The response of shallow mid-and high-latitude mixed-phase clouds to changes in climate remains highly uncertain and is an important factor in the overall cloud feedback uncertainty. The uncertainty highlights the need for in-situ observations of mixed-phase cloud microphysical properties along with aerosol and dynamical parameters. Here, we report datasets associated with two aircraft measurement campaigns that were conducted to collect observations of shallow mixed-phase clouds in two regions with contrasting sea surface temperatures and aerosol loadings. The ACAO (Arctic Cold Air Outbreak) campaign was conducted in March 2022 in the Barents and Norweigian seas, while the M-Phase (resolving climate sensitivity associated with shallow mixed phase cloud in the oceanic mid-to high-latitudes) campaign was conducted during October–November 2022 in the Labrador Sea. During the campaigns, 12 flights were conducted in ACAO and 15 in M-Phase. The focus of both campaigns was to characterize aerosol properties, cloud microphysics, thermodynamic structure, and atmospheric dynamics within cold-air outbreak (CAO) cloud systems, as CAOs offer a natural laboratory to quantify mixed-phase aerosol-cloud interactions and cloud dynamics in a well-defined synoptic flow. Of the flights conducted, 11 captured CAO conditions in ACAO and 12 in M-Phase. This paper describes the datasets and how the data were collected. Information on the structure and contents of the files within the datasets are provided in the paper to aid the accessibility of the datasets to new users. We also provide an overview of the large-scale conditions corresponding to the individual flights, which help data-users to choose which flights to focus on. Data from both campaigns are published online and relevant links are provided in the data accessibility section. These datasets offers an excellent opportunity to enhance our understanding of how aerosol particles interact with mixed-phase cloud microphysics, which is essential to reduce uncertainty in cloud-feedbacks.

Abstract. Shallow, mixed-phase clouds within marine cold air outbreaks (CAOs) frequently form over the North Atlantic. their shortwave radiative effect is modulated as stratocumulus decks break up into cumulus clouds to the south. Microphysical processes controlling their phase remain poorly represented by climate models; of these, secondary ice production (SIP), describing mechanisms producing new ice crystals from existing primary ice, is a major contributor to uncertainties in the mixed-phase cloud response to future warming. We examine in-situ measurements of cloud microphysical properties made using the UK FAAM BAe-146 research aircraft within CAOs over the Labrador Sea as part of the October–November 2022 M-Phase field campaign. Measured ice particle concentrations frequently exceeded ice-nucleating particle (INP) concentrations at all in-cloud temperatures, highlighting the importance of SIP in these clouds. Peak ice concentrations were observed within the Hallett-Mossop (H-M) process temperature range (-3 to -8 °C), four orders of magnitude above expected INP concentrations. SIP regions contained large, rimed columns and graupel mixed with smaller columnar crystals (<200 µm), indicative of the H-M process. Splinter production rate calculations indicated the H-M process could account for most ice production in the largest ice enhancement regions. A secondary zone of SIP activity, between -15 and -18 °C, comprised fragile, branched crystals, aggregates and ice fragments, consistent with laboratory studies of ice-ice collisional breakup. SIP amplified across the stratiform-to-convective regime transition, favouring weak-to-moderate updrafts (0 to +2 m s-1) containing high concentrations of large liquid droplets, suggesting regime-aware SIP schemes would benefit future CAO modelling.

Ice-nucleating particles (INPs), essential for initiating primary ice production in many mixed-phase clouds, have only rarely been measured in air directly relevant for deep convective clouds. In July-August 2022 we used an aircraft to sample aerosol near developing deep convective clouds over Magdalena Mountain, New Mexico, USA. We observed INP concentrations consistently at the upper end of those typically observed in continental air with similar to 0.01-0.5 L-1 at -8 degrees C. Vertically resolved sampling revealed a deep aerosol layer extending 3-4 km above ground level, in which INP concentrations only varied within an order of magnitude. Above this layer, low temperature INP (<-15 degrees C) decreased, but high temperature INP (>-15 degrees C) remained abundant, despite having been collected in air with a lower aerosol concentration. Compositional analysis of filter aerosol showed mineral dusts dominated the surface area of aerosol larger than similar to 800 nm, but the activity of the mineral alone cannot account for the observed INP concentrations. We suggest that these mineral dust particles are internally mixed with ice-nucleating biogenic components, as they would be in organic containing fertile soils, or there is an additional biogenic/biological ice-nucleating component(s) strongly enhancing the INP population over what would be expected for mineral dust alone. Finally, ice particle measurements in rising cloud turrets at progressively colder temperatures reveal that the temperature at which we first observed ice was consistent with the measured INP spectra.

