Abstract. Internal gravity waves have a profound impact on the nucleation of cirrus ice crystals in the stably stratified upper troposphere. Cloud models require the use of parameterization schemes that represent gravity wave-induced variability in vertical wind speeds with high fidelity. Accounting for such variability in microphysical models is a stringent test of their ability to accurately simulate ice nucleation events. However, such schemes are sparse and have not been characterized in detail. This study investigates the impact of several wave-cirrus parameterizations on the formation of tropical tropopause layer cirrus and compares distributions of nucleated ice crystal number concentrations with the statistic obtained directly from long-duration, quasi-Lagrangian superpressure balloon measurements. Fluctuation distribution, autocorrelation function, and power spectral density from both measurements and parameterizations are analyzed. Simulating homogeneous freezing of supercooled aerosols based on the balloon-borne data and on parameterizations with exponentially-distributed vertical wind speed fluctuations yields consistent results. The causes of small discrepancies in ice concentrations are traced back to filtering of the balloon data necessary to remove non-geophysical artifacts and differences in power spectra across the buoyancy frequency between the measurements and models. A brief outline of future research topics motivated by this study offers pointers of how to augment and further improve the representation of gravity wave forcing in both process-based and global models.

Turbulent air motions determine the local environment in which cloud ice crystals form. Homogeneous freezing of aqueous solution droplets is the most fundamental pathway to nucleate ice crystals in cirrus. Lack of knowledge about the role of turbulence in cirrus ice formation limits our understanding of how uncertainties in small-scale cloud processes affect the climatological radiative effect of cirrus. Here we shed first light on how turbulent fluctuations in temperature and supersaturation interact with probabilistic homogeneous freezing. We show that spatial model resolution substantially below 1–10 m is needed to properly simulate homogeneous freezing events. Importantly, microscale turbulence generates large variability in nucleated ice crystal number concentrations. Previous research ascribed this variability to mesoscale dynamical forcing due to gravity waves alone. The turbulence-generated microphysical variability has macrophysical implications. The wide range of predicted cloud radiative heating anomalies in anvil cirrus due to turbulence-ice nucleation interactions, comparable to typical mean values, is potentially large enough to affect the response of tropical cirrus cloud systems to global warming. Our results have ramifications for the multiscale modeling of cirrus clouds and the interpretation of in situ measurements.

Prediction of contrail cirrus persistence is highly problematic for models, in part due to a poor representation of intertwined microphysical and dynamical processes controlling contrail evolution in large-scale ice-supersaturated areas. Knowledge of contrail cirrus lifetimes is required to estimate their lifecycle-average radiative effect, but lifetime statistics inferred from observations are incomplete, hampering model validation. Contrail cirrus ice crystal size distributions are mainly impacted by entrainment and plume dilution, ice deposition growth and sublimation, and gravitational settling. Changes in the size distributions are due to synoptic and mesoscale air motions that affect sign and magnitude of ice supersaturation experienced by contrail particles. Driven by internal gravity waves, rapid growth/sublimation cycles and sedimentation lead to a selection of ice crystal sizes enabling long lifetimes for persistent contrails whose evolution is not limited by synoptic warming. Wave-induced mesoscale supersaturation fluctuations lead to a wide spectrum of lifetimes with mean values of hr and maxima up to 16 hr in large-scale ice-saturated conditions. Even longer maximum lifetimes as seen in some observations are possible within deep ice-supersaturated layers, enhanced microscale turbulent temperature fluctuations, and new ice formation at the top of contrail cirrus. We conceptualize lifetime statistics and define initial adjustment, intermediate stabilization, and final dissipation regimes. Our analysis of contrail cirrus lifetimes and ice crystal size distributions will aid future satellite analyses and model development.

Abstract. Ice nucleation, a critical process in cirrus clouds, remains a challenge in global climate models. To enhance the understanding, a novel ice nucleation parameterization based on the Kärcher (2022) (K22) scheme is introduced into the NCAR Community Atmosphere Model version 6 (CAM6). To investigate ice formation in cirrus clouds, sensitivity tests are conducted to analyze three ice sources: orographic gravity wave (OGW)-induced, convection detrained and turbulence-induced. These tests employ both the K22 scheme and the default Liu and Penner (2005) (LP05) scheme. Model evaluation includes 6-year climatology and nudged simulations representing the Small Particles in Cirrus (SPARTICUS) and O2/N2 Ratio and CO2 Airborne Southern Ocean Study (ORCAS) campaigns. The climatology simulations reveal that both schemes concentrate detrained and turbulence-induced ice crystals in low to mid-latitudes, whereas OGW-induced ice crystals are concentrated in mid- to high latitudes. Compared to the LP05 scheme, the K22 scheme generates a higher number of ice crystals. The simulated cloud microphysical properties using the K22 scheme align well with observations for orographic cirrus during the SPARTICUS campaign. In orographic cirrus, both schemes identify OGW-induced ice crystals as the dominant ice source. However, due to distinct competition parameterizations, the K22 scheme exhibits less competition from minor ice sources (convection detrained and turbulence-induced). This underscores the significance of competition mechanisms within nucleation schemes for accurate cirrus clouds simulation. The application of two distinct nucleation schemes provides valuable insights into the dominant ice sources in cirrus clouds.

