Secondary ice formation has long been a problem in cloud physics. This affects the radiation properties, precipitation development and the lifetime of mixed-phase clouds.  We conducted multiple flights over the Magdalena Mountain region in New Mexico to provide high-resolution information on the spatio-temporal distribution of ice phase evolution and the linkage between convective cloud thermodynamic and secondary ice processes. A combination of high-resolution cloud spectrometers (including 3VCPI, 2DS, HVPS, and CDP) were used to provide measurements of the evolution of cloud particle and precipitation concentrations, sizes, and morphology. Those data were used to identify and assess primary and secondary ice production (SIP) contributions compared with measured INP concentrations to characterise the frequency of SIP events, where precipitation particles first form and how they interact with cloud dynamics. The initial results suggest that most ice enhancement events in these clouds occurred in the temperature range of -5 °C to -10 °C, while occasionally even larger concentrations were observed between -22.5 °C and -25 °C. The results also show that observed secondary ice in the temperature range from -25 °C to -30 °C was more related to the updraft regions. The next step is to produce more detailed explanations and results by examining these data in conjunction with the cloud thermodynamic background.

How to cite: Hu, K., Lloyd, G., Wu, H., Bower, K., Flynn, M., Marsden, N., Choularton, T., Daily, M., Murray, B., Coe, H., Connolly, P., Nott, G., Reed, C., Schledewitz, W., Gallagher, M., and Blyth, A.: Overview of Secondary Ice Production In the Deep Convective Microphysics Experiment (DCMEX), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1493, https://doi.org/10.5194/egusphere-egu24-1493, 2024.

The response of tropical high clouds to global warming has the potential to produce an important climate feedback but remains poorly constrained. To improve our understanding of the tropical high-cloud feedback, we develop a conceptual model of the high-cloud radiative effect as a function of the ice water path (IWP) and surface temperature. This model provides a framework for analysing how changes in IWP distribution and cloud top height with surface warming can generate a tropical high-cloud feedback. By including the entire IWP range, it improves on previous conceptual models that rely on cloud fractions. To parameterize our conceptual model, we use atmospheric profiles from global simulations with the ICOsahedral Nonhydrostatic weather and climate model (ICON) with 5 km horizontal resolution, which are used to calculate the radiative fluxes offline with the line-by-line Atmospheric Radiative Transfer Simulator (ARTS). This setup allows us to “switch off” the high clouds in the radiative transfer calculations to better study the radiative effect of high clouds over low clouds. Our conceptual model represents the main physical processes underlying the high-cloud radiative effect and is able to reproduce the results from the ARTS simulations. It therefore provides a valuable framework for analysing the tropical high-cloud feedback produced by climate models and helps to understand the origin of the associated uncertainties.

How to cite: Deutloff, J., Naumann, A. K., Brath, M., and Buehler, S.: Understanding the tropical high-cloud feedback through the ice-water-path lens, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1580, https://doi.org/10.5194/egusphere-egu24-1580, 2024.

Clouds strongly modulate the top-of-the-atmosphere (TOA) energy budget. While most evidence indicates that changes in cloud-induced radiative anomalies at the TOA likely amplifies warming, the magnitude of this global cloud feedback remains highly uncertain. “Cloud Controlling Factor” (CCF) analysis is an approach that can be used to tackle this uncertainty, deriving relationships between large-scale meteorological drivers and cloud-radiative anomalies which can subsequently be used to constrain cloud feedback. However, the choice of meteorological controlling factors is crucial for a meaningful constraint, and while there is rich literature investigating ideal CCF setups for low-level clouds, there is a distinct lack of analogous research that explicitly targets high clouds.

Here, we use ridge regression to systematically evaluate CCFs that specifically target high cloud formation and cessation using historical data. We evaluate the addition of five candidate CCFs to previously established core CCFs within large spatial domains to predict longwave high-cloud radiative anomalies: upper-tropospheric static stability (S_UT), sub-cloud moist static energy, convective available potential energy, convective inhibition, and upper-tropospheric wind shear. We identify an optimal configuration including S_UT, and show that the spatial distribution of the  S_UT  ridge regression coefficients are congruent with the physical drivers of known high-cloud feedbacks. We further deduce that inclusion of S_UT into observational constraint frameworks may reduce uncertainty associated with changes in anvil cloud amount as a function of climate change. These results highlight upper-tropospheric static stability as an important CCF for high clouds and longwave cloud feedback, which we begin to explore using modelled data under an abrupt quadrupling of CO₂ (abrupt-4xCO₂).

How to cite: Wilson Kemsley, S., Nowack, P., and Ceppi, P.: A systematic evaluation of high-cloud controlling factors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1986, https://doi.org/10.5194/egusphere-egu24-1986, 2024.

