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  • 1
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    Uncertainty in Aerosol Optical Depth From Modern Aerosol‐Climate Models, Reanalyses, and Satellite Products (2022)
    Details
    Publication Date: 2022-04-01
    Description: Despite the implication of aerosols for the radiation budget, there are persistent differences in data for the aerosol optical depth (τ) for 1998–2019. This study presents a comprehensive evaluation of the large‐scale spatio‐temporal patterns of mid‐visible τ from modern data sets. In total, we assessed 94 different global data sets from eight satellite retrievals, four aerosol‐climate model ensembles, one operational ensemble product, two reanalyses, one climatology and one merged satellite product. We include the new satellite data SLSTR and aerosol‐climate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Aerosol Comparisons between Observations and Models Phase 3 (AeroCom‐III). Our intercomparison highlights model differences and observational uncertainty. Spatial mean τ for 60°N – 60°S ranges from 0.124 to 0.164 for individual satellites, with a mean of 0.14. Averaged τ from aerosol‐climate model ensembles fall within this satellite range, but individual models do not. Our assessment suggests no systematic improvement compared to CMIP5 and AeroCom‐I. Although some regional biases have been reduced, τ from both CMIP6 and AeroCom‐III are for instance substantially larger along extra‐tropical storm tracks compared to the satellite products. The considerable uncertainty in observed τ implies that a model evaluation based on a single satellite product might draw biased conclusions. This underlines the need for continued efforts to improve both model and satellite estimates of τ, for example, through measurement campaigns in areas of particularly uncertain satellite estimates identified in this study, to facilitate a better understanding of aerosol effects in the Earth system.
    Description: Plain Language Summary: Aerosols are known to affect atmospheric processes. For instance, particles emitted during dust storms, biomass burning and anthropogenic activities affect air quality and influence the climate through effects on solar radiation and clouds. Although many studies address such aerosol effects, there is a persistent difference in current estimates of the amount of aerosols in the atmosphere across observations and complex climate models. This study documents the data differences for aerosol amounts, including new estimates from climate‐model simulations and satellite products. We quantify considerable differences across aerosol amount estimates as well as regional and seasonal variations of extended and new data. Further, this study addresses the question to what extent complex climate models have improved over the past decades in light of observational uncertainty.
    Description: Key Points: Present‐day patterns in aerosol optical depth differ substantially between 94 modern global data sets. The range in spatial means from individual satellites is −11% to +17% of the multi‐satellite mean. Spatial means from climate model intercomparison projects fall within the satellite range but strong regional differences are identified.
    Description: Hans‐Ertel‐Center for Weather Research
    Description: Collaborative Research Centre 1211
    Description: Max‐Planck‐Institute for Meteorology
    Keywords: ddc:551.5
    Language: English
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  • 2
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    A Lagrangian Perspective on Stable Water Isotopes During the West African Monsoon (2021)
    Details
    Publication Date: 2022-04-01
    Description: We present a Lagrangian framework for identifying mechanisms that control the isotopic composition of mid‐tropospheric water vapor in the Sahel region during the West African Monsoon 2016. In this region mixing between contrasting air masses, strong convective activity, as well as surface and rain evaporation lead to high variability in the distribution of stable water isotopologues. Using backward trajectories based on high‐resolution isotope‐enabled model data, we obtain information not only about the source regions of Sahelian air masses, but also about the evolution of H2O and its isotopologue HDO (expressed as δD) along the pathways of individual air parcels. We sort the full trajectory ensemble into groups with similar transport pathways and hydro‐meteorological properties, such as precipitation and relative humidity, and investigate the evolution of the corresponding paired {H2O, δD} distributions. The use of idealized process curves in the {H2O, δD} phase space allows us to attribute isotopic changes to contributions from (a) air mass mixing, (b) Rayleigh condensation during convection, and (c) microphysical processes depleting the vapor beyond the Rayleigh prediction, i.e., partial rain evaporation in unsaturated and isotopic equilibration in saturated conditions. Different combinations of these processes along the trajectory ensembles are found to determine the final isotopic composition in the Sahelian troposphere during the monsoon. The presented Lagrangian framework is a powerful tool for interpreting tropospheric water vapor distributions. In the future, it will be applied to satellite observations of {H2O, δD} over Africa and other regions in order to better quantify characteristics of the hydrological cycle.
