This year marks the end of the Shedding Light On Cloud Shadows project (SLOCS, 2019-2024). SLOCS aims to understand temporal, spatial, and spectral variability in surface solar irradiance driven by individual clouds from field observations and 3D cloud-resolving large-eddy simulations. In this contribution, we would like to present the highlights of the project and the most important conclusions.

The reason for initiating SLOCS is that clouds trigger large fluctuations in solar surface irradiance, and therefore in surface heat fluxes, but there is still much to be learned about these fluctuations. The incoming radiation in shadows is almost an order of magnitude less than under clear sky, while peaks near clouds shadows can sometimes reach a 50% increase with respect to clear sky, due to scattering of sunlight on clouds. Performing cloud-resolving simulations with realistic surface solar irradiance patterns under broken clouds remains therefore a challenge, and current cloud-resolving models do not capture the radiation-cloud interactions well. 

The Shedding Light On Cloud Shadows (SLOCS) project addresses this challenge by i) performing spatial observations in a spatial grid fine enough (~50 m, 10 Hz) to capture individual clouds using a newly designed instrument, and ii) developing 3D radiative transfer models for cloud-resolving models with optimal balance between detail level and performance. The FESSTVaL, LIAISE, and CloudRoots campaigns provided unique opportunities to measure surface solar irradiance around cloud shadows in different climates. In the campaigns, we performed grid measurements of radiation, while benefiting from complementary boundary-layer and cloud observations.

The most important lessons learned from the field observations are:
1. Scales as small as meters and seconds contribute significantly to fluctuations in surface solar irradiance
2. All broken cloud patterns generate strong peaks, but the underlying mechanisms vary greatly amoung cloud types
3. Spectral variations (in colors of light) are mostly significant under cumulus clouds.

We used those observations to set up a series of cloud-resolving simulations with MicroHH and to evaluate two newly-developed radiative transfer solvers: i) a ray tracer fast enough to be coupled to our cloud-resolving model and ii) a solver that post-processes the outcome of a 1D two-stream solver to emulate 3D effects. Also, we studied the impact of periodic and open lateral boundary conditions. The most important conclusions are:

1. Capturing 3D interactions between clouds and radiation accurately leads to larger clouds with more liquid water compared to those in simulations with conventional 1D methods
2. Post-processing conventional 1D radiation computations allows for simulating surface solar irradiance fields with realistic probability density functions, but inaccurate cloud shadow shape and location.
3. Open lateral boundaries in large-eddy simulations are at least as important as correct radiation-cloud interactions in producing realistic cloud shadows in the range from hectometers to kilometers.

How to cite: van Heerwaarden, C., Mol, W., Veerman, M., van Stratum, B., Tijhuis, M., Heusinkveld, B., Hartogensis, O., Vilà-Guerau de Arellano, J., and Mangan, M.: The Shedding Light On Cloud Shadows project: measuring and simulating surface solar irradiance under broken clouds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18173, https://doi.org/10.5194/egusphere-egu24-18173, 2024.

Most atmospheric models consider radiative transfer only in the vertical direction (1D), as 3D radiative transfer calculations are too costly. Thereby, horizontal transfer of radiation is omitted, resulting in incorrect surface radiation fields. Previous work on 3D radiative effects mainly used uncoupled 3D radiative transfer. In our current work, we study the impact of coupled 3D radiative transfer on the development of clouds, and the resulting impact on the domain average surface solar irradiance.

We performed a series of realistic Large-Eddy simulations with MicroHH. We developed the option to use aerosol data from the CAMS global reanalysis to include the interactions between aerosols and radiation in our LES simulations. This makes sure our simulated radiation is in line with observations. To investigate the impact of 3D radiative transfer, we selected 12 days on which shallow cumulus clouds formed over Cabauw, the Netherlands. For each day, we performed simulations with 1D radiative transfer and with a coupled ray-tracer. The simulations with the coupled ray-tracer also include the results of uncoupled 1D radiative transfer. This allows us to compare the differences between 1D and 3D radiative transfer when the clouds are the same.  

In general, our simulations with coupled 3D radiative transfer have a higher domain average liquid water path compared to our simulations with coupled 1D radiative transfer. The cloud cover is similar in both simulations, but the cloud size is increased in the simulations with coupled 3D radiative transfer. For the domain average radiation, we find that 3D radiative transfer in general decreases the direct radiation and increases the diffuse radiation, but the net effect is on average less than 1 W m^-2. We can explain the differences in radiation when we look separately at the direct and diffuse radiation, the uncoupled 3D effects, and the impact of the change in the clouds. The uncoupled effect of 3D radiative transfer is an increase in global radiation, which is counteracted by a decrease is global radiation caused by the change in clouds.

How to cite: Tijhuis, M., van Stratum, B., and van Heerwaarden, C.: Coupled 3D radiation deepens cumulus clouds without changing the mean surface solar irradiance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5996, https://doi.org/10.5194/egusphere-egu24-5996, 2024.

Investigating solar radiation and its variability due to clouds and aerosol is critical for efficient and reliable solar energy systems. During broken cloud conditions, reflections at cloud edges and changing aerosol properties in the vicinity of clouds affect the surface solar radiation significantly. In these situations, the distinction between clouds and clear skies with aerosol is not always well defined[1]. As seen from the surface, this region exists around cloud core shadows and is called the transition zone. Here, a unique dataset of observations from a dense pyranometer network is used to detect and investigate signatures of shortwave broadband transmittance in the transition zone.

The TROPOS pyranometer network consists of up to 100 individual stations. Data of one campaign is used for this study: 60 stations were distributed over an area of about 6 km² during the S2VSR[2] measurement campaign in 2023 at the ARM Southern Great Planes (SGP) site in Oklahoma, USA. The surface solar irradiance is measured at each station with a time resolution of 10 Hz. The transition zone is detected and characterized by applying a modified clear sky detection algorithm[3] to the data. An additional component of our analysis is the determination of the cloud motion. This vector is determined using the Farneback optical flow algorithm[4] on a cloud shadow mask calculated from the “Clouds Optically Gridded by Stereo” (COGS) product[5].

The study aims to quantify the small-scale effects of the transition zone on surface solar irradiance and potential photo-voltaic yield. This information is valuable for photo-voltaic site planning and provides scientifically relevant insights into the interaction between clouds, aerosol and solar radiation.

How to cite: Witthuhn, J., Deneke, H., and Kalesse-Los, H.: Typical signatures of the transition zone of cumulus cloud shadows in solar radiation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17813, https://doi.org/10.5194/egusphere-egu24-17813, 2024.

