We develop an idealized, physically based model describing combined effects of ice nucleation and sublimation on ice crystal number during persistent contrail formation. Our study represents the ...first effort to predict ice numbers at the point where contrails transition into contrail cirrus—several minutes past formation—by connecting them to aircraft soot particle emissions and atmospheric supersaturation with respect to ice. Results averaged over an observed exponential distribution of ice supersaturation (mean value 15%) indicate that large reductions in soot particle numbers are needed to lower contrail ice crystal numbers significantly for soot emission indices around 1015 (kg fuel)−1, because reductions in nucleated ice number are partially compensated by sublimation losses. Variations in soot particle (−50%) and water vapor (+10%) emission indices at threefold lower soot emissions resulting from biofuel blending cause ice crystal numbers to change by −35% and <5%, respectively. The efficiency of reduction depends on ice supersaturation and the size distribution of nucleated ice crystals in jet exhaust plumes and on atmospheric ice supersaturation, making the latter another key factor in contrail mitigation. We expect our study to have important repercussions for planning airborne measurements targeting contrail formation, designing parameterization schemes for use in large‐scale models, reducing uncertainties in predicting contrail cirrus, and mitigating the climate impact of aviation.
Plain Language Summary
The formation and modification of ice crystals in persistent aircraft condensation trails (contrails) is an important component in evaluating the climate impact of aviation. We connect for the first time contrail ice numbers at the end of the formation stage to aircraft soot emissions and atmospheric ice supersaturation. We offer a framework to estimate changes in ice number and to compare effects of mitigation options. Our results show that large reductions in soot emissions are required to lower contrail ice numbers significantly, and those ice numbers are insensitive to small variations of the amount of water vapor emitted by aircraft jet engines. Our study has important implications for planning field measurements, reducing uncertainties in numerical predictions of contrail cirrus effects on climate and for mitigating this impact.
Key Points
First study to link ice crystal nucleation and sublimation in the contrail formation stage at the process level
Small variations in soot particle and water vapor emissions do not affect initial ice crystal numbers strongly
Conceptual framework useful to compare effects of mitigation options targeting contrail ice formation
The susceptibility of microphysical properties of young contrails to changes in aircraft soot emissions is studied with a microphysical plume model. Liquid plume and ambient particles compete with ...exhaust soot particles for the formation of contrail ice particles, assuming that soot particles are activated into water droplets prior to homogeneous freezing. Soot controls ice formation in contrails for high number emission indices including the range of current global fleet values. A fivefold reduction of soot emissions from average levels of 5 × 1014 − 1015 (kg‐fuel)−1 approximately halves the initial contrail visible optical depth. Further soot reduction reverses this trend at temperatures well below the formation threshold temperature unless emissions of sulfur and organics are cut substantially. Whether and to which degree reductions in soot emissions help mitigate the contrail climate impact depends on subsequent aircraft wake vortex processing of contrails and their development into contrail cirrus.
This study develops an advanced physically‐based parameterization of heterogeneous ice nucleation in cirrus clouds that includes an updated parameterization of stochastic homogeneous freezing of ...supercooled solution droplets. Both components are formulated based on the same methodology and level of approximation, without numerical integration of the underlying ice supersaturation equation. The new scheme includes measured ice nucleation spectra describing deterministic ice activation from an arbitrary number of types of ice‐nucleating particles (INPs), tracks the competition for available water vapor between the different ice nucleation modes, and allows for new ice formation and growth within pre‐existing cirrus clouds. The computationally efficient scheme works with a minimal set of physical input parameters and predicts total nucleated ice crystal number concentrations (ICNCs) along with the maximum ice supersaturation attained during cirrus formation events. Aspects of its implementation into host models are discussed, including the provision of suitably parameterized vertical wind speeds. The parameterization is validated by comparisons to numerical simulations. First off‐line applications to mineral dust and aviation soot particles are presented, including ICNC ensemble statistics resulting from the coupling with statistics of updraft speed variability.