In-cloud measurements of ice crystal concentrations often greatly exceed ice nucleating particle (INP) concentrations. This discrepancy can be accounted for by secondary ice production (SIP), describing mechanisms producing new ice crystals from the presence of existing primary ice particles. As ice particle formation in clouds strongly influences precipitation and earth’s radiative balance, accurate representation of SIP processes is critical for global climate and weather prediction model simulations. However, the dominant SIP mechanisms operating in different cloud systems remain poorly understood. This study aims to improve understanding of SIP within mixed-phase clouds associated with cold air outbreaks (CAOs). We examine in-situ ice particle and ice-nucleating particle measurements made in October-November 2022, using the UK FAAM BAE 146 research aircraft, during a set of CAOs in the North-western Atlantic over the Labrador Sea. This flight campaign comprised part of the M-PHASE project, part of the NERC-funded CloudSense programme, which aims to reduce uncertainties in climate sensitivity due to clouds. Detailed measurements of cloud microphysical properties were made to study the evolution of stratocumulus clouds as they advect southwards before breaking up under increasingly convective conditions.

In-cloud ice crystal concentrations measured with 2D-S (size range 10 - 1280 μm) and HVPS (size range 150 μm - 19.2 mm) optical array probes frequently exceeded INP concentrations measured at the same temperature. Peak ice particle concentrations greater than 200 L^-1 were recorded on numerous flights, several orders of magnitude above INP concentrations. These ice concentration enhancements were observed between -5 and -10 ^oC, within the active temperature range for the Hallett-Mossop SIP process. Analysis of corresponding ice particle imagery from the 2D-S and Cloud Particle Imager instruments shows that small hollow columns, often mixed with larger heavily rimed particles, were the dominant ice crystal habits, providing further evidence of rime splintering. A second ice concentration peak at around -17^oC was also observed. Large irregularly shaped ice crystals were present during this period, suggesting that fragmentation due to ice-ice collisions may be another active SIP mechanism.

We identify a series of SIP events across the flight campaign, with their short-lived nature suggesting ice multiplication is active across limited spatial extents. These segments of elevated ice concentrations are heavily populated by ice crystals of diameter < 100μm. Overall, SIP is observed to increase across convective regions of the CAO, with stratocumulus regions upwind often consisting mainly of supercooled water.

This work provides critical information for numerical modelling studies requiring detailed representation of SIP processes within mixed-phase clouds across the transition region from stratocumulus to convective regimes in CAOs. 

How to cite: Biggart, M., Choularton, T., Gallagher, M., Bower, K., Lloyd, G., and Murray, B.: Secondary ice production within mixed-phase clouds in cold air outbreaks over the North Atlantic. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5449, https://doi.org/10.5194/egusphere-egu24-5449, 2024.

Climate sensitivity changes over time in numerical global climate models (GCMs) due to a so-called “pattern effect”. That is, surface-warming patterns evolve over time to favour different geographical regions giving rise to different climate feedbacks, thus changing climate sensitivity over time.

One of the most important climate feedbacks is the cloud feedback and it has been shown that the pattern effect may strongly impact the strength of this feedback in GCMs. Here we perform slab-ocean model simulations with different versions of the Community Earth System Model (CESM). Different patterns of ocean heat transport convergence (Q-flux) are prescribed, inducing different patterns of surface warming. Notably, the prescribed Q-flux changes average to zero in the global mean, thus introducing no net forcing. We show that (1) net-zero forcing Q-flux changes can have surprisingly large effects on the climate, (2) that the impact strongly depends on the geographic pattern of the Q-flux change and, (3) that different cloud parametrisations may imply different impacts of the same patterns.

While these results may have important implications for the quantification of the pattern effect and climate sensitivity in climate models, we caution against overinterpretation, as preliminary experiments with fully coupled models indicate a weaker sensitivity to similar pattern changes.

How to cite: Eiselt, K.-U. and Graversen, R. G.: Climate sensitivity, the pattern effect, and cloud parametrisation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8556, https://doi.org/10.5194/egusphere-egu24-8556, 2024.

