České vysoké učení technické v Praze Fakulta dopravní 1. listopadu 2018 Praha, Česká republika

Aircraft Contrail Research

Sébastien Láni, Tereza Topkováii, Iveta Kameníkováiii

Abstrakt: Kondenzační stopa je umělý oblak mající podobu pruhu, který se tvoří za letadly. Její životnost může dosahovat několika hodin, přičemž se může dále rozšiřovat a přejít v indukovanou cirrovitou oblačnost. Kondenzační stopy způsobují pozitivní radiační působení a přispívají tak k oteplování atmosféry. Tento článek popisuje vznik kondenzačních stop, jejich dopad na podnebí a výzkum zaměřený na kondenzační stopy, který probíhá na Ústavu letecké dopravy, Fakultě dopravní, Českém vysokém učení technickém v Praze. Výzkum je založený na pozorování kondenzačních stop a využití zpráv ADS- B, zpráv módu S a výsledků aerologického měření. Cílem je zjistit, jak často se kondenzační stopy tvoří, jaká je jich životnost a další vlastnosti v závislosti na meteorologických podmínkách.

Klíčová slova: letadlo, kondenzační stopa, emise, podnebí, radiační působení, ADS-B, Mód S, BDS registr, aerologické měření

Abstract: Contrails are line-shaped clouds formed behind aircraft which may persist for while growing to resemble cirrus clouds. Contrails cause positive radiative forcing which tends to warm the atmosphere. This paper describes the contrail formation, climate effect and contrail research carried out by the Department of Air Transport, Faculty of Transportation Sciences, Czech Technical University in Prague. The research is based on contrail observation, ADS-B messages, Mode S messages and aerological measurement. The aim of the research is to identify the frequency of contrail occurrence, their lifetime and other properties and compare them to meteorological conditions.

Keywords: aircraft, contrail, emission, climate, radiative forcing, ADS-B, Mode S, BDS register, aerological measurement

1. Introduction Condensation trail or contrail is one of the most visible anthropogenic effects on the atmosphere. Contrail is a white strip composed of ice crystals which forms behind an aircraft flying in cold air due to water vapor emissions. It has become a common sight since the 1960s due to civil jet aircraft traffic increase. A contrail forms due to high content of water vapor in the engine exhaust plume. When the air is supersaturated, contrails might cause contrail cirrus. Contrails together with contrail induced cloudiness increase the global cloud coverage and have negative impact on the radiation balance of the Earth-atmosphere system. Therefore, it is important to research contrail formation, properties and techniques to avoid contrails. This paper describes the thermodynamic principle of contrail formation, contrail negative impact on

i Ing. Sébastien Lán, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected] ii Ing. Tereza Topková, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected] iii Mgr. Iveta Kameníková, Czech Technical University in Prague, Faculty of Transportation Sciences, Department of Air Transport, Horská 3, 128 03 Praha 2, Czech Republic, e-mail: [email protected]

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the atmosphere and ongoing scientific research carried out by the Department of Air Transport at the Czech Technical University in Prague [1, 2].

2. Contrail formation

2.1. Principle of contrails formation Contrail is an artificial cloud which looks like cirrus or cirrocumulus and forms behind jet engines in the upper troposphere and lower stratosphere. Contrail is initially 5 m to 10 m wide and forms at a distance of 50 m to 100 m behind an aircraft. Its lifetime is usually less than forty . It forms at altitudes from 7 km to 12 km where the ambient temperature is about -40 ℃ to -50 ℃ [3]. Contrail formation is caused by increase in relative humidity in the engine plume as a result of mixing hot and moist exhaust gases coming out of the engine with cold ambient air. Fuel used by jet aircraft is kerosene, primarily composed of hydrocarbons. There are two main products of kerosene combustion – carbon dioxide and water vapor. The mentioned product has a major impact on contrail formation. The kerosene water vapor emission index is about 1,23. It means that combustion of one kilogram of kerosene gives 1,23 kg of water vapor. The consequence of large value of emission index is high content of water vapor in aircraft engine exhaust [4, 5]. If the ambient air is cold enough the moisture in the engine plume may reach saturation point, i.e. state when the air water content cannot be higher. The moisture may even reach supersaturation point. Supersaturated air contains more water molecules then possible. These molecules tend to go from gaseous state to liquid state and solid state, respectively. When the air is supersaturated condensation (gas to liquid state transition) eventually deposition (gas to solid state transition) begins. Water droplets and ice crystals formation is necessary to achieve equilibrium state. Due to condensation and deposition the ambient air goes from the supersaturated state back to the saturated state. During the condensation process water droplets are being developed. The water vapor condenses mainly on ambient and exhaust aerosols called condensation nuclei. Especially soot particles are very suitable to act as condensation nuclei. Due to very low ambient temperature the water droplets instantly freeze, form icy crystals and grow via deposition. The ice crystals grow as long as the humidity with respect to ice is above the saturation point [1, 4]. Contrail shape and lifetime is dependent on several meteorological factors which include relative humidity, turbulence, atmosphere vertical movements. Contrail eventually dissipates via sublimation (solid to gas state transition) if relative humidity with respect to ice is below the saturation point or by precipitation into non-saturated layers below the flight level [1].

