The Arctic is experiencing a faster warming, known as the Arctic amplification, but climate projections are still uncertain due to the aerosol-cloud interactions (ACI). The impact of aerosols on glaciation temperature, acting as ice nucleating particles, is poorly understood, especially for aerosols from long-range transport. During transport, physico-chemical properties of aerosols evolve due to aging, complicating the quantification of their role.

This study considers satellite observations from POLDER and MODIS instruments with atmospheric chemistry model (GEOS-Chem) and reanalysis datasets (ERA5) to investigate the influence of aerosols on cloud properties and glaciation temperatures. Carbon monoxide (CO) is employed as a passive tracer to aerosols from combustion sources. GEOS-Chem is used to distinguish between biomass burning (BB) and anthropogenic (ANT) emissions and further identify the source regions of air parcels. Spatial and temporal co-localization of cloud, aerosol, and environmental parameter datasets is performed to assess the interplay between meteorological parameters and aerosol properties on cloud properties and on the glaciation process.

Our analysis reveals distinct impacts of aerosols from BB and ANT sources on the glaciation temperature. Preliminary results indicate that pollution plumes from BB are associated with an increase or decrease of about 1.7°C of the glaciation temperature depending on the source region, while ANT pollution plumes suggest an increase of the glaciation temperature between 1.5°C and 2.8°C depending on the transport of air parcels before interacting with the clouds. This work highlights the necessity of considering transport-induced changes in aerosol properties to improve our understanding of the ACI and Arctic climate dynamics.

How to cite: Coopman, Q. and Riedi, J.: Understanding the Impact of Aerosol Transport on the cloud glaciation temperature in the Arctic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9656, https://doi.org/10.5194/egusphere-egu25-9656, 2025.

For most regions of the world, the availability of mineral dust particles drives the production of primary ice in mixed-phase clouds. The mineral dust acts as an ice nucleating particle (INP), which facilitates the freezing of supercooled cloud droplets at higher temperatures than in its absence (~ -38C). Mixed-phase clouds are ubiquitous in the polar regions, but are far from the world’s primary sources of dust emissions in the tropics and subtropics. Secondary sources from high-latitude sources exist but do not fully explain observations of relatively high INP concentrations during the full annual cycle.

In this study we identify a previously overlooked source of low-latitude dust: those from long-lived dusts in smaller size modes that have been in the atmosphere for up to 5 months. Using simulations of the UK Earth System Model (UKESM), we find that although fresh dust contributes most of the dust mass in the polar regions, the INP contribution is weighted towards the older dust particles. In some regions, dust older than 90 days contributes over 50% of the total dust-sourced INP concentration. This occurs due to changes in the dust population size distribution, and has important implications. Close to primary dust sources, the INP concentration is dependent on the super-micron particles, whereas far from the source (i.e. the polar regions and remote oceans) the INP concentration is dependent on the smaller, older, sub-micron particles.

How to cite: Herbert, R., Arnold, S., Murray, B., and Carslaw, K.: Long lived dusts from low latitudes may dominate primary ice production in polar regions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17097, https://doi.org/10.5194/egusphere-egu25-17097, 2025.

Low-level mixed-phase clouds (LLMPCs) containing both ice and supercooled liquid water are ubiquitous in the Arctic and play a crucial role for radiative fluxes. Mixed-phase clouds are inherently instable due to the competition over water between ice crystals and liquid droplets. The persistence of LLMPCs in the Arctic is challenging to reproduce in models, which affects predictions of the energy budget. These challenges stem from current uncertainties related to the complex web of interactions between aerosol particles, cloud microphysics, atmospheric (thermo-)dynamics and surface processes. These uncertainties can be partly attributed to the scarcity of detailed observations of aerosol characteristics and cloud microphysical properties as well as their interactions within clouds.

LLMPCs highly depend on the presence of aerosol particles acting as cloud condensation nuclei (CCN) or ice nucleating particles (INPs). In the Arctic, where aerosol concentrations can be very low, small changes in aerosol properties (e.g., size, chemical composition, hygroscopicity) can significantly impact the radiative properties and lifetime of LLMPCs. Accurate knowledge of aerosol characteristics at cloud level and their influence on cloud properties is crucial for improving the representation of clouds and their radiative behavior.

To address the need for more observations of aerosols and cloud properties at cloud level, a tethered-balloon equipped with the Modular Multiplatform Compatible Air Measurements System (MoMuCAMS) was deployed from the Swedish icebreaker Oden during the Atmospheric River and onset of sea ice melt (ARTofMELT) expedition. The expedition took place in the Fram Strait during the transition from spring to summer (May – June) in 2023. In total, 23 flights up to 645 m above mean sea level were performed, collecting unique and detailed measurements of aerosol and cloud droplet size distributions from 8 nm to 50 µm, below, inside and above LLMPCs. A key aspect addressed with the measurements is how boundary layer and free tropospheric aerosols contribute to the formation of clouds.

Results from of a case study examining the data collected from three consecutive flights through a single cloud layer located between roughly 150 and 350 meters above the surface will be presented. A combination of in situ vertical and surface-based measurements with remote sensing data and modeling studies is used to understand to what extent aerosols from below and above the cloud contribute to the formation of cloud droplets. Results indicate that the cloud is coupled to the surface and profiles of particle number size distributions show a homogenous distribution between the surface and the cloud. A comparison between estimated CCN concentrations below and above the cloud and observed cloud droplet number concentrations suggests however, that entrainment of aerosol from the free troposphere is needed to produce the amount of droplets observed.

These observations suggest that also when clouds are coupled to the surface, a different and significant source of CCN and INPs can feed the cloud from above, which is then not observable from surface-based measurements. This can have important implications for the representation of cloud microphysical and radiative properties based solely on surface-based observations.

How to cite: Pohorsky, R., Calmer, R., Dönmez, B., Brooks, I., Guy, H., Haberstock, L., Kojoj, J., Fauré, N., Murto, S., Mavis, C., Creamean, J., Tjernström, M., Zieger, P., and Schmale, J.: Influence of free tropospheric aerosols on the microphysical properties of a coupled low-level cloud in the central Arctic: a case study from the ARTofMELT expedition. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19436, https://doi.org/10.5194/egusphere-egu25-19436, 2025.

In recent years, there has been a breakthrough in the identification and description of various secondary ice production (SIP) mechanisms that affect mixed-phase clouds. However, SIP is in general not described well in climate and mesoscale models, which leads to notable biases in the representation of warm mixed-phase clouds in terms of ice content, ice number concentration and cloud structure. In this study, the Integrated Community Limited Area Modelling System (ICLAMS) has been utilized to examine the formation and evolution of wintertime orographic clouds over Mt. Helmos, Greece. ICLAMS is a special version of the Regional Atmospheric Modelling System (RAMS). In addition to the Hallett-Mossop process, already included in the model, two additional SIP mechanisms are implemented, a) collisional break-up of ice particles and b) droplet shattering. Model results are evaluated against in-situ and remote sensing observations collected during the CleanCloud CHOPIN campaign (https://go.epfl.ch/chopin-campaign) at Mt. Helmos in the Peloponnese, Greece during Fall 2024 to Spring 2025. Remote sensing (wind lidar and cloud radar) data are used to evaluate model performance, through the application of forward operators (cloud radar simulator) and comparison with radar reflectivity and turbulence parameters. The mesoscale model simulations indicate that SIP enhances aggregation and results in clouds with increased ice crystal number concentrations, aligning more closely with observed distributions. Among the SIP mechanisms, collisional break-up is identified as the dominant contributor to simulated SIP rates, underscoring its critical role in accurately representing orographic mixed-phase clouds.

How to cite: Chaniotis, I., Georgakaki, P., Patlakas, P., Foskinis, R., Clerx, N., Molina, C., Gini, M., Zieger, P., Eleftheriadis, K., Berne, A., Papayannis, A., Komppula, M., Flocas, H., and Nenes, A.: Investigating Secondary Ice Production effects on wintertime orographic clouds using the Regional Atmospheric Modelling System (RAMS), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17259, https://doi.org/10.5194/egusphere-egu25-17259, 2025.

The partitioning between liquid- and ice-phase cloud water content can have a substantial impact on the cloud’s radiative effect and on precipitation. This partitioning is expected to change on average with global warming due to warmer temperatures, more atmospheric moisture, and different cloud nucleating particle distributions, which then feed back into the changing climate. Changes in mixed phase clouds (MPCs)  are difficult to predict due to the complexity of microphysical processes that determine cloud phase partitioning and the uncertainty of future aerosol emissions, both natural and anthropogenic. The characteristics of MPCs are particularly sensitive to the availability of ice-nucleating particles (INPs), which are generally few in number relative to overall aerosol concentrations. Primary bioaerosol emissions are understood to be important to INP availability, making these aerosols and the processes affecting them critical to understanding how clouds and precipitation might change with global warming and with different anthropogenic emissions.

