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Aerosols can cause brightening of stratocumulus clouds, thereby cooling the climate. Observations and models disagree on the magnitude of this cooling, partly because of the aerosol-induced liquid water path (LWP) adjustment, with climate models predicting an increase in the LWP and satellites observing a weak decrease in response to increasing aerosols. With higher-resolution global climate models, which allow the simulation of mesoscale circulations in which stratocumulus clouds are embedded, there is hope to start bridging this gap. In this study, we present boreal summertime simulations conducted with the ICOsahedral Non-hydrostatic (ICON) global storm-resolving model (GSRM). Compared to geostationary satellite data, ICON produces realistic cloud coverage in the stratocumulus regions; however, these clouds look cumuliform, and the sign of LWP adjustments disagrees with observations. We investigate this disagreement with a causal approach, which combines time series with knowledge of cloud processes, allowing us to diagnose the sources of observation–model discrepancies. The positive ICON LWP adjustment results from a superposition of processes, with an overestimated positive response due to (1) precipitation suppression, (2) a lack of wet scavenging, and (3) cloud deepening under a weak inversion, despite (4) small negative influences from cloud-top entrainment enhancement. We also find that precipitation suppression and entrainment enhancement occur at different intensities during the day and the night, implying that daytime satellite studies suffer from selection bias. This causal methodology can guide modelers on how to modify model parameterizations and setups to reconcile conflicting studies concerning the sign and magnitude of LWP adjustments across different data sources. [ABSTRACT FROM AUTHOR]

The ice phase in clouds is essential for precipitation formation over continents. The underlying processes for ice growth are still poorly understood, leading to large uncertainties in precipitation forecasts and climate simulations. One crucial aspect is the Wegener–Bergeron–Findeisen (WBF) process, which describes the growth of ice crystals at the expense of cloud droplets, leading to a partial or full glaciation of the cloud. In the CLOUDLAB project, we employ glaciogenic cloud seeding to initiate the ice phase in supercooled low-level clouds in Switzerland using uncrewed aerial vehicles with the goal of investigating the WBF process. An extensive setup of ground-based remote-sensing and balloon-borne in situ instrumentation allows us to observe the formation and subsequent growth of ice crystals in great detail. In this study, we compare the seeding signals observed in the field to those simulated using a numerical weather model in large-eddy mode (ICON-LEM). We first demonstrate the capability of the model to accurately simulate and reproduce the seeding experiments across different environmental conditions. Second, we investigate the WBF process in the model by comparing the simulated cloud droplet and ice crystal number concentration changes to in situ measurements. In the field experiments, simultaneous reductions in cloud droplet number concentrations with increased ice crystal number concentrations were observed, with periods showing a full depletion of cloud droplets. The model can reproduce the observed ice crystal number concentrations most of the time; however, it cannot reproduce the observed fast reductions in cloud droplet number concentrations. Our detailed analysis shows that the WBF process appears to be less efficient in the model than in the field. In the model, exaggerated ice crystal number concentrations are required to produce comparable changes in cloud droplet number concentrations, highlighting the inefficiency of the WBF process in the numerical weather model ICON. [ABSTRACT FROM AUTHOR]

Aerosol particles influence cloud formation and properties. Hence climate models that aim for a physical representation of the climate system include aerosol modules. In order to represent more and more processes and aerosol species, their representation has grown increasingly detailed. However, depending on one's modelling purpose, the increased model complexity may not be beneficial, for example because it hinders understanding of model behaviour. Hence we develop a simplification in the form of a climatology of aerosol concentrations. In one approach, the climatology prescribes properties important for cloud droplet and ice crystal formation, the gateways for aerosols to enter the model cloud microphysics scheme. Another approach prescribes aerosol mass and number concentrations in general. Both climatologies are derived from full ECHAM-HAM simulations and can serve to replace the HAM aerosol module and thus drastically simplify the aerosol treatment. The first simplification reduces computational model time by roughly 65 %. However, the naive mean climatological treatment needs improvement to give results that are satisfyingly close to the full model. We find that mean cloud condensation nuclei (CCN) concentrations yield an underestimation of cloud droplet number concentration (CDNC) in the Southern Ocean, which we can reduce by allowing only CCN at cloud base (which have experienced hygroscopic growth in these conditions) to enter the climatology. This highlights the value of the simplification approach in pointing to unexpected model behaviour and providing a new perspective for its study and model development. [ABSTRACT FROM AUTHOR]

