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- ItemIncreasing Resolution and Resolving Convection Improve the Simulation of Cloud-Radiative Effects Over the North Atlantic(Hoboken, NJ : Wiley, 2020) Senf, Fabian; Voigt, Aiko; Clerbaux, Nicolas; Hünerbein, Anja; Deneke, HartwigClouds interact with atmospheric radiation and substantially modify the Earth's energy budget. Cloud formation processes occur over a vast range of spatial and temporal scales, which make their thorough numerical representation challenging. Therefore, the impact of parameter choices for simulations of cloud-radiative effects is assessed in the current study. Numerical experiments are carried out using the ICOsahedral Nonhydrostatic (ICON) model with varying grid spacings between 2.5 and 80 km and with different subgrid-scale parameterization approaches. Simulations are performed over the North Atlantic with either one-moment or two-moment microphysics and with convection being parameterized or explicitly resolved by grid-scale dynamics. Simulated cloud-radiative effects are compared to products derived from Meteosat measurements. Furthermore, a sophisticated cloud classification algorithm is applied to understand the differences and dependencies of simulated and observed cloud-radiative effects. The cloud classification algorithm developed for the satellite observations is also applied to the simulation output based on synthetic infrared brightness temperatures, a novel approach that is not impacted by changing insolation and guarantees a consistent and fair comparison. It is found that flux biases originate equally from clear-sky and cloudy parts of the radiation field. Simulated cloud amounts and cloud-radiative effects are dominated by marine, shallow clouds, and their behavior is highly resolution dependent. Bias compensation between shortwave and longwave flux biases, seen in the coarser simulations, is significantly diminished for higher resolutions. Based on the analysis results, it is argued that cloud-microphysical and cloud-radiative properties have to be adjusted to further improve agreement with observed cloud-radiative effects. © 2020. The Authors.
- ItemDetrainment Dominates CCN Concentrations Around Non-Precipitating Convective Clouds Over the Amazon(Hoboken, NJ : Wiley, 2022) Braga, Ramon C.; Rosenfeld, Daniel; Andreae, Meinrat O.; Pöhlker, Christopher; Pöschl, Ulrich; Voigt, Christiane; Weinzierl, Bernadett; Wendisch, Manfred; Pöhlker, Mira L.; Harrison, DanielWe investigated the relationship between the number concentration of cloud droplets (Nd) in ice-free convective clouds and of particles large enough to act as cloud condensation nuclei (CCN) measured at the lateral boundaries of cloud elements. The data were collected during the ACRIDICON-CHUVA aircraft campaign over the Amazon Basin. The results indicate that the CCN particles at the lateral cloud boundaries are dominated by detrainment from the cloud. The CCN concentrations detrained from non-precipitating convective clouds are smaller compared to below cloud bases. The detrained CCN particles from precipitating cloud volumes have relatively larger sizes, but lower concentrations. Our findings indicate that CCN particles ingested from below cloud bases are activated into cloud droplets, which evaporate at the lateral boundaries and above cloud base and release the CCN again to ambient cloud-free air, after some cloud processing. These results support the hypothesis that the CCN around the cloud are cloud-processed.
- ItemThe Dust Emission Potential of Agricultural‐Like Fires—Theoretical Estimates From Two Conceptually Different Dust Emission Parameterizations(Hoboken, NJ : Wiley, 2021) Wagner, R.; Schepanski, K.; Klose, M.Agricultural fires affecting grass-, crop- and shrublands represent a major, mainly anthropogenically driven disturbance of many ecosystems. In addition to emissions of carbonaceous aerosol, they were found to inject also mineral dust particles into the atmosphere. The fires can significantly modulate the near-surface wind patterns so that conditions suitable for dust emission occur. However, the exact emission mechanism has not been investigated so far, but is inevitable for the understanding of its impacts on the Earth system. Here, we test two dust emission parameterizations representing saltation bombardment (SALT) and direct aerodynamic dust entrainment by (convective) turbulence (convective turbulent dust emission, CTDE) in the context of fire-modulated wind patterns using large-eddy simulation with an idealized setup to represent typical agricultural fire settings. Favorable aerodynamic preconditions for the initialization of both emission processes are found, however, with sometimes significant differences in dust emission flux depending on specific wind and fire properties. The strong fire-induced modulations of the instantaneous momentum flux suggest that CTDE can be a very potent emission process in the fire vicinity. Nevertheless, fire impacts on the friction velocity can be significant too, so that dust emission through SALT is facilitated as well. Ultimately, the specific aerodynamic conditions within pyro-convectively modulated wind patterns require the development of a parameterization that can describe these unique fire-related dust emissions and their influencing factors properly. This will finally allow for considering fire-induced dust emissions in aerosol-atmosphere models and an investigation of its atmospheric impacts such as on the radiation budget.
