Reply on RC1

Contrail formation within natural cirrus introduces large perturbations in cirrus ice crystal number concentrations leading to modifications in cirrus microphysical and optical properties. The number of contrail ice crystals formed in an aircraft plume depends on the atmospheric state and aircraft and fuel properties. Our aim is to study the impact of pre-existing cirrus on the contrail formation processes. We analyze contrail ice nucleation within cirrus and the survival of contrail ice crystals within the vortex phase and their change due to the presence of cirrus ice crystals within the high-resolution ICON-LEM at a horizontal resolution of 625 m over Germany. We have selected two different synoptic situations sampling a large range of cirrus cloud properties from very thick cirrus connected with a frontal system to very thin cirrus within a high-pressure system. We find that contrail formation within cirrus often leads to increases in cirrus ice crystal numbers by a few orders of magnitude. Pre-existing cirrus has an impact on contrail ice crystal number concentrations only if the cirrus is optically thick. In thick cirrus, contrail ice nucleation rates and ice crystal survival rates within the vortex phase are both increased. The sublimation of the cirrus ice crystals sucked into and subsequently sublimated within the aircraft’s engine leads to an increase in the contrail formation threshold by up to 0.7 K which causes an increase in the number of nucleated contrail ice crystals. This increase can be large at lower flight levels where ambient temperatures are close to the contrail formation threshold temperature and when the ice water content of the pre-existing cirrus cloud is large. During the contrail’s vortex phase the aircraft plume is trapped within the descending vortices in which the decrease in plume relative humidity leads to the sublimation of contrail ice crystals. This contrail ice crystal loss can be modified by the cirrus ice crystals that are mixed into the plume before the start of the vortex phase. In particular, high ice crystal number concentrations and large ice water content of the pre-existing cirrus cloud or low contrail ice crystal numbers are associated with significant increases in the contrail ice crystal survival rates.

Abstract. In order to study aerosol–cloud interactions in cirrus clouds, we apply a new multiple-mode ice microphysical scheme to the general circulation model ECHAM5-HAM. The multiple-mode ice microphysical scheme allows for analysis of the competition between homogeneous freezing of solution droplets, deposition nucleation of pure dust particles, and immersion freezing of coated dust particles and pre-existing ice. We base the freezing efficiencies of coated and pure dust particles on the most recent laboratory data. The effect of pre-existing ice, which has been neglected in previous ice nucleation parameterizations, is to deplete water vapour by depositional growth and thus prevent homogeneous and heterogeneous freezing from occurring. As a first step, we extensively tested the model and validated the results against in situ measurements from various aircraft campaigns. The results compare well with observations; properties such as ice crystal size and number concentration as well as supersaturation are predicted within the observational spread. We find that heterogeneous nucleation on mineral dust particles and the consideration of pre-existing ice in the nucleation process may lead to significant effects: globally, ice crystal number and mass are reduced by 10 and 5%, whereas the ice crystals' size is increased by 3%. The reductions in ice crystal number are most pronounced in the tropics and mid-latitudes in the Northern Hemisphere. While changes in the microphysical and radiative properties of cirrus clouds in the tropics are mostly driven by considering pre-existing ice, changes in the northern hemispheric mid-latitudes mainly result from heterogeneous nucleation. The so-called negative Twomey effect in cirrus clouds is represented in ECHAM5-HAM. The net change in the radiation budget is −0.94 W m−2, implying that both heterogeneous nucleation on dust and pre-existing ice have the potential to modulate cirrus properties in climate simulations and thus should be considered in future studies.

