Xiaoqin Yan. « Trends in Land–Atmosphere Interactions from CMIP5 Simulations ». Journal of Hydrometeorology 14, no 3 (1 juin 2013) : 829–49. http://dx.doi.org/10.1175/jhm-d-12-0107.1.
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Abstract Data from 15 models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) for preindustrial, historical, and future climate change experiments are examined for consensus changes in land surface variables, fluxes, and metrics relevant to land–atmosphere interactions. Consensus changes in soil moisture and latent heat fluxes for past-to-present and present-to-future periods are consistent with CMIP3 simulations, showing a general drying trend over land (less soil moisture, less evaporation) over most of the globe, with the notable exception of high northern latitudes during winter. Sensible heat flux and net radiation declined from preindustrial times to current conditions according to the multimodel consensus, mainly due to increasing aerosols, but that trend reverses abruptly in the future projection. No broad trends are found in soil moisture memory except for reductions during boreal winter associated with high-latitude warming and diminution of frozen soils. Land–atmosphere coupling is projected to increase in the future across most of the globe, meaning a greater control by soil moisture variations on surface fluxes and the lower troposphere. There is also a strong consensus for a deepening atmospheric boundary layer and diminished gradients across the entrainment zone at the top of the boundary layer, indicating that the land surface feedback on the atmosphere should become stronger both in absolute terms and relative to the influence of the conditions of the free atmosphere. Coupled with the trend toward greater hydrologic extremes such as severe droughts, the land surface seems likely to play a greater role in amplifying both extremes and trends in climate on subseasonal and longer time scales.
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Webb, Mark J., Adrian P. Lock et F. Hugo Lambert. « Interactions between Hydrological Sensitivity, Radiative Cooling, Stability, and Low-Level Cloud Amount Feedback ». Journal of Climate 31, no 5 (mars 2018) : 1833–50. http://dx.doi.org/10.1175/jcli-d-16-0895.1.
Texte intégralRésumé :
Low-level cloud feedbacks vary in magnitude but are positive in most climate models, due to reductions in low-level cloud fraction. This study explores the impact of surface evaporation on low-level cloud fraction feedback by performing climate change experiments with the aquaplanet configuration of the HadGEM2-A climate model, forcing surface evaporation to increase at different rates in two ways. Forcing the evaporation diagnosed in the surface scheme to increase at 7% K−1 with warming (more than doubling the hydrological sensitivity) results in an increase in global mean low-level cloud fraction and a negative global cloud feedback, reversing the signs of these responses compared to the standard experiments. The estimated inversion strength (EIS) increases more rapidly in these surface evaporation forced experiments, which is attributed to additional latent heat release and enhanced warming of the free troposphere. Stimulating a 7% K−1 increase in surface evaporation via enhanced atmospheric radiative cooling, however, results in a weaker EIS increase compared to the standard experiments and a slightly stronger low-level cloud reduction. The low-level cloud fraction response is predicted better by EIS than surface evaporation across all experiments. This suggests that surface-forced increases in evaporation increase low-level cloud fraction mainly by increasing EIS. Additionally, the results herein show that increases in surface evaporation can have a very substantial impact on the rate of increase in radiative cooling with warming, by modifying the temperature and humidity structure of the atmosphere. This has implications for understanding the factors controlling hydrological sensitivity.
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Ustrnul, Zbigniew, Jadwiga Woyciechowska et Agnieszka Wypych. « Relationships between Temperature at Surface Level and in the Troposphere over the Northern Hemisphere ». Atmosphere 14, no 9 (11 septembre 2023) : 1423. http://dx.doi.org/10.3390/atmos14091423.
Texte intégralRésumé :
The thermal structure of the troposphere remains a hot topic, including modelling issues as well as temperature field simulations. This study evaluates the relationship between the air temperature at the Earth’s surface and the temperature of various layers of the troposphere over the Northern Hemisphere, as well as attempts to identify determinants of its variability. Vertical differentiation has been analyzed from the layer σ = 0.995 representing the surface (surface air temperature, SAT), up to an isobaric level of 300 hPa with a focus on the main pressure levels, i.e., 925 hPa, 850 hPa, 700 hPa, 500 hPa. The data were obtained from an NCEP/NCAR reanalysis with a resolution of 2.5 degrees latitude and longitude for the period 1961–2020. The relationship between the SAT and the temperature at each level was expressed using a simple but effective correlation coefficient by Pearson (PCC). These relationships obviously, according to Tobler’s law, weaken with an increasing altitude. However, the distribution of PCC (both horizontal and vertical) proves the impact of geographic factors associated with the relief and also with the surface itself (e.g., land cover). These factors are the main drivers of inversion layers and significantly disturb the straight vertical structure of the atmosphere. The research has shown a significant interannual differentiation of these interactions, as well as their spatial diversity in geographic space. The altitude–temperature relationship becomes weaker in all seasons, but much faster during summer and winter, relative to both spring and autumn.
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Boljka, Lina, et Thomas Birner. « Tropopause-level planetary wave source and its role in two-way troposphere–stratosphere coupling ». Weather and Climate Dynamics 1, no 2 (17 octobre 2020) : 555–75. http://dx.doi.org/10.5194/wcd-1-555-2020.
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Abstract. Atmospheric planetary waves play a fundamental role in driving stratospheric dynamics, including sudden stratospheric warming (SSW) events. It is well established that the bulk of the planetary wave activity originates near the surface. However, recent studies have pointed to a planetary wave source near the tropopause that may play an important role in the development of SSWs. Here we analyze the dynamical origin of this wave source and its impact on stratosphere–troposphere coupling, using an idealized model and a quasi-reanalysis. It is shown that the tropopause-level planetary wave source is associated with nonlinear wave–wave interactions, but it can also manifest as an apparent wave source due to transient wave decay. The resulting planetary waves may then propagate deep into the stratosphere, where they dissipate and may help to force SSWs. Our results indicate that SSWs preceded by both the tropopause and the surface wave-source events tend to be followed by a weakened tropospheric zonal flow several weeks later. However, while in the case of a preceding surface wave-source event this downward impact is found mainly poleward of 60∘ N, it appears to be the strongest between 40 and 60∘ N for SSWs preceded by tropopause wave-source events. This suggests that tropopause wave-source events could potentially serve as an additional predictor of not only SSWs but also their downward impact as well.
