Contrail Cirrus Research Articles

Aerodynamic Contrails: Microphysics and Optical Properties

Abstract Aerodynamic contrails form when air flows across the wings of subsonic aircraft in cruise. During a short adiabatic expansion phase, high supersaturations trigger burstlike homogeneous ice formation on ambient liquid aerosol particles within a wing depth. Small particles freeze first because they equilibrate most rapidly. Ambient temperature is the key determinant of nascent aerodynamic contrail properties. Only above ∼232 K do they become visible (but optically thin). These temperatures are at the high end of those prevailing at tropical upper tropospheric flight levels of subsonic aircraft. In colder midlatitude conditions, aerodynamic contrails stay invisible and the very small ice particles formed quickly evaporate when exposed to small subsaturations, explaining why the formation of these contrails is rarely observed. After formation, aerodynamic contrails develop into contrail cirrus if air is supersaturated with respect to ice. This type of anthropogenic ice cloud adds to contrail cirrus derived from jet exhaust contrails and may become particularly important in the future because air traffic is projected to increase significantly in tropical and subtropical regions. Regardless of whether aerodynamically induced ice formation leads to persistent contrail cirrus, cruising aircraft may act as sources of potent heterogeneous ice nuclei by preactivating the insoluble fraction in atmospheric particle populations. Aerodynamic contrails and aerodynamically induced preactivation should therefore be studied experimentally and with global models to explore their potential to induce climate change.

Role of aircraft soot emissions in contrail 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.

Contrail cirrus supporting areas in model and observations

Contrails form and persist dependent on the surrounding moisture, temperature and pressure fields and on fuel and aircraft specific variables. After formation, contrail persistence requires only supersaturation relative to ice. The fractional area in which contrails can form is called potential contrail coverage. We introduce a potential contrail cirrus coverage equivalent to the cloud free supersaturated area. This field, simulated by the ECHAM4 climate model, agrees fairly well with estimates of supersaturation frequency as inferred from aircraft and satellite measurements. In areas where the two potential coverages are different, especially at lower flight levels, potential contrail coverage is not a valid estimate of maximum attainable contrail cirrus coverage. We parameterize both potential coverages consistently with the ECHAM4 cloud cover parameterization. A comparison of the potential contrail coverage with an earlier estimate reveals substantial differences especially at upper height levels in the tropics.

Overview on Contrail and Cirrus Cloud Avoidance Technology

Greenhouse gas emissions, contrails, and aviation-induced cirrus clouds are the principal pollutants from air traffic, which contribute to the anthropogenic global warming. Recent climate assessments have stressed the importance of contrails and aviation-induced cirrus clouds because they may contribute more than all other aircraft emissions combined. This paper reviews the mechanisms of contrail and aviation-induced cirrus cloud formation from which avoidance strategies are derived and discussed.

A Sensitivity Study of the Effect of Horizontal Photon Transport on the Radiative Forcing of Contrails

Abstract With the rapid growth in air travel, there is concern over the radiative impact of contrails and aircraft-induced cirrus on climate. Previous radiation calculations on contrails have almost all used the independent column approximation, which neglects the transport of photons through the sides of the contrail, but in this study the 3D effects are quantified using the Spherical Harmonic Discrete Ordinate Method (SHDOM). The authors have investigated the dependence of shortwave and longwave radiative forcing on contrail aspect ratio, optical depth, solar zenith angle, solar azimuth angle relative to contrail orientation, particle size, particle habit, surface albedo, and surface temperature. It is found that inclusion of 3D transport results in an increase in the positive longwave radiative forcing of the contrail and either an increase or a decrease in the magnitude of the negative shortwave radiative forcing depending on the orientation of the contrail with respect to the sun. Although these two effects are individually quite modest (of order 10%), the fact that the total shortwave and longwave forcings largely cancel during the day means that the relative change in the net radiative forcing due to the 3D effect is substantial; in some cases this results in a doubling of the net forcing of the contrail, in other cases changing its sign. On a more general note, the relatively simple geometry of contrail cirrus provides an ideal test case for explaining the various mechanisms by which 3D photon transport can change the radiative effect of clouds, which can be rather difficult to visualize for more complex cloud scenarios.

