Eco-efficient flight trajectories - Using a Lagrangian approach in EMAC to investigate contrail formation in the mid latitudes

Air transport has for a long time been linked to environmental issues like pollution, noise and climate change. While CO2 emissions are the main focus in public discussions, non-CO2 emissions of aviation may have a similar impact on the climate as aviation's carbon dioxide, e.g. contrail cirrus, nitrogen oxides or aviation induced cloudiness. While the effects of CO2 on climate are independent of location and situation during release, non-CO2 effects such as contrail formation vary depending on meteorological background. Previous studies investigated the influence of different weather situations on aviation’s climate change contribution, identifying climate sensitive regions and generating data products which enable air traffic management (ATM) to plan for climate optimized trajectories. The research presented here focuses on the further development of methods to determine the sensitivity of the atmosphere to aviation emissions with respect to climate effects in order to determine climate optimized aircraft trajectories. While previous studies focused on characterizing the North Atlantic Flight Corridor region, this study aims to extend the geographic scope by performing Lagrangian simulations in a global climate model EMAC for the northern hemispheric extratropical regions and tropical latitudes. This study addresses how realistically the physical conditions and processes for contrail formation and life cycle are represented in the upper troposphere and lower stratosphere by comparing them to airborne observations (HALO measurement campaign, CARIBIC/IAGOS scheduled flight measurements), examining key variables such as temperature or humidity. Direct comparison of model data with observations using clusters of data provides insight into the extent to which systematic biases exist that are relevant to the climate effects of contrails. We perform this comparison for different vertical resolutions to assess which vertical resolution in the EMAC model is well suited for studying contrail formation. Together with this model evaluation using aircraft measurements, the overall concept for studying the life cycle of contrails in the modular global climate model EMAC is introduced. Hereby, the concept for the development of a MET service that can be provided to ATM to evaluate contrail formation and its impact on the climate along planned aircraft trajectories is presented. Within the ClimOP collaborative project, we can investigate which physical processes determine the effects of contrails on climate and study their spatial and temporal variation. In addition, these climate change functions enable case studies that assess the impact of contrails on climate along trajectories and use alternative trajectories that avoid these regions of the atmosphere that have the potential to form contrails with a large radiative effect. This study is part of the ClimOP project and has received funding from European Union’s Horizo

Feasibility of climate-optimized air traffic routing for trans-Atlantic flights

Current air traffic routing is motivated by minimizing economic costs, such as fuel use. In addition to the climate impact of CO2 emissions from this fuel use, aviation contributes to climate change through non-CO2 impacts, such as changes in atmospheric ozone and methane concentrations and formation of contrail-cirrus. These non-CO2 impacts depend significantly on where and when the aviation emissions occur. The climate impact of aviation could be reduced if flights were routed to avoid regions where emissions have the largest impact. Here, we present the first results where a climate-optimized routing strategy is simulated for all trans-Atlantic flights on 5 winter and 3 summer days, which are typical of representative winter and summer North Atlantic weather patterns. The optimization separately considers eastbound and westbound flights, and accounts for the effects of wind on the flight routes, and takes safety aspects into account. For all days considered, we find multiple feasible combinations of flight routes which have a smaller overall climate impact than the scenario which minimizes economic cost. We find that even small changes in routing, which increase the operating costs (mainly fuel) by only 1% lead to considerable reductions in climate impact of 10%. This cost increase could be compensated by market-based measures, if costs for non-CO2 climate impacts were included. Our methodology is a starting point for climate-optimized flight planning, which could also be applied globally. Although there are challenges to implementing such a system, we present a road map with the steps to overcome these