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Wenta, Marta, Christian M. Grams, Lukas Papritz et Marc Federer. « Linking Gulf Stream air–sea interactions to the exceptional blocking episode in February 2019 : a Lagrangian perspective ». Weather and Climate Dynamics 5, no 1 (8 février 2024) : 181–209. http://dx.doi.org/10.5194/wcd-5-181-2024.
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Abstract. The development of atmospheric blocks over the North Atlantic–European region can lead to extreme weather events like heat waves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air–sea interactions over the Gulf Stream are key processes contributing to the onset, maintenance, and persistence of such flow regimes. To better understand the mechanism connecting air–sea interactions over the Gulf Stream with changes in the large-scale flow, we focus on an episode between 20 and 27 February 2019, when a quasi-stationary upper-level ridge was established over western Europe accompanied by an intensified storm track in the northwestern North Atlantic. During that time, a record-breaking winter warm spell occurred over western Europe bringing temperatures above 20 ∘C to the United Kingdom, the Netherlands, and northern France. The event was preceded and accompanied by the development of several rapidly intensifying cyclones that originated in the Gulf Stream region and traversed the North Atlantic. To explore the mechanistic linkage between the formation of this block and air–sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using kinematic trajectories. This allows us to study the pathways and transformations of air masses that form the upper-level potential vorticity anomaly and interact with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened by the advection of cold air over a warm ocean by one cyclone later ascend into the upper troposphere with the ascending airstream of a subsequent cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region, indicating that their intense coupling between the ocean and atmosphere plays a role in block development. Additionally, they provide a mechanistic pathway linking air–sea interactions in the lower troposphere and the upper-level flow.
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Navea, Juan G., Haihan Chen, Min Huang, Gregory R. Carmichel et Vicki H. Grassian. « A comparative evaluation of water uptake on several mineral dust sources ». Environmental Chemistry 7, no 2 (2010) : 162. http://dx.doi.org/10.1071/en09122.
Texte intégralRésumé :
Environmental context. Dust particles produced from wind blown soils are of global significance as these dust particles not only impact visibility, as evident in the recent 2009 Australian dust storm, but also atmospheric chemistry, climate and biogeochemical cycles. The amount of water vapour in the atmosphere (relative humidity) can play a role in these global processes yet there are few studies and little quantitative data on water-dust particle interactions. The focus of this research is on quantifying water-dust particle interactions for several dust sources including Asia and Africa where dust storms are most prevalent. Abstract. Mineral dust aerosol provides a reactive surface in the troposphere. The reactivity of mineral dust depends on the source region as chemical composition and mineralogy of the aerosol affects its interaction with atmospheric gases. Furthermore, the impact of mineral dust aerosol in atmospheric processes and climate is a function of relative humidity. In this study, we have investigated water uptake of complex dust samples. In particular, water uptake as a function of relative humidity has been measured on three different dust sources that have been characterised using a variety of bulk and surface techniques. For these well-characterised dust samples, it is shown that although there are variations in chemical composition and mineralogy, on a per mass basis, water uptake capacities for the three dusts are very similar and are comparable to single component clay samples. These results suggest that the measured uptake of water of these bulk samples is dominated by the clay component.
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Yang, K., Y. Y. Chen et J. Qin. « Some practical notes on the land surface modeling in the Tibetan Plateau ». Hydrology and Earth System Sciences 13, no 5 (27 mai 2009) : 687–701. http://dx.doi.org/10.5194/hess-13-687-2009.
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Abstract. The Tibetan Plateau is a key region of land-atmosphere interactions, as it provides an elevated heat source to the middle-troposphere. The Plateau surfaces are typically characterized by alpine meadows and grasslands in the central and eastern part while by alpine deserts in the western part. This study evaluates performance of three state-of-the-art land surface models (LSMs) for the Plateau typical land surfaces. The LSMs of interest are SiB2 (the Simple Biosphere), CoLM (Common Land Model), and Noah. They are run at typical alpine meadow sites in the central Plateau and typical alpine desert sites in the western Plateau. The identified key processes and modeling issues are as follows. First, soil stratification is a typical phenomenon beneath the alpine meadows, with dense roots and soil organic matters within the topsoil, and it controls the profile of soil moisture in the central and eastern Plateau; all models, when using default parameters, significantly under-estimate the soil moisture within the topsoil. Second, a soil surface resistance controls the surface evaporation from the alpine deserts but it has not been reasonably modeled in LSMs; an advanced scheme for soil water flow is implemented in a LSM, based on which the soil resistance is determined from soil water content and meteorological conditions. Third, an excess resistance controls sensible heat fluxes from dry bare-soil or sparsely vegetated surfaces, and all LSMs significantly under-predict the ground-air temperature gradient, which would result in higher net radiation, lower soil heat fluxes and thus higher sensible heat fluxes in the models. A parameterization scheme for this resistance has been shown to be effective to remove these biases.
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de Vries, H., et J. D. Opsteegh. « Resonance in Optimal Perturbation Evolution. Part II : Effects of a Nonzero Mean PV Gradient ». Journal of the Atmospheric Sciences 64, no 3 (1 mars 2007) : 695–710. http://dx.doi.org/10.1175/jas3868.1.
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Abstract Optimal perturbations are constructed for a two-layer β-plane extension of the Eady model. The surface and interior dynamics is interpreted using the concept of potential vorticity building blocks (PVBs), which are zonally wavelike, vertically confined sheets of quasigeostrophic potential vorticity. The results are compared with the Charney model and with the two-layer Eady model without β. The authors focus particularly on the role of the different growth mechanisms in the optimal perturbation evolution. The optimal perturbations are constructed allowing only one PVB, three PVBs, and finally a discrete equivalent of a continuum of PVBs to be present initially. On the f plane only the PVB at the surface and at the tropopause can be amplified. In the presence of β, however, PVBs influence each other’s growth and propagation at all levels. Compared to the two-layer f-plane model, the inclusion of β slightly reduces the surface growth and propagation speed of all optimal perturbations. Responsible for the reduction are the interior PVBs, which are excited by the initial PVB after initialization. Their joint effect is almost as strong as the effect from the excited tropopause PVB, which is also negative at the surface. If the optimal perturbation is composed of more than one PVB, the Orr mechanism dominates the initial amplification in the entire troposphere. At low levels, the interaction between the surface PVB and the interior tropospheric PVBs (in particular those near the critical level) takes over after about half a day, whereas the interaction between the tropopause PVB and the interior PVBs is responsible for the main amplification in the upper troposphere. In all cases in which more than one PVB is used, the growing normal mode configuration is not reached at optimization time.