The cloud radiative effect (CRE) of low-level marine clouds has traditionally been expressed primarily as a linear function of the cloud cover. However, recent studies have revealed a substantial change in CRE even at a constant cloud cover. This change is attributed to variations in cloud morphology governed by the horizontal and vertical distribution of cloud water. A unique feature of these morphologies, especially for low marine clouds, is the occurrence of distinct quantities of optically thin clouds. Understanding the impact of these thin clouds on low-level marine CRE is crucial for two reasons. First, spaceborne studies indicate a prevalent occurrence of thin clouds in areas characterized by peak low-level cloud cover. Second, the relationship between thin clouds and CRE may differ from that of their optically thicker counterparts due to their semitransparency to incoming shortwave and outgoing longwave radiations. This study investigates the influence of thin clouds on the CRE of low-level clouds over the Southeast Pacific Ocean using six years of concurrent measurements from MODIS and CERES spaceborne sensors. The results show a substantial influence of thin clouds on the shortwave and longwave components of CRE, as well as the balance them. The findings emphasize the need for a more comprehensive representation of thin clouds and, therefore, cloud morphology in climate models.

How to cite: Choudhury, G. and Goren, T.: Role of thin clouds in modulating the cloud radiative effect of marine low clouds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-288, https://doi.org/10.5194/egusphere-egu24-288, 2024.

There has been significant progress in comprehending the role of characteristic properties of aerosol in cloud droplet formation over the past decade [1]; however, the growth of cloud droplets into rain droplets, initiating precipitation in warm clouds, is still not well understood [2]. Collision and coalescence among droplets are assumed to be responsible for the rapid growth of cloud droplets to rain droplets. Turbulence is believed to play a significant role in the growth of cloud droplets [2]. 

The influence of turbulence on droplet dynamics is nominally characterized by the Stokes number, which decides if the droplet either follows the streamlines or decorrelates from it. Due to their deviation from the streamlines, droplets can form clusters and caustics, thereby increasing the chance of collisions [3]. Thus, the size distribution of droplets can determine the influence of turbulence on droplet collisions. The cloud droplet size distribution depends on several parameters, such as the initial aerosol number concentration, aerosol properties, and the in-cloud supersaturation. Thus, investigating the influence of turbulence on a given droplet size distribution can facilitate a better scientific understanding of the onset of precipitation. 

In the present study, we experimentally investigated the influence of turbulence on different cloud droplet size distributions. We generated homogeneous isotropic turbulence of various intensities in a closed chamber and seeded it with droplets relevant to that observed in clouds originating under different environmental conditions. Using Phase Doppler particle analyzer (PDPA), we measured the droplet size distributions and analyzed the changes with turbulence intensity. Our experiments show significant growth for cloud droplet size distributions with a higher degree of polydispersity than slender droplet size distributions. We attribute this enhancement in collisions to the induced relative velocity between droplets of different Stokes numbers. We observed a positive trend between clustering and droplet size growth, thus indicating the role of clustering in enhancing collisions.

Acknowledgments: We thank Dr. Amit Kumar Patra and Dr. T. Narayana Rao for their valuable suggestions. We acknowledge the ISRO-IITM cell (No. SP/21-22/1197/AE/ISRO/002696) and  IoE initiative (SP22231222CPETWOCTSHOC) for funding this work. 

References:

[1] Gunthe, S.S., King, S.M., Rose, D., Chen, Q., Roldin, P., Farmer, D.K., Jimenez, J.L., Artaxo, P., Andreae, M.O., Martin, S.T. and Pöschl, U., 2009. Cloud condensation nuclei in pristine tropical rainforest air of Amazonia: size-resolved measurements and modeling of atmospheric aerosol composition and CCN activity. Atmospheric Chemistry and Physics, 9(19), pp.7551-7575.

[2] Devenish, B.J., Bartello, P., Brenguier, J.L., Collins, L.R., Grabowski, W.W., IJzermans, R.H.A., Malinowski, S.P., Reeks, M.W., Vassilicos, J.C., Wang, L.P. and Warhaft, Z., 2012. Droplet growth in warm turbulent clouds. Quarterly Journal of the Royal Meteorological Society, 138(667), pp.1401-1429.

[3] Ravichandran, S. and Govindarajan, R., 2015. Caustics and clustering in the vicinity of a vortex. Physics of Fluids, 27(3).

How to cite: Vignesh, S., Sanket, A., Narayanan Unni, A., Sahu, S., S. Gunthe, S., Chaudhuri, S., Govindarajan, R., and I. Sujith, R.: Exploring the Influence of Turbulence on Droplet Size Growth and Precipitation in Warm Clouds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2702, https://doi.org/10.5194/egusphere-egu24-2702, 2024.

A number of studies have reported experimental CCN activation properties of water insoluble particles, mainly various minerals and soots, during the past decade or so (e.g. Kumar et al., 2011; Lathem et al., 2011, Dalirian et al., 2018). A popular theoretical framework for interpreting the results is the FHH adsorption activation theory (Sorjamaa and Laaksonen, 2007), which is a combination of the two-parameter FHH adsorption isotherm model and the Kelvin equation. However, it has become clear that the FHH activation theory tends to overpredict critical supersaturations quite substantially when the FHH parameters are determined from experimental water adsorption isotherms (Laaksonen et al., 2016; Hatch et al., 2019). A possible reason for the discrepancy is surface roughness of the particles, not accounted for in the FHH adsorption activation theory (Laaksonen et al., 2016). One way to quantify the extent of the surface roughness is through the surface fractal dimension, which can be determined e.g. with the help of nitrogen adsorption measurements. We showed earlier (Laaksonen et al., 2016) that employing the surface fractal dimension within the FHH framework does seem to improve the theoretical predictions. However, our data for water and nitrogen adsorption measurements were obtained from literature sources, and therefore the surface properties of a given mineral species employed in the adsorption measurements and in the CCN experiments were not necessarily similar. Therefore, the uncertainty limits of the surface fractal dimension -corrected predictions were rather high. Here, we compare theoretical and experimental critical supersaturations for several metal oxide and mineral aerosols. The materials used in the water and nitrogen adsorption measurements are the same as those used in the CCN experiments, allowing us to improve the reliability of our conclusions regarding the quality of the theoretical predictions.   

Dalirian, M, Ylisirniö, A., Buchholz, A., Schlesinger, D., Ström, J., Virtanen, A. and Riipinen, I. (2018). Cloud droplet activation of black carbon particles coated with organic compounds of varying solubility.  Atmos. Chem. Phys. 18, 12477-12489.

Hatch, C.D., Tumminello, P.R., Cassingham, M.A., Greenaway, A.L., Meredith, R. and Christie, M.J. (2019). Technical note: Frenkel, Halsey and Hill analysis of water on clay minerals: toward closure between cloud condensation nuclei activity and water adsorption. Atmos. Chem. Phys. 19, 13581-13589.

Kumar, P, Sokolik, I.N. and A. Nenes, A (2011). Measurements of cloud condensation nuclei activity and droplet activation kinetics of fresh unprocessed regional dust samples and minerals. Atmos. Chem. Phys. 11, 3527–3541.

Laaksonen, A., Malila, J. and Nenes A (2016). Surface fractal dimension, water adsorption efficiency, and cloud nucleation activity of insoluble aerosol. Sci. Rep. 6, 25504.