- ItemThe Importance of the Representation of DMS Oxidation in Global Chemistry‐Climate Simulations(Hoboken, NJ : Wiley, 2021) Hoffmann, Erik Hans; Heinold, Bernd; Kubin, Anne; Tegen, Ina; Herrmann, HartmutThe oxidation of dimethyl sulfide (DMS) is key for the natural sulfate aerosol formation and its climate impact. Multiphase chemistry is an important oxidation pathway but neglected in current chemistry-climate models. Here, the DMS chemistry in the aerosol-chemistry-climate model ECHAM-HAMMOZ is extended to include multiphase methane sulfonic acid (MSA) formation in deliquesced aerosol particles, parameterized by reactive uptake. First simulations agree well with observed gas-phase MSA concentrations. The implemented formation pathways are quantified to contribute up to 60% to the sulfate aerosol burden over the Southern Ocean and Arctic/Antarctic regions. While globally the impact on the aerosol radiative forcing almost levels off, a significantly more positive solar radiative forcing of up to +0.1 W m−2 is computed in the Arctic (>60°N). The findings imply the need of both further laboratory and model studies on the atmospheric multiphase oxidation of DMS.
- ItemCalifornian Wildfire Smoke Over Europe: A First Example of the Aerosol Observing Capabilities of Aeolus Compared to Ground‐Based Lidar(Hoboken, NJ : Wiley, 2021) Baars, Holger; Radenz, Martin; Floutsi, Athena Augusta; Engelmann, Ronny; Althausen, Dietrich; Heese, Birgit; Ansmann, Albert; Flament, Thomas; Dabas, Alain; Trapon, Dimitri; Reitebuch, Oliver; Bley, Sebastian; Wandinger, UllaIn September 2020, extremely strong wildfires in the western United States of America (i.e., mainly in California) produced large amounts of smoke, which was lifted into the free troposphere. These biomass-burning-aerosol (BBA) layers were transported from the US west coast toward central Europe within 3–4 days turning the sky milky and receiving high media attention. The present study characterizes this pronounced smoke plume above Leipzig, Germany, using a ground-based multiwavelength-Raman-polarization lidar and the aerosol/cloud product of ESA’s wind lidar mission Aeolus. An exceptional high smoke-AOT >0.4 was measured, yielding to a mean mass concentration of 8 μg m−3. The 355 nm lidar ratio was moderate at around 40–50 sr. The Aeolus-derived backscatter, extinction and lidar ratio profiles agree well with the observations of the ground-based lidar PollyXT considering the fact that Aeolus’ aerosol and cloud products are still preliminary and subject to ongoing algorithm improvements.
- ItemHemispheric and Seasonal Contrast in Cloud Thermodynamic Phase From A‐Train Spaceborne Instruments(Hoboken, NJ : Wiley, 2021) Villanueva, Diego; Senf, Fabian; Tegen, InaAerosol-cloud interactions are an important source of uncertainty in current climate models. To understand and quantify the influence of ice-nucleating particles in cloud glaciation, it is crucial to have a reliable estimation of the hemispheric and seasonal contrast in cloud top phase, which is believed to result from the higher dust aerosol loading in boreal spring. For this reason, we locate and quantify these contrasts by combining three different A-Train cloud-phase products for the period 2007–2010. These products rely on a spaceborne lidar, a lidar-radar synergy, and a radiometer-polarimeter synergy. We show that the cloud-phase from the product combination is more reliable and that the estimation of the hemispheric and seasonal contrast has a lower error compared to the individual products. To quantify the contrast in cloud-phase, we use the hemispheric difference in ice cloud frequency normalized by the liquid cloud frequency in the southern hemisphere between −42 °C and 0 °C. In the midlatitudes, from −15 to −30 °C, the hemispheric contrasts increase with decreasing temperature. At −30 °C, the hemispheric contrast varies from 29% to 39% for the individual cloud-phase products and from 52% to 73% for the product combination. Similarly, in the northern hemisphere, we assess the seasonal contrast between spring and fall normalized by the liquid cloud frequency during fall. At −30 °C, the seasonal contrast ranges from 21% to 39% for the individual cloud-phase products and from 54% to 75% for the product combination.