    Description: Key Points: New Lagrangian framework to attribute variability in {H2O, δD} distributions to air mass mixing and phase changes of water. Application to West African Monsoon season 2016 shows characteristic mixing and precipitation effects along trajectories. New framework can be used for the interpretation of satellite and in‐situ observations, and for model validation in future work.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Description: Swiss National Science Foundation
    Description: European Space Agency
    Description: Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347
    Description: Ministerium für Wissenschaft, Forschung und Kunst Baden‐Württemberg (MWK) http://dx.doi.org/10.13039/501100003542
    Keywords: ddc:551.5
    Language: English
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  • 3
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    Observed and Simulated Variability of Droplet Spectral Dispersion in Convective Clouds Over the Amazon (2021)
    Details
    Publication Date: 2022-03-30
    Description: In this study, the variability of the spectral dispersion of droplet size distributions (DSDs) in convective clouds is investigated. Analyses are based on aircraft measurements of growing cumuli near the Amazon basin, and on numerical simulations of an idealized ice‐free cumulus. In cleaner clouds, the relative dispersion ϵ, defined as the ratio of the standard deviation to the mean value of the droplet diameter, is negatively correlated with the ratio of the cloud water content (qc) to the adiabatic liquid water content (qa), while no strong correlation between ϵ and qc/qa is seen in polluted clouds. Bin microphysics numerical simulations suggest that these contrasting behaviors are associated with the effect of collision‐coalescence in cleaner clouds, and secondary droplet activation in polluted clouds, in addition to the turbulent mixing of parcels that experienced different paths within the cloud. Collision‐coalescence simultaneously broadens the DSDs and decreases qc, explaining the inverse relationship between ϵ and qc/qa in cleaner clouds. Secondary droplet activation broadens the DSDs but has little direct impact on qc. The combination of a rather modest DSD broadening due to weak collision‐coalescence with enhanced droplet activation in both diluted and highly undiluted cloud regions may contribute to maintain a relatively uniform ϵ within polluted clouds. These findings can be useful for parameterizing the shape parameter (μ) of gamma DSDs in bulk microphysics cloud‐resolving models. It is shown that emulating the observed μ−qc/qa relationship improves the estimation of the collision‐coalescence rate in bulk microphysics simulations compared to the bin simulations.
    Description: Key Points: Droplet size distribution patterns observed in warm cumuli reflect the roles of collision‐coalescence, secondary activation, and mixing. The intra‐cloud distribution of droplet spectral dispersion varies with aerosol loading. Emulating the observed shape‐parameter improves bulk estimations of collision‐coalescence in models.
    Description: Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) http://dx.doi.org/10.13039/501100001807
    Description: Max Planck Society (MPG)
    Description: U.S. Department of Energy (DOE) http://dx.doi.org/10.13039/100000015
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Description: HALO
    Keywords: ddc:551.5
    Language: English
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  • 4
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    Absorbing aerosol decreases cloud cover in cloud‐resolving simulations over Germany (2021)
    John Wiley & Sons, Ltd | Chichester, UK
    Details
    Publication Date: 2022-03-31
    Description: Aerosol can affect clouds in various ways. Beside the microphysical impact of aerosol particles on cloud formation, the interference of aerosol with atmospheric radiation leads to changes in local heating, surface fluxes and thus mesoscale circulations, all of which may also modify clouds. Rather little is known about these so‐called semi‐direct effects in realistic settings – a reason why this study investigates the impact of absorbing aerosol particles on cloud and radiation fields over Germany. Using advanced high‐resolution simulations with grid spacings of 312 and 625 m, numerical experiments with different aerosol optical properties are contrasted using purely scattering aerosol as a control case and realistic absorbing aerosol as a perturbation. The combined effect of surface dimming and atmospheric heating induces positive temperature and negative moisture anomalies between 800 and 900 hPa, impacting low‐level cloud formation. Decreased relative humidity as well as increased atmospheric stability below clouds lead to a reduction of low‐level cloud cover, liquid water path and precipitation. It is further found that direct and semi‐direct effects of absorbing aerosol forcing have similar magnitudes and contribute equally to a reduction of net radiation at the top of the atmosphere.
    Description: Atmospheric aerosol particles can absorb solar radiation, altering the thermal structure of the atmosphere and surface fluxes. Using advanced high‐resolution simulations over Germany with grid spacings of 312 and 625 m, we find that boundary‐layer absorbing aerosol reduces low‐level cloud cover, liquid water path and precipitation. Direct and semi‐direct effects have similar magnitudes and contribute equally to a positive absorbing aerosol forcing.