Ice and mixed-phase clouds can form at moderate supercooling on seed particles through heterogeneous ice nucleation, but despite numerous experimental and computational investigations, understanding heterogeneous ice nucleation remains one of the great challenges in atmospheric science. While feldspar mineral dust particles have been identified as particularly good ice nucleating particles, they can exhibit different chemical composition and crystal structure, making it difficult to determine the atomistic details of the ice nucleation mechanism, both experimentally, and computationally. Here, we present systematic atomistic molecular dynamics studies of hydration layer structures at the interfaces of K-feldspar maximum microcline (001), (010), and (100) surfaces and water, at room temperature and moderate supercooling. Simulations on the fully hydroxylated α-terminated (001) cleavage plane reveal a complex lateral structure in the first water layer and a less ordered second layer. At room temperature, water exchange within the first hydration layer and between the first and second hydration layers occurs on a sub-nanosecond timescale. We also observe that surface potassium ions can go into solution and return to vacant surface sites on a timescale of tens of nanoseconds, but this causes surprisingly minor perturbations within the first hydration layer if the sampling time is sufficient. Hydration layer structures from simulation are in very good agreement with 3D atomic force microscopy data recently obtained for the first time on a freshly cleaved microcline surface in pure water (Dickbreder et al., 2024) – validating the accuracy of the atomistic model and providing an interpretation of the experimental data. However, the simulated hydration layer structures on the low energy (001) or (010) surfaces do not exhibit a lattice match with faces of cubic or hexagonal ice. Only the higher energy (100) surface with slightly strained lattice parameters can stabilize an ice interface at moderate supercooling in the simulations. Our results confirm previous findings (Kiselev et al., 2017; Soni and Patey, 2019) and indicate that the good ice nucleating properties of feldspars likely result from more complex active sites, possibly involving changes in surface chemistry, or topographic features such as defects, strained lattices, or step edges, which we are currently investigating.

Dickbreder, T., Sabath, F., Reischl, B., Nilsson, R. V. E., Foster, A., Bechstein, R. and Kühnle, A.: Atomic structure and water arrangement on K-feldspar microcline (001), accepted in Nanoscale, DOI:10.1039/d3nr05585j, 2024.

Kiselev, A., Bachmann, F., Pedevilla, P., Cox, S. J., Michaelides, A., Gerthsen, D., and Leisner, T.: Active sites in heterogeneous ice nucleation—the example of K-rich feldspars, Science, 355, 367–371, 2017.

Soni, A. and Patey, G. N.: Simulations of water structure and the possibility of ice nucleation on selected crystal planes of K-feldspar, J. Chem. Phys., 150, 214501, 2019.

How to cite: Reischl, B., Nilsson, R., Foster, A., Sabath, F., Dickbreder, T., Bechstein, R., and Kühnle, A.: Searching for the atomic scale mechanism of ice nucleating particles: hydration layer structures on K-Feldspar microcline surfaces from a combination of atomistic simulation and atomic force microscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8877, https://doi.org/10.5194/egusphere-egu24-8877, 2024.

Cirrus clouds play an important role in the radiation budget of the Earth; nonetheless, the radiative effect of ultra thin cirrus clouds in the tropopause region and in the lowermost stratosphere remains poorly constrained. These clouds have a small vertical extent and optical depth, and are frequently neither observed even by sensitive sensors nor considered in climate model simulations. In addition, their shortwave (cooling) and longwave (warming) radiative effects are often in approximate balance, and their net effect strongly depends on the shape and size of the cirrus particles. However, the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere instrument (CRISTA-2) allows ultra thin cirrus clouds to be detected. Here we use CRISTA-2 observations in summer 1997 in the northern hemisphere midlatitudes together with the Suite Of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES) radiative transfer model to calculate the radiative effect of observed ultra thin cirrus.
Using sensitivity simulations with different ice effective particle size and shape, we provide an estimate for the uncertainty of the radiative effect of ultra thin cirrus in the extratropical lowermost stratosphere and tropopause region during summer and - by extrapolation of the summer results - for winter.
Cloud top height and ice water content are based on CRISTA-2 measurements, while the cloud vertical thickness was predefined to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, their net cloud radiative effect is generally positive (warming). In contrast, assuming aggregates or a hexagonal shape, their net radiative effect is generally negative (cooling) during summer months and very likely positive (warming) during winter. The radiative effect is in the order of +/-(0.1-0.01) W/m² for a realistic global cloud coverage of 10%, similar to the magnitude of the contrail cirrus radiative forcing (of ~0.1 W/m²). The radiative effect is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius of the particle size distribution (e.g. effective radius increase from 5 to 30~microns results in a factor ~6 smaller long and shortwave effect respectively). The properties of ultrathin cirrus clouds in the lowermost stratosphere and tropopause region need to be better observed and ultra thin cirrus clouds need to be evaluated in climate model simulations.

How to cite: Spang, R., Müller, R., and Rap, A.: Radiative effect of thin cirrus clouds in the extratropical lowermost stratosphere and tropopause region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10690, https://doi.org/10.5194/egusphere-egu24-10690, 2024.

Clouds over the Southern Ocean and Antarctica are poorly represented within climate models. It is thought that our poor understanding of aerosol-cloud interaction at these latitudes could play a major role in biasing models towards consistently underpredicting cloud formation in these regions. Unfortunately, there are few studies of aerosols and their impact on clouds at high southern latitudes and those that do exist concentrate on the summer period. Here we present two years of observations from Rothera Station on the Antarctic Peninsula.