The CleanCloud project under which this research is conducted targets these aerosols and MPCs in order to improve our understanding and climate predictions of a post-aerosol drawdown and warmer world with different natural aerosol sources. We present preliminary observations from the CleanCloud Helmos Orographic SIte Experiment (CHOPIN) campaign targeting primary bioaerosols at the Helmos Hellenic Atmospheric Aerosol Climate Change Station (HAC²) on Mount Helmos in the Peloponnese peninsula, southern Greece, carried out between October 2024 and April 2025. At an altitude of 2314 m, atmospheric conditions at HAC² are at times within the planetary boundary layer and otherwise more representative of the free troposphere. During wintertime at this altitude, MPCs can be frequently observed at HAC². We employed a ground-based counterflow virtual impactor (GCVI; Brechtel Industries) in order to sample clouds and observe cloud residuals using a multiparameter bioaerosol spectrometer (MBS; CAIR, University of Herefordshire), scanning electrical mobility spectrometer (SEMS; Brechtel Industries), and a portable ice nucleation experiment (PINE; EPFL). Samples were also taken for offline analysis with transmission electron microscopy (TEM; MRI-JMA).

We present a preliminary overview of fluorescent primary bioaerosol particle (fPBAP) measurements at HAC² in both cloud residuals and whole air as part of the CHOPIN campaign. We relate these to observed INP concentrations measured by the PINE, and to cloud properties measured by a fog monitor (FM120; DMT, Inc.) and a ground-based fog and aerosol sensor (GFAS; DMT, Inc.).

How to cite: Jönsson, A., Zieger, P., Eleftheriadis, K., Gini, M., Kawana, K., Nenes, A., Molina, C., Fetfatzis, P., Foskinis, R., Asplund, J., Haberstock, L., and Adachi, K.: Fluorescent primary bioaerosol particle measurements in mixed phase cloud residuals at Mount Helmos, Greece, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19341, https://doi.org/10.5194/egusphere-egu25-19341, 2025.

Surface-active organics lower the aerosol surface tension (σ_s/a), leading to enhanced cloud condensation nuclei (CCN) activity and potentially exerting impacts on the climate. Quantification of σ_s/a is mainly limited to laboratory or modeling work for particles with selected sizes and known chemical compositions. Inferred values from ambient aerosol populations are deficient. In this study, we propose a new method to derive σ_s/a by combining field measurements made at an urban site in northern China with the κ-Köhler theory. The results present new evidence that organics remarkably lower the surface tension of aerosols in a polluted atmosphere. Particles sized around 40 nm have an averaged σ_s/a of 53.8 mN m^-1, while particles sized up to 100 nm show σ_s/a values approaching that of pure water. The dependence curve of σ_s/a with the organic mass resembles the behavior of dicarboxylic acids, suggesting their critical role in reducing the surface tension. The study further reveals that neglecting the σ_s/a lowering effect would result in lowered ultrafine CCN (diameter < 100 nm) concentrations by 6.8% to 42.1% at a typical range of supersaturations in clouds, demonstrating the significant impact of surface tension on the CCN concentrations of urban aerosols.

How to cite: Fan, T., Ren, J., Liu, C., Li, Z., Liu, J., Sun, Y., Wang, Y., Jin, X., and Zhang, F.: Surface-tension lowering of aerosols by organics in urban atmospheres: implication to cloud condensation nuclei prediction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2203, https://doi.org/10.5194/egusphere-egu25-2203, 2025.

Surface solar irradiance variability is present under all broken clouds, but the patterns, magnitude of variability, and mechanisms behind it vary greatly with cloud type. Most radiative transfer models do not resolve the observed variability, primarily due to limiting radiative transport to two streams (up and down) to save computation time. From observations, we selected a diverse set of surface solar irradiance patterns under various cloud types and modelled these cloud types in combination with a Monte Carlo ray tracer for accurate 3D radiative transfer. Stratus, altocumulus, and cumulus growing into cumulonimbus are among the studied cloud types. The goal of these experiments is to understand through which mechanisms various cloud types generate observed patterns of irradiance variability. 

The results show that we can capture the essence in four mechanisms. We find that for optically thin (optical thickness  <  6) clouds, scattering in the forward direction dominates. In cloud fields with enough optically thin area, such as altocumulus, "forward escape" alone can drive areas of irradiance enhancement of over 50 % of clear-sky irradiance.  For flat, optically thick clouds (optical thickness > 6), irradiance is instead scattered diffusely downward ("downward escape"), and (extreme) enhancements are thus found directly below the cloud rather than in the direction along the solar angle. For vertically structured clouds, "side escape" dominates domain-averaged diffuse irradiance enhancement until anvil clouds form and start shading the updraft.  Lastly, under optically thick cloud cover, surface albedo enhances downward radiative fluxes due to multiple scattering between surface and cloud. This both brightens shadows and contributes 10 to 60 % of the total irradiance enhancement in sunlit areas for respectively low (0.2) to high (0.8) albedo. 

With these four mechanisms, we provide a framework for understanding the vast diversity and complexity found in surface solar irradiance and cloudiness. Such a framework can guide the development of parameterizations that capture 3D solar irradiance effect at a fraction of the computational cost of 3D radiative transfer models, for example.

How to cite: Mol, W. and van Heerwaarden, C.: Mechanisms of surface solar irradiance variability under broken clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8614, https://doi.org/10.5194/egusphere-egu25-8614, 2025.

Accurately estimating cloud microphysics is crucial for reducing uncertainties in weather and climate models. A particular challenge is capturing the interactions between the cloud microphysics and turbulence. The pervasive shallow cumulus clouds over tropical oceans play a critical role in the Earth’s energy budget, making their study crucial for understanding atmospheric dynamics. In this context, the EUREC4A field campaign, conducted over the Atlantic Ocean close to Barbados from January to February 2020, gathered approximately 200 hours of unique observational data from these clouds [1]. The data was collected by two Max Planck CloudKites deployed from ships. A subset of this data includes Particle Image Velocimetry (PIV) and holographic measurements taken within clouds, providing unique insight into cloud dynamics. To our knowledge, our data represent the first application of airborne PIV in atmospheric clouds and provide an unprecedented opportunity to link cloud turbulence and microphysics.

In this study, we explore the feasibility and accuracy of estimating high-resolution turbulence energy dissipation rates within clouds based on the PIV data. We used the approximate 100k PIV image pairs from both precipitating and non-precipitating clouds collected during the EUREC4A campaign. We employed several established dissipation rate estimating methods, including the second-order structure function method [2] and the 2D gradient method used in the field of planar PIV [3, 4]. The turbulence energy dissipation rate across different cloud types (or flight segments) observed during the campaign is computed.

In addition, we have performed a detailed comparative analysis of the dissipation rate estimated with different techniques, including a 1- and 3-dimensional pitot tube. We also investigate the two-dimensional spatial distribution of cloud droplets and its correlation with turbulence features. We believe that these findings will improve our understanding of turbulence in shallow cumulus clouds and its impact on their formation and evolution.

References

[1] Bony, S., Stevens, B., Ament, F., Bigorre, S., Chazette, P., Crewell, S., ... & Wirth, M. EUREC 4 A: A field campaign to elucidate the couplings between clouds, convection and circulation., Surveys in Geophysics, 38, 1529–1568 (2017).

[2] Schröder, M., Bätge, T., Bodenschatz, E., Wilczek, M., & Bagheri, G. Estimating the turbulent kinetic energy dissipation rate from one-dimensional velocity measurements in time., Atmospheric Measurement Techniques, 17, 2, 627-657 (2024).

[3] Tanaka, T. & Eaton, J. K. A correction method for measuring turbulence kinetic energy dissipation rate by PIV: Validated by random Oseen vortices synthetic image test., Experiments in Fluids, 42, 6, 893-902 (2007).

[4] Verwey, C., & Birouk, M. Dissipation rate estimation in a highly turbulent isotropic flow using 2D-PIV. Flow., Turbulence and Combustion, 109, 3, 647-665 (2022).

How to cite: Kim, Y., Bodenschatz, E., and Bagheri, G.:  Estimation of turbulence dissipation rate within shallow cumulus using airborne Particle Image Velocimetry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7034, https://doi.org/10.5194/egusphere-egu25-7034, 2025.

Cloud droplet temperature is an important parameter influencing cloud microphysical and radiative processes. The supercooled droplet temperature and lifetime impact cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be almost equal to the ambient temperature (Ta), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud boundaries.