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Murthy, Varun S., et William R. Boos. « Quasigeostrophic Controls on Precipitating Ascent in Monsoon Depressions ». Journal of the Atmospheric Sciences 77, no 4 (1 avril 2019) : 1213–32. http://dx.doi.org/10.1175/jas-d-19-0202.1.
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Abstract South Asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind. Observations of maximum precipitation downshear of the cyclonic center have led to prior theories of quasigeostrophic (QG) control of moist convection in these storms. This study examines the interaction between adiabatic QG lifting and moist convection in monsoon depressions using an atmospheric reanalysis and idealized model. Inversion of the QG omega equation in the reanalysis shows that in the downshear, heavily precipitating region, adiabatic QG ascent, due to advection of vorticity and temperature, is comparable to diabatic ascent in the lower troposphere, while diabatic ascent dominates in the middle and upper troposphere. The causal influence of adiabatic QG lifting on precipitating ascent in monsoon depressions is then examined in the column QG modeling framework, where moist convection evolves in the presence of vorticity and temperature advection. The heavy observed precipitation rates are only simulated when moist convective heating amplifies QG ascent, with this interaction accounting for roughly 40% of the increase in precipitation relative to the basic state. Another 40% of this increase is produced by enhanced surface wind speed in the surface enthalpy flux parameterization, which represents the effect of cyclonic winds in the monsoon depression. Horizontal advection of the mean-state poleward moisture gradient accounts for the remaining 20% of the precipitation increase. In the upshear region, adiabatic QG subsidence and horizontal moisture advection both suppress precipitation, and are opposed by wind-enhanced surface enthalpy fluxes.
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Parfitt, Rhys, et Young-Oh Kwon. « The Modulation of Gulf Stream Influence on the Troposphere by the Eddy-Driven Jet ». Journal of Climate 33, no 10 (15 mai 2020) : 4109–20. http://dx.doi.org/10.1175/jcli-d-19-0294.1.
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AbstractThis study suggests that the Gulf Stream influence on the wintertime North Atlantic troposphere is most pronounced when the eddy-driven jet (EDJ) is farthest south and better collocated with the Gulf Stream. Using the reanalysis dataset NCEP-CFSR for December–February 1979–2009, the daily EDJ latitude is separated into three regimes (northern, central, and southern). It is found that the average trajectory of atmospheric fronts covaries with EDJ latitude. In the southern EDJ regime (~19% of the time), the frequency of near-surface atmospheric fronts that pass across the Gulf Stream is maximized. Analysis suggests that this leads to significant strengthening in near-surface atmospheric frontal convergence resulting from strong air–sea sensible heat flux gradients (due to strong temperature gradients in the atmosphere and ocean). In recent studies, it was shown that the pronounced band of time-mean near-surface wind convergence across the Gulf Stream is set by atmospheric fronts. Here, it is shown that an even smaller subset of atmospheric fronts—those associated with a southern EDJ—primarily sets the time mean, due to enhanced Gulf Stream air–sea interaction. Furthermore, statistically significant anomalies in vertical velocity extending well above the boundary layer are identified in association with changes in EDJ latitude. These anomalies are particularly strong for a southern EDJ and are spatially consistent with increases in near-surface atmospheric frontal convergence over the Gulf Stream. These results imply that much of the Gulf Stream influence on the time-mean atmosphere is modulated on synoptic time scales, and enhanced when the EDJ is farthest south.
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Zhou, Wenyu, Shang-Ping Xie et Zhen-Qiang Zhou. « Slow Preconditioning for the Abrupt Convective Jump over the Northwest Pacific during Summer ». Journal of Climate 29, no 22 (25 octobre 2016) : 8103–13. http://dx.doi.org/10.1175/jcli-d-16-0342.1.
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Abstract The rapid intensification of convective activity in mid-July over the northwest Pacific marks the final stage of the Asian summer monsoon, accompanied by major shifts in regional rainfall and circulation patterns. An entraining plume model is used to investigate the physical processes underlying the abrupt convective jump. Despite little change in sea surface temperature (SST), gradual lower-troposphere mixing leads to a threshold transition in the model as follows. Before mid-July, although SST is already high (29°C), the convective plume is inhibited by the capping inversion above the trade cumulus boundary layer. As the lower troposphere is gradually mixed, the boundary layer top rises with reduced atmospheric stability and increased humidity in the lower troposphere. These factors weaken the inhibition effect of the inversion on the entraining plume. As soon as the plume is able to overcome the inversion barrier, it can rise all the way to the upper troposphere. This marks an abrupt threshold transition to a deep convection regime with heavy rainfall. The convective available potential energy (CAPE) of the entraining plume is found to be a better indicator of the rainfall intensity compared to the conventional undiluted CAPE. The latter fails to capture the onset by neglecting interactions between convective clouds and the environment. Current general circulation models (GCMs) fail to capture the abrupt convective jump and instead simulate a rather smooth seasonal evolution of rainfall. Compared to observations, GCMs simulate a higher trade cumulus top with excessive mixing in the lower troposphere. Convection is no longer inhibited by the inversion barrier, and rainfall simply follows the smooth variation of SST.
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Teyssèdre, H., M. Michou, H. L. Clark, B. Josse, F. Karcher, D. Olivié, V. H. Peuch et al. « A new tropospheric and stratospheric Chemistry and Transport Model MOCAGE-Climat for multi-year studies : evaluation of the present-day climatology and sensitivity to surface processes ». Atmospheric Chemistry and Physics 7, no 22 (26 novembre 2007) : 5815–60. http://dx.doi.org/10.5194/acp-7-5815-2007.