The East Beach Hut clean air facility at Rothera Station has a comprehensive set of online aerosol instruments measuring size, composition, and the capacity to act as a Cloud Condensation Nuclei (CCN) in addition to offline filter samplers from which the concentration of Ice Nucleating Particle (INP) can be derived. Measurements of the aerosol precursor gas, dimethly sulphide are also available in addition to a micropulse LiDAR to give information on cloud properties. The object of these measurements is to identify the composition and source of the cloud nuclei active at high latitudes so they can be correctly incorporated within climate models through new or revised parameterisations.

Here we report on the first two years of measurements and identify the correlation between chemical composition, biological activity and cloud nuclei activation.. We will present a comparison of aerosol and cloud nuclei during the Antarctic Summer and Winter of 2021-2023, offering an initial assessment of the different sources of CCN and INP observed during these periods.

How to cite: Lachlan-Cope, T., van den Heuvel, F., Flynn, M., Dyson, J., and Smith, D.: Two years of aerosol and cloud observations from the Antarctic Peninsula., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11003, https://doi.org/10.5194/egusphere-egu24-11003, 2024.

Atmospheric and climate models have large biases in their short and long wave radiative fluxes over the Southern Ocean, leading to significant errors in their sea surface temperature, sea ice and large scale circulation. The primary cause for these biases is the representation of low-level clouds, both at the macro- and micro-scale. We assess the performance of a convection-permitting configuration of the Met Office Unified Model (MetUM) over the Southern Ocean using satellite and aircraft observations from the 2023 special observing period of the Southern Ocean Clouds (SOC) field experiment. We focus on the model’s sensitivity to the microphysics schemes. Firstly, the impact of ice nucleating particles (INP) parametrizations via sensitivity experiments using different temperature dependent INP distributions: (i) from Cooper (1986); (ii) as derived for the east Antarctic coast; and (iii) a new distribution derived from observations from the west Antarctic Peninsula during the SOC experiment. Secondly, we examine the impact of the parameterized overlap between ice and water within a grid box (the mixed-phase overlap factor), which modifies mixed-phase process rates, for example the Wegener–Bergeron–Findeisen process and riming.

Reducing the INP concentration to values observed over the Southern Ocean results in top of the atmosphere (TOA) radiative fluxes closer to observations. The lower INP concentrations result in lower ice water content and higher liquid water content, leading to brighter and more widespread cloud; this increases the albedo, resulting in a more accurate simulation of the TOA radiation. Equally, a large sensitivity in the top of the atmosphere fluxes is seen when changing the mixed-phase overlap factor. Decreasing (increasing) the mixed-phase overlap factor results in less (more) ice and more (less) liquid reducing the TOA fluxes. Decreasing the mixed-phase overlap factor results in TOA fluxes closer to the satellite observations. In summary, simulations using INP concentrations suitable for the Southern Ocean result in simulations closer to observed but other parametrizations in the microphysics scheme are equally important for accurate simulations of radiative fluxes.

How to cite: Smith, D., Renfrew, I., van den Heuvel, F., Lachlan-Cope, T., Crawford, I., Bower, K., and Flynn, M.: The impact of mixed-phase cloud processes on radiative fluxes over the Southern Ocean in a convection-permitting model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11320, https://doi.org/10.5194/egusphere-egu24-11320, 2024.

The aggregate state of water in clouds has a fundamental impact on the clouds’ properties such as reflectivity and lifetime. Consequently, it is crucial for the development and improvement of climate models to understand the mechanism of ice nucleation under atmospheric conditions. Most atmospheric ice nucleation is heterogeneous caused by the interaction between water droplets and ice nucleating particles. Under mixed-phase cloud conditions, one of the most important ice nucleating particles are feldspar minerals. Recent scanning electron microscopy studies have shown that ice nucleation on cleavage planes of K-rich feldspars predominantly takes place at step edges and pores (Kiselev, 2017). This has also been confirmed by video and atomic force microscopy on the micrometer scale (Holden, 2019). However, experimental insights into the atomic-scale structure of the most ice-nucleation active K-feldspar microcline are still missing, and, thus, the mechanism behind ice nucleation on feldspar minerals remains elusive. Here, we present high-resolution atomic force microscopy (AFM) data revealing the atomic structure of the microcline (001) surface in its pristine state and in contact with water (Dickbreder, 2024). AFM images of the pristine microcline (001) surface kept under ultrahigh-vacuum conditions, reveal features consistent with a hydroxyl-terminated surface. This finding suggests that water in the residual gas readily reacts with the surface highlighting the high reactivity of the as-cleaved surface. Indeed, corresponding density functional theory calculations confirm a dissociative water adsorption. Three-dimensional AFM measurements performed at the mineral-water interface unravel a layered hydration structure with two features per surface unit cell. Comparison with MD calculations suggest that the structure observed in AFM corresponds to the second hydration layer rather than the first water layer. We are convinced that the combination of structural information of the pristine and water-covered microcline (001) surface will contribute to uncovering the atomic-scale mechanism behind the exceptional ice-nucleation activity of feldspar minerals.