Lathem, T., Kumar, P., Nenes, A., Dufek, J., Sokolik, I.N., Trail, M. and Russell, A. (2011). Hygroscopic properties of volcanic ash. Geophys. Res. Lett. 38, L11802.

Sorjamaa, R. and Laaksonen A. (2007). The effect of H₂O adsorption on cloud drop activation of insoluble particles: a theoretical framework. Atmos. Chem. Phys. 7, 6175–6180.

How to cite: Laaksonen, A., Mustonen, L., Piedehierro, A. A., Viisanen, Y., and Welti, A.: Cloud drop activation of insoluble particles: impact of surface properties, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8917, https://doi.org/10.5194/egusphere-egu24-8917, 2024.

The surface tension of sub-micron aerosol particles is expected to affect their efficiency in becoming cloud droplets. Over the last years the role of surfactants in the activation of atmospheric aerosols into cloud droplets has received a growing interest. However, most of the investigations have focused on mixtures containing only one surfactant, while the composition of atmospheric aerosol is complex. Until now, there was little experimental information on the surface properties of mixtures of surfactants with other aerosol components. In this work pendant droplet tensiometry was used to determine the adsorption isotherms and cmc of aqueous mixtures of amphiphilic surfactants (SDS, Brij35, TritonX100, TritonX114, and CTAC) with inorganic salts (NaCl, (NH₄)₂SO₄) and soluble organic acids (oxalic and glutaric acid). Interestingly, inorganic salts and organic acids systematically enhanced the efficiency of the surfactants by further lowering the surface tension and, in some cases, the CMC. Furthermore, all the mixtures studied were strongly non-ideal, some even displaying some synergism, thus demonstrating that the common assumption of ideality for aerosol mixtures is not valid. The molecular interactions between the mixture components were either in the bulk (salting out), in the mixed surface monolayer (synergy on the surface tension) or in the micelles (synergy on the CMC) and need to be included when describing such aerosol mixtures.

Figure 1: Evolution of the minimal surface tension for mixtures of amphiphilic surfactants and organic acids (left) and two amphiphilic surfactants (right).

How to cite: El Haber, M., Ferronato, C., Fine, L., Girroir-Fendler, A., and Nozière, B.: Salting out, non‑ideality and synergism enhance surfactant efficiency in atmospheric aerosols, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1773, https://doi.org/10.5194/egusphere-egu24-1773, 2024.

Determining number concentrations of cloud condensation nuclei (CCN) is one of the first steps in the chain in analysis of cloud droplet formation, the direct microphysical link between aerosols and cloud droplets, and a process key for aerosol-cloud interactions (ACI). 

Here, we present a new CCN dataset (https://doi.org/10.26050/WDCC/QUAERERE_CCNCAMS_v1) which combines aerosol modeling with observations to better explore magnitude, source, temporal and spatial distribution of CCN numbers. The dataset features 3-D CCN number concentrations of global coverage for various supersaturations and aerosol species covering the years from 2003 to 2021 with daily frequency.

CCN are derived based on aerosol mass mixing ratios from the latest Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA) in a diagnostic model that uses CAMSRA aerosol properties and a simplified kappa-Köhler framework which are suitable for global models. The emitted aerosols in CAMSRA are not only based on input from emission inventories using aerosol observations, they also have a strong tie to satellite-retrieved aerosol optical depth (AOD) as this is assimilated as a constraining factor in the reanalysis. Thus, this dataset is one of its kind as it offers lots of opportunities to be used for evaluation in models and in ACI studies.

We will illustrate the distribution and variability of such derived CCN, evaluate them with observations and have a look at some specific features this dataset provides.

Data description paper (preprint): https://essd.copernicus.org/preprints/essd-2023-172/

How to cite: Block, K., Haghighatnasab, M., Partridge, D. G., Stier, P., and Quaas, J.: Cloud condensation nuclei concentrations derived from the CAMS reanalysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21392, https://doi.org/10.5194/egusphere-egu24-21392, 2024.

Understanding the impact of anthropogenic aerosols on extreme precipitation is of both social and scientific significance. While anthropogenic absorbing aerosols are known to influence Earth's energy balance and atmospheric convection, their role in extreme events remains unclear. This study employs convective-resolving radiative-convective-equilibrium simulations to comprehensively investigate the impact of absorbing aerosols on extreme tropical precipitation. Our findings reveal an underappreciated mechanism whereby absorbing aerosols can, under certain conditions, significantly intensify extreme precipitation despite reducing the mean. Notably, we demonstrate that a mechanism previously observed in much warmer (hothouse) climates—where intense rainfall alternates with multi-day dry spells—can manifest under current realistic conditions due to the influence of absorbing aerosols. This mechanism operates when an aerosol perturbation shifts the lower tropospheric radiative heating rate to positive values, generating a strong inhibition layer. Our work underscores an additional potential effect of absorbing aerosols, with implications for climate change mitigation and disaster risk management.

How to cite: Dagan, G. and Eytan, E.: Absorbing aerosols can strongly enhance extrem precipitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1229, https://doi.org/10.5194/egusphere-egu24-1229, 2024.

The evolution of stratocumulus cloud decks is governed by three timescales: the large-scale evolution of the boundary layer, the mesoscale evolution of liquid water path and cloud fraction, and the microscale processes of cloud microphysics and aerosol-cloud interactions. Our quantitative understanding of aerosol-cloud-climate cooling is especially challenged by the mesoscale response of stratocumulus decks to aerosol perturbations. This response can on the one hand be muted because cloud adjustments partially compensate an initial effect, a feature known as buffering or resilience. On the other hand, stratocumulus may respond by drastic transitions between the closed- and open-cell morphologies or into the shallow-cumulus regime. We will conceptualize this behavior from the perspective of dynamical-systems theory. Our description can be visualized as a quasi-potential landscape. This landscape quantifies the resilience of mesoscale cloud states to perturbations and charts transition pathways. Building on this, we will explore implications for the quantification of adjustments, especially in cloud fraction.

How to cite: Glassmeier, F. and Hernandez, B.: A dynamical-systems perspective on aerosol-stratocumulus interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21400, https://doi.org/10.5194/egusphere-egu24-21400, 2024.

Aerosol-cloud interactions are of central importance for understanding climate processes but remains the largest uncertainty associated with climate change. Hence, the effective radiative forcing (ERF) due to ACI and rapid adjustments (ERF_aci) is still assessed only with medium confidence. An important part of this uncertainty originates from the difficulty of quantifying ACI using observations, especially for ice-containing clouds. In this study, we present a novel Cloud-by-Cloud (CxC) approach for studying ACI in satellite observations that merges properties of individual clouds that have been tracked from geostationary satellite observations with height-resolved concentrations of cloud condensation nuclei (n_CCN) and ice nucleating particles (n_INP) from polar-orbiting lidar data. This approach lays the foundations for better understanding of ACI through a thorough investigation of matched aerosol-cloud cases at cloud level. The methodology is applied to satellite observations over Central Europe and Northern Africa for several years, resulting in a bottom-up dataset of combining parameters that can be stratified accordingly for assessing the impact of changes in cloud-relevant aerosol concentrations on the surrounding quality assured liquid and ice-containing clouds. The first preliminary results of this novel CxC approach are promising and constitute a step forward in the quantification of ERF_aci from space.