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Abstract. We present the configuration of the Météo-France Chemistry and Transport Model (CTM) MOCAGE-Climat that will be dedicated to the study of chemistry and climate interactions. MOCAGE-Climat is a state-of-the-art CTM that simulates the global distribution of ozone and its precursors (82 chemical species) both in the troposphere and the stratosphere, up to the mid-mesosphere (~70 km). Surface processes (emissions, dry deposition), convection, and scavenging are explicitly described in the model that has been driven by the ECMWF operational analyses of the period 2000–2005, on T21 and T42 horizontal grids and 60 hybrid vertical levels, with and without a procedure that reduces calculations in the boundary layer, and with on-line or climatological deposition velocities. Model outputs have been compared to available observations, both from satellites (TOMS, HALOE, SMR, SCIAMACHY, MOPITT) and in-situ instrument measurements (ozone sondes, MOZAIC and aircraft campaigns) at climatological timescales. The distribution of long-lived species is in fair agreement with observations in the stratosphere putting aside the shortcomings associated with the large-scale circulation. The variability of the ozone column, both spatially and temporarily, is satisfactory. However, because the Brewer-Dobson circulation is too fast, too much ozone is accumulated in the lower to mid-stratosphere at the end of winter. Ozone in the UTLS region does not show any systematic bias. In the troposphere better agreement with ozone sonde measurements is obtained at mid and high latitudes than in the tropics and differences with observations are the lowest in summer. Simulations using a simplified boundary layer lead to larger ozone differences between the model and the observations up to the mid-troposphere. NOx in the lowest troposphere is in general overestimated, especially in the winter months over the Northern Hemisphere, which may result from a positive bias in OH. Dry deposition fluxes of O3 and nitrogen species are within the range of values reported by recent inter-comparison model exercises. The use of climatological deposition velocities versus deposition velocities calculated on-line had greatest impact on HNO3 and NO2 in the troposphere.
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Lubis, Sandro W., Katja Matthes, Nili Harnik, Nour-Eddine Omrani et Sebastian Wahl. « Downward Wave Coupling between the Stratosphere and Troposphere under Future Anthropogenic Climate Change ». Journal of Climate 31, no 10 (30 avril 2018) : 4135–55. http://dx.doi.org/10.1175/jcli-d-17-0382.1.
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Abstract Downward wave coupling (DWC) is an important process that characterizes the dynamical coupling between the stratosphere and troposphere via planetary wave reflection. A recent modeling study has indicated that natural forcing factors, including sea surface temperature (SST) variability and the quasi-biennial oscillation (QBO), influence DWC and the associated surface impact in the Northern Hemisphere (NH). In light of this, the authors further investigate how DWC in the NH is affected by anthropogenic forcings, using a fully coupled chemistry–climate model CESM1(WACCM). The results indicate that the occurrence of DWC is significantly suppressed in the future, starting later in the seasonal cycle, with more events concentrated in late winter (February and March). The future decrease in DWC events is associated with enhanced wave absorption in the stratosphere due to increased greenhouse gases (GHGs), which is manifest as more absorbing types of stratospheric sudden warmings (SSWs) in early winter. This early winter condition leads to a delay in the development of the upper-stratospheric reflecting surface, resulting in a shift in the seasonal cycle of DWC toward late winter in the future. The tropospheric responses to DWC events in the future exhibit different spatial patterns, compared to those of the past. In the North Atlantic sector, DWC-induced circulation changes are characterized by a poleward shift and an eastward extension of the tropospheric jet, while in the North Pacific sector, the circulation changes are characterized by a weakening of the tropospheric jet. These responses are consistent with a change in the pattern of DWC-induced synoptic-scale eddy–mean flow interaction in the future.
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Ma, J. « Atmospheric transport of persistent semi-volatile organic chemicals to the Arctic and cold condensation in the mid-troposphere – Part 1 : 2-D modeling in mean atmosphere ». Atmospheric Chemistry and Physics 10, no 15 (9 août 2010) : 7303–14. http://dx.doi.org/10.5194/acp-10-7303-2010.
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Abstract. In the first part of this study for revisiting the cold condensation effect on global distribution of semi-volatile organic chemicals (SVOCs), the atmospheric transport of SVOCs to the Arctic in the mid-troposphere in a mean meridional atmospheric circulation over the Northern Hemisphere was simulated by a two-dimensional (2-D) atmospheric transport model. Results show that under the mean meridional atmospheric circulation the long-range atmospheric transport of SVOCs from warm latitudes to the Arctic occurs primarily in the mid-troposphere. Although major sources are in low and mid-latitude soils, the modeled air concentration of SVOCs in the mid-troposphere is of the same order as or higher than that near the surface, demonstrating that the mid-troposphere is an important pathway and reservoir of SVOCs. The cold condensation of the chemicals is also likely to take place in the mid-troposphere over a source region of SVOCs in warm low latitudes through interacting with clouds. We demonstrate that the temperature dependent vapour pressure and atmospheric degradation rate of SVOCs exhibit similarities between lower atmosphere over the Arctic and the mid-troposphere over a tropical region. Frequent occurrence of atmospheric ascending motion and convection over warm latitudes carry the chemicals to a higher altitude where some of these chemicals may partition onto solid or aqueous phase through interaction with atmospheric aerosols, cloud water droplets and ice particles, and become more persistent at lower temperatures. Stronger winds in the mid-troposphere then convey solid and aqueous phase chemicals to the Arctic where they sink by large-scale descending motion and wet deposition. Using calculated water droplet-air partitioning coefficient of several persistent organic semi-volatile chemicals under a mean air temperature profile from the equator to the North Pole we propose that clouds are likely important sorbing media for SVOCs and pathway of the cold condensation effect and poleward atmospheric transport. The role of deposition and atmospheric descending motion in the cold condensation effect over the Arctic is also discussed.
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Lin, Jonathan, et Kerry Emanuel. « On the Effect of Surface Friction and Upward Radiation of Energy on Equatorial Waves ». Journal of the Atmospheric Sciences 79, no 3 (mars 2022) : 837–57. http://dx.doi.org/10.1175/jas-d-21-0199.1.
