Simon Unterstrasser

The evolution of contrail microphysics in the vortex phase

We investigate the evolution of contrails during the vortex phase using numerical simulations. Emphasis is placed on microphysical properties and on the vertical distribution of ice mass and number concentration at the end of the vortex phase. Instead of using a 3D model which would be preferable but computationally too costly, we use a 2D model equipped with a special tool for controlling vortex decay. We conduct a great number of sensitivity studies for one aircraft type. It turns out that atmospheric parameters, namely supersaturation, temperature, stability and turbulence level have the biggest impact on the number of ice crystals and on the ice mass that survives until vortex breakup and that therefore makes up the persistent contrail in supersaturated air. The initial ice crystal number density and its distribution in the vortex, are of minor importance.

Large‐eddy simulation study of contrail microphysics and geometry during the vortex phase and consequences on contrail‐to‐cirrus transition

Large-eddy simulations (LES) with Lagrangian ice microphysics were used to study the early contrail evolution during the vortex phase. Microphysical and geometrical properties of a contrail produced by a large-sized aircraft (type B777/A340) were investigated systematically for a large parameter range. Crystal loss due to adiabatic heating in the downward moving vortices was found to depend strongly on relative humidity and temperature, qualitatively similar to previous 2-D simulation results. Contrail depth is as large as 450 m for the investigated parameter range and was found to be underestimated in a previous 2-D study. Further sensitivity studies show a nonnegligible effect of the initial ice crystal size distribution and the initial ice crystal number on the crystal loss, whereas the contrail structure and ice mass evolution is only barely affected by these variations. Variation of fuel flow has the smallest effect on crystal loss. At high supersaturations, our choice of contrail spatial initialization may underestimate the ice crystal loss. The set of presented sensitivity studies is a first step toward a quantitative description of young contrails in terms of vertical extent and crystal loss. Concluding contrail-to-cirrus simulations demonstrate the relevance of vortex phase processes and its three-dimensional modeling on the later contrail-cirrus properties.

Aircraft type dependency of contrail evolution

The impact of aircraft type on contrail evolution is assessed using a large eddy simulation model with Lagrangian ice microphysics. Six different aircraft ranging from the small regional airliner Bombardier CRJ to the largest aircraft Airbus A380 are taken into account. Differences in wake vortex properties and fuel flow lead to considerable variations in the early contrail geometric depth and ice crystal number. Larger aircraft produce contrails with more ice crystals (assuming that the number of initially generated ice crystals per kilogram fuel is constant). These initial differences are reduced in the first minutes, as the ice crystal loss during the vortex phase is stronger for larger aircraft. In supersaturated air, contrails of large aircraft are much deeper after 5 min than those of small aircraft. A parameterization for the final vertical displacement of the wake vortex system is provided, depending only on the initial vortex circulation and stratification. Cloud resolving simulations are used to examine whether the aircraft-induced initial differences have a long-lasting mark. These simulations suggest that the synoptic scenario controls the contrail cirrus evolution qualitatively. However, quantitative differences between the contrail cirrus properties of the various aircraft remain over the total simulation period of 6 h. The total extinctions of A380-produced contrails are about 1.5 to 2.5 times higher than those from contrails of a Bombardier CRJ.

Collection/aggregation algorithms in Lagrangian cloud microphysical models: rigorous evaluation in box model simulations

Abstract. Recently, several Lagrangian microphysical models have been developed which use a large number of (computational) particles to represent a cloud. In particular, the collision process leading to coalescence of cloud droplets or aggregation of ice crystals is implemented differently in various models. Three existing implementations are reviewed and extended, and their performance is evaluated by a comparison with well-established analytical and bin model solutions. In this first step of rigorous evaluation, box model simulations, with collection/aggregation being the only process considered, have been performed for the three well-known kernels of Golovin, Long and Hall. Besides numerical parameters, like the time step and the number of simulation particles (SIPs) used, the details of how the initial SIP ensemble is created from a prescribed analytically defined size distribution is crucial for the performance of the algorithms. Using a constant weight technique, as done in previous studies, greatly underestimates the quality of the algorithms. Using better initialisation techniques considerably reduces the number of required SIPs to obtain realistic results. From the box model results, recommendations for the collection/aggregation implementation in higher dimensional model setups are derived. Suitable algorithms are equally relevant to treating the warm rain process and aggregation in cirrus.