How to cite: Alexandri, F., Müller, F., Choudhury, G., Achtert, P., Seelig, T., and Tesche, M.: A cloud-by-cloud approach for studying aerosol-cloud interaction in satellite observations , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11712, https://doi.org/10.5194/egusphere-egu24-11712, 2024.

The estimation of cloud radiative forcing (RFaci), arising from aerosol-cloud interactions (also known as the first indirect effect), relies on approximating cloud albedo susceptibility (β) to changes in droplet concentration. β depends on both cloud albedo and droplet concentration, which are observable through satellite observations. Typically, satellite data is spatially aggregated to coarser resolutions, such as 1 × 1° scenes. However, at these spatial scales, cloud albedo tends to be heterogeneous, while the β approximation assumes homogeneity. This study demonstrates that the common practice of aggregating satellite data and neglecting cloud albedo heterogeneity results in an average overestimation of 10% in previous RFaci estimates.

How to cite: Goren, T., Sourdeval, O., Kretzschmar, J., and Quaas, J.: Spatial Aggregation of Satellite Observations Leads to an Overestimation of the Radiative Forcing due to Aerosol-Cloud Interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12779, https://doi.org/10.5194/egusphere-egu24-12779, 2024.

Aerosol–cloud interactions contribute substantially to uncertainties in anthropogenic forcing, in which the sensitivity of cloud droplet number concentration (N_d) to aerosol plays a central role. Here we use satellite observations to show that the aerosol–N_d relation (in log–log space) is not linear as commonly assumed. Instead, the N_d sensitivity decreases at large aerosol concentrations due to the transition from aerosol-limited to updraft-limited regime, making the widely used linear method problematic. The similar nonlinear behavior is also observed in weekly cycles; specifically, polluted conditions exhibit a reduced amplitude of weekly cycles in N_d compared to clean conditions with similar aerosol perturbations.  A sigmoidal transition is shown to adequately fit the data. When using this revised relationship, the additional warming that arises from air pollution mitigation is delayed by two to three decades in heavily polluted locations, compared to the linear relationship. This cloud-mediated climate penalty will manifest markedly starting around 2025 in China and 2050 in India after applying the strongest air quality policy, underlining the urgency of mitigating greenhouse gas emissions.

How to cite: Jia, H., Quaas, J., and Hasekamp, O.: Nonlinearity of the cloud response postpones climate penalty of mitigating air pollution in polluted regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21153, https://doi.org/10.5194/egusphere-egu24-21153, 2024.

The evolution of cumulus congestus within tropical oceanic and maritime environments is modulated by the interaction of convective dynamics, liquid- and ice-phase microphysical processes, aerosol loading, and entrainment of ambient environmental air. Characterizing this evolution requires robust observational constraints of aerosol properties and cloud macrophysics and microphysics. The NASA Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) field campaign in 2019 targeted growing cumulus congestus clouds using airborne in situ and remote sensing platforms. In situ aircraft microphysical measurements and retrievals from the Research Scanning Polarimeter (RSP) both show that cloud droplet number concentrations decrease and effective radius increases with increasing cloud top height, with droplet size distributions (DSDs) that broaden with height. These observed components are responsive to an active collision-coalescence process that produce millimeter-sized drops, onsetting warm-rain formation. Here, we present an analysis of CAMP2Ex RSP data showing the evolution of droplet number concentrations and DSDs with height and its variation with RSP-retrieved aerosol number concentrations. For one case study (RF14, 9/25/2019), we perform large eddy simulations (LES) at 100-m horizontal grid spacing using bin and bulk microphysics schemes. Detailed multi-modal, vertically resolved aerosol measurements from the Fast Integrated Mobility Spectrometer (FIMS) are used as input. The relative ability of the bulk and bin schemes to produce the observed DSD evolution, from activation to warm-rain production, is evaluated. Sensitivity experiments are performed to assess the roles of height-varying aerosol concentrations, rain-forming collision-coalescence, and entrainment in realizing observed droplet number concentration and effective radius profiles. Additionally, we share the prospect of detailed aerosol properties, droplet number concentrations, and DSDs, similar to as acquired by RSP, becoming available from polarimeters on NASA’s PACE satellite mission, launched in early 2024.

How to cite: van Diedenhoven, B., Stanford, M., Frindlind, A., Ackerman, A., Xiao, Q., Wang, J., Hasekamp, O., Stamnes, S., Cairns, B., Wasilewski, A., and Alexandrov, M.: Evaluating Droplet Size Distribution Evolution in Aerosol-constrained Simulations of Tropical Cumulus Congestus against Airborne Polarimetry Observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12965, https://doi.org/10.5194/egusphere-egu24-12965, 2024.

Fog can substantially impact air traffic by inhibiting or aggravating take-off or landing. This can result in large economical costs and even loss of human lives. In this study we investigate how fog frequency has changed at Swedish airports over time. The large north-south extent of Sweden with strong gradients in aerosol concentrations makes it an interesting study area for aerosol impact on fog. We base the study on visibility data from 14 airports. Most visibility data from the airports start in the 1970s but some stations have data before this and some have measurements that start in the 1980s. The visibility measurements are combined with data on air temperature, wind speed, wind direction, and air pressure from the airports. In the study we also include measurements aerosol proxies, namely SO₂ in the air from 4 stations and SO₄^2- in rainwater from 10 stations in Sweden. Moreover, emission data of SO₂ from Europe from 1970 to present day has been analysed. 
The analysis shows that the fog frequency changes in Sweden vary with location of the airport. At four airports in southern Sweden, the fog frequency show a statistically significant decrease when comparing the periods before and after 1995. The most prominent changes has occurred at the airports close to Malmö and Gothenburg. The annual fog frequency for these stations changes from 5-6 % in the 1980s to 3-4 % in the 2010s with higher changes during winter. For the airports located further north there is no decrease in fog frequency. Some airports in the northern part of Sweden show a statistically significant increase in fog frequency, though the fog frequencies are lower there than in southern Sweden. 
We find that the fog frequency changes in Southern Sweden correlate well with changes in air of SO₂, rainwater SO₄^2- concentrations, which both show strong decreases since the 1970 and 1980s in southern Sweden. The changes in these concentrations are much weaker further north in Sweden. The fog frequency changes in southern Sweden thus seems to be driven by changes in the load of hygroscopic aerosol. The fog changes in northern Sweden correlate well with temperature, which is increasing at all airports. The rising temperatures in the north could contribute with more favorable conditions for fog formation at these airports where it previously was to cold for fog formation during parts of the year. Out results indicate that the work on air pollution mitigation in Europe over the past 50 years has reduced fog impact on air traffic in southern Sweden.    