References:

Atkinson, J. D., Murray, B. J., Woodhouse, M. T., Whale, T. F., Baustian, K. J., Carslaw, K. S., Dobbie, S., O’Sullivan, D., Malkin, T. L., Nature, 498, 355-358, 2013.

Dickbreder, T., Sabath, F., Reischl, B., Nilsson, R. V. E., Foster, A., Bechstein, R. and Kühnle, A., Nanoscale, DOI:10.1039/d3nr05585j, 2024.

Holden, M. A., Whale, T. F., Tarn, M. D., O’Sullivan, D., Walshaw, R. D., Murray, B. J., Meldrum, F. C., Christenson, H. K., Science Advances, 5, 4316, 2019.

Kiselev, A., Bachmann, F., Pedevilla, P., Cox, S. J., Michaelides, A., Gerthsen, D., and Leisner, T., Science, 355, 367–371, 2017.

How to cite: Dickbreder, T., Sabath, F., Reischl, B., Nilsson, R. V. E., Foster, A. S., Bechstein, R., and Kühnle, A.: Atomic structure of pristine and water-covered microcline (001) – A prerequisite for understanding the ice nucleation mechanism on feldspar mineral dust particles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11511, https://doi.org/10.5194/egusphere-egu24-11511, 2024.

Cold-air outbreaks (CAOs) are common high-impact weather events that produce extensive boundary layer clouds that have a substantial influence on our planet’s climate. These clouds are often supercooled and therefore their properties are susceptible to the formation of ice.  The amount of ice in these clouds has been identified as being particularly important for defining the magnitude of the cloud-climate feedback and climate sensitivity.

To address ice production in northern hemisphere CAOs we conducted two contrasting aircraft campaigns in 2022.  One campaign (ACAO, 11 flights) was in March in the Norwegian and Barents Sea where cold air flowed from the ice-covered Arctic Ocean. The other (M-Phase, 12 flights) was in October-November and focused on the Labrador Sea with air coming from the Arctic Archipelago. In both campaigns, we used similar instruments on the FAAM BAe-146 research aircraft designed to probe the aerosol properties, cloud microphysics and atmospheric thermodynamics of the CAO events.  Flight sorties were designed to study aerosol-cloud interactions as the CAO developed through the stratus and into the cumulus regime.

We found that INP concentrations in these Northern Hemisphere CAOs were orders of magnitude greater than CAO events over the Southern Ocean.  The springtime ACAO cases had systematically greater INP (and aerosol) concentrations than the autumnal Labrador Sea M-Phase cases. The presence of substantial amounts of mineral dust in the springtime Arctic, despite all local sources being covered in ice and snow, implies a reservoir of old INPs and aerosol in the springtime Arctic that originated from the low latitudes. This is supported by our global aerosol model. Primary ice production by INPs is shown to define the ice concentrations in the stratus regime in many cases, but in the cumulus regime there are pockets of very high ice concentrations that are indicative of secondary ice production.

Our modelling work has demonstrated that INPs are key to defining the stratus to cumulus transition and the cases are providing an excellent test for the high-resolution regional modelling with the Met Office Unified Model.  We are also using ACAO cases to study how INPs interact with clouds in CAOs, where warm temperature INPs are preferentially lost through nucleation scavenging. Furthermore, we envisage that the data from these campaigns will provide a valuable resource for model development, hypothesis testing and contrasting with other CAO campaigns in other places and times. 

Given the stark contrast of primary ice production in CAO clouds in different locations and times around the globe, we conclude that the primary production of ice in model CAO clouds should be linked to the aerosol properties and knowledge of the local INP population to reduce uncertainty in cloud feedback and climate sensitivity.

How to cite: Murray, B. and the M-Phase Team: Ice production in northern hemisphere cold air-outbreak clouds: two contrasting aircraft campaigns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11598, https://doi.org/10.5194/egusphere-egu24-11598, 2024.

Mineral dust is considered one of the most important seeds for heterogeneous ice nucleation in clouds. In the past decades, several studies have worked on establishing a relationship between mineral dust, number concentration, nucleation temperature, supersaturation, and the number of ice crystals. The explored dust particle-size range was usually limited to a few micrometers for two main reasons: (1) larger and heavier particles are difficult to keep suspended in an experimental setting; and (2) the fraction of coarser aerosol was considered negligible. However, recent studies have shown that dust particles as large as 100 μm or even more can be transported over long distances, leaving a knowledge gap concerning their role as ice-nucleating particles.

In this work, we aim to contribute to closing this gap by investigating the ice nucleation activity for large-size mineral dust particles, extending the studied size range to particles of up to several tens of microns. For this purpose, we used natural dust samples with different mineralogical composition, collected consistently during field campaigns in Morocco and in Iceland, and segregated into five different size classes. In the framework of the MICOS (Dust-induced ice nucleation: effects of Mineralogical COmposition and Size) campaign, we conducted experiments with the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber and with the Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology (INSEKT), in which the size-segregated samples were tested at different temperatures in the range between -16 and -27 °C. The ice nucleation efficiency was quantified in terms of the ice nucleation active surface site (INAS) density approach for the immersion freezing mode. Preliminary results from the AIDA and INSEKT experiments are presented, in which we extended the size range at which cloud chamber experiments are typically conducted.