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.

Marine Arctic clouds greatly influence the radiative balance across the Arctic region and their effectiveness at scattering radiation changes considerably depending on cloud phase. Glaciation of these clouds relies on the presence of ice nucleating particles, which are often limited in number, so often clouds may be liquid even at temperatures well below 0 °C. As the Arctic region warms, cloud feedbacks may accelerate change or lessen absorbed solar radiation. Understanding aerosol-cloud interactions and the sources and pathways of aerosol particles across the Arctic region is central to improving our knowledge of these poorly understood processes. In this paper, aircraft observations of single particle chemical and physical properties are presented and the composition of cloud residuals in both warm and glaciated clouds are examined using a single-particle laser ablation aerosol particle mass spectrometer (LAAPToF). In cloud, the LAAPToF sampled behind a Counterflow Virtual Impactor (CVI) to detect cloud particle residuals, separated into liquid, mixed phase and ice clouds using in situ observations of the fractional ice water content. Three different air mass regimes were sampled: northerly winds in both the marine boundary layer and the lower free troposphere; westerly winds from Canada in both the marine boundary layer and the free troposphere; and periods when the boundary layer winds were northerly but the air immediately above the boundary layer was from continental Canada. When the air in the boundary layer and free troposphere was from the north, most clouds were in the liquid phase, however, considerably more glaciation was observed when the air immediately above the boundary layer clouds was from Canada regardless of the flow direction in the boundary layer. Sea salt particles dominate the observed out of cloud aerosol particle population and liquid cloud particle residuals. However, in the detected mixed phase and ice cloud particle residuals dust and bioaerosol particles were substantial in number. Since these are known to be effective ice-nucleating particles, the observations suggest that long range transport of continental air and entrainment is an important pathway for the supply of aerosol to the remote Arctic boundary layer.

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 -17oC 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.

Mixed‐phase clouds are important for simulating precipitation formation and cloud radiative effects in numerical weather prediction (NWP) and climate models. One challenge to reduce model uncertainties is how best to represent the subgrid distribution of liquid and ice within a model grid box. This is poorly constrained by observations, yet is key for representing microphysical process rates that grow ice crystals at the expense of liquid droplets. This study uses in situ airborne observations from stratiform, shallow cumulus, deep convective, and frontal clouds to investigate the horizontal spatial overlap of liquid and ice phases on length‐scales , which are appropriate for current regional NWP models. We place observational constraints on a simple parametrisation that describes the mixed‐phase fraction as a function of subgrid liquid and ice cloud fractions and demonstrate that most of the observations show that, when ice and liquid are present, they are close to fully overlapped.

Cloud feedbacks associated with deep convective anvils remain highly uncertain. In part, this uncertainty arises from a lack of understanding of how microphysical processes influence the cloud radiative effect. In particular, climate models have a poor representation of microphysics processes, thereby encouraging the collection and study of observation data to enable better representation of these processes in models. As such, the Deep Convective Microphysics Experiment (DCMEX) undertook an in situ aircraft and ground-based measurement campaign of New Mexico deep convective clouds during July–August 2022. The campaign coordinated a broad range of instrumentation measuring aerosol, cloud physics, radar, thermodynamics, dynamics, electric fields, and weather. This paper introduces the potential data user to DCMEX observational campaign characteristics, relevant instrument details, and references to more detailed instrument descriptions. Also included is information on the structure and important files in the dataset in order to aid the accessibility of the dataset to new users. Our overview of the campaign cases illustrates the complementary operational observations available and demonstrates the breadth of the campaign cases observed. During the campaign, a wide selection of environmental conditions occurred, ranging from dry, northerly air masses with low wind shear to moist, southerly air masses with high wind shear. This provided a wide range of different convective growth situations. Of 19 flight days, only 2 d lacked the formation of convective cloud. The dataset presented (https://doi.org/10.5285/B1211AD185E24B488D41DD98F957506C; Facility for Airborne Atmospheric Measurements et al., 2024) will help establish a new understanding of processes on the smallest cloud- and aerosol-particle scales and, once combined with operational satellite observations and modelling, can support efforts to reduce the uncertainty of anvil cloud radiative impacts on climate scales.