This study investigates the evolution of temperature and lifetime of evaporating, supercooled cloud droplets considering initial droplet radius (r0) and temperature (Tr0), and environmental relative humidity (RH), ambient temperature (Ta), and pressure (P). The time (tss) required by droplets to reach a lower steady-state temperature (Tss) after sudden introduction into a new subsaturated environment, the magnitude of ΔT = Ta - Tss, and droplet survival time (tst) at Tss are calculated. The temperature difference (ΔT) is found to increase with Ta, and decrease with RH and P. ΔT values are typically 1–5 K lower than Ta, with highest values (~10.3 K) for very low RH, low P, and Ta closer to 0°C. Results show that tss is < 0.5 s over the range of initial droplet and environmental conditions considered. Tss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The implications for ice nucleation in cloud-top generating cells and near cloud edges are discussed. Using Tss instead of Ta in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2–30, with the greatest increases (>100) coincident with low RH, low P, and Ta closer to 0°C. The findings corroborate the hypothesized mechanism of potential enhancement of ice nucleation at cloud boundaries, such as cloud-top generating cells and for ambient temperatures close to 0°C. The importance of using accurate droplet temperatures to improve existing primary ice nucleation parameterization schemes, especially in sub-saturated environments, is highlighted.

The impacts of droplet evaporative cooling on droplet lifetimes are compared with Maxwellian pure diffusion-limited evaporation approach under similar conditions. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. Larger droplets (r0 ~ 30–50 µm) can survive at Tss for about 5 s to over 10 min, depending on the subsaturation of the environment. The impacts of droplet evaporative cooling on evolution of drop size distributions, using high-resolution direct numerical simulations of moderately supercooled mixed-phase cloud boundaries, are discussed.

How to cite: Roy, P., Rauber, R. M., Di Girolamo, L., Chen, S., Xue, L., and Tessendorf, S. A.: Can evaporative cooling of water droplets play a role in enhancing ice formation at moderately supercooled cloud boundaries?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14843, https://doi.org/10.5194/egusphere-egu25-14843, 2025.

Aerosol-cloud interactions remain one of the largest uncertainties in quantifying anthropogenic impacts on climate, particularly through their influence on cloud liquid water path (LWP), cloud droplet number concentration (CDNC), and cloud susceptibility to aerosol perturbations. This presentation synthesizes insights from satellite observations, large-eddy simulations, global climate models, long-term in-situ observations, and advanced statistical analyses to address biases and uncertainties in these interactions. Satellite-based studies often report a decreasing LWP with increasing CDNC, yet retrieval errors and natural spatial variability can mask positive LWP adjustments, leading to an underestimation of the cooling effects of aerosol-cloud interactions. Large-eddy simulations of marine stratocumulus clouds reveal that assumptions of adiabaticity and spatial variability in cloud properties contribute to biases in satellite-derived LWP-CDNC relationships. However, with careful case selection and well-defined meteorological conditions, satellite-based estimations can be improved. Building on these findings, global climate modeling and machine learning analyses highlight the importance of updraft velocity and aerosol size distributions in shaping the CCN-CDNC relationship. Advanced methods such as Elastic Net Regression isolate these confounding factors, refining susceptibility estimates and enhancing consistency with physical expectations. Further, long-term in-situ observations of aerosols and clouds at high-latitude locations reveal that the susceptibility of CDNC to CCN is significantly higher for low-level stratiform clouds than suggested by global oceanic satellite data. This implies stronger aerosol radiative forcing than current satellite-based estimates assume. Comparisons with Earth system models reveal large inter-model variability in susceptibility, driven by differences in sub-grid scale updraft velocities and aerosol size distributions. Even models with relatively accurate susceptibility values exhibit unrealistic underlying physics, highlighting areas for improvement in model representation. Lastly, combining satellite, reanalysis, and in-situ ACTRIS observations, we evaluate the roles of aerosol size distributions and updrafts in warm cloud formation, bridging gaps between microphysical processes and large-scale variability. This comprehensive approach emphasizes the need for integrating multi-platform observations with advanced modeling and statistical methods to reduce biases and improve the fidelity of aerosol-cloud interaction estimates. These advancements are crucial for more accurately quantifying aerosol radiative forcing and its implications for climate prediction.

How to cite: Kokkola, H., Romakkaniemi, S., Virtanan, A., Partridge, D., Blichner, S., Mielonen, T., Calderón, S., Irfan, M., Lipponen, A., Virtanen, T., Holopainen, E., Kolmonen, P., Raj Jallu, P., Moisseev, D., Mom, B., and Arola, A.: Reconciling aerosol-cloud interactions through multiscale observations, modeling, and statistical techniques, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16481, https://doi.org/10.5194/egusphere-egu25-16481, 2025.

Satellite sensors provide global monitoring of cloud properties, such as cloud effective radius and cloud optical thickness, which have been extensively used to quantify the radiative forcing due to aerosol-cloud interactions. These cloud properties are simultaneously retrieved from a pair of reflectance measurements using a bi-spectral retrieval algorithm. However, the algorithm’s solution space is limited, and retrievals often fail when observations fall outside this space. Upon analyzing five years of quality-constrained liquid-cloud pixels observed by MODIS aboard Aqua, we find that a significant 10% of cloudy pixels experience retrieval failure, primarily because the observations correspond to an effective radius exceeding MODIS’s upper retrieval limit of 30 µm. The omission of these cloudy pixels introduces a sampling bias in aggregated mean gridded cloud properties, affecting, among other things, radiative forcing calculations. To address this, we restore the failed cloud retrievals in MODIS using two reconstruction algorithms: (1) a conservative approach that assigns a fixed minimum effective radius to failed pixels, and (2) a realistic approach that uses extreme effective radius distributions from spaceborne radar measurements. Our findings reveal that MODIS-derived cloud droplet number concentration is positively biased, while liquid water path is negatively biased. Accounting for this bias increases the magnitude of cloud water adjustments, highlighting the crucial need to expand the solution space in MODIS and similar sensors.

How to cite: Choudhury, G. and Goren, T.: Sampling bias from satellite retrieval failure of cloud properties and its implications for aerosol-cloud interactions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9585, https://doi.org/10.5194/egusphere-egu25-9585, 2025.

The difficulty in establishing causality between stratocumulus cloud droplet number concentration (Nd) and liquid water path (LWP) is well established. Recent studies on the satellite observations of the development of clouds over short time scales to examine the role of Nd perturbations in LWP variations demonstrated that LWP evolved differently depending on the initial Nd. This highlighted the need to consider the temporal development rather than the instantaneous measurements.

Here we characterise the dependence of this short timescale behaviour on the local meteorological environment, with aerosol production, entrainment from the free troposphere and wet scavenging all acting to modify the Nd. Many of these effects act to further steepen the Nd–LWP relationship and obscure the causal Nd impact on LWP. The multi-dimensional process space to represent stratocumulus is reduced to the two-dimensional Nd-LWP state space. The role of different physical processes is investigated and process-level fingerprints are extracted in this space.

How to cite: Nair, V. and Gryspeerdt, E.: Short time-scale evolution of aerosol-cloud interactions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19417, https://doi.org/10.5194/egusphere-egu25-19417, 2025.

Relationships between variables derived from satellite observations can be interpreted as causal connections between explanatory and response variables. When physical processes support the observed relationship, it appears more likely to represent a true causal link. A notable example is the observed relationship between liquid water path and droplet number concentration in marine low clouds, which aligns with the physical mechanisms involved. However, a closer examination reveals that the observed relationships may actually be driven by co-variability between meteorological conditions and aerosol levels, reflecting the climatological evolution of stratocumulus clouds. We therefore suggest that the aerosol influence on marine low clouds should be separated into two pathways: (1) the influence of background aerosol levels on the clouds' climatology, which overlays the co-variability, and (2) the causal response, as seen in the case of ship tracks.

How to cite: Goren, T., Choudhury, G., Kretzschmar, J., and McCoy, I.: When Co-Variability Mimics Causal Aerosol-Cloud Interactions in Satellite Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10728, https://doi.org/10.5194/egusphere-egu25-10728, 2025.

Deep convective clouds (DCCs) are crucial in the Earth’s energy budget. Although the abundant DCC-generated ice-phase anvil and cirrus theoretically have a warming effect, the reported observations of their cloud radiative effect (CRE) by previous studies are unexpectedly negative. Here, based on five years of global satellite data analysis, we find that the apparent contradiction between theory and observations resulted from neglecting the radiative contribution of background underlying clouds based on active and passive satellite observations. The probability of underlying clouds vertically below the anvils is up to 2/3. They can contribute up to 70% of the observed total shortwave cooling effect when they fully overlap with anvils. After excluding the effect of underlying clouds, most of the anvil CRE changes sign from negative to positive, increasing by over +25 W/m², especially over land. This revelation suggests a substantially underestimated warming effect of DCC anvils and cirrus in previous observations. Also, it may imply an underestimated aerosol-driven positive radiative forcing on DCC, which has been estimated as neutral previously.

