129 research outputs found

    The Sensitivity of Moisture Flux Partitioning in the Cold‐Point Tropopause to External Forcing

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    AbstractThe dryness of the stratosphere is the result of air entering through the cold tropical tropopause layer (TTL). However, our understanding of the moisture flux partitioning into water vapor and frozen hydrometeors is incomplete. This raises concerns regarding the ability of General Circulation Models to accurately predict changes in stratospheric water vapor following perturbations in the radiative budget due to volcanic aerosol or stratospheric geoengineering. We present the first results using a global storm‐resolving model investigating the sensitivity of moisture fluxes within the TTL to an additional heating source. We address the question how the partitioning of moisture fluxes into water vapor and frozen hydrometeors changes under perturbations. The analysis reveals the resilience of the TTL, keeping the flux partitioning constant even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.Plain Language Summary: The stratosphere is a dry region since moisture entering it from below has to pass the cold‐point, a temperature minimum between troposphere and stratosphere. The low temperatures lead to ice formation and sedimentation of moisture. Frozen moisture within clouds rising above the cold‐point tropopause can pass this temperature barrier and be injected into the stratosphere, where temperatures increase again, promoting the melting and sublimation of ice crystals. However, little is known about the sensitivity of the split of moisture entering the stratosphere into frozen and non‐frozen moisture, especially under external influences, like heating by volcanic aerosol or stratospheric geoengineering efforts. Convective parameterizations in conventional simulations can lead to biases. The emerging km‐scale simulations, which explicitly resolve the physical processes, offer the unique possibility to study moisture fluxes under external forcing while circumventing the downsides of parameterizations. Here, the sensitivity of the moisture flux partitioning into non‐frozen and frozen components to an additional heating source is studied for the first time in global storm‐resolving simulations. The analysis reveals an unaltered flux partitioning even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.Key Points:Water vapor dominates the stratospheric moisture budget with a contribution of around 80% in global storm‐resolving simulation. The partitioning of stratospheric moisture fluxes into vapor and frozen hydrometeors remains stable under large temperature perturbations.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Fueglistaler Grou

    How Moisture Shapes Low‐Level Radiative Cooling in Subsidence Regimes

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    Abstract Radiative cooling of the lowest atmospheric levels is of strong importance for modulating atmospheric circulations and organizing convection, but detailed observations and a robust theoretical understanding are lacking. Here we use unprecedented observational constraints from subsidence regimes in the tropical Atlantic to develop a theory for the shape and magnitude of low‐level longwave radiative cooling in clear‐sky, showing peaks larger than 5–10 K/day at the top of the boundary layer. A suite of novel scaling approximations is first developed from simplified spectral theory, in close agreement with the measurements. The radiative cooling peak height is set by the maximum lapse rate in water vapor path, and its magnitude is mainly controlled by the ratio of column relative humidity above and below the peak. We emphasize how elevated intrusions of moist air can reduce low‐level cooling, by sporadically shading the spectral range which effectively cools to space. The efficiency of this spectral shading depends both on water content and altitude of moist intrusions; its height dependence cannot be explained by the temperature difference between the emitting and absorbing layers, but by the decrease of water vapor extinction with altitude. This analytical work can help to narrow the search for low‐level cloud patterns sensitive to radiative‐convective feedbacks: the most organized patterns with largest cloud fractions occur in atmospheres below 10% relative humidity and feel the strongest low‐level cooling. This motivates further assessment of favorable conditions for radiative‐convective feedbacks and a robust quantification of corresponding shallow cloud dynamics in current and warmer climates

    The El Niño–Southern Oscillation pattern effect

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    El Niño–Southern Oscillation (ENSO) variability is accompanied by out-of-phase anomalies in the top-of-atmosphere tropical radiation budget, with anomalous downward flux (i.e., net radiative heating) before El Niño and anomalous upward flux thereafter (and vice versa for La Niña). Here, we show that these radiative anomalies result mainly from a sea surface temperature (SST) “pattern effect,” mediated by changes in tropical-mean tropospheric stability. These stability changes are caused by SST anomalies migrating from climatologically cool to warm regions over the ENSO cycle. Our results are suggestive of a two-way coupling between SST variability and radiation, where ENSO-induced radiative changes may in turn feed back onto SST during ENSO

    Polar Stratospheric Clouds: Satellite Observations, Processes, and Role in Ozone Depletion