Contrails are estimated to account for the majority of the present-day warming by the aviation industry. Their formation relies on the availability of aerosol in the exhaust plume, upon which water vapour can condense and subsequently freeze to form contrail ice crystals Most modern aircraft operate in the soot-rich regime, releasing soot particles with a number emission index (EIn) of between 1014 and 1016 (kg-fuel)-1. Under these conditions, the number concentration of soot particles and contrail ice crystals scales linearly. For this reason, existing global contrail simulations typically assume that the number concentration of ice crystals and soot particles are equivalent. However, the use of alternative fuels such as sustainable aviation fuel (SAF) and liquid hydrogen, and the adoption of cleaner lean-burn combustors in the existing fleet are likely to drive the soot EIn into the soot-poor regime < 1013 (kg-fuel)-1. Here, (semi) volatile material and entrained ambient particles can compete with soot for plume supersaturation and the relationship between the number concentration of soot particles and contrail ice crystals is non-linear. These effects are not currently accounted for in existing contrail models used to simulate regional and global contrail climate forcing.   In this work, we extend the parcel model proposed by Kärcher et al. [1] to account for the activation of volatile particulate matter (vPM) in the soot-poor regime and integrate this into the contrail cirrus prediction model (CoCiP) [2]. We explore the relationship between the soot EIn and the apparent ice emissions index (AEI) in the soot-rich and soot-poor regimes, evaluating the model’s sensitivity to different aerosol properties, including particle hygroscopicity and characteristics of the particle size distribution. Preliminary results show a linear relationship between the soot EIn and AEI in the soot-rich regime, consistent with previous work [1]. However, in the soot-poor regime, the AEI: (i) could be up to two orders of magnitude larger than the soot EIn; (ii) increases with decreasing ambient temperatures and (iii) depends on the assumed particle properties of the (semi) volatile and ambient particle modes. These results suggest that existing global contrail simulations may underestimate the contrail climate forcing for a small subset of flights with soot EIn < 1014 (kg-fuel)-1. The model developed in this work will be implemented in a global contrail simulation, incorporating activation of vPM. A sensitivity analysis will also be performed, and the results validated with in-situ measurements from the recent Emissions and Climate Impact of Alternative Fuels (ECLIF) III experimental campaign [3].

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

In addition to their formation mechanisms, the origin of cirrus clouds (liquid-origin or in situ) can significantly influence their microphysical and radiative properties. Liquid-origin cirrus, which form through the freezing of water droplets from the mixed-phase region upon reaching cirrus temperatures (below -38°C), are typically characterized by high ice crystal concentrations and are associated with a strong cooling effect. In contrast, in situ-origin cirrus consist of ice crystals that form directly within the cirrus regime via homogeneous or heterogeneous freezing. Large cloud systems often comprise a mixture of both types. However, the global occurrence, distribution, and environmental conditions associated with these cirrus types remain poorly understood.This study investigates cirrus clouds by integrating satellite observations with reanalysis data. Observations from lidar-radar satellite instruments (DARDAR-Nice) provide detailed retrievals of cirrus microphysical properties, including ice water content and ice crystal number concentration. To trace the origins of cirrus clouds, we employ Lagrangian air mass trajectories based on ERA5 reanalyses, using the FLEXPART Lagrangian model. This analysis is conducted for every satellite retrieval pixel, with a horizontal resolution of 1.7 km and a vertical resolution of 60 m. Environmental conditions at and preceding cirrus formation, as well as those in the mixed-phase regime for liquid-origin cirrus, are examined along the trajectories. Aerosol data from CAMS reanalyses are also included to assess their influence.A combined cloud-aerosol dataset, derived from satellite observations and reanalysis tools, is compiled for one year of global data. The global occurrence of liquid-origin cirrus is analyzed in relation to ice crystal formation drivers and the resulting microphysical and radiative properties of these clouds, as retrieved by the DARDAR-Nice product. The relative occurence of insitu- and liquid-origin ice in cirrus is also assessed. The role of aerosols in cirrus formation processes will briefly be explored.