How to cite: Pan, Z., Yin, J., Liu, F., Zang, L., Mao, F., and Rosenfeld, D.: Vertical Structure of Deep Convective Clouds and Their Large Radiative Warming Masked by Background Water Clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20156, https://doi.org/10.5194/egusphere-egu25-20156, 2025.

The Amazon rainforest plays an important role in global climate systems, particularly in regional precipitation patterns, atmospheric circulation, and Earth's energy balance. Convective systems in this region are intricately linked to these broader climatic processes. Through a combination of ground-based, satellite, and aircraft observations, we find that Amazonian convective clouds are particularly sensitive to aerosol concentrations, being highly aerosol-limited. This study explores the relationship between cloud droplet concentrations and ambient aerosol particles, both within the Amazon and in broader regions, evaluating various parameterizations commonly used in global climate models. Machine learning methods were used to capture the relationships between various aerosol, cloud, and meteorological parameters in the Amazon rainforest. To gain deeper insight into the microphysical processes within individual clouds, we examine the evolution of the cloud droplet effective radius (rₑ) as a function of cloud temperature (T), looking into the vertical structure of deep convective cumulus clouds. 

How to cite: Pöhlker, M. L., Romshoo, B., Lauer, O., Maddali, J., Liznerski, P., Henkes, A., Quaas, J., Seifert, P., Gaudek, T., Meller, B., Raj, S., Machado, L., Cecchini, M., Albrecht, R., Atabakhsh, S., Artaxo, P., Yang, Y., Babu Suja, A., Kloft, M., and Pöhlker, C.: Convective clouds over the Amazon rainforest – aerosol dependence and microphysical features, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10796, https://doi.org/10.5194/egusphere-egu25-10796, 2025.

Last year, we presented the Cloud-by-Cloud (CxC) approach for studying aerosol-cloud interactions (ACI) through a combination of observations with polar-orbiting and geostationary satellites. Specifically, cloud-relevant aerosol concentrations at cloud level are matched to individual clouds that are observed throughout their life time.

The methodology has now been applied to 11 years of data from MSG-SEVIRI and the CALIPSO lidar over Europe and northern Africa. The resulting data set of several thousand matched aerosol-cloud cases provides a first satellite-based assessment of ACI in warm and cold clouds in which the aerosol component is expressed in actual number concentrations of cloud condensation nuclei (n_CCN) and ice nucleating particles (n_INP) at cloud level. We present findings of the aerosol impact on cloud droplet number concentration, effective radius, liquid water path and cloud phase for different aerosol types and discuss differences to the conventional data-aggregation approach in which aerosols are expressed through aerosol optical depth.

How to cite: Alexandri, F., Müller, F., Choudhury, G., Seelig, T., Tesche-Achtert, P., and Tesche, M.: Combining spaceborne observations of CCN, INP, and cloud development for assessing ACI in liquid and ice-containing clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18210, https://doi.org/10.5194/egusphere-egu25-18210, 2025.

The first indirect effect of aerosols on cloud reflectivity, primarily through changes in cloud droplet number concentration (N_d) or effective radius (R_eff), remains one of the most uncertain components of anthropogenic radiative forcing. The strength of the first aerosol indirect effect (AIE) is quantified using relationships between aerosol proxies and N_d/R_eff. For large-scale assessments, these relationships have historically been observed via satellites and serve as critical constraints for climate models calculating radiative forcing from aerosol-cloud interactions (ACIs). They have often relied on observations of aerosol optical depth or aerosol index, which are column-integrated proxies for Cloud Condensation Nuclei (CCN) concentration that may not be directly relevant for studying ACIs. Additionally, these proxies are influenced not only by particle concentration but also by size distribution, composition, and relative humidity. Since CCN represents only a fraction of the aerosol size distribution, there may not always be an obvious correlation between CCN and optical properties, introducing uncertainties in estimating indirect effects when using aerosol optical properties.

To address this issue, we developed a machine learning approach to estimate the vertical profile of CCN concentration at 0.4% supersaturation using airborne High Spectral Resolution Lidar Generation-2 (HSRL-2) data and collocated in situ CCN, the latter as truth to train a neural network model. Reanalysis data were used to enhance model performance. Our algorithm predicts vertically resolved CCN concentration within a mean relative uncertainty of 20% and is applicable to EarthCARE/ATLID measurements.  Utilizing this new CCN product derived from the full suite of HSRL-2 extinction and backscatter measurements and reanalysis data of relative humidity and temperature in ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) along with collocated cloud properties retrieved from Research Scanning Polarimeter data, we investigate the first AIE over the western North Atlantic Ocean. Our preliminary findings indicate that the new CCN product consistently constrains the relationships between CCN and N_d/R_eff, for a wide range of cloud liquid water paths. The separation of indirect effects for different aerosol types indicates the expected differences in aerosol properties relevant for ACI. Overall, our approach using ML-derived CCN yields tighter constraints and physically more plausible insights into ACIs than vertically-resolved aerosol extinction, vertically-resolved aerosol index (extinction multiplied by Angstrom exponent), or column-integrated aerosol optical depth. We will conclude our presentation by illustrating that the aerosol vertical distribution and hygroscopic growth characteristics are the primary reasons why aerosol optical properties are inadequate for directly constraining the first AIE in the western North Atlantic Ocean.

How to cite: Redemann, J., Gao, L., Lenhardt, E., Burton, S., Crosbie, E., Fenn, M., Ferrare, R., Hair, J., Hostetler, C., Nehrir, A., Shingler, T., Cairns, B., and Sorooshian, A.: Machine Learning derived CCN concentrations provide better constraints on the first aerosol indirect effect than aerosol optical properties, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11812, https://doi.org/10.5194/egusphere-egu25-11812, 2025.

The vertical structures of precipitation serve as a direct link between clouds and surface precipitation, reflecting the processes of cloud and precipitation formation and development. However, their significance remains debatable, particularly concerning the aerosol effects on these structures. We utilize data from PM2.5 station measurements and the GPM 2A DPR to investigate the impact of aerosols on the vertical structure of precipitation and its microphysical characteristics.

Our findings show that the raindrop size in cold-topped (storm top height more than freezing level) precipitation exhibits three distinct trends from the storm top height to the surface: raindrop size decreases or shows no significant change in the upper layers, increases rapidly in the middle layers, and slightly decreases in the lower layers. Based on the observed turning points in raindrop size changes, we conducted a study on the influence of aerosols on the vertical structure of precipitation.

This study reveals phenomena different from previous views. Conventional wisdom suggests that higher and more extensive cloud development leads to greater surface precipitation intensity. However, our results indicate that for local cold-topped convective precipitation, the correlation between storm top height and precipitation rate at different altitudes decreases gradually with decreasing altitude. Absorbing aerosols are identified as a significant factor exacerbating the heterogeneity of precipitation vertical structures.

Within clouds, aerosols act as cloud condensation nuclei (CCN), influencing the growth of cloud droplets and ice crystals through microphysical effects. The competition between latent heat released and evaporative cooling initially increases storm top height but subsequently reduces it. Above the freezing level, precipitation rates and raindrop sizes remain highly consistent with storm top height. Below the freezing level, however, the vertical structure of precipitation is altered by evaporation. Larger raindrops and higher proportions of absorbing aerosols enhance evaporation, leading to a complex relationship where surface precipitation rates first decrease and then increase with increasing aerosol concentration. This response to aerosol is almost opposite to that observed above the freezing level.

This indicates that aerosols significantly exacerbate the heterogeneity of precipitation vertical structures through both microphysical and radiative effects.

How to cite: Sun, Y. and Zhao, C.: Aerosols exacerbating the heterogeneity of precipitation vertical structures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7763, https://doi.org/10.5194/egusphere-egu25-7763, 2025.

We present the findings from our in-situ holographic measurements conducted in shallow cumulus clouds, using the tethered aerostat Max Planck CloudKite (MPCK) during the EUREC4A campaign. The MPCK+ instrument is equipped with a holographic system sampling cloud droplets  8µm diameter or larger in a 10cm^3 three-dimensional volume every 12cm. This unprecedentedly small inter-sample spacing for holographic measurements is achieved by combining two variables: the high sampling frequency of our MPCK+ holographic setup, set at 75 Hz, and the low true airspeed of the aerostat.

The microphysical characteristics of clouds, such as droplet concentration, size distribution and liquid water content can be calculated with sub-meter spatial resolution. This allows us to obtain a detailed horizontal snapshot of a cloud's microphysics. The three-dimensional nature of holographic droplet data also allows a direct calculation of the radial distribution function (RDF), and the high measurement cadence of the MPCK+ data invites a spatially localized investigation of cloud droplet clustering.