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    Polar stratospheric clouds (PSCs) play important roles in stratospheric ozone depletion during winter and spring at high latitudes (e.g., the Antarctic ozone hole). PSC particles provide sites for heterogeneous reactions that convert stable chlorine reservoir species to radicals that destroy ozone catalytically. PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas‐phase HNO3 and H2O by sedimentation of large nitric acid trihydrate (NAT) and ice particles. Contemporary observations by the spaceborne instruments Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS), and Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) have provided an unprecedented polar vortex‐wide climatological view of PSC occurrence and composition in both hemispheres. These data have spurred advances in our understanding of PSC formation and related dynamical processes, especially the firm evidence of widespread heterogeneous nucleation of both NAT and ice PSC particles, perhaps on nuclei of meteoritic origin. Heterogeneous chlorine activation appears to be well understood. Reaction coefficients on/in liquid droplets have been measured accurately, and while uncertainties remain for reactions on solid NAT and ice particles, they are considered relatively unimportant since under most conditions chlorine activation occurs on/in liquid droplets. There have been notable advances in the ability of chemical transport and chemistry‐climate models to reproduce PSC temporal/spatial distributions and composition observed from space. Continued spaceborne PSC observations will facilitate further improvements in the representation of PSC processes in global models and enable more accurate projections of the evolution of polar ozone and the global ozone layer as climate changes.Plain Language Summary: Polar stratospheric clouds (PSCs) occur during winter and early spring in the polar stratosphere, when temperatures are low enough to enable cloud formation despite the extremely dry conditions. Ground‐based PSC sightings date back to the late 19th century, but they were little more than a scientific curiosity until the discovery of the Antarctic ozone hole in 1985. Soon thereafter, it was shown that PSCs play a crucial role in converting stable halogen (mainly chlorine) species of anthropogenic origin into reactive gases that rapidly destroy ozone. Considerable progress was made over the next two decades in quantifying these processes through laboratory studies, field campaigns, and limited spaceborne observations. We are now reaping the benefits of new PSC observations over the entire polar regions from three complementary 21st century spaceborne instruments. This study reviews these instruments and highlights new findings on PSC occurrence and composition. These datasets have also triggered advances in understanding how PSCs form and the influence of atmospheric dynamics, as well as improvements in how detailed cloud processes are approximated in global models. This will ultimately lead to better predictions of how quickly the stratospheric ozone layer will recover from human influence as global climate changes in the future.Key Points: We provide a new vortex‐wide climatology of polar stratospheric cloud occurrence and composition based on 21st century satellite data. We review advances in understanding cloud formation, the role of dynamical processes, and heterogeneous chlorine activation. We highlight improvements in techniques for parameterizing polar stratospheric clouds and their effects in global models.Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659NASA CALIPSO‐CloudSat Science TeamNASA Contract NNL11AA10DAustralian Antarctic Science projectNASA contract with Jet Propulsion LaboratoryKarlsruhe Institute of Technology (KIT) http://dx.doi.org/10.13039/100009133Forschungszentrum Jülich, Institut für Energie und Klimaforschung, Stratosphäre, IEK‐7NERC SISLAC grantRoyal Society Wolfson Merit AwardKarlsruhe House of Young ScientistsETH ZurichItalian National Research Council, CNR; Institute of Atmospheric Sciences and Climate ISACNASA Atmospheric Composition Modeling and Analysis Program and Aura Science TeamMax Planck Institute for ChemistryBritish Antarctic Surve

    Multitimescale variations in modeled stratospheric water vapor derived from three modern reanalysis products

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    Stratospheric water vapor (SWV) plays important roles in the radiation budget and ozone chemistry and is a valuable tracer for understanding stratospheric transport. Meteorological reanalyses provide variables necessary for simulating this transport; however, even recent reanalyses are subject to substantial uncertainties, especially in the stratosphere. It is therefore necessary to evaluate the consistency among SWV distributions simulated using different input reanalysis products. In this study, we evaluate the representation of SWV and its variations on multiple timescales using simulations over the period 1980–2013. Our simulations are based on the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by horizontal winds and diabatic heating rates from three recent reanalyses: ERA-Interim, JRA-55 and MERRA-2. We present an intercomparison among these model results and observationally based estimates using a multiple linear regression method to study the annual cycle (AC), the quasi-biennial oscillation (QBO), and longer-term variability in monthly zonal-mean H2O mixing ratios forced by variations in the El Niño–Southern Oscillation (ENSO) and the volcanic aerosol burden. We find reasonable consistency among simulations of the distribution and variability in SWV with respect to the AC and QBO. However, the amplitudes of both signals are systematically weaker in the lower and middle stratosphere when CLaMS is driven by MERRA-2 than when it is driven by ERA-Interim or JRA-55. This difference is primarily attributable to relatively slow tropical upwelling in the lower stratosphere in simulations based on MERRA-2. Two possible contributors to the slow tropical upwelling in the lower stratosphere are suggested to be the large long-wave cloud radiative effect and the unique assimilation process in MERRA-2. The impacts of ENSO and volcanic aerosol on H2O entry variability are qualitatively consistent among the three simulations despite differences of 50&thinsp;%–100&thinsp;% in the magnitudes. Trends show larger discrepancies among the three simulations. CLaMS driven by ERA-Interim produces a neutral to slightly positive trend in H2O entry values over 1980–2013 (+0.01&thinsp;ppmv&thinsp;decade−1), while both CLaMS driven by JRA-55 and CLaMS driven by MERRA-2 produce negative trends but with significantly different magnitudes (−0.22 and −0.08&thinsp;ppmv&thinsp;decade−1, respectively).</p