Abstract. In this paper measurements are presented of the observed properties of aerosols and microphysics of clouds associated with the characteristics of precipitation in convective clouds that formed off the east coast of Barbados during EUREC4A. Most data were gathered by the instrumented British Antarctic Survey Twin Otter aircraft supported by detailed in-situ aerosol measurements at the Ragged Point observatory on Barbados as well as HALO and PoldiRad radars, dropsonde and satellite data. The development of precipitation was studied in the three aerosol regimes previously reported, i.e. one low aerosol regime and two containing desert dust that had been advected across the Atlantic Ocean. The later dust event also contained evidence of biomass burning aerosol. Results showed that the maximum intensity of rain was similar for all the aerosol regimes. Clouds that developed in an environment with high aerosol loading tended to be deeper than those that developed in the clean environment. It was also found that the greatest intensities occurred in clouds that had aggregated, in agreement with previous work.

New observations of anomalously high cloud ice crystal concentrations at the Jungfraujoch research station (Switzerland, 3.5 km a.s.l.) are presented. High-resolution measurements of these ice crystals using a high-speed 2D imaging cloud particle spectrometer confirm that the concentrations far exceed those expected from any known primary ice production mechanisms and are at temperatures well below those for known secondary ice production processes to contribute. The most likely explanation is due to a strong surface source generated by the interaction of turbulent deposition of supercooled droplets to fragile ice-covered snow surfaces. This process enhances the detachment of crystal fragments wherein the smaller size mode is turbulently re-suspended even at low wind speeds below expected blowing snow thresholds. These then continue to grow, adding significantly to the ice crystal number concentrations whose size and habit is determined by the transport time between the ice crystal source and measurement location and liquid water profile within the cloud. We confirm, using eddy covariance measurements of ice particle number fluxes, that the likely source is significantly far upwind to preclude flow distortion effects such that the source plume has homogenised by the time they are measured at the mountain top summit.

&lt;p&gt;In-Service Aircraft for a Global Observing System (IAGOS) is a European research infrastructure that uses the infrastructure of commercial aviation to make in-situ measurements of the atmosphere. We present data from the cloud sensing instrument installed on these aircraft between 2011 and 2021. This includes 1000s of flights across the globe that detect the concentration of cloud particles over the range 5-75 um and this provides information about seasonal variation in cloud frequency across different parts of the globe. From these measurements we are able to estimate properties such as Liquid/Ice Water Content (LWC/IWC), The Effective Diameter (ED) and Mean Volume Diameter (MVD).&lt;/p&gt;

Eight years of upper-tropospheric (UT) ice crystal measurements with the backscatter cloud probe (BCP), installed on commercial aircraft operated as part of the In-Service Aircraft for a Global Observing System (IAGOS), have been analyzed to assess the frequency and characteristics of extreme ice crystal events (EIEs), defined in this study as encounters with clouds that have number concentrations exceeding 5000 L−1. A total of 3196 events, in clouds of horizontal extent ≥ 2.5 km, were identified during the period from December 2011 to March 2020 in the latitude band between 30∘ S and 30∘ N. Regions of anthropogenic sources of carbon monoxide, with particles that can alter cloud microphysics, were attributed to these EIEs in UT clouds using the SOFT-IO model. The evaluation of low- and upper-level kinematic variables from the European Centre for Medium-Range Weather Forecasts (ERA5) reanalysis, combined with spatial distributions of aerosol optical depth and regions of biomass burning, highlights the physical mechanisms by which the particles are lofted to flight levels in regions of deep convection. The maps of lightning frequency, derived from the World Wide Lightning Location Network (WWLLN), provide additional evidence of the role of deep convection in transporting aerosol particles, cloud hydrometeors and carbon monoxide to aircraft cruising altitudes. The evaluation of aerosol particle mass concentrations and composition from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) contributes additional evidence for a link between regions of EIEs and surface emissions of dust, black carbon (BC), organic carbon (OC) and sulfate particles. Given the composition of the source aerosols and the role of deep convection in their transport to the UT, the sampled ice clouds likely originate from the homogeneous or heterogeneous freezing of droplets formed on these particles, as has been reported in previous studies. The results from this study, which have been obtained from a large sample of measurements, have ramifications related to satellite measurement validation, weather forecasting and climate change. In addition, over 2000 of the randomly sampled clouds had derived ice water contents larger than 1 g m−3, a concentration that is considered potentially hazardous to commercial aircraft operations.