Global simulations suggest the mean annual contrail cirrus net radiative forcing is comparable to that of aviation's accumulated CO2 emissions. Currently, these simulations assume non-volatile particulate matter (nvPM) and ambient particles are the only source of condensation nuclei, omitting activation of volatile particulate matter (vPM) formed in the nascent plume. Here, we extend a microphysical model to include vPM and benchmark this against a more advanced parcel model (pyrcel) modified to treat contrail formation. We explore how the apparent emission index (EI) of contrail ice crystals (AEI(ice)) scales with EInvPM, vPM properties, ambient temperature, and aircraft/fuel characteristics. We find model agreement within 10 %-30 % in the previously defined "soot-poor" regime. However, discrepancies increase non-linearly (up to 60 %) in the "soot-rich" regime, due to differing treatment of droplet growth. Both models predict that, in the "soot-poor" regime, AEI(ice) approaches 10(16) kg(-1) for low ambient temperatures (< 210 K) and sulfur-rich vPM, which is comparable to estimates in the "soot-rich" regime. Moreover, our sensitivity analyses suggest that the point of transition between the "soot-poor" and "soot-rich" regimes is a dynamic threshold that ranges from 10(13)-10(16) kg(-1) and depends sensitively on ambient temperature and vPM properties, underlining the need for vPM emission characterisation measurements. We suggest that existing contrail simulations omitting vPM activation may underestimate AEI(ice), especially for flights powered by lean-burn engines. Furthermore, our results imply that, under these conditions, AEI(ice) might be reduced by (i) reducing fuel sulfur content, (ii) minimising organic emissions, and/or (iii) avoiding cooler regions of the atmosphere.

Ice crystal formation in cirrus clouds is poorly understood, and its representation remains a challenge in global climate models. To enhance the understanding, a novel ice nucleation parameterization based on the Kärcher (2022) (K22) scheme is introduced into the NCAR Community Atmosphere Model version 6 (CAM6). To investigate ice formation in cirrus clouds, sensitivity tests are conducted to analyze three ice sources from orographic gravity wave (OGWs), convective detrainment, and turbulence. These tests employ both the K22 scheme and the default Liu and Penner (2005) (LP05) scheme. Model evaluation includes 6-year climatology and nudged simulations representing the Small Particles in Cirrus (SPARTICUS) and O2 / N2 Ratio and CO2 Airborne Southern Ocean Study (ORCAS) campaigns. Both schemes simulate that convection detrained and turbulence-induced ice crystals are concentrated in low- to mid-latitudes, whereas OGW-induced ice crystals are concentrated in mid- to high latitudes. Compared to the LP05 scheme, the K22 scheme generates a higher number of ice crystals. The simulated cloud microphysical properties using the K22 scheme align well with observations for orographic cirrus during the SPARTICUS campaign. In orographic cirrus over high terrains at mid- to high latitudes, both schemes identify OGW-induced ice crystals as the dominant ice source. Due to its distinct competition parameterizations, the K22 scheme exhibits less contribution from minor ice sources (convection detrained and turbulence-induced). This underscores the significance of competition mechanisms within ice nucleation schemes and helps clarify regional and dynamical controls on ice sources in cirrus clouds. The application of two distinct nucleation schemes provides valuable insights into the dominant ice sources in cirrus clouds.

Aircraft contrails, formed largely on soot particles in current flights, are important for aviation's non-CO2 climate impact. Here we show that the activation of nonvolatile soot particles during contrail formation is likely determined by the sizes of primary soot particles rather than the effective sizes of soot aggregates as assumed in previous studies, which can explain less-than-unity fractions of soot particles forming contrail ice particles as recently observed during ECLIF (Emission and CLimate Impact of alternative Fuels) campaigns. The smaller soot primary sizes compared to aggregate sizes delay the onset of contrail ice formation, increase the maximum plume supersaturation reached in the contrail plume, and thus increase the probability of small volatile particles contributing to the total contrail ice particle number. This study suggests that the range of conditions for volatile plume particles to contribute significantly to the contrail ice number budget is wider than previously thought. As the aviation industry is moving toward sustainable aviation fuel and/or lean-burning engine technology, which is expected to reduce not only the emission index of nonvolatile soot particles but also the sizes of primary soot particles, this study highlights the need to better understand how the combined changes may affect contrail formation, contribution of volatile particles, and climate impacts.