How to cite: Sporre, M. K., Hartman, L., Singh, S., and Friberg, J.: Long term changes in fog frequency at Swedish airports and its potential drivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1828, https://doi.org/10.5194/egusphere-egu24-1828, 2024.

Anthropogenic aerosols and their interactions with clouds play a crucial role in regulating the Earth's radiation balance and introduce significant uncertainties in climate change projection. The effective radiative forcing due to aerosol-cloud interactions (ERFaci) is particularly difficult to quantify, leading to uncertainties in model projections of cloud feedback and climate sensitivity. Analysis of CMIP6 model simulations indicate that models with a strongly-positive cloud feedback tend to be offset with strongly negative ACI, leading to similar projections of global mean temperatures during the historical period. However, because anthropogenic aerosol primarily occur in the Northern Hemisphere, the hemispheric asymmetry in warming (NH-SH) differs significantly between low and high ACI models, with observed trends being more consistent with low ACI (weak cloud feedback) models. However, recent satellite estimates of ERFaci based on cloud controlling factors (CCF) is more consistent with high ACI models.  We evaluate the CCF approach using a series of perfect model experiments. The magnitude of ERFaci depends on two factors: the amount of aerosol loading between the pre-industrial and present day, and the susceptibility of cloud albedo and cloud lifetime to that aerosol loading. By comparing observationally-constrained estimates of ERFaci with CMIP6 model simulations, we quantify the contributions of aerosol loading differences and cloud susceptibility to the inter-model spread. We find that explicitly accounting for the role of aerosol activation on cloud droplet formation is essential to obtaining accurate estimates of ERFaci, and when this is done, the satellite constrained estimates of ERFaci are more consistent with low ACI models.

How to cite: soden, B. and park, C.: Reconciling Top-Down and Bottom-Up Estimates of the Effective Radiative Forcing from Aerosol-Cloud Interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3278, https://doi.org/10.5194/egusphere-egu24-3278, 2024.

Determining the susceptibilities of cloud properties to perturbations in aerosols has been a persisting challenge in climate research. For example, satellite-retrieved susceptibility of cloud droplet number concentration (CDNC) to changes in cloud condensation nuclei (CCN) varies regionally, also having opposite correlations over land and ocean. Over the oceans, the correlation between CCN and CDNC is positive and in many cases, with a proper regression method, the dlogCDNC/dlogCCN exceeds 1. On the other hand, over land, many studies have found a negative correlation. As our preliminary global climate model simulations give qualitatively similar results to satellite retrievals, we have used the model together with reanalysis data of aerosol, meteorological properties, and cloud properties to interpret how other parameters such as cloud activation updrafts and vertical mixing of aerosol affect the satellite derived CCN vs CDNC correlations. In this study, we focus on ocean regions determining how these different atmospheric properties affect the derived slope between CCN and CDNC. Our results indicate that as satellite derived CCN is a columnar value, does not properly represent the true variability of cloud base CCN. Thus, the mixing of aerosol as well as cloud activation updrafts cause biases in the satellite determined CCN vs CDNC correlations.

How to cite: Kokkola, H., Irfan, M., Lipponen, A., Calderon, S., and Arola, A.: Using a global aerosol model to interpret satellite-derived aerosol-cloud interactions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15150, https://doi.org/10.5194/egusphere-egu24-15150, 2024.

This study employed the WRF-Chem-SBM model which couples spectral-bin cloud microphysics (SBM) and online aerosol module (MOSAIC) to investigate aerosol-cloud interactions in liquid-phase clouds over eastern China and its adjacent oceans. The results indicate that with an increase in aerosol number concentration (N_a), cloud droplet number concentration (N_d) exhibits a trend of initially increasing and then decreasing, both over land and ocean. The difference lies in the stronger convective and land surface effects over land, leading to more intense activation, while over the ocean, weaker supersaturation and richer water vapor content result in weaker activation but more favorable conditions for cloud droplet growth. Cloud processes over land are more intense than over the ocean, but the cloud liquid water content (CLWC) in both regions shows a similar trend with the variation of N_d. In precipitating clouds with richer water content and stronger intracloud processes, as N_d increases, the cloud droplet effective radius decreases, and CLWC exhibits a gradual increase followed by a rapid decrease. In non-precipitating clouds with lower water content and weaker intracloud processes, the increase in N_d leads to a more gradual growth of CLWC, and the subsequent decrease in CLWC is also more subtle. Furthermore, this study discusses the impact of meteorological and aerosol conditions on aerosol activation and cloud development. Environments with a moderate N_a are more conducive to aerosol activation, while in environments with low to moderate N_a, CLWC exhibits faster growth. Compared to humidification, cooling has a more significant effect on aerosol activation and CLWC growth.

How to cite: Zhao, J., Ma, X., Quaas, J., and Jia, H.: Exploring aerosol-cloud interactions in liquid-phase clouds over eastern China and its adjacent ocean using the WRF-Chem-SBM model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9292, https://doi.org/10.5194/egusphere-egu24-9292, 2024.

The important role of warm clouds in regulating the regional energy balance and ocean temperature, that are directly linked to the thermal coral bleaching events, has been increasingly recognised over the Great Barrier Reef (GBR). These shallow clouds, however, are by their nature sensitive to perturbations in both their thermodynamic environment and microphysical background. In this study, we employ the Weather Research and Forecasting (WRF) model with a convection-permitting configuration at 1 km resolution to examine the interactions between the warm clouds and different local forcings over the GBR. A range of local forcings including local aerosol loading, coastal topography, and sea surface temperature (SST) is examined.

Our simulations show a strong response of cloud microphysical properties, including cloud droplet number concentration (CDNC), liquid water path (LWP), and precipitation to the changes in atmospheric aerosol population over the GBR. Higher CDNC and LWP correlated to increased aerosol number concentration leads to a rise in shortwave cloud radiative effect, though the magnitude is small, over both the mountains and upwind over the GBR. While cloud fraction shows little responses, a slight deepening of the simulated clouds is evident over the upwind region in correspondence to the increased aerosol number concentration. A downwind effect of aerosol loading on simulated cloud and precipitation properties is further noted. In consideration of the coastal topography, cloud fraction and accumulated precipitation are strongly sensitive to orographic forcing over the GBR. Orographic lifting and low-level convergence are found to be crucial in explaining the cloud and precipitation features over the coastal mountains downwind of the GBR. However, clouds over the upwind ocean are more strongly constrained by the trade wind inversion, whose properties are, in part, regulated by the coastal topography. Finally, on the scales considered in our study, the warm cloud fraction and the ensuant precipitation over the GBR show only a small response to the local SST forcing, with this response being tied to the simulated cloud type.

How to cite: Zhao, W., Huang, Y., Siems, S., Manton, M., and Harrison, D.: Interactions of warm cloud, precipitation, and local forcings over the Great Barrier Reef: Insights from convection-permitting simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2062, https://doi.org/10.5194/egusphere-egu24-2062, 2024.