Cloud resolving modeling of contrail evolution

Contrails are ice clouds that form behind aircraft. As a result of burning kerosene in the engines, water vapor is emitted that rapidly freezes and forms ice crystals. If the atmosphere is sufficiently moist and cold, these contrails expand and persist for many hours. This chapter describes the numerical modeling of contrails on a local scale with cloud resolving simulations. Emphasis is put on the description of microphysical modeling. With this methodological approach valuable information on contrail evolution for a multitude of atmospheric and aircraft parameters can be obtained.

Conference on Transport, Atmosphere and Climate (TAC)

In the light of the Paris COP21 agreement, i.e. to limit global warming to 2 °C by the end of the century, the transport sector is more and more in the focus of scientific research and political debates. Emissions of equivalent CO2 from the transport sector are growing faster than any other mature sector of human activity. However, estimating the climate impact of transport is more complicated than the already difficult task of determining emission inventories. Beyond the direct emission of long-lived greenhouse gases (those considered in the Kyoto Protocol), there are several other ways how transport can impact climate, some of them being unique. Transport also emits precursors of greenhouse gases, such as NOx, which changes the atmospheric abundances of ozone and methane. Transport results in a change in the atmospheric abundances of particles, directly by particle emission and indirectly by emission of precursors. Finally, transport, in particular aviation and shipping, triggers additional clouds from its water vapour emissions (contrails and contrail cirrus) and particle emissions (ship tracks), and transport-induced aerosols modify cloudiness. All these cloud effects have an impact on climate. Many of the transport emissions are short-lived; hence, they never become homogeneously distributed in the atmosphere. The same is true for emission products such as ozone or secondary aerosol. Therefore, the location and altitude of emissions are of particular importance. Most aircraft emissions occur at altitudes (around the altitude of the extra-tropical tropopause) where no other anthropogenic sources directly emit and where short-lived species have longer lifetimes. Similarly, a significant fraction of ship emissions occurs at locations distant from land. Because of the distinct background chemical composition of the marine boundary layer, different amounts of secondary products are formed than would occur for the same emissions on land. All these facets make the investigation of the impact of transport on atmospheric composition and climate particularly complex but also interesting. Scientists working in this field are not only helping to increase our understanding of the relevant science, but they also are providing necessary information to the policy community, which needs to consider how emissions may be regulated in order to prevent dangerous anthropogenic interference with our climate system. An important question is, ‘how does the total impact of transport compare with other sectors, and how do the various modes of transport (road, rail, aviation, and shipping) differ in their impacts’? As a follow-up of the European Conference on Aviation, Atmosphere and Climate held at Friedrichshafen (Lake Constance, Germany) in 2003, a series of International Conferences on Transport, Atmosphere and Climate (TAC) have been held since then in order to update our knowledge on the atmospheric impacts of aviation, and to also include all other modes of transport. The various TAC conferences took place in 2006 at Oxford (United Kingdom), in 2009 at Aachen (Germany)/Maastricht (Netherlands), in 2012 at Prien am Chiemsee (Germany) and finally in 2015 at Bad Kohlgrub (Germany). This issue of METEOROLOGISCHE ZEITSCHRIFT comprises 8 papers from contributions of the latest TAC conference which all cover aviation related topics. Several papers deal with detailed simulations of contrails, hereby focusing either on the formation stage (Khou, 2017), the early phase of contrail-to-cirrus transition (Paoli et al., 2017) or differences between contrail-cirrus and natural cirrus (Unterstrasser et al., 2017a,b). On the other hand, Pitari et al. (2017) focuses on chemistry effects of aviation and quantifies the radiative forcing by aircraft NOx emissions. Grewe and Linke (2017) study how both, economic and environmental aspect of aviation can be studied within a common framework. Linke et al. (2017) discuss the climate benefits of intermediate stop operations and their usefulness as an operational mitigation strategy. Grewe et al. (2017) assess the climate benefits of a novel aircraft design, a multi-fuel blended wing body aircraft. Rick Miake-Lye (Aerodyne Research Inc., Billerica, MA, USA) and Darrel Baumgardner (Droplet Measurement Technologies Inc., Longmont, CO, USA) served as guest editors of this special issue of Meteorologische Zeitschrift. We would like to take this opportunity to thank them as well as the Editor-in Chief Stefan Emeis for their commitment and diligent work.