    Stratospheric sudden warmings in an idealized GCM

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    PublishedJournal ArticleAn idealized general circulation model (GCM) with an analytically described Newtonian cooling term is employed to study the occurrence rate of sudden stratospheric warmings (SSWs) over a wide range of parameters. In particular, the sensitivity of the SSW occurrence rates to orographic forcing and both relaxation temperature and damping rate is evaluated. The stronger the orographic forcing and the weaker the radiative forcing (in both temperature and damping rate), the higher the SSW frequency. The separate effects of the damping rates at low and high latitudes are somewhat more complex. Generally, lower damping rates result in higher SSW frequency. However, if the low- and high-latitude damping rates are not the same, SSW frequency tends to be most sensitive to a fractional change in the lower of the two damping rates. In addition, the effect of the damping rates on the stratospheric residual circulation is investigated. It is found that higher high-latitude damping rate results in deeper but narrower circulation, whereas higher low-latitude damping rates cause strengthening of the stream function in the tropical midstratosphere to upper stratosphere. Finally, the relation between easily measured and compared climatological fields and the SSW occurrence rate is determined. The average stratospheric polar zonal mean zonal wind shows a strong anticorrelation with the SSW frequency. In the troposphere, there is a high correlation between the meridional temperature gradient and SSW frequency, suggesting that the strength of synoptic activity in the troposphere may be an important influence on SSW occurrence.National Science FoundationSwiss National Science Foundatio

    Advancements in decadal climate predictability: the role of nonoceanic drivers

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    We review recent progress in understanding the role of sea ice, land surface, stratosphere, and aerosols in decadal-scale predictability and discuss the perspectives for improving the predictive capabilities of current Earth system models (ESMs). These constituents have received relatively little attention because their contribution to the slow climatic manifold is controversial in comparison to that of the large heat capacity of the oceans. Furthermore, their initialization as well as their representation in state-of-the-art climate models remains a challenge. Numerous extraoceanic processes that could be active over the decadal range are proposed. Potential predictability associated with the aforementioned, poorly represented, and scarcely observed constituents of the climate system has been primarily inspected through numerical simulations performed under idealized experimental settings. The impact, however, on practical decadal predictions, conducted with realistically initialized full-fledged climate models, is still largely unexploited. Enhancing initial-value predictability through an improved model initialization appears to be a viable option for land surface, sea ice, and, marginally, the stratosphere. Similarly, capturing future aerosol emission storylines might lead to an improved representation of both global and regional short-term climatic changes. In addition to these factors, a key role on the overall predictive ability of ESMs is expected to be played by an accurate representation of processes associated with specific components of the climate system. These act as “signal carriers,” transferring across the climatic phase space the information associated with the initial state and boundary forcings, and dynamically bridging different (otherwise unconnected) subsystems. Through this mechanism, Earth system components trigger low-frequency variability modes, thus extending the predictability beyond the seasonal scale

    Aortic atherosclerosis as an embolic source

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    Stroke is the third leading cause of death in several industrial countries and cardiogenic embolism accounts for 15\u201330 % of ischaemic strokes [1\u20135]. The diagnosis of a cardioembolic source of stroke is frequently uncertain and relies on the identification of a potential cardiac source of embolism in the absence of significant autochthonous cerebrovascular occlusive disease. In this regard, echocardiography (either transthoracic \u2013 TTE or Transoesophageal \u2013 TEE) serves as a cornerstone in the evaluation and diagnosis of these patients [6, 7]

    Departure from Clausius- Clapeyron scaling of water entering the stratosphere in response to changes in tropical upwelling

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    Water entering the stratosphere ([H2O]entry) is strongly constrained by temperatures in the tropical tropopause layer (TTL). Temperatures at tropical tropopause levels are 15–20 K below radiative equilibrium. A strengthening of the residual circulation as suggested by general circulation models in response to increasing greenhouse gases is, based on radiative transfer calculations, estimated to lead to a temperature decrease of about 2 K per 10% change in upwelling (with some sensitivity to vertical scale length). For a uniform temperature change in the inner tropics, [H2O]entry may be expected to change as predicted by the temperature dependence of the vapor pressure, referred here as “Clausius-Clapeyron (CC) scaling.” Under CC scaling, this corresponds to ∼1 ppmv change in [H2O]entry per 10% change in upwelling. However, the change in upwelling also changes the residence time of air in the TTL. We show with trajectory calculations that this affects [H2O]entry, such that [H2O]entry changes ∼10 % less than expected from CC scaling. This residence time effect for water vapor is a consequence of the spatiotemporal variance in the temperature field. We show that for the present-day TTL, a little more than half of the effect is due to the systematic relation between flow and temperature field. The remainder can be understood from the perspective of a random walk problem, with slower ascent (longer path) increasing each air parcel's probability to encounter anomalously low temperatures. Our results show that atmospheric water vapor may depart from CC scaling with mean temperatures even when all physical processes of dehydration remain unchanged
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