The role of aerosol and aerosol-cloud interactions (ACI) in the Earth system is a major source of uncertainty in projections of Earth’s future climate and in interpreting how the climate has evolved in the past. The “Enabling Aerosol-cloud interactions at GLobal convection-permitting scalES (EAGLES)” project aims to achieve unprecedented realism in predictions of aerosol and ACI in the next-generation Earth system models. The effort includes improving the representation of aerosol and ACI processes with physics-based or data-driven methods, readying the parameterizations for kilometer-scale simulations, and constraining the model using process-oriented diagnostics based on both satellite and in-situ measurements. By combining process models, large-eddy simulations, and observational data from ARM, satellites, and other sources, stubborn model biases associated with resolution and physics have been addressed accordingly. We demonstrate that improved atmospheric simulations can be achieved with better physics, better integration with data, and better software. We also identify new opportunities for improvements.

How to cite: Ma, P.-L.: Enabling Aerosol-cloud interactions at GLobal convection-permitting scalES (EAGLES), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12164, https://doi.org/10.5194/egusphere-egu24-12164, 2024.

Aerosol effects on convective clouds and climate mediated via radiative and microphysical perturbations remain highly uncertain. Microphysical perturbations are generally not included in current climate models due to the simplified representation of convective clouds in existing parameterisations. Progress has been made through regional cloud resolving modelling, however such simulations often neglect energy and water budget constraints and the coupling to larger scales.

The emergence of global km-scale climate models provides a significant opportunity to advance our understanding of aerosol-convection interactions. Here we present results from a hierarchy of global km-scale atmospheric model simulations using ICON, investigating aerosol effects on convective clouds. Idealised model simulations, in which aerosols are prescribed as fixed plumes of radiative properties, with an optional associated semi-empirical scaling of droplet number perturbations, provide fascinating insights into the physical processes underlying aerosol effects on convection and into the interaction of local perturbations with the larger scale dynamics – but neglect key aerosol-convection interactions. These simulations highlight the importance of the radiatively mediated pathway for tropical convective clouds, with significant impacts on the diurnal cycle of cloud properties and precipitation over the Amazon and the Congo basin – and interactions with the large-scale dynamics for perturbations over the Pacific warm pool region.

We contrast our results from idealised simulations with simulations including explicit aerosols, enabled by a novel reduced complexity aerosol scheme suitable for global km-scale models, HAM-Lite. Comparison of the idealised simulations with prescribed aerosol perturbations and the simulations with explicit aerosols, provides new insights into the complexity of aerosol-convection interactions. This study provides a testbed for a future global km-scale model intercomparison project focusing on aerosol effects as part of the GEWEX Aerosol Precipitation (GAP) initiative.

How to cite: Stier, P., Weiss, P., Herbert, R., and Sela, M.: Aerosol effects on convective clouds in global km-scale models – from idealised aerosol perturbations to explicit aerosol modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20572, https://doi.org/10.5194/egusphere-egu24-20572, 2024.

Aerosol particles can affect the formation and properties of clouds by acting as cloud condensation nuclei (CCN) and ice nucleating particles (INP). The accurate representation of aerosol size distribution and composition along with cloud nucleating properties play an important role in describing aerosol-cloud interactions. The Deep Convective Microphysics Experiment (DCMEX) is a project aimed at improving the representation of microphysical processes in deep convective clouds. The DCMEX campaign (July to Aug 2022) was conducted using the UK FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 Atmospheric Research Aircraft and characterized the aerosol-cloud system over the isolated Magdalena Mountain region in New Mexico. The aircraft was equipped with a range of online instruments to measure aerosol chemical composition (i.e., Aerosol Mass Spectrometry, AMS; Laser Ablation Aerosol Particle Time of Flight mass spectrometry, LAAPToF) and aerosol size distributions, as well as cloud microphysics.

A 6-days backward dispersion analysis of this region shows that the air source flow transferred from Northwest (NW, California coast) to Southeast (SE, Gulf of Mexico) during the campaign period. This air mass source change coincided with changes in meteorological parameters including such as enhancement of convection available potential energy (CAPE), decreased cloud-base height, and increased boundary layer humidity. The aerosol size distribution and chemical composition in out-of-cloud runs also show variations under different air mass source conditions. Larger sulphate and lower organic contributions were observed in the sub-micron (<1 μm) aerosol mass fraction in the SE airflow when compared to flow from the NW, with the organic components more oxidized. The LAAPToF single particle measurements (0.5-2.5 μm) indicate more aged sea salt in number fraction within the SE ocean flow. The calculated kappa values suggest more hygroscopic aerosols with the source transfer. Number size distributions indicate enhanced Aiken-mode particles when the air mass source changed.

A bin-microphysics model was employed to simulate the warm cloud development in this convective system. The simulation results show that both the change of aerosol characteristics and cloud-base conditions affect the warm cloud development, which follow the trends seen in the cloud microphysics observations. Initial cloud base conditions (i.e., initial temperature and relative humidity) mainly affected cloud properties by altering the water mixing ratios while aerosol characteristics mainly affected the initial cloud droplet number concentrations.

Next, we will combine these online aerosol measurements with detailed cloud microphysical measurements and offline INP analysis, to investigate the contributory effect of aerosols on primary ice formation in this deep-convection system and their relationship to secondary ice production processes.

How to cite: Wu, H., Marsden, N., Connolly, P., Flynn, M., Williams, P., Nott, G., Hu, K., Finney, D., Thamban, N., Bower, K., Blyth, A., Choularton, T., Gallagher, M., and Coe, H.: Observations of ambient aerosol and warm cloud formation in a New Mexico summer deep-convection system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6816, https://doi.org/10.5194/egusphere-egu24-6816, 2024.

Aerosol-cloud interactions remain among the most uncertain key parameters in the fast-changing Arctic climate system. Arctic clouds often consist of both liquid droplets and ice crystals, the abundance of which is constrained by the availability of ice nucleating particles (INP). We present observations of fluorescent primary biological aerosol particles (fPBAP), shown to be potent INP, obtained during the Arctic Ocean 2018 expedition onboard the Swedish icebreaker Oden in August- September of 2018, at the North Pole. The fPBAP were recorded on a single-particle level using a Multiparameter Bioaerosol Spectrometer, as a part of a complete setup for measuring physical and chemical aerosol properties.  Potential sources of fPBAP during an extended period of high concentrations are investigated using a combination of auxiliary measurements, trajectory analysis, remote sensing data, ocean biogeochemistry reanalysis data, and model experiments with WRF-Chem. Our evidence suggests that the observed case of increased fPBAP concentration at the North Pole was caused by transport of fPBAP enriched marine aerosol from a source within the Arctic region, but in open water south of the pack ice. We also highlight how future interdisciplinary efforts can be used more efficiently to improve the source mapping of Arctic fPBAP, which is needed to assess their overall climate-relevance in the polar regions.