Texte intégralRésumé :
Abstract In theoretical models of tropical dynamics, the effects of both surface friction and upward wave radiation through interaction with the stratosphere are oft-ignored, as they greatly complicate mathematical analysis. In this study, we relax the rigid-lid assumption and impose surface drag, which allows the barotropic mode to be excited in equatorial waves. In particular, a previously developed set of linear, strict quasi-equilibrium tropospheric equations is coupled with a dry, passive stratosphere, and surface drag is added to the troposphere momentum equations. Theoretical and numerical model analysis is performed on the model in the limits of an inviscid surface coupled to a stratosphere, as well as a frictional surface under a rigid lid. This study confirms and extends previous research that shows the presence of a stratosphere strongly shifts the growth rates of fast-propagating equatorial waves to larger scales, reddening the equatorial power spectrum. The growth rates of modes that are slowly propagating and highly interactive with cloud radiation are shown to be negligibly affected by the presence of a stratosphere. Surface friction in this model framework acts as purely a damping mechanism and couples the baroclinic mode to the barotropic mode, increasing the poleward extent of the equatorial waves. Numerical solutions of the coupled troposphere–stratosphere model with surface friction show that the stratosphere stratification controls the extent of tropospheric trapping of the barotropic mode, and thus the poleward extent of the wave. The superposition of phase-shifted barotropic and first baroclinic modes is also shown to lead to an eastward vertical tilt in the dynamical fields of Kelvin wave–like modes.
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Zhao, Wenhui, Yi Huang, Steven Siems, Michael Manton et Daniel Harrison. « Interactions between trade wind clouds and local forcings over the Great Barrier Reef : a case study using convection-permitting simulations ». Atmospheric Chemistry and Physics 24, no 9 (17 mai 2024) : 5713–36. http://dx.doi.org/10.5194/acp-24-5713-2024.
Texte intégralRésumé :
Abstract. Trade wind clouds are ubiquitous across the subtropical oceans, including the Great Barrier Reef (GBR), playing an important role in modulating the regional energy budget. These shallow clouds, however, are by their nature sensitive to perturbations in both their thermodynamic environment and microphysical background. In this study, we employ the Weather Research and Forecasting (WRF) model with a convection-permitting configuration at 1 km resolution to examine the sensitivity of the trade wind clouds to different local forcings over the GBR. A range of local forcings including coastal topography, sea surface temperature (SST), and local aerosol loading is examined. This study shows a strong response of cloud fraction and accumulated precipitation to orographic forcing both over the mountains and upwind over the GBR. Orographic lifting, low-level convergence, and lower troposphere stability are found to be crucial in explaining the cloud and precipitation features over the coastal mountains downwind of the GBR. However, clouds over the upwind ocean are more strongly constrained by the trade wind inversion, whose properties are, in part, regulated by the coastal topography. On the scales considered in this study, the warm-cloud fraction and the ensuant precipitation over the GBR show only a small response to the local SST forcing, with this response being tied to the surface flux and lower troposphere stability. Cloud microphysical properties, including cloud droplet number concentration, liquid water path, and precipitation, are sensitive to the changes in atmospheric aerosol population over the GBR. While cloud fraction shows little responses, a slight deepening of the simulated clouds is evident over the upwind region in correspondence to the increased aerosol number concentration. A downwind effect of aerosol loading on simulated cloud and precipitation properties is further noted.
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Johansson, E., A. Devasthale, T. L'Ecuyer, A. M. L. Ekman et M. Tjernström. « The vertical structure of cloud radiative heating over the Indian subcontinent during summer monsoon ». Atmospheric Chemistry and Physics Discussions 15, no 4 (25 février 2015) : 5423–59. http://dx.doi.org/10.5194/acpd-15-5423-2015.
Texte intégralRésumé :
Abstract. Every year the monsoonal circulation over the Indian subcontinent gives rise to a variety of cloud types that differ considerably in their ability to heat or cool the atmosphere. These clouds in turn affect monsoon dynamics via their radiative impacts, both at the surface and in the atmosphere. New generation of satellites carrying active radar and lidar sensors are allowing realistic quantification of cloud radiative heating (CRH) by resolving the vertical structure of the atmosphere in an unprecedented detail. Obtaining this information is a first step in closing the knowledge gap in our understanding of the role that different clouds play as regulators of the monsoon and vice versa. Here, we use collocated CloudSat-CALIPSO data sets to understand following aspects of cloud-radiation interactions associated with Indian monsoon circulation. (1) How does the vertical distribution of CRH evolve over the Indian continent throughout monsoon season? (2) What is the absolute contribution of different clouds types to the total CRH? (3) How do active and break periods of monsoon affect the distribution of CRH? And finally, (4) what are the net radiative effects of different cloud types on surface heating? In general, the vertical structure of CRH follows the northward migration and the retreat of monsoon from May to October. It is found that the alto- and nimbostratus clouds intensely warm the middle troposphere and equally strongly cool the upper troposphere. Their warming/cooling consistently exceeds ±0.2 K day−1 (after weighing by vertical cloud fraction) in monthly mean composites throughout the middle and upper troposphere respectively, with largest impact observed in June, July and August. Deep convective towers cause considerable warming in the middle and upper troposphere, but strongly cool the base and inside of the tropical tropopause layer (TTL). Such cooling is stronger during active (−1.23 K day−1) monsoon conditions compared to break periods (−0.36 K day−1). The contrasting warming effect of high clouds inside the TTL is found to be double in magnitude during active conditions compared to break periods. It is further shown that stratiform clouds (combining alto- and nimbostratus clouds) and deep convection significantly cool the surface with net radiative effect in the order of −100 and −400 W m−2, respectively, while warming the atmosphere in the order of 40 and 150 W m−2. While deep convection produces strong cooling at the surface during active periods of monsoon, the importance of stratiform clouds, on the other hand, increases during break periods. The contrasting CREs in the atmosphere and at surface, and during active and break conditions, have direct implications for monsoonal circulation.
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Ren, Lili, Yang Yang, Hailong Wang, Rudong Zhang, Pinya Wang et Hong Liao. « Source attribution of Arctic black carbon and sulfate aerosols and associated Arctic surface warming during 1980–2018 ». Atmospheric Chemistry and Physics 20, no 14 (30 juillet 2020) : 9067–85. http://dx.doi.org/10.5194/acp-20-9067-2020.