Impact of H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O coating and ice crystal size on radiative properties of sub-visible cirrus

Abstract. Recent laboratory experiments showed that at conditions resembling those near the tropopause region, small ice particles can be coated by a liquid H2SO4/H2O over-layer formed after the freezing of diluted sulfuric acid/water aerosol drops. Here, idealized radiative transfer tests are conducted to evaluate the impact that such an over-layer would have on the radiative effects produced by sub-visible cirrus clouds (SVCs). Spherical particle shape is assumed to keep the problem tractable. The calculations show that the over-layer increases both the shortwave (SW) and longwave (LW) cloud radiative effects (CRE), but the impact is small: ~0.02 W m−2, or even less, for the total (LW+SW) CRE at the top of the atmosphere. For the smallest ice particles, for which the over-layer is thickest, the fractional change in CRE can, however, reach ~20% for the SW CRE and over 50% for the LW CRE. The dependence of LW and SW CRE on particle size is also studied in the paper. Calculations for spherical and spheroidal uncoated ice particles show that even for high, optically thin cirrus, the total CRE can be negative, if the diameter of the particles is smaller than about 3–4 μm. Apart from the SVCs, this result could be relevant for contrail cirrus clouds, which are believed to consist of large numbers of very small ice particles.

Potential of the cryoplane technology to reduce aircraft climate impact: A state-of-the-art assessment

The potential reduction in climate impact due to a switch from kerosene supported aviation to liquid hydrogen (LH 2) supported aviation (cryoplanes) is assessed. Different scenarios for a respective gradual technology transition between 2015 and 2050 are provided. State-of-the-art simulations of three-dimensional (3D) radiative forcing (RF) distributions are compiled and complemented for the various agents determining aviation climate impact. They are utilized to quantify the globally averaged transient climate response and respective differences between the transition scenarios. A climate impact reduction of between 15% and 50% in terms of RF is indicated for the cryoplane scenarios at the 2050 time slice, with a best estimate near 30% in case of a swift transition. The respective reduction range is between 5% and 15% in terms of surface temperature change, with a best estimate of about 10%. The environmental benefit of a cryoplane transition further increases if time horizons beyond 2050 are considered. Besides known uncertainties in quantifying aviation climate impacts, crucial limitations of our assessment are insufficient knowledge on contrail cirrus induced by cryoplanes and on the feasibility to produce large amounts of LH 2 from renewable energy sources.