2.2. Schmidt-Appleman criterion The basic rule defining whether a contrail will occur or not is the Schmidt-Appleman criterion. According to this criterion contrail forms if the humidity in the engine plume reaches liquid saturation. Saturation with respect to ice is not sufficient for contrail formation. When the air is supersaturated with respect to liquid water, the water vapor starts to condense [6, 7].

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The Schmidt-Appleman criterion is met when the ambient temperature is below the threshold temperature. The threshold temperature sometimes called critical temperature is the highest temperature which allows contrail formation for a given ambient water vapor partial pressure, exhaust gases temperature and exhaust water vapor partial pressure [7]. Jet aircraft with overall propulsion efficiency of 0,3 cause contrails as follows. If the ambient temperature corresponds to the standard atmosphere and the relative humidity with respect to liquid water is 100 % (the air is saturated), aircraft cause contrails within the altitude range from 8,2 km to 19 km. If the relative humidity with respect to liquid water is 0 % (the air is completely dry), aircraft cause contrails within the altitude range from 10,2 km to 14 km [4]. The ambient air temperature is very low in places of high altitude. The lower the temperature is, the lower the water vapor partial pressure sufficient for contrail formation is. Hence, contrails form more easily at high altitudes where the ambient temperature is low.

3. Climate impact of contrails Aircraft emit gases (mainly greenhouse gases carbon dioxide and water vapor) and particles in the upper troposphere and lower stratosphere. These substances cause changes in greenhouse gases concentrations, may trigger contrails formation, increase cirrus coverage and change other clouds properties, and hence they may contribute to climate change [8].

3.1. Contrail coverage Contrails formed in a dry air dissipate very quickly. The global short-lived contrail coverage is very low and has a negligible impact on the climate. If there is a high content of water vapor and low ambient temperature, contrails might persist for hours. During their lifetime they might spread and become contrail cirrus. Contrail induced cirrus can form more easily than natural cirrus because the formation of cirrus requires higher humidity than for contrail persistence and spreading [5]. Unlike the line shaped contrails, it is very difficult to determine the coverage of contrail induced cloudiness. Contrail cirrus looks like a natural cirrus, so it is hard to distinguish a contrail cirrus from natural cloudiness. The largest coverage of persistent contrails and contrail induced cloudiness has been found in regions with high density of air traffic, namely over central Europe, over the east coast of the United States of America and over the east coast of southeast Asia [9, 10].

3.2. Radiative forcing Contrails and contrail cirrus affect the cloudiness of Earth’s atmosphere and therefore might affect the atmospheric temperature and climate. They reflect some solar (short-wave) radiation that would otherwise warm the Earth-atmosphere system and absorb some infrared (long-wave) radiation that cools the system. The overall radiative impact depends on many properties which include the contrail lifetime, optical thickness, ice crystal concentration, size of ice crystals, , , contrast between the contrail and its background, etc. Depending on these parameters the result can be either positive or negative [1, 4]. Contrails reduce the amount of incoming solar radiation reaching the Earth and reduce the outgoing infrared radiation leaving the Earth to . The second effect prevails, so the global

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radiative impact is positive. The highest radiative forcing has been found in central Europe and eastern USA. It corresponds to large contrail coverage caused by high air traffic density [9, 10].

4. Observation In order to find the climate impact of condensation trails, it is necessary to identify the frequency of contrail occurrence, their lifetime and other properties and compare them to meteorological conditions. The Department of Air Transport at the Czech Technical University in Prague has started a project regarding contrails monitoring. The contrail monitoring process consists of several steps. The first one is contrail observation. For this purpose, a recording system has been installed on the top of the building of the Faculty of Transportation Sciences in Decin. The position of the system is 50,77899° N 14,21602° E. It has been chosen for its advantageous position in the proximity of several flight routes (see Fig. 1). There are three cameras which are set up to the sky. They automatically record and save video recordings each day from 04:15 till 16:00 UTC.