    Description: German Ministry for Education and Research EU Horizon 2020 project CONSTRAIN
    Description: https://cera-www.dkrz.de/WDCC/ui/cerasearch/entry?acronym=DKRZ_LTA_1174_ds00001
    Keywords: ddc:551.5
    Language: English
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  • 5
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    Targeted covariance inflation for 3D‐volume radar reflectivity assimilation with the LETKF (2021)
    John Wiley & Sons, Ltd. | Chichester, UK
    Details
    Publication Date: 2022-03-31
    Description: The local ensemble transform Kalman filter (LETKF) suggested by Hunt et al., 2007 is a very popular method for ensemble data assimilation. It is the operational method for convective‐scale data assimilation at Deutscher Wetterdienst (DWD). At DWD, based on the LETKF, three‐dimensional volume radar observations are assimilated operationally for the operational ICON‐D2. However, one major challenge for the LETKF is the situation where observations show precipitation (reflectivity) whereas all ensemble members do not show such reflectivity at a given point in space. In this case, there is no sensitivity of the LETKF with respect to the observations, and the analysis increment based on the observed reflectivity is zero. The goal of this work is to develop a targeted covariance inflation (TCI) for the assimilation of 3D‐volume radar data based on the LETKF, adding artificial sensitivity and making the LETKF react properly to the radar observations. The basic idea of the TCI is to employ an additive covariance inflation as entrance point for the LETKF. Here, we construct perturbations to the simulated observation which are used by the core LETKF assimilation step. The perturbations are constructed such that they exhibit a correlation between humidity and reflectivity. This leads to a change in humidity in such a way that precipitation is more likely to occur. We describe and demonstrate the theoretical basis of the method. We then present a case study where targeted covariance inflation leads to a clear improvement of the LETKF and precipitation forecast. All examples are based on the German radar network and the ICON‐D2 model over Central Europe.
    Description: The goal of this work is to develop a targeted covariance inflation (TCI) for the assimilation of 3D‐volume radar data based on the local ensemble transform Kalman filter (LETKF), adding artificial sensitivity and making the LETKF react properly to the radar observations. Perturbations to the simulated observations are constructed such that they exhibit an empirically derived correlation between humidity and reflectivity. This leads to a change in humidity in such a way that precipitation is more likely to occur.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:551.5
    Language: English
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  • 6
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    Changes in the Tropical Lapse Rate due to Entrainment and Their Impact on Climate Sensitivity (2021)
    Details
    Publication Date: 2022-03-31
    Description: The tropical temperature in the free troposphere deviates from a theoretical moist‐adiabat. The overall deviations are attributed to the entrainment of dry surrounding air. The deviations gradually approach zero in the upper troposphere, which we explain with a buoyancy‐sorting mechanism: the height to which individual convective parcels rise depends on parcel buoyancy, which is closely tied to the impact of entrainment during ascent. In higher altitudes, the temperature is increasingly controlled by the convective parcels that are warmer and more buoyant because of weaker entrainment effects. We represent such temperature deviations from moist‐adiabats in a clear‐sky one‐dimensional radiative‐convective equilibrium model. Compared with a moist‐adiabatic adjustment, having the entrainment‐induced temperature deviations lead to higher clear‐sky climate sensitivity. As the impact of entrainment depends on the saturation deficit, which increases with warming, our model predicts even more amplified surface warming from entrainment in a warmer climate.
    Description: Plain Language Summary: The tropical temperature structure is determined by regions with deep convection, which is believed to be moist‐adiabatic. However, both models and observations show that the temperature deviates from moist‐adiabats. This is because convective parcels often mix with dry environmental air during ascent, pushing the temperature away from the moist‐adiabatic structure. More importantly, the tropical temperature is not dominated by one or a few strongest convective plumes, but rather controlled by the combined effect of many convective plumes of different strengths and depths. Therefore, the tropical temperature structure reflects the composition of convection happening at different values of boundary‐layer energy and mixing processes of variable efficiency with the environment. Using an idealized model, we find that representing such a deviation in the temperature structure increases the surface warming, because the resulting temperature lapse rate (LR) is more similar to a constant LR, showing less temperature increases higher than a moist‐adiabatic LR. This effect is likely amplified in a warmer climate due to this mixing process becoming more efficient in pushing the temperature further away from moist‐adiabats.