A prerequisite to applying 10Be in natural archives for solar and geomagnetic reconstructions is to know how 10Be deposition reflects atmospheric production changes. However, this relationship remains debated. To address this, we use two state‐of‐the‐art global models GEOS‐Chem and ECHAM6.3‐HAM2.3 with the latest beryllium production model. During solar modulation, both models suggest that 10Be deposition reacts proportionally to global production changes, with minor latitudinal deposition biases (<5%). During geomagnetic modulation, however, 10Be deposition changes are enhanced by ∼15% in the tropics and attenuated by 20%–35% in subtropical and polar regions compared to global production changes. Such changes are also hemispherically asymmetric, attributed to asymmetric production between hemispheres. For the solar proton event in 774/5 CE, 10Be shows a 15% higher deposition increase in polar regions than in tropics. This study highlights the importance of atmospheric mixing when comparing 10Be from different locations or to independent geomagnetic field records. Plain Language Summary: The cosmogenic radionuclide beryllium‐10 (10Be) deposition in natural archives can be used to reconstruct solar and geomagnetic changes in the past. Understanding how 10Be deposition reflects atmospheric production rate changes is crucial for these applications. However, this relationship remains debated. To address this issue, we use two state‐of‐the‐art global models, GEOS‐Chem 14.1.1 and ECHAM6.3‐HAM2.3, along with the latest beryllium production model (CRAC: Be). When responding to solar modulation, both models indicate that 10Be deposition corresponds proportionally to global production rate changes, with a minor latitudinal bias. However, during geomagnetic modulation, 10Be deposition changes significantly compared to global production rate changes. 10Be deposition also shows varying hemispheric responses to geomagnetic modulation, attributed to the asymmetric production between hemispheres. For the extreme solar proton event in 774/5 CE, 10Be shows a higher deposition flux increase in the polar regions compared to the tropics. These findings underscore the need to account for atmospheric mixing on 10Be deposition from different locations, especially for the changes due to the geomagnetic field variations. Key Points: We used two state‐of‐the‐art global models incorporating the latest beryllium production rates to study the sources of 10Be deposition10Be deposition shows strong regional bias compared to the global signal in response to geomagnetic modulation but not to solar modulation10Be deposition shows varying hemispheric responses to geomagnetic modulation, attributed to the asymmetric production between hemispheres [ABSTRACT FROM AUTHOR]

Since pre-industrial times, aerosol emissions have caused brightening of stratocumulus clouds, thereby cooling the climate. However, observational studies and climate models disagree on the magnitude of this cooling, in particular because of the liquid water path (LWP) response of stratocumulus clouds to increasing aerosols, with climate models predicting an increase in LWP, and satellites observing a weak decrease. With higher-resolution global climate models, there is hope to bridge this gap. In this study, we present simulations conducted with the ICOsahedral Non-hydrostatic climate model (ICON) used as a global storm-resolving model (GSRM) with 5 km horizontal resolution. We compare the model outputs with geostationary satellite data, and we observe that, while ICON produces realistic low-cloud cover in the stratocumulus regions, these clouds look cumuliform and the sign of LWP adjustments to aerosols disagrees with satellite data. We evaluate this disagreement with a causal approach, which combines time series with knowledge of cloud processes in the form of a causal graph, allowing us to diagnose the sources of discrepancies between satellite and model studies. We find that the positive LWP adjustment to increasing aerosols in ICON results from a superposition of processes, with an overestimated positive response due to precipitation suppression and cloud deepening under a weak inversion, despite small negative influences from cloud-top entrainment enhancement. Such analyses constitute a methodology that can guide modelers on how to modify model parameterizations and set-ups to reconcile conflicting studies concerning the sign and magnitude of LWP adjustments across different data sources. [ABSTRACT FROM AUTHOR]