Fig. 1 Monitored area [11] Saved recordings are manually filtered and analyzed. In order to observe and measure contrails, the sky has to be clear. During cloudy days it is not possible to see contrails as they are formed at high altitudes above the nature cloudiness. Therefore, recordings taken during cloudy days are deleted. Each discovered contrail is written down. For further analysis it is important to record the time when the contrail was observed, its lifetime, number of relevant camera and eventually make a note in cases when dissipation of the contrail cannot be observed, in other words when the exact contrail lifetime cannot be measured. Such a situation occurs when contrail is covered by nature cloudiness or blown out of the picture.

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5. Flight data processing The next step is flight data processing which is aimed for adding information about the aircraft which created a contrail observed in the camera recordings. The data is permanently received by four Radarcape receivers operated by the Laboratory of ATM Systems in the Prague area. Based on the condition of the receivers, any data transmitted on 1090 MHz frequency in their range (approximately 200 NM) may be received. In other words, they may provide ADS-B data as well as replies on interrogation of a Mode S secondary surveillance radar. The Ethernet interface enables to collect data in hexadecimal format with a time stamp. The received messages are saved into dataset containing record from all receivers within twenty minutes [12]. On condition that the aircraft is equipped by appropriate Mode S transponder with Enhanced Surveillance and ADS-B 1090 MHz extended squitter, and SSR elicits replies on Mode S EHS addressed surveillance, different information from on-board of the airplane may be passively received. In order to assign flight information to the corresponding aircraft, a script in MATLAB was created. It can obtain coordinates, altitude and ground speed from ADS-B messages and heading, true airspeed and Mach number from EHS BDS registers. For purpose of weather condition research, the MRAR BDS registers were also included into data processing. Unfortunately, only a small amount of aircraft provides this data and it is very unlikely that the aircraft in the recorded area would send it. Due to this fact, the area for processing meteorological BDS register was extended [13].

5.1. Data pairing with recorded area The created program in MATLAB is suitable for processing already received datasets in a longer time period (e.g. , ). It works with the offline saved dataset containing a , a receiver ID, a hexadecimal time stamp and 112 bits of the message in hexadecimal system. The first for loop ensures that all data from one file are loaded and subsequently processed by the script. While data is recorded when the cameras are switched off this dataset is skipped automatically. Program chooses only data with downlink format 17, 20 or 21 which is primarily tested to reduce the number of examined messages. According to the information about the position of the aircraft obtained from ADS-B messages, the unique 24 bit ICAO address is saved into temporary variable and all information from the received data is being saved into a new file for a period of time, when the aircraft still flies in the observed area. The new created file in .csv format is specified by the date and time when it intersected a border of the monitored area and by the ICAO address of the aircraft for facilitating assignment to a contrail. According to the type of the messages, aforementioned data is decoded and saved into the appropriate file created for one aircraft. The program processed is characterized in the Fig. 2. Different condition has to be met for collecting more meteorological data. Providing that a meteorological BDS register is contained within the tested data, and the aircraft with the corresponding ICAO address has flown in the area of circle with radius 100 km and the centre calculated from the observed polygons, the meteorological data is saved together with other flight information into a new file. These files are characterised by “meteo” and the aircraft ICAO address.

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Fig. 2 Data processing workflow

6. Meteorological data The main source of meteorological data at high altitudes is aerological measurement. Aerology is one of the fields of meteorology that deals with the observation of the atmosphere using of balloons, radiosondes etc. The basic and most frequent measured elements are the air temperature, atmospheric pressure, humidity and wind speed and wind direction. The radiosonde is launched into air 3 a day and may rise up to 35 km. Measurements are made at 00, 06 and 12 UTC. The GPS navigation system is used to determine its location. The results of aerological measurements are available from two stations – Praha Libus and Prostejov in the Czech Republic [14]. VAISALA's METGRAPH program evaluates measured and calculated data at standard pressure levels and also determines significant levels in the temperature, humidity and wind profile. This data is encoded in the form of the TEMP meteorological report and the PILOT message. These reports are used not only for predictive meteorological purposes. There are only 26 meteorological stations conducting aerological measurements within Europe. The closest

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stations to our borders are situated in German Kummersbruck, Munich, Lindenberg, Polish Wroclaw, Austrian Linz and Vienna [14].

6.1. Area for receiving meteorological reports Due to the fact that the surface of the Czech Republic is mostly flat, there is no significant change of individual meteorological elements with a relatively large distance (circuit with a radius of 100 km) at higher altitudes above the territory of our republic. That is why only 2 stations are set for aerological measurements, which cover the entire territory of the Czech Republic at higher altitudes. In the free atmosphere there is no immediate interaction between the atmosphere and the Earth's surface. These interactions include vertical transmission of momentum, thermal energy and humidity, friction of flowing air with the Earth's surface, and specific daily and annual course of thermal stratification. The daily flow of temperature and humidity parameters in the boundary layer is influenced by the substrate. The height of the boundary layer of the atmosphere increases with the increasing surface roughness, wind speed and increasing instability of thermal stratification. It follows from the above that significant changes in the meteorological elements in the upper troposphere can be recorded at a relatively large distance of hundred kilometers (except for the situation where atmospheric fronts pass through our territory), such as applied to the meteorological data processing.