We present an overview of our holographic data showcasing the structure of shallow cumulus clouds as well as an analysis of cloud droplet clustering in a short horizontal cloud snapshot.

How to cite: Thiede, B., Larsen, M. L., Schlenczek, O., Nordsiek, F., Bodenschatz, E., and Bagheri, G.: A Closer Look into Shallow Cumulus Clouds: Investigating Cloud Microphysics and Droplet Clustering Using the Max Planck Cloudkite+, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13932, https://doi.org/10.5194/egusphere-egu25-13932, 2025.

Earth system models struggle to accurately simulate aerosol’s interactions with weather and climate. This is largely attributed to structural uncertainty including insufficient process representation and model resolution due to limited computer power, and model tuning has become a low-cost remedy for improving performance. With unprecedented computational capability, improved understanding, modern software, and novel machine learning algorithms, high-resolution Earth system modeling with accurate and yet expensive process representations has become possible. In this study, we quantify the impacts of longstanding structural uncertainty on aerosol effective radiative forcing (ERF) by incorporating much more sophisticated process representations in U.S. Department of Energy’s Energy Exascale Earth System Model (E3SM). The ERF associated with aerosol-cloud interactions is further decomposed into the Twomey effect, liquid water path (LWP) adjustment, and cloud fraction adjustment using a satellite-based radiative kernel so that the impacts of each new process representation on aerosol ERF can be evaluated against observations. We find that while increasing model resolution to kilometer scale changes aerosol ERF by 30%, model physics representations (aerosol mixing assumption, condensational growth, secondary organic and sulfate aerosol formation, aerosol optics, aerosol activation, emission, giant aerosol, chemistry, warm rain process, and aerosol-turbulence coupling) contributes to a factor-of-two variation in aerosol ERF. As opposed to model tuning, this approach improves understanding and increases confidence in simulations as they are traceable to physics. Furthermore, even though the model’s total aerosol ERF or the Twomey effect alone can be brought to agree well with satellite estimate, significant biases in LWP and cloud fraction adjustments remain, highlighting the importance of improving aerosol interactions with cloud macrophysics in the model.

How to cite: Ma, P.-L., Fast, J., Geiss, A., Hassan Mozumder, M. T., Huang, M., Qin, Y., Wu, M., Zaveri, R., Zhang, K., Kang, L., Marchand, R., Larson, V., Morrison, H., Zhao, B., Wall, C., and Yao, Y.: Advancing understanding and predictability of aerosol effective radiative forcing due to structural uncertainty in an Earth system model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7303, https://doi.org/10.5194/egusphere-egu25-7303, 2025.

Aerosol-cloud interactions are a persistent source of uncertainty in climate research. This study presents findings from a model intercomparison project examining the impact of aerosols on clouds and climate in "cloud-resolving" Radiative-Convective Equilibrium (RCE) simulations. Specifically, 11 different models conducted RCE simulations under varying aerosol concentrations, domain configurations, and sea surface temperatures (SSTs). We analyze the response of domain-mean cloud and radiative properties to imposed aerosol concentrations across different SSTs. Additionally, we explore the potential impact of aerosols on convective aggregation and large-scale circulation in large-domain simulations.

The results reveal that the cloud and radiative responses to aerosols vary substantially across models. However, a common trend across models, SSTs, and domain configurations is that increased aerosol loading tends to suppress warm rain formation, enhance cloud water content in the mid-troposphere, and consequently increase mid-tropospheric humidity and upper-tropospheric temperature, impacting static stability. The warming of the upper troposphere can be attributed to reduced entrainment effects due to the higher environmental humidity in the mid-troposphere. However, examining high percentiles of vertical velocities at the mid troposphere do not demonstrate convective invigoration. In large-domain simulations, where convection tends to self-organize, aerosol loading does not influence self-organization but tends to reduce the intensity of large-scale circulation forming between convective clusters and dry regions. This reduction in circulation intensity can be explained by the increase in static stability.      

How to cite: Dagan, G., van den Heever, S. C., Stier, P., Abbott, T. H., Barthlott, C., Chaboureau, J.-P., de Roode, S., Fan, J., Gasparini, B., Hoose, C., Jansson, F., Kulkarni, G., Leung, G., Prabhakaran, T., Romps, D. M., Shum, D., Tijhuis, M., van Heerwaarden, C. C., Wing, A., and Yunpeng, S.: RCEMIP-ACI: Aerosol-Cloud Interactions in a Multimodel Ensemble of Radiative-Convective Equilibrium Simulations , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9604, https://doi.org/10.5194/egusphere-egu25-9604, 2025.

How to cite: Tippett, A., Gryspeerdt, E., and Field, P.: Simulating shipping impacts on clouds in a high-resolution weather model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18825, https://doi.org/10.5194/egusphere-egu25-18825, 2025.

Cloud-water adjustments are a part of aerosol-cloud interactions, affecting the ability of clouds to reflect shortwave radiation by processes altering the vertically integrated cloud water content L in response to changes in the droplet concentration N. In this study, we utilize a simple entrainment parameterization for mixed-layer models to determine entrainment-mediated cloud-water adjustments in non-precipitating stratocumulus. At lower N, L decreases due to an increase in entrainment in response to an increase in N suppressing the stabilizing effect of evaporating precipitation (virga) on boundary layer dynamics. At higher N, the cessation of cloud-droplet sedimentation sustains more liquid water at the cloud top, and hence stronger preconditioning of free-tropospheric air, which increases entrainment with N. Overall, cloud-water adjustments are found to weaken distinctly from dln(L)/dln(N) = -0.48 at N = 100 cm^-3 to -0.03 at N = 1000 cm^-3, indicating that a single value to describe cloud-water adjustments in non-precipitating clouds is insufficient. Based on these results, we speculate that cloud-water adjustments at lower N are associated with slow changes in boundary layer dynamics, while a faster response is associated with the preconditioning of free-tropospheric air at higher N.

How to cite: Hoffmann, F., Chen, Y.-S., and Feingold, G.: On the Processes Determining the Slope of Cloud-Water Adjustments in Non-Precipitating Stratocumulus, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3708, https://doi.org/10.5194/egusphere-egu25-3708, 2025.

An understanding of aerosol-cloud interactions is key to many areas of climate science due to the diversity of mechanisms by which clouds interact with the Earth’s climate. One such area of growing interest is Climate Intervention – deliberately altering the climate to counteract anthropogenic Climate Change. Many Climate Intervention techniques concern aerosol-cloud interactions in some way, Marine Cloud Brightening (MCB) – whereby aerosols are injected into the marine boundary layer to increase the albedo of low clouds – does in particular. Developing a comprehensive understanding of MCB requires modelling across all scales. The focus of this work is modelling with large-eddy simulations (LES) and parcel models, performing a variety of experiments such as step forcing and continuous injection of aerosols in different models of the same class. Most the LES experiments have been performed in MONC (Met Office NERC Cloud) and compared to DALES (Dutch Atmospheric LES). Model intercomparison is key to understanding MCB; thus far there is disagreement among LES models over some important details of MCB, such the impact of small injected aerosols on the entrainment of dry air. It is hoped that better comparison of models can address this.

How to cite: Smith, W. M.: Modelling marine cloud brightening in large-eddy simulation and parcel model intercomparison projects, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19507, https://doi.org/10.5194/egusphere-egu25-19507, 2025.

Aerosol-cloud interactions are complex processes that contribute significantly to uncertainties in predicting global climate. Cloud droplet formation is influenced by factors such as the hygroscopicity of soluble aerosol particles, aerosol processing within clouds, aerosol particle number concentration, and updraft velocity. Accurately understanding these factors is crucial for better representing cloud droplet numbers on a global scale. Updraft velocity is characterized by the probability density function (PDF) of the vertical velocity distribution. Earth system models (ESMs) typically assume a constant standard deviation (σ_w) for the vertical velocity PDF (van Noije et al. 2021; Mulcahy et al. 2020). However, this assumption may be inaccurate, as it has been shown that the σ_w exhibits significant diurnal variability (Bougiatioti et al. 2020). In this study, we implemented the turbulent kinetic energy (TKE) calculations to OpenIFS cycle 48r1 (OIFS48r1) following Bastak Duran et al. (2018). In addition, we implemented Morales and Nenes (2014) activation parameterization (M&N) to OIFS48r1 including the TKE to calculate the σ_w instead of using a constant σ_w for the vertical velocity PDF. OIFS48r1 is derived from the Integrated Forecasting System (IFS) developed by European Centre for Medium-Range Weather Forecasts (ECMWF) and it will be the main atmospheric model used in the newest version of European Community ESM (EC-Earth 4). First, we investigated the effects of using constant σ_w values of 0 and 0.8 m/s on the TKE-derived σ_w and their impact on column averaged cloud droplet number concentrations. The results showed that using σ_w of 0 m/s led to very low droplet numbers, as weaker updraft variability resulted in smaller supersaturations, causing fewer particles to activate. On the other hand, using σ_w of 0.8 m/s produced stronger activation, particularly in regions with high aerosol particle number concentrations. When using the TKE-derived σ_w, the differences in droplet numbers compared to the activation scheme with σ_w of 0.8 m/s were minimal. Next, we compared the monthly column-averaged and boundary-layer (BL) average number of activated particles obtained from the TKE-derived σ_w activation routine with the pre-existing Abdul-Razzak and Ghan (2000) activation parameterization (AR&G). The results showed that the AR&G scheme produced stronger activation than the M&N scheme, both in terms of the total column average and the BL-averaged regions. One of the reasons for this difference could be due to the activation calculations in the M&N scheme, which are only performed in areas where clouds are present, while in the AR&G scheme, activation is calculated for every gridbox.