Editorial

Aviation can impact climate in several ways: (1) Aircraft emit greenhouse gases, in particular CO2 and water vapour. (2) They emit precursors of greenhouse gases, like NOx, which impacts the atmospheric abundances of ozone and methane. (3) Aircraft engines emit particles and their precursors; the corresponding aerosols are radiatively active and modify the optical and micro-physical properties of clouds. (4) Aviation triggers additional clouds (contrails and contrail cirrus) from its water vapour emissions. In all cases the result is a radiative forcing that eventually leads to climate change. Due to the particular location of aircraft emissions, i.e., in the upper troposphere / lower stratosphere, aviation’s contribution to anthropogenic climate change is significantly larger than would be expected from its CO2 emissions alone. Furthermore, aviation is one of the fastest growing industrial sectors, e.g., in the first half of 2005 the number of revenue passengers travelling from German airports increased by 6.9 % relative to the same period of the previous year, according to a recent press release3 of the Arbeitsgemeinschaft Deutscher Verkehrsflughafen (ADV). Therefore, the impact of aviation on the composition of the global atmosphere and on climate has attracted a particular scientific interest, which resulted in a Special Report of the Intergovernmental Panel on Climate Change: “Aviation and the Global Atmosphere” (IPCC, 1999). Research has continued since the publication of this Special Report and the scientific community has continued in its endeavours to increase our knowledge and to reduce uncertainties. The topic has been further explored with laboratory measurements, large field campaigns, theoretical studies, analyses of satellite data and numerical simulations. During recent years many European projects (e.g. AEROCHEM-2, TRADEOFF, PARTEMIS, MOZAIC, AERO2K, SCENIC, INCA) and many national projects have been devoted to the topic of “Aviation, Atmosphere and Climate”. Significant scientific progress has been achieved: the aviation impact on the atmospheric concentrations of ozone and methane and their uncertainties have been better quantified; a consistent quantification of the radiative forcing from linear contrails has been provided; the interaction between aerosols emitted by aircraft and clouds is now better understood; first quantitative estimates of the radiative forcing from contrail cirrus are now available; and we have learned more about the potential mitigation of aircraft effects on climate. The European Conference on Aviation, Atmosphere and Climate (AAC)4 in Friedrichshafen (Lake Constance, Germany), 30 June to 3 July 2003, was initiated by the European Commission to update our knowledge on the atmospheric impacts of aviation. This Special Issue of METEOROLOGISCHE ZEITSCHRIFT comprises 15 papers from this conference. It covers a wide range of topics: aviation inventories (GARBER et al.; BUKOVNIK and KALIVODA), chemical reaction rates (SOMNITZ et al.), particles from aviation (PETZOLD et al.), ice nucleation (MOHLER et al.; MANGOLD et al.; LIU and PENNER), contrails (MINNIS et al.; PALIKONDA et al.; DUDA et al.), contrail cirrus (MANNSTEIN and SCHUMANN), radiative forcing from aviation (SAUSEN et al.), and mitigation of aircraft environment effects (FICHTER et al.; MARQUART et al.; GREEN).

Aircraft induced contrail cirrus over Europe

Condensation trails (contrails) and aircraft induced cirrus are nowadays a common feature at the mid latitude skies. Previously the impact of aircraft induced cirrus changes has been roughly estimated from observed decadal trends in cirrus cover but the direct attribution of observed cirrus changes to changes in aviation activity remains uncertain. In this paper the amount of additional cirrus induced from spreading contrails in humid air is estimated from the direct correlation between observed cirrus cover derived with suitable methods from METEOSAT data and aviation flight density reported by EUROCONTROL at high spatial and temporal resolution from June 22 to July 27, 1998 and September 27 to October 21, 2000. The results indicate that the aircraft induced cirrus cover over Europe is about ten times larger than that of linear contrails in the same region. Radiative forcing from the additional cirrus may be more than 10 times higher than that of linear contrails and aviation induced CO 2 increases.

Impact of air traffic on cirrus coverage

TOVS Path‐B satellite data provide information on effective high cloud amount and relative humidity for the period 1987 to 1995. Differences in trends of seasonal mean effective high cloud amount between situations favorable for contrails and for cirrus are analyzed in regions with high and low air traffic density. In regions with especially high air traffic density, a significantly stronger increase of effective high cloud amount is found for situations with sufficiently cold and humid air masses favorable for contrails than for all situations in general or for situations favorable for cirrus. Situations of potential contrails occur in about 5 to 10% of all situations, and their seasonal effective high cloud amount averages lie between about 7% and 22%. Indicators of cirrus increase due to air traffic corresponding to the difference in trends of effective high cloud amount between potential contrail situations and cirrus or all situations are about 2.8%–3.5% and 1.6%–4.7% per decade over Europe and the North Atlantic flight corridor, respectively. Weighted by frequency of potential contrail occurrence, the overall increase amounts to at least 0.20%–0.25% and 0.08%–0.24% per decade over regions with very high air traffic (Europe and the North Atlantic flight corridor, respectively).

A note on how to avoid contrail cirrus

Aircraft induced cirrus clouds have a major effect on climate. Here we use operational radiosonde data with high vertical resolution to estimate the effect of a small change in flight altitudes on the contrail and cirrus formation. It is shown that a substantial fraction of contrails and contrail induced cirrus can be avoided by relatively small changes in flight level, due to the shallowness of ice-super-saturation layers.