    Description: Key Points: The tropical temperature profile in the free troposphere deviates from that following a moist‐adiabatic lapse rate (LR). The deviations from the moist‐adiabatic LR can be explained by entrainment with a buoyancy‐sorting mechanism. The temperature deviations from moist‐adiabats increase climate sensitivity.
    Description: https://doi.org/10.5281/zenodo.1313687
    Description: https://cds.climate.copernicus.eu/cdsapp#%21/dataset/reanalysis-era5-pressure-levels-monthly-means?tab=overview
    Description: https://esgf-data.dkrz.de/projects/cmip6-dkrz/
    Description: http://hdl.handle.net/21.11116/0000-0008-FDA6-0
    Keywords: ddc:551.5
    Language: English
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  • 7
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    Consequences of regularizing the Sawyer–Eliassen equation in balance models for tropical cyclone behaviour (2019)
    Details
    Publication Date: 2022-03-25
    Description: The consequences of regularizing the Sawyer–Eliassen equation to calculate the stream function for the axisymmetric secondary circulation of a tropical cyclone are explored. Regularization is an ad hoc procedure in which the coefficients of the equation are suitably modified to replace negative values of the discriminant by small positive values, thereby ensuring that the equation is globally elliptic. The consequences of the procedure may be understood in terms of the analogue behaviour of a stretched membrane subject to a particular force distribution. Several regularization procedures are assessed by comparing the azimuthally averaged radial flow from a three-dimensional numerical simulation of a tropical cyclone with that from an axisymmetric balance calculation of the Sawyer–Eliassen equation, forced by diabatic and frictional terms diagnosed from the simulation. The comparison shows that the largest challenge for regularization occurs in regions of inertial instability, especially when the diagnosed forcing overlaps with such regions. In the example shown, the diagnosed balanced flow is sensitive to the particular regularization procedure and none of the procedures examined give a flow that is structurally and quantitatively close to that obtained from the numerical solution in and near the region of regularization. The flow in regions of large vertical shear that are common in the lower part of the boundary layer is less sensitive to the regularization procedure, even though such regions are ones in which there is (frictional) forcing. Nevertheless, there are comparatively large differences between the low-level inflow in the azimuthally averaged numerical solution and the axisymmetric balance solution. These differences can be attributed to the intrinsic lack of balance in the boundary layer. This finding, together with the issues associated with regularization, is further confirmation that balance dynamics is unable to adequately capture the flow in the boundary layer, contrary to recent claims.
    Keywords: ddc:551.5
    Language: English
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  • 8
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    The generalized Ekman model for the tropical cyclone boundary layer revisited: The myth of inertial stability as a restoring force (2020)
    Details
    Publication Date: 2022-03-25
    Description: We revisit the linear boundary-layer approximation that expresses a generalized Ekman balance and use it to clarify a range of interpretations in the previous literature on the tropical cyclone boundary layer. Some of these interpretations relate to the reasons for inflow in the boundary layer and others relate to the presumed effects of inertial stability on boundary-layer dynamics. Inertial stability has been invoked, for example, to explain aspects of boundary-layer behaviour, including the frontogenetic nature of the boundary layer and its relationship to vortex spin-up. Our analysis exposes the fallacy of invoking inertial stability as a resistance to radial inflow in the boundary layer. The analysis shows also that the nonlinear acceleration terms become comparable to the linear Coriolis acceleration terms in relatively narrow vortices that are inertially stable above the boundary layer. Estimates of the nonlinear accelerations using the linear solutions are expected to underestimate the actual contribution in a nonlinear boundary-layer model, cautioning against neglecting the nonlinear terms in diagnostic or prognostic models.