How to cite: Asplund, J., Ekman, A. M., Freitas, G., Granskog, M. A., Heutte, B., Lapere, R., Muilwijk, M., Naakka, T., Schmale, J., Thomas, J., and Zieger, P.: Observation of fluorescent primary biological particles at the North Pole: A case of inter-coupled system behaviour?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16865, https://doi.org/10.5194/egusphere-egu24-16865, 2024.

Effusive volcanic eruptions are known to impact climate through the emission of sulphur dioxide and subsequent formation of sulphate aerosols. These aerosols affect radiative transfer in the atmosphere, both directly by scattering sunlight and indirectly through aerosol-cloud interactions. By scattering sunlight, the direct aerosol effect leads to surface cooling. Changes in cloud properties as a result aerosol-cloud interactions, on the other hand, lead to both reflection of sunlight and trapping of outgoing thermal emissions from the ground. Clouds, therefore, have the potential to either cause surface warming or cooling, depending on factors such as the cloud response to the volcanic aerosols and the availability of sunlight.

We perform a series of simulations using the Community Earth System Model with the Community Atmosphere Model (CESM2-CAM6) to simulate the climate impacts of northern hemisphere, high latitude, effusive volcanic eruptions. We construct a standard eruption scenario, using the 2014-15 Holuhraun eruption in Iceland as a reference. The Holuhraun eruption released up to 9.6 Tg SO₂ over a period of six months, from September 2014 to February 2015, with the emission rate gradually decreasing over time. We apply several different magnitude scalings to this standard scenario and vary the timing of the eruption. This allows us to analyse the climate response as a function of both the eruption size and season.

For eruptions starting in winter, we find significant surface warming in our simulations as a result of trapping of outgoing thermal emissions in the absence of sunlight. This warming is mainly confined to the Arctic but also appears over parts of northern Eurasia and North-America, albeit to a lesser extent. This is consistent with our previous work on the Holuhraun eruption where we found evidence for winter surface warming over the Greenland Sea as a result of that eruption, both in model simulations and observations.

Conversely, we find surface cooling during summertime eruptions. The spatial distribution of the cooling pattern is different from the winter warming as the cooling predominantly occurs over the Eurasian and North-American continents and is hardly visible in the Arctic. Furthermore, based on preliminary results, whereas the Arctic winter warming is mainly due to aerosol-cloud interactions, the continental summer cooling stems mostly from the direct aerosol effect. Our results indicate a non-linear relationship between the surface air temperature response and the eruption size.

How to cite: Zoëga, T., Storelvmo, T., and Krüger, K.: Surface climate response to the size and season of northern hemisphere, high latitude, effusive volcanic eruptions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11095, https://doi.org/10.5194/egusphere-egu24-11095, 2024.

The Arctic is currently warming rapidly, at a rate four times higher than the global average. This warming has significant consequences, leading to increased precipitation in the Arctic. Aerosols play a crucial role in cloud formation, cloud condensation nuclei (CCNs) and ice-nucleating particles (INPs), influencing rain and snowfall. However, uncertainties remain in the modelling of aerosols and their impact on precipitation due to a lack of high-resolution spatio-temporal observations. This is particularly the case in the central Arctic cryosphere due to the presence of extensive cold, bright snow and ice surfaces coupled with widespread cloud cover.

This study addresses the observational data gap and provides an opportunity to refine model simulations at different spatio-temporal scales. We achieve this by using total aerosol optical depth (AOD) datasets generated by the AEROSNOW algorithm over the extensive central Arctic cryosphere. AEROSNOW retrieves AOD data using top-of-atmosphere reflectance measurements obtained through the Advanced Along-Track Scanning Radiometer (AATSR) aboard the ENVISAT satellite, spanning from 2003 to 2011. AEROSNOW integrates an aerosol retrieval algorithm with a rigorous cloud masking scheme and intro-
duces a novel quality flagging methodology tailored for the central Arctic region (≥ 72°N).

Using the AEROSNOW retrieved dataset for the central Arctic, we evaluate different models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Our results show significant differences in the spatio-temporal aerosol load and its annual and seasonal variations with precipitation. In particular, there is a decrease in aerosol loading that coincides with increased precipitation along the northern periphery of Alaska and the Bering Sea.

Significant discrepancies and variations of up to 6.2 mm/day in precipitation are observed between models, with higher aerosol loading leading to lower precipitation and vice versa. Furthermore, the spatially averaged multi-model mean overestimates aerosol concentrations in spring and underestimates them in summer compared to satellite observations. The CMIP6 models do not reproduce the seasonal variations in aerosol distribution seen with AEROSNOW, particularly an increase in aerosol loading during the summer coinciding with the sea ice retreat cycle. These discrepancies may be due to the lack of advanced natural aerosol formation mechanisms in the models, as a consequence of Arctic warming, and exposure to open
ocean emissions.

In summary, our study has led us to speculate that as model sophistication increases, modelled aerosol processes become increasingly uncertain. Ultimately, this investigation has the potential to elucidate the critical link between aerosols and the prevailing rain-dominated Arctic conditions under ongoing Arctic warming in future CMIP projects.

How to cite: Swain, B., Marco, V., Adrien, D., Luca, L., Sachin S., G., Hartmut, B., and John P., B.: Assessing Model simulation for central Arctic aerosol load by usingAEROSNOW dataset: Relevance for precipitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19084, https://doi.org/10.5194/egusphere-egu24-19084, 2024.

Arctic clouds are still poorly represented in climate models. An important reason for this is our lack of knowledge regarding the various sources of natural aerosol in the high Arctic. Recent field campaigns have provided evidence that over sea ice blowing snow can act as a source of sea salt aerosol (SSA). This source can account for the maximum in SSA that occurs in the Polar Regions during winter and spring. SSA can influence the regional climate through the indirect radiative effect, but also through the role it plays as nucleation particle in cloud formation. Its contribution to and potential as ice nucleating particle (INP) is still largely unknown though. 

 Here we will present offline samples of airborne aerosol taken in the Central Arctic during MOSAiC focussing on the transition period from winter to spring. The samples comprise of quasi-ciontinuous low-volume air filter samples taken in the British Antarctic Survey’s aerosol lab container on board of RV Polarstern, weekly snow samples from the ice floe, and filter samples taken by tethered balloon. These samples were analysed for their ice nucleating characteristics using a peltier cold stage and applying a machine learning algorithm to the images taken during the cooling process.

Initial results confirm an increased presence of INP in both the airborne and snow samples at the turn from winter to spring. 

How to cite: Kirchgaessner, A., Frey, M., van den Heuvel, F., Lachlan-Cope, T., Ranjithkumar, A., and Yang, X.: Ice nucleating properties of air filter and snow samples taken in the Central Arctic during MOSAiC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17179, https://doi.org/10.5194/egusphere-egu24-17179, 2024.