How to cite: Holopainen, E. and Nenes, A.: Aerosol processes and activation in OpenIFS cycle 48r1 portable global aerosol-climate and weather prediction model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9397, https://doi.org/10.5194/egusphere-egu25-9397, 2025.

We have developed a double moment aerosol model appropriate for numerical weather prediction (NWP) within the Met Office’s Unified Model framework. Our aerosol model (Sol-Insol) is fully coupled to a double moment cloud microphysics model (CASIM) permitting aerosol-cloud interactions to be resolved at high-resolution. As a test we investigate how our setup performs against traditional ACI parametrizations used for NWP by comparing model results to observations from the Café-Brazil field campaign over the Amazon. Our simulations contribute to the CleanCloud model intercomparison project (MIP) aimed at improving aerosol-cloud interactions across timescales. As a case study, we use 14th January 2023, a date selected due to HALO aircraft observations being recorded alongside a pronounced mesoscale squall line.

The MIP protocol sets out 3 regional domains surrounding the ATTO site - a 6.6km resolution grid over the Northern part of South America into Central America, a 3.3km resolution grid inside this and then a 1.6km resolution grid covering the Amazon. Three model configurations were selected to isolate the impacts of aerosol on cloud and precipitation: a control run with no aerosol scheme, fixed cloud CDNC and a Cooper ICNC vs T relation (CONTROL); a coupled aerosol-cloud run with arbitrary initialised aerosols (ARB-AER); and a coupled aerosol-cloud run with aerosol initialised to CAMS reanalysis data (CAM-AER).

We constrain the simulated aerosol, cloud and radiation properties using a range of co-located satellite, surface (ATTO) and air (HALO) observations. Our results show that initialising the aerosol to CAMS reanalysis concentrations leads to slightly improved results relative to CONTROL while ARB-AER performs significantly worse when looking at observed top of the atmosphere radiation, highlighting the high sensitivity of cloud to ambient aerosol. Further experiments comparing ground and flight observations to model metrics help to support the conclusion that CAM-AER is performing better than ARB-AER and highlights potential improvements to NWP hydro-forecasts from using realistic aerosol properties.

How to cite: Carton-Kelly, J., Jones, A., and Field, P.: Investigating a double moment, fully coupled aerosol-cloud model over the Amazon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3245, https://doi.org/10.5194/egusphere-egu25-3245, 2025.

A major uncertainty in the cloud-feedback to a warming climate can be attributed to shallow convection in the trades. With recent advances in observational and computational resources, the potential impact of the mesoscale organization of these clouds became apparent. While the processes leading to the mesoscale organization are not resolved in current climate models, observations of these features and more importantly their interaction with different scales became available through field campaigns like EUREC4A.

This study quantifies the ability of large-domain large-eddy simulations to represent the observed mesoscale variability in cloudiness. By using forward operators to mimic the observations, we show that the stratiform cloud amount and precipitation frequency remain challenging to simulate at hectometre resolutions. Despite these challenges, the simulations show a similar sensitivity in cloud distribution to environmental conditions.

By perturbing the 41 days simulation with a 13-fold increase in CCN concentration we quantify the sensitivity of the mesoscale cloud organisation to changes in precipitation and show the strong influence on the cold-pool driven cloud formations. We further emphasise the importance to correctly represent the mesoscale processes in climate simulations by showing that changes in cloud-radiative effects due to aerosol changes vary day-by-day with varying contributions from changes in cloud albedo and cloud fraction.

How to cite: Schulz, H., Wood, R., and Stevens, B.: Quantification of aerosol influence on day-to-day mesoscale variability of shallow convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18015, https://doi.org/10.5194/egusphere-egu25-18015, 2025.

Anthropogenic aerosol emissions have had a major influence on global climate over the industrial era, counteracting some of the warming and precipitation increases due to rising greenhouse gas concentrations. These greenhouse gas driven changes are now rapidly being unmasked, through a range of national and regional policies aimed at improving air quality. However, unlike greenhouse gases, aerosol emissions have strongly regional response patterns, and influence the climate both near to, and far from emission sources. These regional influences have to date not been quantified in a consistent, multi-model framework.

Here, we present results from the Regional Aerosol Model Intercomparison Project (RAMIP). Ten CMIP6 era models have performed 10- member ensemble simulations investigating the climate response to aerosol emissions, separately, from South Asia, East Asia, Africa and the Middle East, and Europe and North America. All RAMIP experiments are based on two CMIP6-era SSPs: SSP3-7.0 (strong GHG increases, minimal aerosol reductions) and a hybrid SSP370-126aer (anthropogenic emissions of SO2, organic carbon, and black carbon are rapidly reduced following SSP1-2.6, either globally or regionally).

We find a rapid surface warming in response to aerosol reductions, and strong precipitation increases, but with marked regional differences in the magnitude of the response. While there are many robust responses, strong inter-model differences in the pattern and strength of the responses in some regions highlights where aerosol related uncertainty is large in the near-future.

We discuss the linearity of the effective radiative forcing and climate responses from regional aerosol perturbations, and demonstrate that emission location is key to the amplitude and extent of the response. In particular, emission changes in East Asia and North America and Europe have a larger global temperature impact than those over South Asia due to their influence on Pacific clouds. This initial analysis demonstrates the need for aerosol awareness in the design of future scenario ensembles, and in climate risk and impact studies both near to and far from aerosol emission sources.

How to cite: Wilcox, L., Samset, B., Stjern, C., and Allen, R. and the RAMIP Team: Distinct climate responses to regional aerosol emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6015, https://doi.org/10.5194/egusphere-egu25-6015, 2025.

The rate of global warming has seemingly increased since around 2010, relative to the previous 30-year period, leading up to the recent strong records in global surface temperature anomalies set in 2023 and 2024. Concurrently, Chinese emissions of SO₂ have dropped dramatically as a consequence of strong national air quality policies. This has, in turn, led to marked reductions in atmospheric sulphate aerosol loadings over, and downwind from, the Chinese mainland, as well as strong improvements in air quality. To date, the contribution of Chinese emissions reductions to the intensification of global warming, through unmasking of greenhouse gas driven climate change, has however not been quantified.

Here, we use simulations from the Regional Aerosol Model Intercomparison Project (RAMIP) to investigate the climate response to strong reductions in Chinese SO₂ emissions, closely analogous to real-world changes since 2010. We use 10-member ensembles of fully coupled simulations from ten CMIP6 era Global Climate Models, to quantify the global and regional, seasonally resolved influences on temperature and precipitation in a total of 100 ensemble members.

Overall, we find a warming due to recent reductions in SO₂ emissions from China of 0.07 ± 0.05 ºC. Assuming that this has happened since 2010 leads to a warming rate of 0.05 ºC / decade. Recent observations of global surface temperature anomalies indicate a warming rate increase of 0.07 ºC / decade, when filtered for the effects of interannual variability in sea surface temperature patterns. Hence, our results indicate a strong contribution of aerosol emissions reductions to this elevated warming rate. This conclusion is supported by the geographical pattern of elevated warming, as well as correspondence between modelled and observed top-of-atmosphere radiative imbalance changes.

How to cite: Samset, B. H., Wilcox, L. J., and Allen, R. J. and the RAMIP team: Strong contribution of SO2 emissions reductions from China to global warming intensification since 2010, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6060, https://doi.org/10.5194/egusphere-egu25-6060, 2025.

The quantification of aerosol-induced radiative forcing remains a significant challenge in climate modeling, primarily due to the complex interplay of aerosol and clouds in a warming world. Traditional approaches often rely on either bottom-up process-based models, difficult to constrain against present-day observations, or top-down methods that lack the ability to capture the underlying processes accurately.