A concept for multi-criteria environmental assessment of aircraft trajectories

Comprehensive assessment of the environmental aspects of flight movements is of increasing interest to the aviation sector as a potential input for developing sustainable aviation strategies that consider climate impact, air quality and noise issues simultaneously. However, comprehensive assessments of all three environmental aspects do not yet exist and are in particular not yet operational practice in flight planning. The purpose of this study is to present a methodology which allows to establish a multi-criteria environmental impact assessment directly in the flight planning process. The method expands a concept developed for climate optimisation of aircraft trajectories, by representing additionally air quality and noise impacts as additional criteria or dimensions, together with climate impact of aircraft trajectory. We present the mathematical framework for environmental assessment and optimisation of aircraft trajectories. In that context we present ideas on future implementation of such advanced meteorological services into air traffic management and trajectory planning by relying on environmental change functions (ECFs). These ECFs represent environmental impact due to changes in air quality, noise and climate impact. In a case study for Europe prototype ECFs are implemented and a performance assessment of aircraft trajectories is performed for a one-day traffic sample. For a single flight fuel-optimal versus climate-optimized trajectory solution is evaluated using prototypic ECFs and identifying mitigation potential. The ultimate goal of such a concept is to make available a comprehensive assessment framework for environmental performance of aircraft operations, by providing key performance indicators on climate impact, air quality and noise, as well as a tool for environmental optimisation of aircraft trajectories. This framework would allow studying and characterising changes in traffic flows due to environmental optimisation, as well as studying trade-offs between distinct strategic measure

Case Study for Testing the Validity of NOx-Ozone Algorithmic Climate Change Functions for Optimising Flight Trajectories

One possibility to reduce the climate impact of aviation is the avoidance of climate-sensitive regions, which is synonymous with climate-optimised flight planning. Those regions can be identified by algorithmic Climate Change Functions (aCCFs) for nitrogen oxides (NO x x), water vapour (H 2 2O) as well as contrail cirrus, which provide a measure of climate effects associated with corresponding emissions. In this study, we evaluate the effectiveness of reducing the aviation-induced climate impact via ozone (O 3) formation (resulting from NO x x emissions), when solely using O 3 3 aCCFs for the aircraft trajectory optimisation strategy. The effectiveness of such a strategy and the associated potential mitigation of climate effects is explored by using the chemistry–climate model EMAC (ECHAM5/MESSy) with various submodels. A summer and winter day, characterised by a large spatial variability of the O 3 3 aCCFs, are selected

Climate Change Functions Update: Geographical extension, refinement and comparison with aCCFs

For the planning of eco-efficient flight trajectories detailed knowledge on the climate impact in response to local aviation emissions is a major premise. For this purpose, so-called climate change functions (CCFs) were calculated by means of a Lagrangian approach within the atmospheric chemistry climate model system EMAC (ECHAM5/MESSy Atmospheric Chemistry Model). The CCFs contain temporally and spatially resolved information on the climate impact of standardized non-CO2 aviation emissions

Comparing and combining different climate mitigation measures in aviation

In the context of aviation’s significant contribution to anthropogenic climate change from CO2 and non-CO2 emissions, ambitious climate goals have been defined for the aviation industry that require an implementation of extensive measures from technical, regulatory and operational perspectives. While technical innovations including new aircraft designs and alternative fuels are expected to contribute significantly in the long run as they are associated with late entry-into-service, operational measures can benefit from their fast implementation with the current world fleet. Regulatory implementation enablers can further support the required changes to the air transport system. While the current state of research comprises a broad variety of studies on individual climate mitigation measures, a direct comparison and combination of the achieved results is typically not directly possible due to different reference cases, application scopes, and modelling assumptions as well as different maturities and expected realization times. However, a direct comparability is required to identify especially effective and efficient measures as well as to combine individual approaches in order to compare the resulting potentials towards the defined climate goals. This study aims to address the lack of comparability by developing an approach to compare and combine different climate mitigation measures. We consider different concepts addressing technical, operational as well as regulatory aspects. Based on the individual assessment of climate mitigation measures, we expand a previously developed generalization approach to scale individual results from measures-specific studies to a comparable scope considering varying traffic samples, assessment methods as well as temporal and spatial boundary conditions of the individual studies. Differences in maturities and possible entry-into-service times are also incorporated. Hence, different combinations of mitigation measures can be analysed regarding their climate mitigation potential in terms of temperature change as well as their operational applicability. Finally, possible combinations of the selected measures can be contrasted with defined climate goals