How to cite: Vergara Palacio, S., Vogel, F., Fösig, R., González-Romero, A., Kandler, K., Querol, X., Umo, N. S., Hoose, C., Möhler, O., Pérez García-Pando, C., and Klose, M.: Extending measurements of ice nucleation activity to large-size mineral dust particles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12083, https://doi.org/10.5194/egusphere-egu24-12083, 2024.

Due to the changing climate, wildfires globally have been increasing in size and intensity. With the increase of these biomass burning events there is a surge of organic aerosols present in the atmosphere. Recent evidence from our group and the community suggests that organic aerosols can catalyze heterogeneous ice nucleation.^1–3 Currently, heterogeneous ice nucleation is the largest source of uncertainty in climate models as it governs the formation of mixed-phase clouds, important climate regulators linked to annual precipitation and global cloud coverage. We are interested in what impact an increase in atmospheric biomass burning aerosols will have on mixed-phase cloud formation.

An important component of organic biomass aerosols is lignin, a macromolecule which provides strength and structure to vascular plants. Lignin has been measured as a notably recalcitrant component of organic aerosols following biomass burning events.^4,5 To elucidate the role of morphology and size of biomass burning organic aerosols in ice nucleation, we synthesized nanoparticles from commercially available Kraft lignin via a facile nanoprecipitation process.^6,7 The nanoparticles were centrifugally separated by size, characterized by dynamic light scattering (DLS) and by transmission electron microscopy (TEM), then tested for their freezing ability in our home-built Freezing Ice Nuclei Counter (FINC).⁸ Next, the freezing mechanism and location of onset freezing for lignin was investigated using a high-speed camera on a cryo-microscope.⁹ Cylindrical droplets, between two glass slides, were frozen to localize the onset location of freezing at the air-water interface (AWI) or in the bulk of the droplets. Videos of single freezing events were recorded with a time resolution of over 2000 frames per second.

Our preliminary results suggest that lignin nanoparticles ranging in size from 50 – 500 nm in diameter are ice active at -15 ºC, well above the background freezing of the instrument (-25 °C). Normalizing the freezing data to mass and surface area suggests that aggregation facilitates ice nucleation. Moreover, the high-speed videos suggest that lignin’s ice nucleation activity is higher closer to the AWI of a droplet, indicating that hydrophobic interactions could be responsible for the aggregation of lignin and adsorption at the AWI, similar to the behavior of surfactants. These findings help understand how lignin within biomass burning organic aerosols are able to nucleate ice and hence impact the ice crystal concentration in mixed-phase clouds.

References:

(1)        Bogler, S.; Borduas-Dedekind, N. Atmospheric Chem. Phys. 2020, 20 (23), 14509–14522.

(2)        Knopf, D. A. et al., Atmos Chem Phys 2014, 14 (16), 8521–8531.

(4)        Shakya, K. M. et al., Environ. Sci. Technol. 2011, 45 (19), 8268–8275.

(5)        Myers-Pigg, A. N. et al., Environ. Sci. Technol. 2016, 50 (17), 9308–9314.

(6)        Lievonen, M. et al., Green Chem. 2016, 18 (5), 1416–1422.

(7)        Zou, T. et al., J. Phys. Chem. B 2021, 125 (44), 12315–12328.

(8)        Miller, A. J. et al., Atmospheric Meas. Tech. 2021, 14 (4), 3131–3151.

(9)        Bieber, P.; Borduas-Dedekind, N. ChemRxiv 2023. (preprint)

How to cite: Bieber, P., Zeleny, A., and Borduas-Dedekind, N.: Investigating lignin’s ice nucleation mechanisms by applying nano-particle synthesis and high-speed cryo-microscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13392, https://doi.org/10.5194/egusphere-egu24-13392, 2024.

The Deep Convective Microphysics EXperiment (DCMEX) was held in and around the convective clouds that formed and grew steadily over the Magdalena Mountains near Socorro, New Mexico during July and August, 2022. The overall goal of DCMEX is to reduce the uncertainty in cloud feedbacks associated with deep convection by improving the representation of microphysical processes in the UM/CASIM model. It is part of the NERC CloudSense programme that aims to reduce the uncertainty in climate sensitivity due to clouds. The aim of the field campaign was to make observations of the aerosols, ice nucleating particles (INPs), and the microphysics and dynamics of the clouds in order to both make new discoveries and to provide novel measurements to improve models. Measurements were made with the FAAM aircraft, ground-based aerosol instruments, radars and routinely with the NEXRAD radars and GOES-17 satellite instruments. In this talk, we will present an overview of the project and of the progress that has been made so far towards the overall goals, such as a new representation of INP in CASIM based on the observations and good measurements of the ice concentrations at several temperatures and stages of development. We will also present results on the observations of primary ice in the context of the measured INPs

How to cite: Blyth, A. and Finney, D. and the DCMEX team: Overview of DCMEX project, progress made towards goals, and measurements of primary ice particles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15019, https://doi.org/10.5194/egusphere-egu24-15019, 2024.