Texte intégralRésumé :
Abstract. Observations show that the concentrations of Arctic sulfate and black carbon (BC) aerosols have declined since the early 1980s. Previous studies have reported that reducing sulfate aerosols potentially contributed to the recent rapid Arctic warming. In this study, a global aerosol–climate model (Community Atmosphere Model, version 5) equipped with Explicit Aerosol Source Tagging (CAM5-EAST) is applied to quantify the source apportionment of aerosols in the Arctic from 16 source regions and the role of aerosol variations in affecting changes in the Arctic surface temperature from 1980 to 2018. The CAM5-EAST simulated surface concentrations of sulfate and BC in the Arctic had a decrease of 43 % and 23 %, respectively, in 2014–2018 relative to 1980–1984 mainly due to the reduction of emissions from Europe, Russia and local Arctic sources. Increases in emissions from South and East Asia led to positive trends in Arctic sulfate and BC in the upper troposphere. All aerosol radiative impacts are considered including aerosol–radiation and aerosol–cloud interactions, as well as black carbon deposition on snow- and ice-covered surfaces. Within the Arctic, sulfate reductions caused a top-of-atmosphere (TOA) warming of 0.11 and 0.25 W m−2 through aerosol–radiation and aerosol–cloud interactions, respectively. While the changes in Arctic atmospheric BC has little impact on local radiative forcing, the decrease in BC in snow and ice led to a net cooling of 0.05 W m−2. By applying climate sensitivity factors for different latitudinal bands, global changes in sulfate and BC during 2014–2018 (with respect to 1980–1984) exerted a +0.088 and 0.057 K Arctic surface warming, respectively, through aerosol–radiation interactions. Through aerosol–cloud interactions, the sulfate reduction caused an Arctic warming of +0.193 K between the two time periods. The weakened BC effect on snow–ice albedo led to an Arctic surface cooling of −0.041 K. The changes in atmospheric sulfate and BC outside the Arctic produced a total Arctic warming of +0.25 K, the majority of which is due to the midlatitude changes in radiative forcing. Our results suggest that changes in aerosols over the midlatitudes of the Northern Hemisphere have a larger impact on Arctic temperature than other regions through enhanced poleward heat transport. The combined total effects of sulfate and BC produced an Arctic surface warming of +0.297 K, explaining approximately 20 % of the observed Arctic warming since the early 1980s.
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Holdsworth, G., et E. Peake. « Acid Content of Snow from a Mid-Troposphere Sampling Site on Mount Logan, Yukon Territory, Canada ». Annals of Glaciology 7 (1985) : 153–60. http://dx.doi.org/10.3189/s0260305500006091.
Texte intégralRésumé :
An ice core 103 m long was extracted in 1980 from an altitude of 5340 m on the icefield plateau of Mount Logan, Yukon Territory (lat 60°35ˈN, long 140°30ˈW). The firn-ice transition occurs at a depth of 65 m, corresponding to about the year 1880.The chemistry of this upper 65 m is apparently dominated by acid-ion species, the peaks in which are provisionally identified with several documented volcanic events. Although the analyses cover only selected discontinuous intervals, it appears that there is no significant long-term trend in the background acidity level of the precipitation at this location over the past century, in contrast to the results from the North American Arctic and Greenland.Nitrate ion concentration shows pseudo-seasonal variations, which may be associated with stratospheric-tropospheric interactions, although other seasonally linked mechanisms are possible. This result has also been reported for ice-core sequences from Greenland. Other nitrate pulses are tentatively associated with local volcanic events and a possible meteorite event (the entry of Tunguska in 1908). One of the largest short-term sources of sulfate ions is probably from volcanic activity on the north Pacific rim. Background volcanically-quiet nitrate and sulfate ion concentrations are compared with similar Greenland data in an attempt to throw further tight on the origin of the acids.Since the moisture for this precipitation originates primarily in the Gulf of Alaska, the data has particular relevance to that region. Short-term climatic changes, as reflected by the oxygen isotope (δl8O) record, show some response to the major volcanic-acid events. The influences affecting the δl8O record are listed but not discussed.
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de Szoeke, Simon P. « Variations of the Moist Static Energy Budget of the Tropical Indian Ocean Atmospheric Boundary Layer ». Journal of the Atmospheric Sciences 75, no 5 (mai 2018) : 1545–51. http://dx.doi.org/10.1175/jas-d-17-0345.1.
Texte intégralRésumé :
The atmospheric circulation depends on poorly understood interactions between the tropical atmospheric boundary layer (BL) and convection. The surface moist static energy (MSE) source (130 W m−2, of which 120 W m−2 is evaporation) to the tropical marine BL is balanced by upward MSE flux at the BL top that is the source for deep convection. Important for modeling tropical convection and circulation is whether MSE enters the free troposphere by dry turbulent processes originating within the boundary layer or by motions generated by moist deep convection in the free troposphere. Here, highly resolved observations of the BL quantify the MSE fluxes in approximate agreement with recent cloud-resolving models, but the fluxes depend on convective conditions. In convectively suppressed (weak precipitation) conditions, entrainment and downdraft fluxes export equal shares (60 W m−2) of MSE from the BL. Downdraft fluxes are found to increase 50%, and entrainment to decrease, under strongly convective conditions. Variable entrainment and downdraft MSE fluxes between the BL and convective clouds must both be considered for modeling the climate.
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Ma, J. « Atmospheric transport of persistent semi-volatile organic chemicals to the Arctic and cold condensation at the mid-troposphere – Part 1 : 2-D modeling in mean atmosphere ». Atmospheric Chemistry and Physics Discussions 10, no 1 (12 janvier 2010) : 453–89. http://dx.doi.org/10.5194/acpd-10-453-2010.