Plain Language Summary
Two decades after introduction of the first parameterization of cirrus cloud formation by freezing of ubiquitous liquid solution droplets, an improved version is developed based on the latest experimental findings regarding solid ice‐nucleating particles, a small subset of the atmospheric aerosol. The new scheme allows to predict ice crystal formation in cirrus from competing homogeneous freezing and heterogeneous ice activation more realistically and with greater computational efficiency. It considers new developments regarding the properties of vertical wind speeds (triggering ice formation) and the molecular kinetics of water vapor uptake onto ice crystals (controlling ice growth). This study explains the foundation of cirrus ice formation and growth based on cloud physical theory, derives and explains the parameterization, discusses its use in host models to facilitate applications, checks its performance by comparison to comprehensive numerical simulations, and presents first results involving mineral dust and aircraft‐emitted soot particles as examples for good and poor atmospheric ice‐nucleating particles, respectively.
Key Points
Competing ice nucleation processes in cirrus are predicted reliably and efficiently
Partial activation of dust particles may occur frequently in cirrus formation
Nucleation of ice within already‐existing cirrus requires high updraft speeds
Dust ice nuclei effects on cirrus clouds Kuebbeler, M; Lohmann, U; Hendricks, J ...
Atmospheric chemistry and physics,
03/2014, Letnik:
14, Številka:
6
Journal Article
Recenzirano
Odprti dostop
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.
Effects of a spectrum of mesoscale gravity waves on homogeneous aerosol freezing in midlatitude cirrus are studied by means of parcel model simulations that are driven by random vertical wind speeds ...constrained by balloon measurements. Stochastic wave forcing with mean updraft speeds of 5–20 cm/s leads to substantial nucleated ice crystal number concentrations (ICNC) of 0.1–1 cm−3 in situations with slow large‐scale cooling, which by itself would generate fewer ice crystals. The stochastic nature of wave‐driven air parcel temperatures enhances ICNC even further, but the times required to reach freezing conditions unsupported by large‐scale cooling may vary widely. In the presence of wave forcing, ice crystals with low ICNC (<1–10 L−1) are also generated by homogeneous freezing, albeit only rarely. Comparisons with aircraft measurements suggest significant influences of heterogeneous ice‐nucleating particles and ice crystal sedimentation on ICNC, but quantifying their individual contributions remains elusive.
Plain Language Summary
Spontaneous freezing of airborne, water‐containing particles below −38 °C is a fundamental pathway to form ice crystals in high‐altitude cirrus clouds. This ice formation process has been well researched and was the first represented in weather forecast and climate models to advance cirrus predictions. One key characteristic is its strong dependence of the number of ice crystals formed on the cooling rate of air. Recent observations show that rapid cooling rates are generated by ubiquitous gravity waves. Here, we explore the rich suite of phenomena taking place during cirrus formation caused by a spectrum of gravity waves. We find that wave effects should be considered in future model simulations, when comparing model results with observations, and in parameterizations of cloud ice crystal formation.
