Simulations of Contrail Optical Properties and Radiative Forcing for Various Crystal Shapes

The aim of this study is to investigate the sensitivity of radiative-forcing computations to various contrail crystal shape models. Contrail optical properties in the shortwave and longwave ranges are derived using a ray-tracing geometric method and the discrete dipole approximation method, respectively. Both methods present good correspondence of the single-scattering albedo and the asymmetry parameter in a transition range (3–8 µm). There are substantial differences in single-scattering properties among 10 crystal models investigated here (e.g., hexagonal columns and plates with different aspect ratios, and spherical particles). The single-scattering albedo and the asymmetry parameter both vary by up to 0.1 among various crystal shapes. The computed single-scattering properties are incorporated in the moderate-resolution atmospheric radiance and transmittance model(MODTRAN) radiative transfer code to simulate solar and infrared fluxes at the top of the atmosphere. Particle shapes have a strong impact on the contrail radiative forcing in both the shortwave and longwave ranges. The differences in the net radiative forcing among optical models reach 50% with respect to the mean model value. The hexagonal-column and hexagonal-plate particles show the smallest net radiative forcing, and the largest forcing is obtained for the spheres. The balance between the shortwave forcing and longwave forcing is highly sensitive with respect to the assumed crystal shape and may even change the sign of the net forcing. The optical depth at which the mean diurnal radiative forcing changes sign from positive to negative varies from 4.5 to 10 for a surface albedo of 0.2 and from 2 to 6.5 for a surface albedo of 0.05. Contrails are probably never that optically thick (except for some aged contrail cirrus), however, and so will not have a cooling effect on climate

H-alpha features with hot onsets. II. A contrail fibril

The solar chromosphere observed in H-alpha consists mostly of narrow fibrils. The longest typically originate in network or plage and arch far over adjacent internetwork. We use data from multiple telescopes to analyze one well-observed example in a quiet area. It resulted from the earlier passage of an accelerating disturbance in which the gas was heated to high temperature as in the spicule-II phenomenon. After this passage a dark H-Halpha fibril appeared as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset and subsequent visibilities in various diagnostics and conclude that such H-alpha fibrils can indeed be contrail phenomena, not indicative of the thermodynamic and magnetic environment when they are observed but of more dynamic happenings before. They do not connect across internetwork cells but represent launch tracks of heating events and chart magnetic field during launch, not at present.Comment: Accepted for Astronomy & Astrophysic

Aircraft clouds: from chemtrail pseudoscience to the science of contrails

The most frequent statements and arguments found in pseudoscience websites and forums supporting the existence of so-called aircraft chemtrails can be refuted with a scientific explanation of the processes resulting in the formation of condensation or deposition trails, known as contrails. Thus, the hypothesis that chemtrails exist is disproven by the scientific literature that shows that they are the exact same entity as contrails: They are hydrological phenomena which result from a physical process referenced in the many studies carried out since the beginning of the age of aviation, in the early twentieth century. Hence, in this paper we conclude that pseudoscience’s chemtrails are nothing more than the contrails described by science.Peer ReviewedVersió final publicad

Contrail microphysics in the near wake of a realistic wing through RANS simulations

This paper focuses on Steady Reynolds Average Navier Stokes simulations (RANS) of ice particles growth in the near field of a wing-injector configuration. The multiphysics multiphase flow solver CEDRE, enriched with a microphysical model, has been developed in order to simulate the impact of a more real aircraft geometry in contrail formation studies. As a first evaluation case, a simplified aircraft description, i.e. a NACA0012 2D wing with two injectors, has been used. Ice formation has been simulated by assuming water condensation and instantaneous freezing on activated soot particles, initially emitted by aircraft engines. Our investigation focuses on the near field, extending from the nozzle exit until eight wing spans. Although the main goal is to address the question of ice formation, the aerodynamic flow field has been investigated and numerical results compared with existing experimental data. The first results indicate that the exhaust jet is correctly wrapped around the vortex and that the pattern of dilution qualitatively matches observations in the near field. Sensitivity studies to humidity and to the initial soot particle radius have also been performed