Organic aerosols make up a considerable mass fraction of atmospheric particulate matter, and impact air quality and climate. In the atmosphere, organic aerosols are exposed to different relative humidities (RH), often ranging between 20% to 100% RH. Gas-particle partitioning of water equilibrates the aerosol particles with the ambient RH, forming aqueous organic aerosols. When exposed to solar radiation, photochemical reactions can occur within the aqueous organic aerosol particles. Such photochemical interactions are often enhanced at the interface formed between the aqueous organic phase and the surrounding air. Depending on the changes in composition these photochemical reactions can induce phase transitions of the particles, including liquid-liquid phase separation, resulting in aqueous organic aerosols with multiple condensed phases. Understanding of the interfacial photochemical reaction and impacts on the number of phases in aqueous organic aerosols remains poor but is critical to assess the impacts of aqueous organic aerosols on air pollution and climate. For example, the number of phases in aqueous organic aerosol particles impacts their reactivity and cloud formation potential, with important implications for air quality and climate.

Here, we propose how the combination of spectroscopy and microscopy tools can be exploited to address this issue: Sum-frequency generation, a surface-sensitive, nonlinear optical spectroscopy method, is used to investigate bulk laboratory proxies of atmospheric aqueous organic aerosols and study changes in their chemical surface composition, as a function of solar irradiation. In addition, we use optical microscopy, to directly study the number of condensed phases in individual particles of the same aerosol system. The combined methods provide microscopic- and molecular-level insights how photochemical reactions impact the phase behavior of aqueous organic aerosols.

How to cite: Abdelmonem, A. and Mahrt, F.: Combining surface spectroscopy and optical microscopy can provide evidence of phase separation induced by photochemical aging of organic aerosol, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15080, https://doi.org/10.5194/egusphere-egu24-15080, 2024.

Heterogeneous ice nucleation is a ubiquitous process in the natural and built environment. Deposition ice nucleation, according to the traditional view,, occurs in a subsaturated water vapor environment without the presence of supercooled water on the solid, ice-forming surface. This process is notably significant among the various ice formation mechanisms in high-altitude cirrus and mixed-phase clouds. Despite its significance, our understanding of the microscopic mechanism of deposition ice nucleation remains quite limited. This study introduces an adsorption-based mechanism for deposition ice nucleation through results from a combination of atomistic simulations, experiments and theoretical modeling.

Silver iodide (AgI) particles prove highly efficient as ice-nucleating particles (INPs), commonly employed in rain seeding, and stand as one of the most potent laboratory surrogates for ice nucleation. In this study, AgI is used as a substrate for the simulations. The study involves a combination of grand canonical Monte Carlo and molecular dynamics (GCMC/MD) techniques to investigate deposition ice nucleation on AgI. We find that water initially adsorbs in clusters which merge and grow over time to form layers of supercooled water. Ice nucleation on silver iodide requires at minimum the adsorption of 4 molecular layers of water. Guided by the simulations we propose the following fundamental freezing steps: 1) Water molecules adsorb on the surface, forming nanodroplets. 2) The supercooled water nanodroplets merge into a continuous multilayer when they grow to about 3 molecular layers thick. 3) The layer continues to grow until the critical thickness for freezing is reached. 4) The critical ice cluster continues to grow.

How to cite: Roudsari, G., Lbadaoui-Darvas, M., Welti, A., Nenes, A., and Laaksonen, A.: Investigating the Molecular-Scale Mechanism of Deposition Ice Nucleation on Silver Iodide Surfaces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15843, https://doi.org/10.5194/egusphere-egu24-15843, 2024.

A wide range of aerosols, including dust, soot, and biological particles, can serve as ice nuclei, initiating the freezing of supercooled cloud droplets. This process significantly impacts cloud characteristics, and consequently, weather and climate. Among biological ice nuclei, some exhibit exceptionally high nucleation temperatures. While Ice Nucleating Macromolecules (INMs) found on pollen are typically not among the most active ice nuclei, they are abundant, as evidenced by their presence throughout the tissues of trees. Notably, recent studies have shown that certain tree-based INMs, such as those from Betula pendula, demonstrate ice nucleation activity above -10°C. These findings suggest that INMs emitted from the biosphere could play a more significant role in atmospheric processes than previously understood.