Tropical cirrus clouds, i.e., any type of ice cloud with tops above 400 hPa, play a critical role in the climate system and are a major source of uncertainty in our understanding of global warming. Tropical cirrus clouds involve processes spanning a wide range of spatial and temporal scales, from ice microphysics on cloud scales to mesoscale convective organization and planetary wave dynamics. This complexity makes tropical cirrus clouds notoriously difficult to model and has left many important questions stubbornly unanswered. At the same time, their multi-scale nature makes them well-positioned to benefit from the rise of global, high-resolution simulations of Earth's atmosphere and a growing abundance of remotely sensed and in situ observations. Rapid progress on our understanding of tropical cirrus requires coordinated efforts to take advantage of these modern computational and observational abilities. In this opinion paper, we review recent progress in cirrus studies, highlight important unanswered questions, and discuss promising paths forward. Significant progress has been made in understanding the life cycle of convectively generated “anvil” cirrus and the response of their macrophysical properties to large-scale controls. On the other hand, much work remains to be done to fully understand how small-scale anvil processes and the climatological anvil radiative effect will respond to global warming. Thin, in situ formed cirrus clouds are now known to be closely tied to the thermal structure and humidity of the tropical tropopause layer, but microphysical uncertainties prevent a full understanding of this link, as well as the precise amount of water vapor entering the stratosphere. Model representation of ice-nucleating particles, water vapor supersaturation, and ice depositional growth continue to pose great challenges to cirrus modeling. We believe that major advances in the understanding of tropical cirrus can be made through a combination of cross-tool synthesis and cross-scale studies conducted by cross-disciplinary research teams.

Abstract. Tropical cirrus clouds play a critical role in the climate system and are a major source of uncertainty in our understanding of global warming. Tropical cirrus are affected by processes spanning a wide range of spatial and temporal scales, from ice microphysics on cloud scales to mesoscale convective organization and planetary wave dynamics. This complexity makes tropical cirrus clouds notoriously difficult to model and has left many important questions stubbornly unanswered. At the same time, their multi-scale nature makes them well positioned to benefit from the rise of global, high-resolution simulations of Earth's atmosphere and a growing abundance of remotely sensed and in situ observations. Rapid progress requires coordinated efforts to take advantage of these modern computational and observational abilities. In this Opinion, we review recent progress in cirrus studies, highlight important questions that remain unanswered, and discuss promising paths forward. We find that significant progress has been made in understanding the life cycle of convectively generated ``anvil" cirrus and how their macrophysical properties respond to large-scale controls. On the other hand, much work remains to be done to understand how small-scale anvil processes and the climatological anvil radiative effect may respond to global warming. Thin, in situ-formed cirrus are now known to be closely tied to the thermal structure and humidity of the tropical tropopause layer (TTL), but uncertainty at the microphysical scale remains a significant barrier to understanding how these clouds regulate the TTL moisture and temperature budgets, as well as the mixing ratio of water vapor entering the stratosphere. Model representation of ice-nucleating particles, water vapor supersaturation, and ice depositional growth continue to pose great challenges to cirrus modeling. We believe that major advances in the understanding of tropical cirrus can be made through a combination of cross-tool synthesis and cross-scale studies conducted by cross-disciplinary research teams.

The homogeneous nucleation of ice in supercooled liquid-water clouds is characterized by time-dependent freezing rates. By contrast, water phase transitions induced heterogeneously by ice-nucleating particles (INPs) are described by time-independent ice-active fractions depending on ice supersaturation (s). Laboratory studies report ice-active particle number fractions (AFs) that are cumulative in s. Cloud models budget INP and ice crystal numbers to conserve total particle number during water phase transitions. Here, we show that ice formation from INPs with time-independent nucleation behavior is overpredicted when models budget particle numbers and at the same time derive ice crystal numbers from s-cumulative AFs. This causes a bias towards heterogeneous ice formation in situations where INPs compete with homogeneous droplet freezing during cloud formation. We resolve this issue by introducing differential AFs, thereby moving us one step closer to more robust simulations of aerosol–cloud interactions.

Fully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft-emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process-based underpinning for improved climate model parametrizations and targeted field observations.

Many climate intervention (CI) methods have been proposed to offset greenhouse gas-induced global warming, but the practicalities regarding implementation have not received sufficient attention. Stratospheric aerosol injection (SAI) involves introducing large amounts of CI material well within the stratosphere to enhance the aerosol loading, thereby increasing reflection of solar radiation. We explore a delivery method termed solar-powered lofting (SPL) that uses solar energy to loft CI material injected at lower altitudes accessible by conventional aircraft. Particles that absorb solar radiation are dispersed with the CI material and heat the surrounding air. The heated air rises, carrying the CI material to the stratosphere. Global model simulations show that black carbon aerosol (10 microgram per cubic meter) is sufficient to quickly loft CI material well into the stratosphere. SPL could make SAI viable at present, is also more energy efficient, and disperses CI material faster than direct stratospheric injection.