    Keywords: ddc:551.5
    Language: English
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  • 9
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    Intercalibration of the Plasma Density Measurements in Earth's Topside Ionosphere (2021)
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    Publication Date: 2022-03-25
    Description: Over the last 20 years, a large number of instruments have provided plasma density measurements in Earth's topside ionosphere. To utilize all of the collected observations for empirical modeling, it is necessary to ensure that they do not exhibit systematic differences and are adjusted to the same reference frame. In this study, we compare satellite plasma density observations from Gravity Recovery and Climate Experiment (GRACE), Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), CHAllenging Minisatellite Payload (CHAMP), Swarm, and Communications/Navigation Outage Forecasting System (C/NOFS) missions. Electron densities retrieved from GRACE K‐Band Ranging (KBR) system, previously shown to be in excellent agreement with incoherent scatter radar (ISR) measurements, are used as a reference. We find that COSMIC radio occultation (RO) densities are highly consistent with GRACE‐KBR observations showing a mean relative difference of 〈2%, and therefore no calibration factors between them are necessary. We utilize the outstanding three‐dimensional coverage of the topside ionosphere by the COSMIC mission to perform conjunction analysis with in situ density observations from CHAMP, C/NOFS, and Swarm missions. CHAMP measurements are lower than COSMIC by ∼11%. Swarm densities are generally lower at daytime and higher at nighttime compared to COSMIC. C/NOFS ion densities agree well with COSMIC, with a relative bias of ∼7%. The resulting cross‐calibration factors, derived from the probability distribution functions, help to eliminate the systematic leveling differences between the data sets, and allow using these data jointly in a large number of ionospheric applications.
    Description: Key Points: A systematic comparison of the plasma density data from CHAMP, C/NOFS, GRACE, COSMIC, and Swarm missions is performed. Electron densities retrieved from COSMIC‐RO agree well with GRACE‐KBR observations showing a relative difference of less than 2%. Intercalibration factors, allowing to eliminate the systematic offsets between the considered data sets, are presented.
    Description: Helmholtz Pilot Projects Information & Data Science II, MAchine learning based Plasma density model project
    Description: National Center for Atmospheric Research http://dx.doi.org/10.13039/100005323
    Description: Air Force Office of Scientific Research http://dx.doi.org/10.13039/100000181
    Description: National Science Foundation http://dx.doi.org/10.13039/100000001
    Keywords: ddc:538.76 ; ddc:551.5
    Language: English
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  • 10
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    Paleoclimate Evolution on Titan After Episodic Massive Methane Outgassing Simulated by a Global Climate Model (2021)
    Details
    Publication Date: 2022-03-25
    Description: Titan's paleoclimate after the onset of the putative last major methane outgassing event 700 Myr ago is simulated by a global climate model. If the atmosphere was methane‐depleted prior to outgassing, outgassed methane initially causes warming due to increased greenhouse effect. Further outgassing leads to methane snowfall, which in turn cools the troposphere and surface by an ice‐albedo feedback and thereby initiates a lengthy ice age. Formation of ice sheets begins in the polar region, but with increasing methane inventory the entire globe is eventually covered by surface methane frost as thick as 100 m, with local accumulation on elevated terrains. Among various time‐dependent input parameters the methane inventory by far exerts the greatest control over the climate evolution. As Titan's climate transitions from a dry state via a partially ice‐covered state to a globally ice‐covered state, the circulation and precipitation pattern change profoundly and the tropospheric temperature further decreases. Globally ice‐covered snowball Titan is characterized by weak meridional circulation, weak seasonality and widespread snowfall. Frost ablation begins after the end of outgassing due to photochemical destruction of atmospheric methane. It is conceivable that Titan's polar seas resulted from melting of the polar caps within the past 10 Myr and subsequent drainage to the polar basins. Surface methane frost could only melt when the frost retreated to the polar region, which led to global warming by lowering of the surface albedo at low latitudes and increased greenhouse effect.
    Description: Plain Language Summary: Saturn's moon Titan may have experienced long periods of cold climate in the past when the nitrogen atmosphere contained no methane unlike the present atmosphere. We simulated how Titan's climate may have changed when large amounts of methane were outgassed into such a cold atmosphere as indicated by models of Titan's evolution. The atmosphere can hold a certain amount of methane but the vast majority of outgassed methane condenses out as snow and is deposited on the surface. Bright methane snow on the surface keeps the surface cold and thereby prevents efficient greenhouse warming. Initially, surface methane frost is confined to high latitudes, but eventually the entire globe will be ice‐covered under the assumed total amount of outgassed methane. The seasonal and global pattern of atmospheric circulation and snowfall strongly depend on the degree of frost coverage. The surface frost sublimes away long after outgassing has ceased because methane is destroyed in the atmosphere by photochemistry. Eventually, the polar caps melt, leaving behind the observed polar seas.
    Description: Key Points: Massive methane outgassing into Titan's atmosphere should have caused global ice sheets if the atmosphere was previously depleted in methane. Climate of methane snowball Titan is characterized by weak circulation, low temperature, weak seasonality and widespread snowfall. Melting polar caps in geologically recent past may have resulted in polar seas.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:523 ; ddc:551.5
    Language: English
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