Concentrations of ice-nucleating particles (INPs) were measured in springtime cold-air outbreaks over the Norwegian and Barents Seas using filter samples taken on board the FAAM BAe-146 aircraft. These measurements of INP concentrations were comparable to the highest INP concentrations previously observed in the Arctic and were similar to typical terrestrial midlatitude INP concentrations. This is important because shallow cloud systems such as those in mid- to high-latitude cold-air outbreaks are highly sensitive to INPs and are a highly uncertain contributor to cloud feedbacks. 

To investigate the types of aerosol responsible for this high INP concentration, we used aerosol-size data from underwing optical probes to derive an active site density of the INP samples. By comparing to laboratory derived active site densities of different aerosol types, this suggested that sea spray was unlikely to be a dominant INP type and that there were likely to be strong biological and dust components to the INP population. Scanning electron microscopy with energy-dispersive spectroscopy used on selected filters revealed that sub-micron particles were dominantly sulphates and carbonaceous, while super-micron particles were dominantly mineral dust.

Samples taken above the cloud decks had greater active site densities than those below, and back-trajectory analysis and meteorological conditions suggested a lack of obvious local INP sources. We hypothesise that the high INP concentration is most likely to be associated with aged aerosol that has accumulated over the Arctic (Arctic Haze).  These high INP concentrations imply that these clouds may have a more negative cloud-phase feedback than their Southern Ocean equivalents.

How to cite: Raif, E., Barr, S., Tarn, M., McQuaid, J., Daily, M., Abel, S., Barrett, P., Bower, K., Field, P., Carslaw, K., and Murray, B.: Ice-nucleating particles in springtime cold-air outbreaks associated with Arctic haze, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4358, https://doi.org/10.5194/egusphere-egu24-4358, 2024.

The Southern Ocean is a key component of the climate system, where clouds especially matter. Therefore, it is important to correctly simulate clouds in climate models. Even though there has been substantial improvement, climate models still struggle in their representation of cloud microphysical properties.

In this study, based on data from the Antarctic Circumnavigation Expedition in 2026/17, we explore environmental factors, such as stable water isotopes in atmospheric water vapor, cyclones and boundary layer stability, that influence the abundance of aerosols and their size distribution, the most important variables for particles to act as cloud condensation nuclei (CCN), along a latitudinal gradient from 35°S to 75°S. Moreover, we use a cloud parcel model to estimate the cloud droplet number concentration and cloud maximum supersaturation (SS) based on the particles’ size distribution, hygroscopicity and measured updraft velocities.

Based on the latitudinal gradient of observed CCN, which features a distinct minimum around 60°S, and the carbon monoxide mixing ratios, which reach background levels south of 60°S indicating absence of anthropogenic influence, we compare aerosol properties north and south of this latitude. The northern aerosol population features two distinct Aitken modes, a nucleation mode and a mode with a Hoppel minimum around 60 nm. The presence of cyclones reduces the particle number concentrations over all diameters. We also observe a stronger Aitken mode presence in unstable boundary layer conditions, where downward mixing of freshly formed particles in the outflow of clouds in the free troposphere can occur. The southern population features only three modes, a nucleation mode and two distinct bimodal distributions with Hoppel minima around 70 nm. Only in stable boundary layer conditions an Aitken mode emerges in the 75^th percentile that is larger in particle number than the accumulation mode, pointing towards a potential source of condensable vapors from the ocean surface that grow the Aitken mode, leading to observably higher kappa values. The Aitken mode is further associated with air masses with relatively less depletion in d¹⁸O, pointing towards a marine source further north.

The cloud droplet number concentration simulations feature the same latitudinal pattern as the measured CCN with the “dip” around 60°S. This is consistent with droplet observations from satellites. Interestingly, the simulated cloud maximum SS tends to increase with latitude, from roughly 0.27% at 40°S towards 0.43% at 75°S. To estimate the sensitivity of clouds towards available aerosol particles, we form the ratio of the particle number concentration larger than the observed Hoppel minimum over the simulated cloud droplet number concentrations. We find that clouds north and south of 60°S experience elevated sensitivity (ratio < 1) to aerosol concentrations in 23 % and 27 % of the time, respectively. This demonstrates that the Southern Ocean cloud regime is indeed sensitive to aerosol number and size distributions, which in turn are influenced by synoptic features (e.g., cyclones) and marine boundary layer stability. On the other hand, frequent occurrence of low SS, demonstrates that cloud formation is also often updraft limited.

How to cite: Schmale, J., Nenes, A., Thurnherr, I., Henning, S., Tatzelt, C., Baccarini, A., and Gysel-Beer, M.: Aerosol size distribution variability over the Southern Ocean: implications for cloud droplet number concentrations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5460, https://doi.org/10.5194/egusphere-egu24-5460, 2024.

Atmospheric composition plays an important role in present and near-future climate change. Airborne particles exert direct and indirect radiative impacts and can serve as cloud condensation and ice nuclei, having therefore a strong influence on cloud formation and precipitation. Furthermore, a detailed understanding of present-day atmospheric transport pathways of particles from source to deposition in Antarctica remains essential.

Since 2010, the aerosol total number and size distribution, aerosol absorption coefficient and mass concentration of light-absorbing aerosols and the aerosol total scattering coefficient have been monitored at the Belgian research station Princess Elisabeth Antarctica (PEA). The station is situated in Dronning Maud Land, East Antarctica (71.95° S, 23.35° E, 1390 m asl). Besides these instruments, a cloud condensation nuclei counter was operated during three austral summers. Meteorological data come from an automatic weather station. In this work, we investigate the climatology of the particle properties with respect to the air mass origin. To that end, we used the FLEXTRA trajectory model to investigate transport pathways into Antarctica. The model was driven with ECMWF ERA-5 meteorological fields. 10-days 3D backward trajectories, starting from PEA, were calculated for the period 01/01/2010 to 31/12/2020, in 3-hour-intervals. A k-means cluster analysis has been done based on latitude, longitude and altitude, resulting in four clusters of air mass origin.

We will present results for the climatology of particle properties and the air mass origin. In addition, the backward trajectories have been combined with measured atmospheric particle properties and parameters like potential vorticity and exposure to sunshine duration, showing the distribution of the measured atmospheric particle properties between and within the air mass origin clusters. Some distinct features could be seen in the air mass origin clustering. Source regions from South America, Southern Africa and Australia, New Zealand were limited and the Southern Ocean was a main source region, as was the Antarctic continent itself. For each season, the dominating cluster represented mainly air masses of Antarctic continental origin with a large influence of upper tropospheric air. We will show further results of our analysis on air mass origin and atmospheric and particle properties, with respect to differentiations between seasons, clusters, continental and maritime origin and source altitude compartments.

How to cite: Mangold, A., De Causmaecker, K., Laffineur, Q., Van Overmeiren, P., Deramaix, C., Walgraeve, C., Mattielli, N., and Delcloo, A.: Atmospheric aerosol characterization at Princess Elisabeth station, East Antarctica and identifying source regions using backward trajectory modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17439, https://doi.org/10.5194/egusphere-egu24-17439, 2024.