Predicting the climate impact of aviation for en-route emissions: the algorithmic climate change function submodel ACCF 1.0 of EMAC 2.53

Using climate-optimized flight trajectories is one essential measure to reduce aviation's climate impact. Detailed knowledge of temporal and spatial climate sensitivity for aviation emissions in the atmosphere is required to realize such a climate mitigation measure. The algorithmic Climate Change Functions (aCCFs) represent the basis for such purposes. This paper presents the first version of the Algorithmic Climate Change Function submodel (ACCF 1.0) within the European Centre HAMburg general circulation model (ECHAM) and Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model framework. In the ACCF 1.0, we implement a set of aCCFs (version 1.0) to estimate the average temperature response over 20 years (ATR20) resulting from aviation CO2 emissions and non-CO2 impacts, such as NOx emissions (via ozone production and methane destruction), water vapour emissions, and contrail cirrus. While the aCCF concept has been introduced in previous research, here, we publish a consistent set of aCCF formulas in terms of fuel scenario, metric, and efficacy for the first time. In particular, this paper elaborates on contrail aCCF development, which has not been published before. ACCF 1.0 uses the simulated atmospheric conditions at the emission location as input to calculate the ATR20 per unit of fuel burned, per NOx emitted, or per flown kilometre. In this research, we perform quality checks of the ACCF 1.0 outputs in two aspects. Firstly, we compare climatological values calculated by ACCF 1.0 to previous studies. The comparison confirms that in the Northern Hemisphere between 150–300 hPa altitude (flight corridor), the vertical and latitudinal structure of NOx-induced ozone and H2O effects are well represented by the ACCF model output. The NOx-induced methane effects increase towards lower altitudes and higher latitudes, which behaves differently from the existing literature. For contrail cirrus, the climatological pattern of the ACCF model output corresponds with the literature, except that contrail-cirrus aCCF generates values at low altitudes near polar regions, which is caused by the conditions set up for contrail formation. Secondly, we evaluate the reduction of NOx-induced ozone effects through trajectory optimization, employing the tagging chemistry approach (contribution approach to tag species according to their emission categories and to inherit these tags to other species during the subsequent chemical reactions). The simulation results show that climate-optimized trajectories reduce the radiative forcing contribution from aviation NOx-induced ozone compared to cost-optimized trajectories. Finally, we couple the ACCF 1.0 to the air traffic simulation submodel AirTraf version 2.0 and demonstrate the variability of the flight trajectories when the efficacy of individual effects is considered. Based on the 1 d simulation results of a subset of European flights, the total ATR20 of the climate-optimized flights is significantly lower (roughly 50 % less) than that of the cost-optimized flights, with the most considerable contribution from contrail cirrus. The CO2 contribution observed in this study is low compared with the non-CO2 effects, which requires further diagnosis

The shortwave to longwave ratio in contrail radiative forcing as evident in two radiation schemes used for global GCMs