- ItemFormation of Toxic Unsaturated Multifunctional and Organosulfur Compounds From the Photosensitized Processing of Fluorene and DMSO at the Air-Water Interface(Hoboken, NJ : Wiley, 2020) Mekic, Majda; Zeng, Jiafa; Jiang, Bin; Li, Xue; Lazarou, Yannis G.; Brigante, Marcello; Herrmann, Hartmut; Gligorovski, SashoPolycyclic aromatic hydrocarbons and dimethyl sulfoxide (DMSO) are ubiquitous at the sea surface. Photochemistry at the air-sea interface is a potentially important source of volatile organic compounds, but the relevant chemical processes are currently not well known. When aqueous solutions containing a mixture of fluorene (FL) and DMSO are irradiated with actinic radiation, a large suite of unsaturated high molecular weight compounds appear in the aqueous phase; a broad variety of saturated and unsaturated oxygenated multifunctional compounds are also observed in the gas phase, most of which are more toxic than FL. A possible sequence of steps leading to some of the observed compounds in aqueous solution as well as in the gas phase is proposed. The reaction pathways initiated by excited triplet state of FL (3FL*) are supported by theoretical calculations of the reaction Gibbs energies. The formation of organosulfur compounds has been observed to occur in the gas and the aqueous phases initiated by the reaction between 3FL* and DMSO. The aforementioned photosensitized chemistry at the water surface can have an important impact on the formation of secondary organic aerosol in marine boundary layer as polycyclic aromatic hydrocarbons and DMSO enriched at the water surface are ubiquitous. ©2020. American Geophysical Union. All Rights Reserved.
- ItemSources, Occurrence and Characteristics of Fluorescent Biological Aerosol Particles Measured Over the Pristine Southern Ocean(Hoboken, NJ : Wiley, 2021) Moallemi, Alireza; Landwehr, Sebastian; Robinson, Charlotte; Simó, Rafel; Zamanillo, Marina; Chen, Gang; Baccarini, Andrea; Schnaiter, Martin; Henning, Silvia; Modini, Robin L.; Gysel-Beer, Martin; Schmale, JuliaIn this study, we investigate the occurrence of primary biological aerosol particles (PBAP) over all sectors of the Southern Ocean (SO) based on a 90-day data set collected during the Antarctic Circumnavigation Expedition (ACE) in austral summer 2016-2017. Super-micrometer PBAP (1-16 µm diameter) were measured by a wide band integrated bioaerosol sensor (WIBS-4). Low (3σ) and high (9σ) fluorescence thresholds are used to obtain statistics on fluorescent and hyper-fluorescent PBAP, respectively. Our focus is on data obtained over the pristine ocean, that is, more than 200 km away from land. The results indicate that (hyper-)fluorescent PBAP are correlated to atmospheric variables associated with sea spray aerosol (SSA) particles (wind speed, total super-micrometer aerosol number concentration, chloride and sodium concentrations). This suggests that a main source of PBAP over the SO is SSA. The median percentage contribution of fluorescent and hyper-fluorescent PBAP to super-micrometer SSA was 1.6% and 0.13%, respectively. We demonstrate that the fraction of (hyper-)fluorescent PBAP to total super-micrometer particles positively correlates with concentrations of bacteria and several taxa of pythoplankton measured in seawater, indicating that marine biota concentrations modulate the PBAP source flux. We investigate the fluorescent properties of (hyper-)fluorescent PBAP for several events that occurred near land masses. We find that the fluorescence signal characteristics of particles near land is much more variable than over the pristine ocean. We conclude that the source and concentration of fluorescent PBAP over the open ocean is similar across all sampled sectors of the SO.