EUREC4A is an international project that aims to address the current lack of understanding of the processes controlling the response of trade-wind cumulus clouds to changing environmental conditions in a warmer climate. The radiative properties of the trade-wind cumulus clouds have a major influence on the Earth's radiation budget. The response to global warming of these clouds is therefore critical for global mean cloud feedbacks. The EUREC4A field campaign took place in the vicinity of Barbados during January and February, 2020. The BAS Twin Otter aircraft was deployed in the project to make measurements of aerosols, cloud microphysics and boundary-layer processes in the life cycle of the clouds. In-situ measurements were made of the cloud droplet size distributions and the development of warm rain in multiple cases at different altitudes. We found significant variability in the development of precipitation between cases. The cloud structure appeared to have a significant impact on the precipitation, while the aerosol concentrations in the boundary layer were strongly related to the initial droplet number concentration at cloud base. We will present these findings by highlighting a number of cases with different cloud types and aerosol properties.

This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the Facility for Airborne Atmospheric Measurements (FAAM) aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 ∘C. Sensitivity studies were performed using existing parameterization schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these increased the secondary ice particle concentration but not by a sufficient amount. Simulations that involved only dust particles as ice-nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice-nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement of the freezing efficiency or relaxation of the onset temperature produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water in the Hallett–Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. It is possible of course that several mechanisms, some of them only recently discovered, may be responsible for producing the ice particles in clouds. This study highlights the fact that the dynamics of the clouds likely play an important role in producing high concentrations of secondary ice particles in clouds.

Abstract. The science guiding the EUREC4A campaign and its measurements are presented. EUREC4A comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic – eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice.

Ground based and airborne observations of ice crystal concentrations are often found to exceed the concentration of ice nucleating particles by many orders of magnitude. This discrepancy between the expected ice particle concentrations formed through primary ice nucleation and observed ice particle concentration has led to the search for missing physical processes capable of creating new ice crystals. Secondary ice production (SIP) is a mechanism that produces new ice crystals without requiring the action of an ice nucleating particle. Evidence has now been found for several of these Increasingly sophisticated cloud microphysical representations are being used in Numerical Weather Prediction and climate models to provide more realistic simulations of clouds. This drive towards greater complexity is motivated by the recognition of the importance of microphysical processes to the evolution of clouds, precipitation and the atmospheric environment. One important challenge for the successful implementation of cloud microphysics is the prediction of ice crystal concentrations, these influence the water budget of the cloud s through precipitation processes and the radiative properties of clouds especially when the ice crystals are in the majority over water droplets. The understanding and quantification of primary ice nucleation has grown in recent years, secondary ice production processes have received relatively little attention but are potentially very important for controlling the ice concentrations found in some types of clouds. In this stalk a number of SIP mechanisms will be discussed: The Hallett-Mossop process, by far the most powerful mechanism when conditions are right; the fracture on freezing of supercooled raindrops, the fragmentation of falling snow flakes; the detachment of frost crystals from a surface.

Observations of high-altitude cirrus clouds are reported from measurements made during the routine monitoring of cloud properties on commercial aircraft as part of the In-Service Aircraft for a Global Observing System. The increasing global scale of the measurements is revealed, with 7 years of in situ data producing a unique and rapidly growing dataset. We find that cloud fractions measured ≥ 10 km at aircraft cruise altitude are representative of seasonal trends associated with the mid-latitude jet stream in the Northern Hemisphere, and the relatively higher cloud fractions are found in tropical regions such as the Inter-Tropical Convergence Zone and South East Asia. Both stratospheric and tropospheric data were used to calculate the cloud fractions routinely experienced by commercial aircraft. Further work is needed for a direct comparison with previous studies that limit cloud fraction calculations to tropospheric data only. The characteristics of these clouds are discussed and the potential different formation mechanisms in different regions assessed.