Here, we present an approach that combines both bottom-up process-based constraints and top-down energetic constraints of aerosol forcing and cloud feedbacks simultaneously to achieve a more comprehensive understanding of aerosol impacts on clouds and the climate. We generate one million samples of parametric uncertainty of aerosol forcing and cloud feedback and estimate the historic temperatures they would have produced using an impulse-response model. Both the temperature trajectories and the associated microphysical properties (such as the hemispheric contrast in cloud droplet number concentration) can then be compared to observations simultaneously.

Applying the new method to the Community Atmosphere Model v6, we infer narrower parameter ranges for key process parameters, a reduced effective radiative forcing of -1.08 [-1.29 – -0.77] Wm^-2, and hence 66% more precise future projections.

How to cite: Watson-Parris, D.: Integrating Bottom-Up Process-Based Constraints with Top-Down Energetic Constraints of Historic Warming for More Accurate Future Projections, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21790, https://doi.org/10.5194/egusphere-egu25-21790, 2025.

The western North Pacific Subtropical High (WNPSH) significantly influences East Asian weather. In the Northwest Pacific where sea‐salt aerosols (SSAs) are abundant and the large‐scale environment is dominated by the dry subsidence of the WNPSH during summer, inhomogeneous SSAs form as a product of the environment. However, the extent to which inhomogeneous SSAs affect the WNPSH remains unclear. This study investigates the radiative effects of SSAs through numerical simulations, revealing a novel mechanism for the strengthening of the WNPSH. The results demonstrate that inhomogeneous SSAs enhance the WNPSH by generating diabatic cooling in the upper troposphere and associated unstable subsidence motion. Further considering the radiative hysteresis effects of inhomogeneous SSAs, the WNPSH further strengthens under the combined dynamic and thermodynamic influences associated with upper‐level radiative cooling. Inhomogeneous SSAs not only enhance the WNPSH but also influence the location where the central area of high pressure intensifies.

How to cite: Shu, S.: Inhomogeneous Sea‐Salt Aerosols—A New StrengtheningMechanism for the Western North Pacific Subtropical High, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14328, https://doi.org/10.5194/egusphere-egu25-14328, 2025.

Anthropogenic aerosol emissions are projected to decline significantly by 2050, with major implications for regional climate. However, unlike greenhouse gases, aerosol impacts are spatially heterogeneous and can influence climate both near emission sources and through remote teleconnections. This is particularly important for the Indian monsoon system, where both local and remote aerosol changes can significantly affect precipitation patterns.

Using the Regional Aerosol Model Intercomparison Project (RAMIP) framework, we examine how local and remote aerosol emission changes influence Indian climate across both pre-monsoon and monsoon seasons. Our analysis employs 10-member ensembles from multiple CMIP6-era models to compare three experiments: Global, South Asian, and East Asian aerosol reductions relative to a high-emission baseline (SSP3-7.0). This experimental design allows us to isolate and quantify the distinct impacts of regional emission changes. Initial results reveal that global aerosol reductions lead to more widespread and intense precipitation changes compared to regional reductions alone, with South Asian aerosol reductions largely mirroring the global response pattern while East Asian emissions play an additional role in modulating monsoon circulation. The Western Ghats and Indo-Gangetic Plains show particularly strong responses. We find significant inter-model diversity in the spatial patterns and magnitudes of these changes, highlighting key areas of uncertainty in aerosol-monsoon interactions. Through detailed analysis of circulation patterns and moisture transport, we investigate the mechanisms driving these precipitation responses and their implications for future climate projections.

Our findings provide insights into the complex relationship between regional aerosol emissions and monsoon dynamics, with important implications for both climate prediction and air quality management in South Asia.

How to cite: Bhandekar, A., Lawrence, B., Abraham, L., O’Connor, F., and Maynard, C. and the RAMIP Team: Indian Summer Monsoon Response to Regional Aerosol Emission Changes: RAMIP insights, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4280, https://doi.org/10.5194/egusphere-egu25-4280, 2025.

Attributing variations in the Asian summer monsoon to aerosol forcing is crucial for reducing uncertainties in future regional water availability projections. This is essential for effective risk management and adaptation planning in this densely populated region. However, accurately simulating the monsoon remains a significant challenge for climate models due to persistent biases, which compromise their reliability in attributing anthropogenic influences. In this study, we analyze a set of climate model experiments to uncover the connection between these biases and the monsoon’s response to Asian aerosols, focusing on the underlying physical mechanisms, including large-scale circulation changes. The impact of aerosols on monsoon precipitation and circulation is strongly shaped by a model's ability to represent the spatio-temporal variability in climatological monsoon winds, clouds, and precipitation across Asia. This variability modulates the magnitude and efficacy of aerosol–cloud–precipitation interactions, a key component of the total aerosol response. Our findings reveal a strong interplay between South Asia and East Asia monsoon precipitation biases, with their relative dominance significantly influencing the overall monsoon response. Notably, there is a sharp contrast between aerosol-driven changes during early and late summer, which can be attributed to the opposing signs and seasonal evolution of biases in these two regions. Realistically simulating the progression of large-scale atmospheric circulation is essential to fully capture the aerosol impact across Asia. These insights have significant implications for improving the understanding and reducing inconsistencies in model responses to aerosol changes over Asia, both in historical simulations and future projections.

How to cite: Liu, Z., Bollasina, M., and Wilcox, L.: Impact of Asian aerosols on the summer monsoon strongly modulated by regional precipitation biases, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16348, https://doi.org/10.5194/egusphere-egu25-16348, 2025.

Rapid urbanization and industrialization lead to significant changes in land use and land cover (LULC), particularly in agricultural activities over the Indo-Gangetic Plain (IGP), which in turn impacts the distribution of aerosol loading. In the present study, climatological and spatio-temporal trends of aerosol optical depth (AOD) and aerosol sources are investigated over the IGP region from 2001 to 2022. Climatological analysis of the AOD over the upper, central, and lower IGP regions was conducted using satellite-based MODIS data. LULC analysis over the IGP region during the study period showed a significant increase in crop land and built-up area, mainly replacing the vegetation. It is hypothesized that changes in LULC patterns, especially the expansion of cropland and crop residue burning (CRB), are major drivers of aerosol trends. The results reveal significant AOD increases, particularly in the lower IGP, which experienced extensive vegetation loss replaced by cropland, intensifying CRB and aerosol emissions. In contrast, the upper IGP exhibited reduced CRB activity and even declining AOD trends during the second half of the study period (2012–2022), indicating improved air quality. The findings highlight a direct relationship between LULC changes, CRB, and AOD trends, emphasizing the need for sustainable agricultural practices and emission control measures to mitigate aerosol pollution and improve air quality across the IGP.

How to cite: Kumar Singh, R. and Satyanarayana, A. N. V.: Impact of Land Use Changes during Agricultural Cycles on AOD Trends over the Indo-Gangetic Plain during Last Two Decades, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2136, https://doi.org/10.5194/egusphere-egu25-2136, 2025.

Precipitation changes over sub-Saharan Africa linked to remote aerosol emissions have severely impacted agriculture, ecosystems, and livelihoods historically in the region. Established links between aerosol emissions and precipitation responses impact future projections of sub-Saharan precipitation, which remain uncertain due to differences in model representations of aerosol, aerosol-precipitation interactions, and unclear future aerosol emission pathways. Ongoing large reductions in aerosol emissions from East Asia, combined with uncertainty in future aerosol emissions for India and Africa, indicate that aerosol changes are likely to play an important role in African climate in the near-term future.
In this presentation, we identify regional African precipitation responses to local and remote aerosol emission changes, and establish mechanisms behind them. We focus on responses in the East and West African monsoons, including changes to the intensity, timing, spatial pattern, and variability of rainfall. We also demonstrate the sensitivity of the responses to aerosol emission region, to determine whether local or remote emission changes dominate rainfall responses on seasonal timescales. Using the Regional Aerosol Model Intercomparison Project experiments, we quantify the role of regional aerosol emission changes in near-term African precipitation responses. This allows us to determine the aerosol emission regions which dominate the African precipitation responses, while also exploring sensitivities to absorbing and scattering species of aerosol emissions.
Current analysis has determined that reductions in global aerosol emissions cause West Africa to become significantly hotter and wetter, with a northward shift in precipitation found in some models; this change is strongest along the coastline in most models, though there is considerable diversity in the magnitude of modelled responses.
This work highlights the role of changing aerosol emissions on African precipitation patterns, providing essential information for near-term climate adaptation strategies.

How to cite: Toolan, C., Turner, A., and Adabouk Amooli, J. and the RAMIP Team: Sub-Saharan African Precipitation Responses to Aerosol Emission Changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6953, https://doi.org/10.5194/egusphere-egu25-6953, 2025.