Atmospheric ice formation in cirrus clouds is often initiated by aerosol particles that act as ice-nucleating particles. The aerosol–cloud interactions of soot and associated feedbacks remain uncertain, in part because a coherent understanding of the ice nucleation mechanism and activity of soot has not yet emerged. Here, we provide a new framework that predicts ice formation on soot particles via pore condensation and freezing (PCF) that, unlike previous approaches, considers soot particle properties, capturing their vastly different pore properties compared to other aerosol species such as mineral dust. During PCF, water is taken up into pores of the soot aggregates by capillary condensation. At cirrus temperatures, the pore water can freeze homogeneously and subsequently grow into a macroscopic ice crystal. In the soot-PCF framework presented here, the relative humidity conditions required for these steps are derived for different pore types as a function of temperature. The pore types considered here encompass n-membered ring pores that form between n individual spheres within the same layer of primary particles as well as pores in the form of inner cavities that form between two layers of primary particles. We treat soot primary particles as perfect spheres and use the contact angle between soot and water (θsw), the primary particle diameter (Dpp), and the degree of primary particle overlap (overlap coefficient, Cov) to characterize pore properties. We find that three-membered and four-membered ring pores are of the right size for PCF, assuming primary particle sizes typical of atmospheric soot particles. For these pore types, we derive equations that describe the conditions for all three steps of soot PCF, namely capillary condensation, ice nucleation, and ice growth. Since at typical cirrus conditions homogeneous ice nucleation can be considered immediate as soon as the water volume within the pore is large enough to host a critical ice embryo, soot PCF becomes limited by either capillary condensation or ice crystal growth. We use the soot-PCF framework to derive a new equation to parameterize ice formation on soot particles via PCF, based on soot properties that are routinely measured, including the primary particle size, overlap, and the fractal dimension. These properties, along with the number of primary particles making up an aggregate and the contact angle between water and soot, constrain the parameterization. Applying the new parameterization to previously reported laboratory data of ice formation on soot particles provides direct evidence that ice nucleation on soot aggregates takes place via PCF. We conclude that this new framework clarifies the ice formation mechanism on soot particles in cirrus conditions and provides a new perspective to represent ice formation on soot in climate models.

During the coronavirus pandemic, a simplistic model has been developed as an attempt of a non-specialist scientist in lockdown to better understand the evolution of infectious diseases. The model is based on Monte Carlo simulation and applied to a closed, homogeneous population. Basic characteristics of epidemics dynamics are analyzed and compared to results from a deterministic model. Sensitivity studies corroborate key recommendations of public health officials.

Recent laboratoryexperiments have advanced our understanding of heterogeneous ice nucleation at low temperatures. We use these laboratory results, along with field measurements of aerosol composition, to constrain a parameterization of heterogeneous nucleation rate dependence on temperature and supersaturation. We apply this nucleation parameterization in detailed simulations of transport and cloud formation in the Tropical Tropopause Layer (TTL, ≃14–18 km) constrained by high‐altitude aircraft measurements of TTL cirrus microphysical properties and supersaturation. The model results indicate that if the abundant TTL organic‐containing aerosols were effective glassy ice nuclei (IN), as indicated by laboratory experiments with simple sugars/acids, then heterogeneous nucleation would prevent the occurrences of large ice concentrations and large ice supersaturations, both of which are clearly indicated by the in situ observations. If glassy organic aerosols are relatively poor IN, as indicated by laboratory experiments with more atmospherically relevant secondary organic aerosol‐like composition, then the simulated ice concentrations and supersaturations are in reasonable agreement with the observations. Concentrations of effective mineral dust or ammonium sulfate IN larger than about 50/L can drive significant changes in cirrus microphysical properties and occurrence frequency. The cloud occurrence frequency can either increase or decrease, depending on the efficacy and abundance of IN added to the TTL. Relatively low TTL concentrations of mineral dust particles (≤10 L−1 indicated by the limited available field measurements) have negligible impact on cirrus frequencies or microphysical properties. Given the assortment of uncertainties, meaningful estimates of TTL aerosol‐cirrus indirect effects on radiative forcing are likely not possible at this time.

Aircraft-produced contrail cirrus clouds contribute to anthropogenic climate change. Observational data sets and modelling approaches have become available that clarify formation pathways close to the source aircraft and lead to estimates of the global distribution of their microphysical and optical properties. While contrail cirrus enhance the impact of natural clouds on climate, uncertainties remain regarding their properties and lifecycle. Progress in representing aircraft emissions, contrail cirrus and natural cirrus in global climate models together with tighter constraints on the sensitivity of the climate system will help judge efficiencies of and trade-offs between mitigation options.