Secondary ice production (SIP) is a crucial phenomenon for explaining the formation of ice crystal clouds, especially when addressing the discrepancies between observed ice crystal number concentrations and ice nucleating particles (INPs). In this study, we investigate parameterizations of three SIP processes (Hallett-Mossop, fragmentation of freezing drops, and fragmentation due to ice–ice collision) by simulating a deep convective cloud observed during the HAIC/HIWC campaign with the 3D bin microphysics scheme DESCAM (DEtailed SCAvening and Microphysics model). The simulated mean cloud properties, including particle size distributions and ice crystal number concentration are compared with in situ probe observations obtained during the campaign. Simulation excluding SIP shows a large underestimation of small ice crystals (< 1 mm diameter) for temperatures warmer than ‐30∘C. In our results, incorporating Hallett-Mossop and fragmentation due to ice–ice collision processes leads to ice crystal number concentrations close to observed values, thereby reducing discrepancies by two orders of magnitude. Our simulations also indicates that fragmentation of freezing drops affect minimally the properties of the cloud at its mature stage. Furthermore, we investigate the impact of fragments sizes resulting from SIP processes and show that the size of fragments generated from fragmentation due to ice–ice collision significantly influences the shape of ice particle size distribution. Employing various parameterizations of the ice crystal sticking efficiency reveals a notable impact on cloud properties. This study shows that SIP mechanisms are important and have to be considered for cold and mixed-phase clouds. However their parameterization lack reliability, highlighting the need for better quantifying these mechanisms. The companion paper, investigates the effects of SIP processes on the formation and the evolution of the deep convective system.

Analyzing the formation of contrails on emissions from the most recent generation of aircraft engines is key to understand the climate impact from aviation. As for conventional Rich-Quench-Lean engines contrail ice crystals mainly form on a high number of emitted soot particles, the question arises which and how many particles are activated during the contrail formation process if soot emissions are strongly reduced through the Lean Combustion technology.The ecoDemonstrator experiment is a collaboration between Boeing, GE, NASA, DLR and other international partners and took place in October 2023. For the first time, we present in-flight measurements of contrail ice crystals in the exhaust of the ultra-low soot emitting CFM International LEAP-1B engine. This experiment combines the lean-burning engine technology with the use of Sustainable Aviation Fuels (SAF) and ultra-low sulfur kerosene (LS-Jet-A) in newly manufactured engines to achieve close to soot-free emissions. During the experiment, we performed measurements on board the NASA DC-8 research aircraft, sampling emissions behind a Boeing 737 MAX 10 aircraft. In addition to gas and particle emissions, we also measured the number concentrations of ice crystals that formed behind the 737 MAX 10 at a distance between 3 and 8 kilometers with two Cloud and Aerosol Spectrometers (CAS) that were positioned on the upper and lower fuselage of the DC-8. We show a first analysis of the contrail ice data for selected flights and present apparent ice emission indices (AEI) in relation to ambient conditions.

Cirrus clouds are of great importance to the global climate, with their net radiative forcing strongly dependent on the microphysical properties that are related to the ice‐nucleating regime. However, the influence of long‐range transport of dust on primary ice formation in cirrus clouds is limitedly understood, specifically over the clean remote ocean regions. Here, two case studies show that transpacific Asian dust can impact the ice formation of cirrus clouds over the central Pacific based on Cloud‐Aerosol Lidar with Orthogonal Polarization and Cloud Profiling Radar (CPR, CloudSat) observations. One case shows a well‐developed horizontally extended cirrus embedded in a pure dust layer, with an average dust‐related ice‐nucleating particle concentration (INPC) of 7 L−1 and 96 L−1 for an ice saturation ratio Si of 1.15 and 1.25, respectively; ice crystal number concentration (ICNC) with diameters &gt;25 and 100 μm (denoted as nice,25 μm and nice,100 μm) are 64 L−1 and 7 L−1, respectively. Another case shows that cirrus clouds with a much smaller horizontal extent appeared in the vicinity of polluted dust, with an average INPC of 42–310 L−1 for the typical higher Si of 1.25–1.35 by considering a tenfold reduction of the ice nucleation efficiency of ice crystals; nice,25 μm and nice,100 μm are 168 L−1 and 20 L−1, respectively. The estimated INPC and ICNC values suggest the dominance of ice formation by dust‐induced heterogeneous nucleation, proving that the long‐range transport of dust toward the upper troposphere and the potential influence on cirrus formation over the central Pacific should be well considered in atmospheric models.