Figure SM1.Comparison of the fire radiative power derived from MODIS on the a) Aqua (nadir pass at 1:30pm LT) and b) Terra (nadir pass at 10:30amLT) satellites for July through December (2011-2019).The orange markers show the location of Extreme Ice Events (EIE) in cirrus observed in each month.(Logarithmic scale?)

IAGOS (www.iagos.org) is a European Research Infrastructure using commercial aircraft (Airbus A340, A330, and soon A350) for automatic and routine measurements of atmospheric composition including reactive gases (ozone, carbon monoxide, nitrogen oxides, volatile organic compounds), greenhouse gases (water vapour, carbon dioxide, methane), aerosols and cloud particles along with essential thermodynamic parameters. The main objective of IAGOS is to provide the most complete set of high-quality essential climate variables (ECV) covering several decades for the long-term monitoring of climate and air quality. The observations are stored in the IAGOS data centre along with added-value products to facilitate the scientific interpretation of the data. IAGOS began as two European projects, MOZAIC and CARIBIC, in the early 1990s. These projects demonstrated that commercial aircraft are ideal platforms for routine atmospheric measurements. IAGOS then evolved as a European Research Infrastructure offering a mature and sustainable organization for the benefits of the scientific community and for the operational services in charge of air quality and climate change issues such as the Copernicus Atmosphere Monitoring Services (CAMS) and the Copernicus Climate Change Service (C3S). IAGOS is also a contributing network of the World Meteorological Organization (WMO). IAGOS provides measurements of numerous chemical compounds which are recorded simultaneously in the critical region of the upper troposphere – lower stratosphere (UTLS) and geographical regions such as Africa and the mid-Pacific which are poorly sampled by other means. The data are used by hundreds of groups worldwide performing data analysis for climatology and trend studies, model evaluation, satellite validation and the study of detailed chemical and physical processes around the tropopause. IAGOS data also play an important role in the re-assessment of the climate impact of aviation. Most important in the context of weather-related research, IAGOS and its predecessor programmes provide long-term observations of water vapour and relative humidity with respect to ice in the UTLS as well as throughout the tropospheric column during climb-out and descending phases around airports, now for more than 25 years. The high quality and very good resolution of IAGOS observations of relative humidity over ice are used to better understand the role of water vapour and of ice-supersaturated air masses in the tropopause region and to improve their representation in numerical weather and climate forecasting models. Furthermore, CAMS is using the water vapour vertical profiles in near real time for the continuous validation of the CAMS atmospheric models.

Abstract. Eight years of upper tropospheric (UT) ice crystal measurements with the Backscatter Cloud Probe (BCP), operated as part of the In-Service Aircraft for a Global Observing System (IAGOS), have been analyzed to assess the frequency and characteristics of extreme ice crystal events (EIE), defined in this study as encounters with clouds that have number concentrations exceeding 5000 L−1. A total of 6490 events, in clouds of horizontal extent ≥ 2.5 km, were identified during the period from December 2011 to March 2020 in the global band between 30° S and 30° N. Evaluation of in situ measurements of carbon monoxide in these UT clouds, combined with back-trajectories and carbon monoxide emission inventories, allow the identification of regional anthropogenic sources. An evaluation of low- and upper-level kinematic variables from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), combined with spatial distribution of aerosol optical depth and regions of biomass burning, reveal the physical mechanism by which the particles are lofted to flight levels in regions of deep convection. The maps of lightning frequency derived from the World Wide Lightning Location Network (WWLLN) contribute additional evidence of the role of deep convection lofting ice forming aerosols to the UT. The MERRA-2 analysis shows clear spatial correlations that link dust, black carbon (BC), organic carbon (OC) and sulfate particles with regions of EIE. Given the composition of the source aerosols and the nature of their transport to the UT, the ice clouds are likely of liquid origin whereby droplets formed on these particles are lofted and freeze heterogeneously as have been reported in previous studies. The results from this study have important ramifications related to climate change, satellite measurement validation, weather forecasting and aircraft operations.