Key Points
We present a systematic process study of effects of a spectrum of gravity waves on homogeneous ice nucleation in cirrus through ensemble simulations
High cooling rates have disproportionately large impact on nucleated ice crystal number concentrations at low background updraft speeds
Analysis of midlatitude continental cirrus measurements suggests impact of heterogeneous nucleation and sedimentation on total ice numbers
A multiple‐mode ice microphysical scheme is applied in the European Centre/Hamburg (ECHAM) general circulation model to simulate effects of aerosol‐ice interactions on global cirrus properties. The ...different ice modes represent cirrus ice formed by homogeneous freezing of liquid aerosols and heterogeneous nucleation on mineral dust or black carbon particles. A fourth ice mode represents ice from other sources. The competition of these modes for available water is realized in a physical parameterization scheme considering also the effect of preexisting ice on the ice nucleation process. The model is applied to analyze the global characteristics of ice formed by the different aerosol types and to study potential global effects of mineral dust and black carbon particles on cirrus microphysical parameters. The simulations reveal that, on average, ice from heterogeneous nucleation shows fewer but larger crystals and has a smaller contribution to the mean cirrus ice water content than ice from homogeneous freezing. However, heterogeneous ice nuclei may have important effects on the overall cirrus properties. Reductions in zonal mean annual average cirrus ice particle number concentrations induced by heterogeneous nucleation of up to 20% in the tropics and 1%–10% in the midlatitudes are simulated. The effect is further amplified by ice formation on aircraft‐generated soot. Significant reductions in the mean ice water content are modeled, which likely result from efficient sedimentation and precipitation of large ice particles generated by heterogeneous nucleation. This leads to reductions in the zonal mean annual average water vapor mixing ratio of up to 5% at cirrus levels.
Key Points
Aerosol‐cirrus interactions are simulated in a global climate model
Mineral dust and black carbon aerosol can affect cirrus globally
Cirrus ice types generated by different aerosol types are characterized
We investigate homogeneous freezing of aqueous aerosol particles, a fundamental ice formation process in cirrus clouds. We estimate freezing time scales and vertical extensions of freezing layers, ...demonstrating that such freezing events are highly transient and localized. While time scales decrease with increasing vertical velocity driving ice nucleation, layer depths are weak functions of the vertical velocity. Our results are used to discuss possible effects of turbulent diffusion and entrainment‐mixing on homogeneous freezing in cirrus. Large turbulent diffusivity acts to broaden water vapor‐depleted freezing layers and facilitate sedimentation of freshly nucleated ice crystals out of them into ice‐supersaturated air. Homogeneous freezing events could be affected by microscale turbulence in episodes of intense turbulence dissipation rates, although such episodes are rare. We conjecture that freezing layers are broader in the case of heterogeneous ice nucleation and effects of sedimentation on nucleation increase in importance. Our findings point to the difficulty of inferring nucleated cirrus ice crystal numbers from measurements and place tight constraints on cirrus models with regard to spatial and temporal resolution.
Key Points
Homogeneous freezing events in cirrus are highly transient and localized
Strong diffusion and turbulence affects the number of nucleated ice crystals
Modeling homogeneous freezing requires high temporal and spatial resolution
Homogeneous droplet freezing in the warm cirrus regime (230–240 K) is investigated along idealized convective cloud trajectories using a spectral parcel model developed to track droplet freezing ...events accurately. The novel model is described and used to study ice formation from rapidly ascending (vertical velocity 0.6–6 m s−1) air parcels containing cloud condensation nuclei (CCN) and liquid water droplets. Homogeneous freezing events in warm cirrus are affected by latent heat exchange and produce a mode of small ice crystals with maximum dimensions 10–100 μm after initial supersaturation quenching. During the formation stage, ice‐crystal number concentrations formed homogeneously in convective cloud outflow are hardly affected by ice‐crystal settling and depend sensitively on vertical velocity. In the case of CCN activation into cloud water droplets prior to or along with freezing, relative humidity variations also result in widely varying ice numbers that are insensitive to CCN solubility. These results offer pointers on how further progress can be achieved in simulating and better understanding the formation of upper tropospheric ice clouds originating from convective detrainment zones.
Homogeneous droplet freezing in the warm cirrus regime (230–240 K) is investigated along idealized convective cloud trajectories using a spectral microphysical parcel model. In cold convective detrainment zones, homogeneously nucleated ice‐crystal number concentrations are hardly affected by ice‐crystal settling and depend sensitively on vertical velocity and CCN load. Homogeneous freezing temperatures depend on whether CCN activation into cloud water droplets occurs prior to or along with cloud‐droplet freezing.