Contrail radiative forcing is difficult to obtain, even if contrail parameters like coverage, ice water content, crystal size etc. are known. Respective results as documented in literature hint to a substantial uncertainty. One key problem is the considerable degree of cancellation between the positive (warming) component from the contrails’ greenhouse effect and the negative (cooling) component from backscattering of solar irradiance. The longwave/shortwave cancellation depends on ambient parameters like contrail temperature, co-existing natural clouds, and surface albedo. High demands are set for any radiative transfer model aiming at reliable results of the net radiative forcing. Climate models are optimally suited to provide a representation of the required variety of ambient parameters for a climatological estimate of contrail radiative forcing. However, comprehensive global climate models use simplified radiative transfer schemes for reasons of computational economy. Hence, a dedicated test of these schemes is required. We present a comparison of contrail radiative forcing estimates from two global climate models with different radiation schemes. The first estimate results from the ECHAM4 model that has been frequently used over the last ten years for contrail climate impact calculations. The second estimate originates from the more recent ECHAM5 model (also being part of the EMAC/MESSy model system) that is used in current and future studies. Use is made of the so-called "Myhre benchmark test" with specified contrail parameters. Beyond global annual means, emphasis is also given to longwave/shortwave forcing ratios for different seasons and to daytime/nighttime differences

Importance of representing optical depth variability for estimates of global line-shaped contrail radiative forcing

Estimates of the global radiative forcing by line-shaped contrails differ mainly due to the large uncertainty in contrail optical depth. Most contrails are optically thin so that their radiative forcing is roughly proportional to their optical depth and increases with contrail coverage. In recent assessments, the best estimate of mean contrail radiative forcing was significantly reduced, because global climate model simulations pointed at lower optical depth values than earlier studies. We revise these estimates by comparing the probability distribution of contrail optical depth diagnosed with a climate model with the distribution derived from a microphysical, cloud-scale model constrained by satellite observations over the United States. By assuming that the optical depth distribution from the cloud model is more realistic than that from the climate model, and by taking the difference between the observed and simulated optical depth over the United States as globally representative, we quantify uncertainties in the climate model’s diagnostic contrail parameterization. Revising the climate model results accordingly increases the global mean radiative forcing estimate for line-shaped contrails by a factor of 3.3, from 3.5 mW∕m2 to 11.6 mW∕m2 for the year 1992. Furthermore, the satellite observations and the cloud model point at higher global mean optical depth of detectable contrails than often assumed in radiative transfer (off-line) studies. Therefore, we correct estimates of contrail radiative forcing from off-line studies as well. We suggest that the global net radiative forcing of line-shaped persistent contrails is in the range 8–20 mW∕m2 for the air traffic in the year 2000

Performance of two GCM borne radiation schemes in calculating contrail radiative forcing

Contrail radiative forcing is difficult to determine even if the numerous key parameters like coverage, ice water content, crystal size etc. are known. One reason is the considerable amount of cancellation between the positive (warming) component from the contrails' greenhouse effect and the negative (cooling) component from backscattering of solar irradiance. This sets high demands on the abilities of the radiative transfer model used to calculate the forcing, and a substantial respective uncertainty has been documented in literature. To complicate things further, the longwave/shortwave cancellation depends on some ambient parameters like ambient temperature, co-existing natural clouds, surface albedo and some others. Climate models are optimally suited to provide a representation of the required variety of ambient parameters for a climatological estimate of contrail radiative forcing. However, comprehensive global climate models have to use simplified radiative transfer schemes for reasons of computational economy. Hence, a dedicated test of these schemes is always indicated. We present a comparison of contrail radiative forcing between two contrail radiative forcing estimates from global climate models. The first is yielded with the ECHAM4 model frequently used for this purpose over the last ten years, the second by the more recent ECHAM5/EMAC model to be applied in the coming years. Use is made of the so-called "Myhre benchmark test" with specified contrail parameters. The ratio of longwave/shortwave cancellation for various seasons and the daytime/nighttime difference are features of particular focus. Contrail radiative forcing is difficult to determine even if the numerous key parameters like coverage, ice water content, crystal size etc. are known. One reason is the high degree of cancellation between the positive (warming) component from the contrails' greenhouse effect and the negative (cooling) component from backscattering of solar irradiance. This sets high demands on the abilities of the radiative transfer model, and a considerable uncertainty of respective calculations has been documented in literature. Furthermore, the longwave/shortwave cancellation is sensitive to several ambient parameters like temperature, co-existing natural clouds, and surface albedo