Texte intégralRésumé :
Abstract. In the first part of this study for revisiting the cold condensation effect on global distribution of semi-volatile organic chemicals (SVOCs), the atmospheric transport of SVOCs to the Arctic at the mid-troposphere in a mean meridional atmospheric circulation over Northern Hemisphere was simulated by a two-dimensional atmospheric transport model. Results show that under the mean meridional atmosphere the long-range atmospheric transport of SVOCs from warm latitudes to the Arctic occurs primarily at the mid-troposphere. Accordingly, the cold condensation of the chemicals is likely also to take place at the mid-troposphere over a source region of the chemicals in warm low latitudes. We demonstrate that the temperature dependent vapour pressure and atmospheric degradation rate of SVOCs exhibit similarities between lower atmosphere over the Arctic and the mid-troposphere over a tropical region. Frequent occurrence of atmospheric ascending motion and convection over warm latitudes carry the chemicals to a higher altitude where some of these chemicals may condense/partition to particle or aqueous phase through the interaction with atmospheric aerosols, cloud water droplets and ice particles, and become more persistence in the lower temperatures. Stronger winds at the mid-troposphere then convey the condensed chemicals to the Arctic where they are brought down to the surface by large-scale descending motion and wet deposition. Using calculated water droplet-air partitioning coefficient of several persistent organic semi-volatile chemicals under a mean air temperature profile from the equator to the North Pole we propose that clouds are likely important sorbing media for SVOCs and pathway of the cold condensation effect and poleward atmospheric transport. The role of deposition and atmospheric descending motion in the cold condensation effect over the Arctic was also discussed.
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Wulfmeyer, Volker, David D. Turner, B. Baker, R. Banta, A. Behrendt, T. Bonin, W. A. Brewer et al. « A New Research Approach for Observing and Characterizing Land–Atmosphere Feedback ». Bulletin of the American Meteorological Society 99, no 8 (août 2018) : 1639–67. http://dx.doi.org/10.1175/bams-d-17-0009.1.
Texte intégralRésumé :
AbstractForecast errors with respect to wind, temperature, moisture, clouds, and precipitation largely correspond to the limited capability of current Earth system models to capture and simulate land–atmosphere feedback. To facilitate its realistic simulation in next-generation models, an improved process understanding of the related complex interactions is essential. To this end, accurate 3D observations of key variables in the land–atmosphere (L–A) system with high vertical and temporal resolution from the surface to the free troposphere are indispensable.Recently, we developed a synergy of innovative ground-based, scanning active remote sensing systems for 2D to 3D measurements of wind, temperature, and water vapor from the surface to the lower troposphere that is able to provide comprehensive datasets for characterizing L–A feedback independently of any model input. Several new applications are introduced, such as the mapping of surface momentum, sensible heat, and latent heat fluxes in heterogeneous terrain; the testing of Monin–Obukhov similarity theory and turbulence parameterizations; the direct measurement of entrainment fluxes; and the development of new flux-gradient relationships. An experimental design taking advantage of the sensors’ synergy and advanced capabilities was realized for the first time during the Land Atmosphere Feedback Experiment (LAFE), conducted at the Atmospheric Radiation Measurement Program Southern Great Plains site in August 2017. The scientific goals and the strategy of achieving them with the LAFE dataset are introduced. We envision the initiation of innovative L–A feedback studies in different climate regions to improve weather forecast, climate, and Earth system models worldwide.
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Marshall, Andrew G., Oscar Alves et Harry H. Hendon. « An Enhanced Moisture Convergence–Evaporation Feedback Mechanism for MJO Air–Sea Interaction ». Journal of the Atmospheric Sciences 65, no 3 (1 mars 2008) : 970–86. http://dx.doi.org/10.1175/2007jas2313.1.
Texte intégralRésumé :
Abstract Simulations using an atmospheric model forced with observed SST climatology and the same atmospheric model coupled to a slab-ocean model are used to investigate the role of air–sea interaction on the dynamics of the MJO. Slab-ocean coupling improved the MJO in Australia’s Bureau of Meteorology atmospheric model over the Indo-Pacific warm pool by reducing its period from 70–100 to 45–70 days, thereby showing better agreement with the 30–80-day observed oscillation. Air–sea coupling improves the MJO by increasing the moisture flux in the lower troposphere prior to the passage of active convection, which acts to promote convection and precipitation on the eastern flank of the main convective center. This process is triggered by an increase in surface evaporation over positive SST anomalies ahead of the MJO convection, which are driven by the enhanced shortwave radiation in the region of suppressed convection. This in turn generates enhanced convergence into the region, which supports evaporation–wind feedback in the presence of weak background westerly winds. A subsequent increase in low-level moisture convergence acts to further moisten the lower troposphere in advance of large-scale convection in a region of reduced atmospheric pressure. This destabilizing mechanism is referred to as enhanced moisture convergence–evaporation feedback (EMCEF) and is utilized to understand the role of air–sea coupling on the observed MJO. The EMCEF mechanism also reconciles traditionally opposing ideas on the roles of frictional wave–conditional instability of the second kind (CISK) and wind–evaporation feedback. These results support the idea that the MJO is primarily an atmospheric phenomenon, with air–sea interaction improving upon, but not critical for, its existence in the model.
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Muthers, Stefan, Christoph C. Raible, Eugene Rozanov et Thomas F. Stocker. « Response of the AMOC to reduced solar radiation – the modulating role of atmospheric chemistry ». Earth System Dynamics 7, no 4 (11 novembre 2016) : 877–92. http://dx.doi.org/10.5194/esd-7-877-2016.
Texte intégralRésumé :
Abstract. The influence of reduced solar forcing (grand solar minimum or geoengineering scenarios like solar radiation management) on the Atlantic Meridional Overturning Circulation (AMOC) is assessed in an ensemble of atmosphere–ocean–chemistry–climate model simulations. Ensemble sensitivity simulations are performed with and without interactive chemistry. In both experiments the AMOC is intensified in the course of the solar radiation reduction, which is attributed to the thermal effect of the solar forcing: reduced sea surface temperatures and enhanced sea ice formation increase the density of the upper ocean in the North Atlantic and intensify the deepwater formation. Furthermore, a second, dynamical effect on the AMOC is identified driven by the stratospheric cooling in response to the reduced solar forcing. The cooling is strongest in the tropics and leads to a weakening of the northern polar vortex. By stratosphere–troposphere interactions, the stratospheric circulation anomalies induce a negative phase of the Arctic Oscillation in the troposphere which is found to weaken the AMOC through wind stress and heat flux anomalies in the North Atlantic. The dynamic mechanism is present in both ensemble experiments. In the experiment with interactive chemistry, however, it is strongly amplified by stratospheric ozone changes. In the coupled system, both effects counteract and weaken the response of the AMOC to the solar forcing reduction. Neglecting chemistry–climate interactions in model simulations may therefore lead to an overestimation of the AMOC response to solar forcing.