Hydrogen-powered aircraft have the potential to reduce CO2 emissions to zero. However, a significant portion of the global warming attributed to aviation arises from non-CO2 effects, including contrails. The thermodynamic state and microphysical pathways that form these contrails differ substantially between hydrogen and conventional kerosene combustion. Therefore, the overall climate impact of contrails formed by hydrogen combustion is not yet known and needs to be assessed by Global Circulation Models (GCMs). The contrail parametrization in a GCM cannot resolve the contrail formation processes. However, these early processes have a large influence on the contrail life cycle and should therefore be included in the contrail initialization of a GCM. Here, a crucial ingredient is the number of ice crystals formed during the jet phase.In this study, we present a parametrization that provides a link between the outcome of a high-resolution model and the contrail initialization in a GCM. For that, we performed contrail formation simulations with the particle-based Lagrangian Cloud Module (LCM) in a box model approach. We assume that contrail droplets and ice crystals form solely on entrained ambient aerosols. With our simulation setup, we aim to cover the entire parameter space relevant for contrail formation in the case of hydrogen combustion. This involves varying background meteorological conditions, ambient aerosol properties, and engine exit conditions, resulting in more than 20,000 simulations.The simulation results show that the number of formed ice crystals is mostly sensitive to a variation of the ambient aerosol background concentration, followed by a variation of the ambient temperature. We identify a parameter subspace where the number of ice crystals becomes almost independent of the size and chemical composition of the ambient aerosols.Furthermore, we performed simulations with two coexisting background aerosol ensembles differing in mean size and/or solubility. The simulation results show that coarse mode particles have neither a direct nor an indirect influence on the number of formed ice crystals if their number concentration is 2-3 orders of magnitude lower than that of a coexisting Aitken/accumulation mode. Furthermore, the ice crystal number from simulations with the two coexisting background aerosol ensembles can be reconstructed by a weighted mean of two single simulations, each containing only one of the two aerosol ensembles. This allows to construct a simpler parametrization still covering the case of two coexisting aerosol ensembles.We used the simulation results to train a shallow feed-forward neural network that maps the box model input parameters to the number of formed ice crystals. This neural network serves as a fit function of our simulation results, which can be implemented in a GCM for contrail initialization.This work contributes to the collaborative effort of the German Aerospace Center (DLR) and Airbus in assessing the climate impact of H2 contrails.

In-situ aircraft observations of aerosol and cloud microphysical characteristics of mixed-phase clouds over the North China Plain

Cirrus clouds impact the planetary energy balance and upper-tropospheric water vapor transport and are therefore relevant for climate. In this study cirrus clouds at temperatures colder than −40°C simulated by the ECHAM–Hamburg Aerosol Module (ECHAM-HAM) general circulation model are compared to Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations ( CALIPSO) satellite data. The model captures the general cloud cover pattern and reproduces the observed median ice water content within a factor of 2, while extinction is overestimated by about a factor of 3 as revealed by temperature-dependent frequency histograms. Two distinct types of cirrus clouds are found: in situ–formed cirrus dominating at temperatures colder than −55°C and liquid-origin cirrus dominating at temperatures warmer than −55°C. The latter cirrus form in anvils of deep convective clouds or by glaciation of mixed-phase clouds, leading to high ice crystal number concentrations. They are associated with extinction coefficients and ice water content of up to 1 km−1 and 0.1 g m−3, respectively, while the in situ–formed cirrus are associated with smaller extinction coefficients and ice water content. In situ–formed cirrus are nucleated either heterogeneously or homogeneously. The simulated homogeneous ice crystals are similar to liquid-origin cirrus, which are associated with high ice crystal number concentrations. On the contrary, heterogeneously nucleated ice crystals appear in smaller number concentrations. However, ice crystal aggregation and depositional growth smooth the differences between several formation mechanisms, making the attribution to a specific ice nucleation mechanism challenging.

The representation of cloud processes in models is one of the largest sources of uncertainty in weather forecast and climate projections. While laboratory settings offer controlled conditions for studying cloud processes, they cannot reproduce the full range of conditions and interactions present in natural cloud systems. To bridge this gap, here we leverage weather modification, specifically glaciogenic cloud seeding, to investigate ice growth rates within natural clouds. Seeding experiments were conducted in supercooled stratus clouds (at to C) using an uncrewed aerial vehicle, and the created ice crystals were measured 4-10 min downwind by in situ and ground-based remote sensing instrumentation. We observed substantial variability in ice crystal growth rates within natural clouds, attributed to variations in ice crystal number concentrations and in the supersaturation, which is difficult to reproduce in the laboratory and which implies faster precipitation initiation than previously thought. We found that for the experiments conducted at C, the ice crystal populations grew nearly linearly during the time interval from 6 to 10 min. Our results demonstrate that the targeted use of weather modification techniques can be employed for fundamental cloud research (e.g. ice growth processes, aerosol-cloud interactions), helping to advance cloud microphysics parameterizations and to improve weather forecasts and climate projections.