Evaluation of Contrail Reduction Strategies Based on Aircraft Flight Distances

This paper evaluates a set of contrail reduction strategies based on the flight range of aircraft as contrail reduction strategies have different impacts on aircraft depending on how they plan to fly. In general, aircraft with longer flight distances cruise at the altitudes where contrails are more likely to form. The concept of the contrail frequency index is used to quantify contrail impacts. The strategy for reducing the persistent contrail formation is to minimize the contrail frequency index by altering the aircraft's cruising altitude. A user-defined factor is used to trade off between contrail reduction and extra CO2 emissions. A higher value of tradeoff factor results in more contrail reduction and extra CO2 emissions. Results show that contrail reduction strategies using various tradeo factors behave differently from short-range flights to long-range ights. Analysis shows that short-distance flights (less than 500 miles) are the most frequent flights but contribute least to contrail reduction. Therefore these aircraft have the lowest priority when applying contrail reduction strategies. Medium-distance flights (500 to 1000 miles) have a higher priority if the goal is to achieve maximum contrail reduction in total; long-distance flights (1000 to 1500 miles) have a higher priority if the goal is to achieve maximum contrail reduction per flight. The characteristics of transcontinental flights (greater than 1500 miles) vary with different weather days so the priority of applying contrail reduction strategies to the group needs to be evaluated based on the locations of the contrail areas during any given day. For the days tested, medium-distance ights contribute up to 42.6% of the reduction among the groups during a day. The contrail frequency index per 1,000 miles for medium-distance, long-distance, and transcontinental flights can be reduced by an average of 75%. The results provide a starting point for developing operational policies to reduce the impact of aviation on climate based on aircraft flight distances

H-alpha features with hot onsets III. Fibrils in Lyman-alpha and with ALMA

In H-alpha most of the solar surface is covered by dense canopies of long opaque fibrils, but predictions for quiet-Sun observations with ALMA have ignored this fact. Comparison with Ly-alpha suggests that the large opacity of H-alpha fibrils is caused by hot precursor events. Application of a recipe that assumes momentary Saha-Boltzmann extinction during their hot onset to millimeter wavelengths suggests that ALMA will observe H-alpha-like fibril canopies, not acoustic shocks underneath, and will yield data more interesting than if these canopies were transparent.Comment: Accepted for Astronomy & Astrophysics; Figure 1 correcte

Global contrail radiative forcing and the impact of diurnal variations of air the impact of diurnal variations of air

International audienceWe combined high resolution aircraft flight data from the EU Fifth Framework Programme project AERO2k with analysis data from the ECMWF's integrated forecast system to calculate diurnally resolved 3-D contrail cover. Calibrating for the 1992 contrail cover in the Bakan area (eastern-Atlantic/western-Europe), we obtained a global, annual mean contrail cover due to persistent, line-shaped contrails of 0.04%. Adopting a contrail visible optical depth of 0.1, this contrail cover results in a global, annual mean radiative forcing of 2.0 mW/m2 for all-sky and 2.1 mW/m2 for clear sky conditions. Less than 40% of the global distance travelled by aircraft is due to flights during local night time. Yet, due to the cancellation of shortwave and longwave effects during daytime, night-flights contribute a disproportional 60 to 76% to the annual mean forcing. In general, regions with a significant local contrail radiative forcing are also regions for which night time flights amount to less than half of the daily total of flights. Neglecting diurnal variations in air traffic/contrail cover by assuming a diurnal mean contrail cover can therefore increase the global mean radiative forcing by up to 30%. Scaling the 1992 forcing for the year 2000 fuel usage and accounting for differences in contrail optical depth, our forcing estimate is at the lower end but within the range of the most recent results. This reinforces the finding that some earlier published estimates of contrail radiative forcing are likely to be too large. Our study builds confidence in the calculation of contrail radiative forcing. Once the amount and optical properties of contrails are known there is relatively little uncertainty about their radiative effects. However, global model calculations of contrail radiative forcing crucially rely on scaling their contrail cover with observations. We therefore see the urgent need for an update of area mean contrail cover values derived from multi-year analyses of observational data

Numerical simulations of contrail-to-cirrus transition – Part 2: Impact of initial ice crystal number, radiation, stratification, secondary nucleation and layer depth

Simulations of contrail-to-cirrus transition were performed with an LES model. In Part 1 the impact of relative humidity, temperature and vertical wind shear was explored in a detailed parametric study. Here, we study atmospheric parameters like stratification and depth of the supersaturated layer and processes which may affect the contrail evolution. We consider contrails in various radiation scenarios herein defined by the season, time of day and the presence of lower-level cloudiness which controls the radiance incident on the contrail layer. Under suitable conditions, controlled by the radiation scenario and stratification, radiative heating lifts the contrail-cirrus and prolongs its lifetime. The potential of contrail-driven secondary nucleation is investigated. We consider homogeneous nucleation and heterogeneous nucleation of preactivated soot cores released from sublimated contrail ice crystals. In our model the contrail dynamics triggered by radiative heating does not suffice to force homogeneous freezing of ambient liquid aerosol particles. Furthermore, our model results suggest that heterogeneous nucleation of preactivated soot cores is unimportant. Contrail evolution is not controlled by the depth of the supersaturated layer as long as it exceeds roughly 500 m. Deep fallstreaks however need thicker layers. A variation of the initial ice crystal number is effective during the whole evolution of a contrail. A cut of the soot particle emission by two orders of magnitude can reduce the contrail timescale by one hour and the optical thickness by a factor of 5. Hence future engines with lower soot particle emissions could potentially lead to a reduction of the climate impact of aviation