We study losses of ice crystals in a persistent, soot‐rich contrail in the wake behind a medium‐sized aircraft at cruise. Constraining a model covering ice nucleation, growth, and sublimation phases ...with an aircraft data set, we track the sublimation history over 2 min of contrail age and relate ice crystal numbers to the number of soot particles emitted by the aircraft engines. We analyze the observed vertical distribution of ice numbers, estimating an exponential scale height in the range 50–100 m and wake‐averaged ice numbers (1.3–1.7) × 1015 (kg‐fuel)−1 after sublimation, removing 60% of the ice crystals that originally nucleated on emitted soot particles. We define soot emission‐ and ice supersaturation‐dependent contrail depths, affecting estimates of horizontal spreading rates of contrails. Our findings have ramifications for the representation of long‐lived contrails in global models.
Plain Language Summary
Contrails are climate‐forcing agents, but their overall climatic effect is difficult to quantify. Airborne measurements quantify properties of the initially line‐shaped ice clouds forming behind cruising jet aircraft. Exploring the formation stage of contrails reveals something about their properties and enables us to predict how they evolve in the atmosphere. We elucidate processes in a contrail that remove within few minutes more than half of the ice crystals generated from the engine emissions. Ice crystal number in, and vertical extension of, young contrails are fundamental determinants of processes governing their life cycle and climate impact. Our results highlight the need for focused observational studies and pave the way for improvements in the representation of contrails and the clouds evolving from them in climate models.
Key Points
We analyze the sublimation of contrail ice crystals in the aircraft wake vortex regime
Wake‐average ice numbers increase with soot emissions despite significant sublimation losses
Inhomogeneous vertical distributions of ice numbers affect processes controlling contrail life cycles
Ubiquitous mesoscale gravity waves generate high cooling rates important for cirrus formation. We make use of long‐duration balloon observations to devise a probabilistic model describing mesoscale ...temperature uctuations (MTF) away from strong wave sources. We define background conditions based on observed probability distributions of temperature and underlying vertical wind speed fluctuations. We show theoretically that MTF are subject to damping at a rate near the Coriolis frequency when the vertical wind speed fluctuations are autocorrelated over a fraction of a Brunt‐Väisälä period. We find that for background wave activity, a decrease in temperature of 1K translates into cooling rate standard deviations and mean updraft speeds of 4–8Kh−1 and ≈ 10–20 cms−1, respectively, depending on latitude and stratification. We introduce an effective Coriolis frequency to generate cooling rates in equatorial regions consistent with balloon data. Above ice saturation, MTF are large enough to affect ice crystal nucleation. Our results help constrain uncertainty in aerosol‐cirrus interactions, provide insights to better meet challenges in comparing measurement data with model simulations, and support the development of cutting‐edge ice cloud schemes in global models.
Plain Language Summary:
The limited scientific understanding of pure ice clouds (cirrus)—and therefore the difficulty to account for them in models—causes substantial uncertainty in climate projections. Two research issues important for cirrus formation continue to form a roadblock on the path of scientific progress: the dynamical forcing driving cirrus ice crystal formation and the ice‐forming properties of solid atmospheric particles. Long‐duration balloons floating in the high atmosphere have quantified key properties of gravity waves that generate vertical air motions (cooling rates) crucial for ice formation in cirrus. Only when occurrence and magnitude of cooling rates are well understood can effects of different solid and liquid ice‐forming particles during cirrus formation be predicted with confidence. Blending insights obtained from the research balloon measurements with theoretical methods developed in statistical physics, our study elaborates on the dynamical forcing issue by devising a model that represents air parcel cooling rates on a probabilistic basis. We thereby hope to contribute significantly to a comprehensive process understanding and, ultimately, to removing one of the roadblocks in cloud research.
Key Points
A probabilistic model for gravity wave‐induced mesoscale temperature fluctuations is developed based on observations
A relationship between wave‐induced temperature and associated cooling/heating rate fluctuation amplitudes is derived
It is shown how wave effects can be included in microphysical simulation models and global model parameterizations
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