Our research delves into the properties of Betula pendula INMs through comprehensive ice-nucleation assays. We explore the stability of these INMs and the factors influencing their ice nucleation activity. Our approach integrates experimental data with size measurements and chemical analyses to better comprehend the underlying mechanisms.

Our findings reveal that Betula pendula INMs comprise three distinct classes active at -6°C, -15°C, and -18°C, each present in varying concentrations. We observed that freeze-drying and freeze-thaw cycles markedly alter their ice nucleation capacity. Additionally, heat treatments and chemical analysis suggest that these INM classes may be size-varying aggregates, with larger aggregates being more efficient at nucleating ice. This hypothesis aligns with previous studies on fungal and bacterial ice nucleators. Our research highlights the significance of birch INMs in atmospheric ice nucleation, not only because of their prevalence but also due to their occasional but notable high nucleation temperatures.

How to cite: Reyzek, F., Bothen, N., Schwidetzky, R., Seifried, T., Bieber, P., Pöschl, U., Meister, K., Bonn, M., Fröhlich-Nowoisky, J., and Grothe, H.: Exploring the role of aggregation in ice-nucleating macromolecules of Betula pendula pollen, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15951, https://doi.org/10.5194/egusphere-egu24-15951, 2024.

This study explores the seasonal dynamics of ice-nucleating particles (INPs) in Helsinki over a year. Using an automatic sampler, we collected atmospheric particle samples daily onto filters, which were subsequently analyzed offline through drop freezing experiments in our laboratory. The year-long measurements are used to study the temporal variations and seasonal patterns of INP concentrations in Helsinki. Measurements of different meteorological variables are also considered for the study. Additionally, we present case studies with higher temporal resolution. The offline laboratory analysis of the collected filters enables the characterization of INP concentrations in an urban environment with changing aerosols sources in different seasons. The presented results contribute to the understanding of the variation of INPs in an environment where anthropogenic activity is a main contributor to the present aerosol load.

How to cite: Perez Fogwill, G., Welti, A., Nontasin, P., Mustonen, L., Álvarez Piedehierro, A., and Lehtipalo, K.: Insights from a Year-long Study on Ice-Nucleating Particles in Helsinki, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16314, https://doi.org/10.5194/egusphere-egu24-16314, 2024.

Barnes and Sänger (1961) suggested that a substance only becomes active at nucleating ice when the adsorbed water on the surface is in an ice-like state, and that there should be a correspondence between the temperature of ice nucleation and the “freezing” of adsorbed water.
We present spectroscopic measurements that allow to simultaneously determine the amount of adsorbed water and whether the adsorbed water is liquid or ice-like. These measurements are conducted using a new setup that allows to expose test substances to a broad temperature and humidity range while recording the diffuse infrared reflectance spectrum of the adsorbed water. The phase state of the adsorbed water with decreasing temperature is then compared to the ice nucleation temperature of the test substance, which is measured using a continuous flow diffusion chamber.

References:

Barnes, G. T., and Sänger, R., ZAMP 12, 159 (1961).

How to cite: Welti, A., Viisanen, Y., Piedehierro, A. A., and Laaksonen, A.: Phase state of water adsorbed on ice nucleating particles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16526, https://doi.org/10.5194/egusphere-egu24-16526, 2024.

Climate models remain the best tools for predicting the impact of climate change on quantities relevant to human activity, such as precipitation, surface temperature and occurrence of severe weather events. Since many of these changes scale with the models equilibrium climate sensitivity, it is crucial to understand the differences in climate sensitivity between the models, which are primarily driven by inter-model differences in cloud feedbacks.

Inter-model differences in cloud feedbacks are largest in the equatorial Pacific. Focussing on the area from 10°S - 10°N, and 160°E – 270°E, we find an inter-model standard deviation in cloud feedback of ~1.36 W m-2 K-1. Using appropriate weighting to account for the area of this region, this equates to a contribution to the global mean cloud feedback uncertainty of ~ 0.07 W m-2 K-1, which represents approximately 20% of the inter-model spread in global mean cloud feedback. Local differences in cloud feedback between models in this region are even larger and may have implications for regional circulation and precipitation changes. This region is also notable as an exception to the high correlation in cloud feedbacks between coupled and atmosphere-only models.

In this presentation we will describe analysis of the causes of the inter-model spread in cloud feedbacks in this region. We shall demonstrate that the spread in domain-mean feedback in this region is due to inter-model differences in both dynamic and thermodynamic cloud feedbacks and show how this relates to changes in the properties of different cloud types amongst different models. We will also describe the use of empirical orthogonal function analysis to identify consistent cloud feedback patterns in this region across the ensemble of models and explain the causes of these patterns.

How to cite: Hill, P., Finney, D., and Zelinka, M.: Causes of large climate model spread in equatorial Pacific cloud feedback, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18918, https://doi.org/10.5194/egusphere-egu24-18918, 2024.