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Sen Gupta, Alexander, et Matthew H. England. « Coupled Ocean–Atmosphere Feedback in the Southern Annular Mode ». Journal of Climate 20, no 14 (15 juillet 2007) : 3677–92. http://dx.doi.org/10.1175/jcli4200.1.
Texte intégralRésumé :
Abstract Previous studies have demonstrated that while the Southern Annular Mode (SAM) is an intrinsic feature of the atmosphere, it projects strongly onto the ocean and sea ice properties and circulation. This study investigates the extent of “back interaction” whereby these oceanic SAM anomalies feed back to the atmosphere. A comparison between atmosphere-only and full coupled climate models demonstrates that air–sea interactions in the coupled system act to increase the persistence of the SAM in the atmosphere. To identify the nature of feedback from the ocean to the atmosphere, ensemble experiments are carried out in both atmosphere-only and full coupled models whereby a continuous SAM-like sea surface temperature (SST) anomaly is imposed. Both coupled and uncoupled experiments show a direct thermal response that affects the lower-tropospheric temperature and surface meridional winds. An indirect upper troposphere–wide response is also seen whose characteristics are sensitive to the coupling. For the uncoupled experiment a negative-phase SAM SST perturbation produces an indirect atmospheric response that projects strongly onto the SAM. A positive-phase anomaly, however, shows little robust response away from the local heating at the surface. The coupled experiments, however, do show linearity with respect to the sign of the anomaly. However, the response is considerably weaker than the uncoupled case and the projection of the response onto the SAM mode is poorer. Nonetheless the authors find a clear persistence of the SAM at interseasonal time scales that relies on air–sea coupling and cannot be reproduced in unforced atmosphere-only experiments. This demonstrates that the ocean plays a role in modulating the Southern Annular Mode at these time scales.
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Allen, G., H. Coe, A. Clarke, C. Bretherton, R. Wood, S. J. Abel, P. Barrett et al. « Southeast Pacific atmospheric composition and variability sampled along 20° S during VOCALS-REx ». Atmospheric Chemistry and Physics Discussions 11, no 1 (10 janvier 2011) : 681–744. http://dx.doi.org/10.5194/acpd-11-681-2011.
Texte intégralRésumé :
Abstract. The VAMOS Ocean-Climate-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from 15 October to 15 November 2008 in the South East Pacific region to investigate interactions between land, sea and atmosphere in this unique tropical eastern ocean environment and to improve the skill of global and regional models in representing the region. This study synthesises selected aircraft, ship and surface site observations from VOCALS-REx to statistically summarise and characterise the atmospheric composition and variability of the Marine Boundary Layer (MBL) and Free Troposphere (FT) along the 20° S parallel between 70° W and 85° W. Significant zonal gradients in mean MBL sub-micron aerosol particle size and composition, carbon monoxide, ozone and sulphur dioxide were seen over the campaign, with a generally more variable and polluted coastal environment and a less variable, more pristine remote maritime regime. Gradients are observed to be associated with strong gradients in cloud droplet number. The FT is often more polluted in terms of trace gases than the MBL in the mean; however increased variability in the FT composition suggests an episodic nature to elevated concentrations. This is consistent with a complex vertical interleaving of airmasses with diverse sources and hence pollutant concentrations as seen by generalised back trajectory analysis, which suggests contributions from both local and long-range sources. Furthermore, back trajectory analysis demonstrates that the observed zonal gradients both in the boundary layer and the free troposphere are characteristic of marked changes in airmass history with distance offshore – coastal boundary layer airmasses having been in recent contact with the local land surface and remote maritime airmasses having resided over ocean for in excess of ten days. Boundary layer composition to the east of 75° W was observed to be dominated by coastal emissions from sources to the west of the Andes, with evidence for diurnal pumping of the Andean boundary layer above the height of the marine capping inversion. The climatology presented here aims to provide a valuable dataset to inform model simulation and future process studies, particularly in the context of aerosol-cloud interaction and further evaluation of dynamical processes in the SEP region for conditions analogous to those during VOCALS-REx.
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Liu, Boqi, Guoxiong Wu, Jiangyu Mao et Jinhai He. « Genesis of the South Asian High and Its Impact on the Asian Summer Monsoon Onset ». Journal of Climate 26, no 9 (26 avril 2013) : 2976–91. http://dx.doi.org/10.1175/jcli-d-12-00286.1.
Texte intégralRésumé :
Abstract The formation of the South Asian high (SAH) in spring and its impacts on the Asian summer monsoon onset are studied using daily 40-yr ECMWF Re-Analysis (ERA-40) data together with a climate-mean composite technique and potential vorticity–diabatic heating (PV–Q) analysis. Results demonstrate that, about 2 weeks before the Asian summer monsoon onset, a burst of convection over the southern Philippines produces a negative vorticity source to its north. The SAH in the upper troposphere over the South China Sea is then generated as an atmospheric response to this negative vorticity forcing with the streamline field manifesting a Gill-type pattern. Afterward, the persistent rainfall over the northern Indochinese peninsula causes the SAH to move westward toward the peninsula. Consequently, a trumpet-shaped flow field is formed to its southwest, resulting in divergence pumping and atmospheric ascent just over the southeastern Bay of Bengal (BOB). Near the surface, as a surface anticyclone is formed over the northern BOB, an SST warm pool is generated in the central–eastern BOB. This, together with SAH pumping, triggers the formation of a monsoon onset vortex (MOV) with strong surface southwesterly developed over the BOB. Enhanced air–sea interaction promotes the further development and northward migration of the MOV. Consequently, the wintertime zonal-orientated subtropical anticyclone belt in the lower troposphere splits, abundant water vapor is transported directly from the BOB to the subtropical continent, and heavy rainfall ensues; the atmospheric circulation changes from winter to summer conditions over the BOB and Asian summer monsoon onset occurs.