This study assessed Africa’s climate response to different injection materials for stratospheric aerosol geoengineering (SAG) using simulations from the SOCOLv4 model, which provides a set of SAG simulations following the G6sulfur experiment in the Geoengineering Multi-model Intercomparison Project (GeoMIP). We analysed four SAG experiments, which used four injection materials (sulfur, alumina, calcite, and diamond) referred to as G6sulfur, G6alumina, G6calcite, and G6diamond, respectively. The SAG experiments used a high-emission pathway (SSP5-8.5) as baseline in which each material was injected into the equatorial stratosphere to keep global warming levels similar to an intermediate emission pathway (SSP2-4.5). We assessed Africa’s climate response to these SAG materials by quantifying the end-of-century (2080-2099) mean changes in minimum and maximum temperatures and precipitation relative to SSP2-4.5. Our findings suggest that all injection materials show a cooling potential by keeping annual and seasonal minimum and maximum temperatures below SSP2-4.5 across most parts of the continent. Maximum and minimum temperatures could decrease the most between 10°S and 10°N, along the Guinean coast of west Africa and parts of Central Africa, by up to -2°C and -4°C, respectively. This SAG-induced cooling remains partial over north Africa where a residual warming of about 1°C could persist at the end of the century relative to the SSP2-4.5, irrespective of the injection material. On the other hand, the impact on precipitation is less linear and spatially heterogeneous. However, SAG could reverse the SSP5-8.5 projected mean continental and regional (especially over Central Africa) increases in annual and seasonal precipitation, inducing a dryer future under SAG across the continent, irrespective of the injection material. Our results further suggest that, relative to SSP2-4.5, G6alumina could cause the largest precipitation decrease and G6diamond the slightest decrease at the end of the century. In summary, our results show that irrespective of the injection material, SAG could significantly decrease temperatures across Africa, however lower warming and drying could still be achieved under SSP2-4.5, over parts of Africa.

How to cite: Odoulami, R. C., Patel, T. D., Sukhodolov, T., Egbebiyi, T. S., Vattioni, S., Chiodo, G., Lennard, C. J., Abiodun, B. J., and New, M. G.: Africa’s climate response to stratospheric aerosol injection materials, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13980, https://doi.org/10.5194/egusphere-egu25-13980, 2025.

Since the 1980s, the increase in greenhouse gases and the decrease in anthropogenic aerosols (AA) over Europe and the Mediterranean (MED), driven by air pollution control policies, along with anthropogenic land-use changes, have been linked to the enhanced regional warming trend in the MED, particularly during boreal summer (JJA). However, the effect of AA changes on the temperature trend over this region requires further investigation. This study examines temperature changes attributed to AA over the MED during the historical period (1850-2014) with respect to changes in effective radiative forcing (ERF) based on experiments from 11 CMIP6 Earth System Models. The transient shortwave ERF due to AA is assessed using the “histSST” and “histSST-piAer” experiments, which share the same historical forcings driven by prescribed sea surface temperatures and sea ice from the corresponding coupled models except that “histSST-piAer” uses pre-industrial aerosol precursor emissions from the year 1850. Shortwave ERFs due to aerosol-radiation interactions and aerosol-cloud interactions, as quantified by the approximate partial radiative perturbation (APRP) method,  present a negative peak in 1965-1984 relative to 1850 over the MED region (multi-model means of -2.17±0.82 W m^-2 and -3.08±1.85 W m^-2, respectively), exhibiting trends towards less negative values in recent past (1995-2014) relative to the 1965-1984 period (changing by 0.94±0.37 W m^-2 and 0.51±1.02 W m^-2, respectively). Furthermore, we quantify the response in near-surface air temperature (monthly mean, and daily maximum and minimum) caused by AA on an annual and seasonal basis using the “historical” and “hist-piAer” experiments, which are driven by historical forcings except that “hist-piAer” uses fixed pre-industrial AA and aerosol precursor emissions. Consistent with the negative ERF changes, we find a surface cooling of -1.23±0.56 K in the 1965-1984 period relative to 1850 on an annual basis. During 1995-2014 there is an annual mean increase of 0.25±0.36 K relative to 1965-1984 pointing towards an amplification of warming due to AA reduction. The regional surface warming in 1995-214 relative to 1965-1984 is more prominent in JJA (0.26±0.34 K) than in boreal winter (0.18±0.57 K) as models show lower agreement on the sign of change during wintertime. Similar results are derived for maximum and minimum temperatures regarding the magnitude and the trend of changes over the MED region.

This work is implemented within the research project REINFORCE (impRovEments in the simulation of aerosol clImate liNkages in earth system models: From glObal to Regional sCalEs) in the framework of HFRI call “Basic Research Financing (Horizontal Support of all Sciences)” under the National Recovery and Resilience Plan “Greece 2.0” funded by the European Union – NextGenerationEU (HFRI; project no. 15155).

How to cite: Kalisoras, A., Georgoulias, A. K., Akritidis, D., Allen, R. J., Naik, V., and Zanis, P.: Anthropogenic aerosol-induced changes in radiative forcing and temperature over the Mediterranean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16293, https://doi.org/10.5194/egusphere-egu25-16293, 2025.

The freshwater resources of the Southwestern United States (SWUS) largely depend on wintertime precipitation, which has declined since 1980. During this period, the tropical Pacific sea surface temperature (SST) has exhibited a multi-decadal La Niña-like trend, inducing a teleconnection that reduces SWUS precipitation. However, the extent to which this SST trend is driven by internal variability (i.e., natural fluctuations) versus anthropogenic radiative forcings remains uncertain. This study explores two aspects of the impact of anthropogenic aerosols on the Pacific sector and their influence on the wintertime SWUS precipitation trend. First, we demonstrate that the La Niña-like SST trend is partially forced by anthropogenic aerosols, as evidenced by simulations from the Community Earth System Model version 2 (CESM2) large ensemble and its single-forcing large ensemble. This forced La Niña-like SST trend, in turn, drives a decline in SWUS precipitation through its teleconnection. Second, we show that the teleconnection pattern associated with internal Pacific decadal variability during the post-1980 period differs from the pre-industrial condition. Specifically, using a hierarchy of model simulations, we find that even under El Niño-like SST trends, there is a tendency toward a North Pacific anticyclonic circulation trend and reduced SWUS precipitation during post-1980 — contrary to the canonical El Niño teleconnection. This unintuitive yet robust circulation change arises from nonadditive responses to tropical mean warming and radiative effects from anthropogenic aerosols. As the forced SWUS precipitation decline combines with anthropogenic warming, the post-1980 period exhibits the most rapid SWUS soil moisture drying among past and future periods of similar length. Although future projected El Niño-like warming and aerosol emission reductions could potentially reverse the precipitation trend, these changes are unlikely to mitigate the currently projected drought risk in the region.

How to cite: Kuo, Y.-N., Lehner, F., Simpson, I., Deser, C., Phillips, A., Newman, M., Shin, S.-I., Wong, S., Arblaster, J., and Kim, H.: Evidence of Anthropogenic Aerosols Impacts on the Southwestern U.S. Droughts since 1980, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15483, https://doi.org/10.5194/egusphere-egu25-15483, 2025.

Wildfire activity in boreal regions is changing, due to climate change and other anthropogenic drivers. Given the high climate sensitivity of the Arctic and boreal regions, it is important to explore the impacts of these changes. There are also region-specific impacts of biomass burning particular to high latitude regions, such as black carbon (BC) deposition on snow. While many sources of atmospheric pollutions are being mitigated, fires are emerging as a growing contributor to poor air quality, both locally to the fire emissions source and across wider regions.

Here we investigate the climate and atmospheric impacts of several idealised biomass burning perturbations, focusing on aerosols. We present initial results from a new set of multi-model experiments (including CESM2, NorESM2 and EC-Earth), where biomass burning emissions are perturbed in several idealised experiments. The species concerned are: BC, SO4, organic carbon, SO2, DMS, and secondary organic aerosol precursors. We first perturb all boreal biomass burning emissions, and then smaller regions of interest individually (boreal North America, East Siberia and West Siberia). These experiments use 2005-2014 as a baseline period, and use the sum of this period as the perturbation, giving an approximately x10 perturbation in the regions of interest, in both fixed SST (30 years) and coupled (200 years) simulations. The strength and location of the aerosol changes studied here (when comparing aerosol optical depth) are comparable to the recent trends in aerosols between 2015-2024 and 2005-2014.

We will present the modelled atmospheric composition response both globally and in the focus regions described above, including teleconnections to other regions. This includes aerosol optical depth, aerosol absorption, and a breakdown by aerosol species. We will also highlight the climate response to the biomass burning perturbations, including effective radiative forcing (ERF) and fully-coupled climate response estimates.

How to cite: Staniaszek, Z., Samset, B., Lund, M. T., and Ekman, A.: Regional to global impacts of boreal biomass burning emissions changes: a multi-model study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2559, https://doi.org/10.5194/egusphere-egu25-2559, 2025.