On contrail climate sensitivity

In equilibrium climate change simulations with a global climate model we estimate the climate sensitivity parameter to contrail cirrus to be 0.43 K/(Wm−2), only about 60% of the corresponding value for a CO2 forcing. The spatial pattern of the surface temperature response is much smoother than the forcing pattern of contrails with little correlation between both. As the thermal inertia of the climate system causes a marked delay of the transient response, only about 30% of the equilibrium surface warming to be expected from some aircraft induced radiative forcing is actually realised for aviation increase rates typical for the 1990s. Our findings do not confirm recent results of a dominating influence of contrail cirrus on temperature change over the United States between 1975 and 1994.

Formation, properties and climatic effects of contrails

Condensation trails (contrails) are aircraft induced cirrus clouds, which may persist and grow to large cirrus cover in ice-supersaturated air, and may cause a warming of the atmosphere. This paper describes the formation, occurrence, properties and climatic effects of contrails. The global cover by lined-shaped contrails and the radiative impact of line-shaped contrails is smaller than that assessed in an international assessment in 1999. Contrails trigger contrail cirrus with far larger coverage than observed for line-shaped contrails, but still unknown radiative properties. Some model simulations indicate an impact of particles and particle precursors emitted from aircraft engines on cirrus clouds properties. However, the magnitude of this effect cannot yet be assessed. Contrail formation can be avoided only by flying in sufficiently warm and dry air. The formation of contrail cirrus can be reduced by avoiding flights in ice-supersaturated regions of the atmosphere, e.g., by raising the flight level into the lower-most stratosphere. To cite this article: U. Schumann, C. R. Physique 6 (2005).

Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds.

In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (Delta-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Delta-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Delta-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.

Evidence of impact of aviation on cirrus cloud formation

Abstract. This work examines changes in cirrus cloud cover (CCC) in possible association with aviation activities at congested air corridors. The analysis is based on the latest version of the International Satellite Cloud Climatology Project D2 data set and covers the period 1984-1998. Over the studied areas, the effect of large-scale modes of natural climate variability such as ENSO, QBO and NAO as well as the possible influence of the tropopause variability, were first removed from the cloud data set in order to calculate long-term changes of observed cirrus cloudiness. The results show increasing trends in (CCC) between 1984 and 1998 over the high air traffic corridors of North America, North Atlantic and Europe. Of these upward trends, only in the summertime over the North Atlantic and only in the wintertime over North America are statistically significant (exceeding +2.0% per decade). Over adjacent locations with low air traffic, the calculated trends are statistically insignificant and in most cases negative both during winter and summer in the regions studied. These negative trends, over low air traffic regions, are consistent with the observed large scale negative trends seen in (CCC) over most of the northern middle latitudes and over the tropics. Moreover, further investigation of vertical velocities over high and low air traffic regions provide evidence that the trends of opposite signs in (CCC) over these regions, do not seem to be caused by different trends in dynamics. It is also shown that the longitudinal distribution of decadal changes in (CCC) along the latitude belt centered at the North Atlantic air corridor, parallels the spatial distribution of fuel consumption from highflying air traffic, providing an independent test of possible impact of aviation on contrail cirrus formation. The correlation between the fuel consumption and the longitudinal variability of (CCC) is significant (+0.7) over the middle latitudes but not over the tropics. This could be explained by the fact that over the tropics the variability of (CCC) is dominated by dynamics while at middle latitudes microphysics explain most of its variability. Results from this study are compared with other studies and for different periods of records and it appears that there exists general agreement as to the evidence of a possible aviation effect on high cloud positive trends over regions with congested air traffic.

In situ observations of contrail microphysics and implications for their radiative impact

In this study we present the microphysical characteristics of 21 jet contrail clouds sampled in situ and examine the possible effects of exhaust on natural cirrus and radiative effects of contrails. Microphysical samples were obtained with Particle Measuring Systems (PMS) 2D‐C, 1D‐C, and FSSP probes. About one half of the study contrails were generated by the sampling aircraft, a Cessna Citation, primarily at times of 3–15 min after generation; the source and age of the others is unknown. On average, the contrails contained particles of mean diameter of the order of 10 μm in concentrations exceeding 10,000 L−1. Contrails embedded in natural cirrus appeared to have little effect on the natural cloud microphysics. Anomalous diffraction theory was used to model radiative properties of sampled contrails. The contrail cirrus showed considerably more spectral variation in extinction and absorption efficiencies than natural cirrus because of the large numbers of small crystals in contrails. Embedded contrails also displayed greater emissivity and emission than natural cirrus and a greater spectral variation in transmission.

