Lisa Bock

The Microphysical Pathway to Contrail Formation

A conceptual framework to predict microphysical and optical properties of contrail particles within a wingspan behind the source aircraft is developed. Results from two decades of contrail observations and numerical simulations are reviewed forming the basis of theoretical model development. The model utilizes cloud theory applied to the dynamics and thermodynamics of jet aircraft exhaust plumes in upper tropospheric conditions. Droplet nuclei include soot particles emitted from aircraft engines and atmospheric particles entrained into the plume. These precursor particles activate into copious homogeneously freezing water droplets as the plume relative humidity rises beyond liquid water saturation. A unimodal size spectrum of ice particles develops wherein ice particles grow to micrometer mean sizes. Contrail particle formation is analyzed over a wide range of soot emissions relating to conventional jet fuels as well as to alternative aviation fuels producing much less soot and volatile particle emissions. For current aviation fuels and propulsion technology, the number of contrail ice particles scales roughly in proportion to the number of emitted soot particles that act as water condensation nuclei despite their poor hygroscopicity. Close to the contrail formation threshold, only few plume particles can be water activated and freeze. Implications for effects of alternative fuels on contrails, an arena for future scientific exploration, are outlined.

Reassessing properties and radiative forcing of contrail cirrus using a climate model

Contrail cirrus is the largest known component contributing to the radiative forcing associated with aviation. Despite major advances simulating contrail cirrus, their microphysical and optical properties and the associated radiative forcing remain largely uncertain. We use a contrail cirrus parameterization in a global climate model which was extended to include a microphysical two-moment scheme. This allows a more realistic epresentation of microphysical processes, such as deposition and sedimentation, and therefore of the microphysical and optical properties of contrail cirrus. The simulated contrail microphysical and optical properties agree well with in situ and satellite observations. As compared to estimates using an older version of the contrail cirrus scheme, the optical depth of contrail cirrus is significantly higher, particularly in regions with high air traffic density, due to high ice crystal number concentrations on the main flight routes. Nevertheless, the estimated radiative forcing for the year 2002 supports our earlier results. The global radiative forcing of contrail cirrus for the year 2006 is estimated to be 56mW/m2. A large uncertainty of the radiative forcing estimate appears to be connected with the, on average, very small ice crystal radii simulated in the main air traffic areas, which make the application of a radiative transfer parameterization based on geometric optics questionable.

The temporal evolution of a long‐lived contrail cirrus cluster: Simulations with a global climate model

The representation of contrail cirrus in climate models has advanced in the last years tremendously. Nevertheless, uncertainties in particular regarding the representation of contrail microphysics still remain. Properties of young contrail cirrus differ from those of natural cirrus due to the large ice crystal number concentration common in contrails. Consequently, microphysical process rates in contrail cirrus, which control its lifetime, can be very different to those in natural cirrus. We extend a contrail cirrus scheme within a climate model by implementing a microphysical two-moment scheme and study the life cycle of a contrail cirrus cluster. In an idealized experiment we study the properties and microphysical process rates of a contrail cirrus cluster in a large and long-lived ice supersaturated region. We find that at flight level contrail cirrus display their typical high ice crystal number concentration (of about 10–100 cm−3) for a few hours with far lower densities in lower levels caused by sedimentation. After about 7 h contrail cirrus have spread considerably so that even at flight level associated ice crystal number concentrations have dropped to values that prohibit fast relaxation of ice supersaturation. The reduced ice crystal number and the resulting limited water uptake in the contrail cirrus limit the lifetime of the contrail cirrus cluster to about 10 h even though surrounding conditions would be still favorable for contrail cirrus persistence. In our case studies, contrail cirrus resembles natural cirrus regarding their ice crystal number concentration and size after 5–7 h.

Synoptic Control of Contrail Cirrus Life Cycles and Their Modification Due to Reduced Soot Number Emissions

The atmospheric state, aircraft emissions and engine properties determine formation and initial properties of contrails. The synoptic situation controls microphysical and dynamical processes and causes a wide variability of contrail cirrus life cycles. A reduction of soot particle number emissions, resulting e.g. from the use of alternative fuels, strongly impacts initial ice crystal numbers and microphysical process rates of contrail cirrus. We use a climate model including a contrail cirrus scheme, ECHAM5-CCMod, studying process rates, properties and life cycles of contrail cirrus clusters within different synoptic situations. The impact of reduced soot number emissions is approximated by a reduction in the initial ice crystal number, exemplarily studied for 80%. Contrail cirrus microphysical and macrophysical properties can depend much more strongly on the synoptic situation than on the initial ice crystal number. They can attain a large cover, optical depth and ice water content in long-lived and large-scale ice-supersaturated areas, making them particularly climate relevant. In those synoptic situations, the accumulated ice crystal loss due to sedimentation is increased by around 15% and the volume of contrail cirrus, exceeding an optical depth of 0.02, and their short-wave radiative impact are strongly decreased due to reduced soot emissions. These reductions are of little consequence in short-lived and small-scale ice-supersaturated areas, where contrail cirrus stay optically very thin and attain a low cover. The synoptic situations in which long-lived and climate relevant contrail cirrus clusters can be found over the eastern USA occur in around 25% of cases.

Mitigating the contrail cirrus climate impact by reducing aircraft soot number emissions

Contrail cirrus are a major component of the climate forcing due to air traffic. For a given contrail cirrus cover, ice water content and ice crystal shape, their impact on radiation is dependent on the number and size of ice crystals. Here we use a global climate model to study the impact of a reduction in initially formed ice crystal numbers, as may be caused by reduced soot number emissions. We find that for reduced initial ice crystal numbers the ice water content is decreased and ice crystal sizes increased, leading to a reduction in contrail cirrus optical depth and doubling the fraction of contrail cirrus that cannot be detected by satellite remote sensing. Contrail cirrus lifetimes and coverage are strongly reduced leading to significant reductions in contrail cirrus radiative forcing. The global climate impact of contrail cirrus is nonlinearly dependent on the reduction in initial ice crystal numbers. A reduction in the initial ice crystal number of 80% leads to a decrease in contrail cirrus radiative forcing by 50%, whereas a twofold reduction leads to a decrease in radiative forcing by approximately 20%. Only a few contrail cirrus outbreaks explain a large percentage of the climate impact. The contrail cirrus climate impact can be effectively mitigated by reducing initial ice crystal concentrations in such outbreak situations. Our results are important for assessments dealing with mitigating the climate impact of aviation and discussions about the use of alternative fuels or lean combustion in aviation.Atmospheric science: alternative aviation fuel helps mitigate the contrail climate impactReducing soot emissions from aircraft by changing the composition of aviation fuels can effectively mitigate the climate impact of contrail cirrus. Ulrike Burkhardt and colleagues from the German Aerospace Centre use a global climate model to estimate the impacts of reduced aircraft soot emissions on properties and the climate forcing of contrail cirrus. Under the low emission condition in which the number of initial ice crystals formed from soot particles is reduced by 80%—a level of reduction that could be obtained using a blend of biofuel and conventional jet fuels—the climate impact of contrail cirrus is estimated to be reduced by 50%. The mitigation effect occurs predominantly in weather conditions that are favourable for contrail cirrus outbreaks: with the same level of reduction in the initial ice crystal number, capturing 25% of those events can reduce 30% of the climate forcing caused by contrail cirrus.