Uncrewed aerial vehicles (UAVs) have become widely used in a range of atmospheric science research applications. Because of their small size, flexible range of motion, adaptability, and low cost, multirotor UAVs are especially well-suited for probing the lower atmosphere. However, their use so far has been limited to conditions outside of clouds, first because of the difficulty of flying beyond visual line of sight and second because of the challenge of flying in icing conditions in supercooled clouds. Here, we present two UAVs for cloud microphysical research: one UAV (the measurement UAV) equipped with a Portable Optical Particle Spectrometer (POPS) and meteorological sensors to probe the aerosol and meteorological properties in the boundary layer and one UAV (the seeding UAV) equipped with seeding flares to produce a plume of particles that can nucleate ice in supercooled clouds. A propeller heating mechanism on both UAVs allows for operating in supercooled clouds with icing conditions. These UAVs are an integral part of the CLOUDLAB project in which glaciogenic cloud seeding of supercooled low stratus clouds is utilized for studying aerosol–cloud interactions and ice crystal formation and growth. In this paper, we first show validations of the POPS on board the measurement UAV, demonstrating that the rotor turbulence has a small effect on measured particle number concentrations. We then exemplify the applicability for profiling the planetary boundary layer, as well as for sampling and characterizing aerosol plumes, in this case, the seeding plume. We also present a new method for filtering out high-concentration data to ensure good data quality of POPS. We explain the different flight patterns that are possible for both UAVs, namely horizontal or vertical leg patterns or hovering, with an extensive and flexible parameter space for designing the flight patterns according to our scientific goals. Finally, we show two examples of seeding experiments: first characterizing an out-of-cloud seeding plume with the measurement UAV flying horizontal transects through the plume and, second, characterizing an in-cloud seeding plume with downstream measurements from a POPS and a holographic imager mounted on a tethered balloon. Particle number concentrations and particle number size distributions of the seeding plume from the experiments reveal that we can successfully produce and measure the seeding plume, both in-cloud (with accompanying elevated ice crystal number concentrations) and out-of-cloud. The methods presented here will be useful for probing the lower atmosphere, for characterizing aerosol plumes, and for deepening our cloud microphysical understanding through cloud seeding experiments, all of which have the potential to benefit the atmospheric science community. [ABSTRACT FROM AUTHOR]

Aerosol particles influence cloud formation and properties. Hence climate models that aim for a physical representation of the climate system include aerosol modules. In order to represent more and more processes and aerosol species, their representation has grown increasingly detailed. However, depending on one's modeling purpose, the increased model complexity may not be beneficial, for example because it hinders understanding of model behaviour. Hence we develop a simplification in the form of a climatology of aerosol concentrations. In one approach, the climatology prescribes properties important for cloud droplet and ice crystal formation, the gateways for aerosols to enter the model cloud microphysics scheme. Another approach prescribes aerosol mass and number concentrations in general. Both climatologies are derived from full ECHAMHAM simulations and can serve to replace the HAM aerosol module and thus drastically simplify the aerosol treatment. The first simplification reduces computational model time by roughly 65%. However, the naive mean climatological treatment needs improvement to give results that are satisfyingly close to the full model. We find that mean CCN concentrations yield an underestimation of CDNC in the Southern Ocean, which we can reduce by allowing only CCN at cloud base (which have experienced hygroscopic growth in these conditions) to enter the climatology. This highlights the value of the simplification approach in pointing to unexpected model behaviour and providing a new perspective for its study and model development. [ABSTRACT FROM AUTHOR]

The ice phase impacts many cloud properties as well as cloud lifetime. Ice particles that sediment into a lower cloud from an upper cloud (external seeder-feeder process) or into the mixed-phase region of a deep cloud from cirrus levels (internal seeder-feeder process) can influence the ice phase of the lower cloud, amplify cloud glaciation and enhance surface precipitation. Recently, numerical weather prediction modeling studies have aimed at representing the ice crystal number concentration in mixed-phase clouds more accurately by including secondary ice formation processes. The increase in the ice crystal number concentration can impact the number of ice particles that sediment into the lower cloud and alter its composition and precipitation formation. In the Swiss Alps, the orography permits the formation of orographic clouds, making it ideal for studying the occurrence of multi-layered clouds and the seeder-feeder process. We present results from a case study on May 18, 2016, showing the occurrence frequency of multi-layered clouds and the seeder-feeder process. About half of all observed clouds were categorized as multi-layered, and the external seeder-feeder process occurred in 10% of these clouds. In between cloud layers, ≈ 60% of the ice particle mass was lost due to sublimation or melting. The external seeder-feeder process was found to be more important than the internal seeder-feeder process with regard to the impact on precipitation. In the case where the external seeder-feeder process was inhibited, the average surface precipitation and riming rate over the domain were both reduced by 8.5% and 3.9%, respectively. When ice-graupel collisions were allowed, further large reductions were seen in the liquid water fraction and riming rate. Inhibiting the internal seeder-feeder process enhanced the liquid water fraction by 6% compared to a reduction of 5.8% in the cloud condensate and, therefore, pointing towards the deamplification in cloud glaciation and a reduction in surface precipitation. Adding to the observational evidence of frequent seeder-feeder situations at least over Switzerland Proske et al. (2021), our study highlights the extensive influence of sedimenting ice particles on the properties of feeder clouds as well as on precipitation formation. [ABSTRACT FROM AUTHOR]