On the effect of cloud inhomogeneity an area averaged radiative properties of contrails

Certainly cloud inhomogeneity is not only important for cumulus or stratocumulus clouds on which the focus of most of the current studies is. To show that the impact of contrail cirrus microphysical inhomogeneity on cloud albedo can be deduced, fields of ice water content, effective radius, number density, and size distribution of ice particles generated by large eddy simulations (LESs) have been used within two‐dimensional radiative transfer schemes to elucidate the effect of cloud inhomogeneity on area average radiative properties of contrails at evolution stages in the late dispersion regime. The results obtained indicate that differences in shortwave cloud albedo between homogeneous and inhomogeneous conditions remain small for fresh contrails but can become important if contrails would attain larger optical depths. In that case larger differences were obtained at which the albedo for horizontal homogeneous conditions is always larger than the corresponding value for inhomogeneous conditions.

Evolution of the morphology and microphysics of contrail cirrus from airborne remote sensing

Contrail cirrus were observed by airborne lidar and multispectral infrared remote sensing. Results of microphysical analysis were obtained for two cases of persistent contrail cirrus which form in the same area but of differing maturity. The cross section area of the older contrail was 18 times greater than the newer case. In both cases the inferred number of particles per distance along the flight track dimension of contrails was nearly similar, close to 26 ×1011/m. The average liquid water concentration of the two cases was similar, but the amount of water per distance along the flight line dimension was much greater for the older contrail, 34 g/cm and 680 g/cm. In both cases the amount of liquid water in the contrail was much greater than the amount generated by the aircraft which produced the contrail. These results are consistent with a model of contrail evolution where the effect of the jet exhaust is to generate a dense concentration of small ice particles. Mixing then maintains the total number of particles, and particles grow to the extent of the available vapor in the ambient air.

On the radiative properties of contrail cirrus

Using the observed ice crystal size distribution in contrail cirrus from SUCCESS, we have carried out the scattering and absorption calculations based on a unified theory for light scattering by ice crystals covering all sizes and shapes. We illustrate the effects of ice crystal size and surface roughness on the scattering phase function features for remote sensing applications. The extinction coefficient and single‐scattering albedo exhibit a minimum feature at 2.85 µm, referred to as the Christiansen effect, which is particularly pronounced for clouds consisting of a significant number of small ice crystals. Based on a line‐by‐line equivalent solar model, we show from spectral curves that cloud reflection increases as ice crystal sizes become smaller, but the cloud absorption increase is only evident for wavelengths longer than about 2.7 µm. The ice crystal shape has a substantial effect on the cloud reflection and absorption for a given size; more complex ice particles reflect more solar radiation. Finally, we propose a contrail cirrus cloud model consisting of a combination of bullet rosettes (50%), hollow columns (30%), and plates (20%), with sizes ranging from 1 to 90 µm in association with radiation perturbation studies.

A Simulation Model for Studying Possible Modification of Surface Temperature

Abstract A simulation model is presented which hopefully is consistent with known physical and statistical properties of atmospheric events, and consistent with criteria that might be applied in the management of actual experiments in the atmosphere. The process being simulated is the possible modification of daytime surface temperatures during the summer in central Missoui through the generation of contrail cirrus clouds. Monte Carlo techniques are used in the model to allow for the likelihood of failure of the experiment on any particular day, and to allow for variations in the degree of success on days when the experiment is considered to not be a failure. The model is applied to an observed time series (1946-1965) of surface and upper air observations from Columbia, Mo. Estimates of the results are based on analysis of the relationship between temperatures on the cirrus and cirrus-free days. If it can be assumed that it is possible to create enough contrial cirrus to reduce the per cent of possible su...