A method of retrieving cloud microphysical properties using combined observations from both cloud radar and lidar is introduced. The description of the lidar‐radar model accounts for nonspherical effects of the ice crystals, with a treatment for multiple scattering and Mie effects. This retrieval makes use of an improvement to the traditional optimal estimation retrieval method, whereby a series of corrections are applied to the state vector during the search for an iterative solution. This allows faster convergence to a solution and is less processor intensive. The retrieval method is applied to radar and lidar observations from the CRYSTAL‐FACE experiment, and vertical profiles of ice crystal characteristic diameter, number concentration, and ice water content are retrieved for a cirrus cloud layer observed 1 day of that experiment. Empirical relationships between ice water content and radar reflectivity as well as between particle number concentrations and characteristic diameter are also examined. The results indicate that a distinct and robust relationship exists between the latter two parameters, offering insight into the nature of cirrus microphysical processes.

Abstract. The number of ice crystals formed during the contrail's jet phase has a long-lasting impact on the life cycle and radiative forcing of contrail cirrus clouds. For conventional kerosene combustion, suitable parameterizations for early ice crystal number have been developed and employed in general circulation models that are used to estimate the climate impact of contrail cirrus. However, a parameterization for the number of ice crystals formed is lacking for hydrogen combustion. To develop such a parameterization, we present a comprehensive set of contrail formation simulations using the particle-based Lagrangian Cloud Module in a box model approach. Unlike kerosene combustion, no soot particles are emitted. Thus, ice crystals are assumed to form on ambient aerosols entrained into the exhaust plume. The total number of entrained particles primarily governs the nonlinear depletion of water vapor. Consequently, the impact of coarse-mode particles is negligible due to their low abundance. Additionally, ice crystal formation from multiple aerosol populations can be reconstructed from single-population simulations using population-specific properties (size and hygroscopicity) and the total number concentration. We also identify atmospheric conditions where homogeneous droplet nucleation can be safely neglected as potential ice formation pathway. Based on more than 20 000 simulations covering a broad range of atmospheric conditions and aerosol properties, we identify a regime where ice crystal formation becomes nearly independent of ambient relative humidity, aerosol size, and hygroscopicity. Our results provide a basis for a data-driven parameterization of ice crystal number in contrails from hydrogen combustion, to be presented in a companion paper.

Abstract. Microphysical processes in cold clouds which act as sources or sinks of hydrometeors below 0 ∘C control the ice crystal number concentrations (ICNCs) and in turn the cloud radiative effects. Estimating the relative importance of the cold cloud microphysical process rates is of fundamental importance to underpin the development of cloud parameterizations for weather, atmospheric chemistry, and climate models and to compare the output with observations at different temporal resolutions. This study quantifies and investigates the ICNC rates of cold cloud microphysical processes by means of the chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) and defines the hierarchy of sources and sinks of ice crystals. Both microphysical process rates, such as ice nucleation, aggregation, and secondary ice production, and unphysical correction terms are presented. Model ICNCs are also compared against a satellite climatology. We found that model ICNCs are in overall agreement with satellite observations in terms of spatial distribution, although the values are overestimated, especially around high mountains. The analysis of ice crystal rates is carried out both at global and at regional scales. We found that globally the freezing of cloud droplets and convective detrainment over tropical land masses are the dominant sources of ice crystals, while aggregation and accretion act as the largest sinks. In general, all processes are characterized by highly skewed distributions. Moreover, the influence of (a) different ice nucleation parameterizations and (b) a future global warming scenario on the rates has been analysed in two sensitivity studies. In the first, we found that the application of different parameterizations for ice nucleation changes the hierarchy of ice crystal sources only slightly. In the second, all microphysical processes follow an upward shift in altitude and an increase by up to 10 % in the upper troposphere towards the end of the 21st century.