6.2. Meteorological sensors of the aircraft However, there is the possibility to obtain meteorological data from the airplane directly or indirectly. The first pre-requisite for obtaining meteorological data is that the onboard sensors are essential able to measure meteorological data. The basic sensors include the Pitot-static tube and the total air temperature probe (thermometer). The measured data can be further processed and deduced other elements such as wind speed and wind direction, pressure altitude, etc. The amount of data which can be got by using a SSR Mode S technology is much larger and cheaper when compared to a radiosonde. It can be expected the number of meteorological data gained from the aircraft will increase in hand with the modernization of aircraft that will be better equipped with meteorological sensors in the . The main advantage of this method is that meteorological BDS registers sent by the aircraft already contain direct information on wind and temperature which is obtained directly from the sensors installed on the airframe.

7. Conclusion Aircraft emit gases and particles directly into the upper troposphere and lower stratosphere. These gases and particles have an impact on the atmosphere. They alter the concentration of greenhouse gases, cause contrail formation and increase cloud coverage. Contrails cause positive radiative forcing which tends to warm the Earth-atmosphere system. The highest coverage and radiative forcing have been found over areas with high air traffic density, namely over central Europe, eastern North America and southeast Asia. Many papers and researches about contrails have been done but the level of scientific understanding is still very low. Especially the impact on environment and potential regional temperature change are not well understood and further research needs to be carried out.

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The Department of Air Transport has started its own research based on contrail observation around Decin. The aim of the ongoing research is to collect data (video recordings, ADS-B messages and meteorological data) within one . Then it would be possible to get consistent database of observed contrails and all required parameters in order to determine frequency of contrail occurrence in the Czech Republic airspace, conditions under which persistent contrails are formed and in order to compare contrail lifetime with aircraft type.

References [1] MINNIS, Patrick. Contrails. In: Encyclopedia of atmospheric sciences. Amsterdam; Boston: Academic Press, 2003, p. 509-520. ISBN 01-222-7090-8. [2] BOUCHER, Olivier. Seeing through contrails. Nature Climate Change [online]. 2011, 1(1), 24-25. DOI: 10.1038/nclimate1078. ISSN 1758-678X. Available at: http://www.nature.com/articles/nclimate1078 [3] ČESKÁ METEOROLOGICKÁ SPOLEČNOST (ČMeS), Meteorologický slovník výkladový a terminologický (eMS) [online]. 2017. Available at: http://slovnik.cmes.cz/ [4] SCHUMANN, Ulrich. Formation, properties and climatic effects of contrails. Comptes Rendus Physique [online]. 2005, 6(4-5), 549-565. DOI: 10.1016/j.crhy.2005.05.002. ISSN 16310705. Available at: http://linkinghub.elsevier.com/retrieve/pii/S1631070505000563 [5] SCHUMANN, Ulrich. Aircraft Emissions. In: Encyclopedia of global environmental change. New York: Wiley, 2002, p. 178-186. ISBN 0-471-97796-9. [6] APPLEMAN, Herbert. The Formation of Exhaust Condensation Trails by Jet Aircraft. Bulletin American Meteorological Society. 1953, (34), 14-20. [7] SCHUMANN, Ulrich, Kaspar GRAF and Hermann MANNSTEIN. Contrails: Visible Aviation Induced Climate Impact. In: Atmospheric physics: Background, Methods, Trends. New York: Springer, 2012, p. 239-253. Research topics in aerospace. ISBN 978-3-642-30182- 7. [8] INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE. IPCC Special Report Aviation and the Global Atmosphere: Summary for Policymakers. Geneva, 1999. Available at: https://www.ipcc.ch/pdf/special-reports/spm/av-en.pdf [9] BURKHARDT, Ulrike and Bernd KÄRCHER. Global radiative forcing from contrail cirrus. Nature Climate Change [online]. 2011, 1(1), 54-58. DOI: 10.1038/nclimate1068. ISSN 1758- 678X. Available at: http://www.nature.com/articles/nclimate1068 [10] SCHUMANN, Ulrich and Kaspar GRAF. Aviation-induced cirrus and radiation changes at diurnal timescales. Journal of Geophysical Research: Atmospheres [online]. 2013, 118(5), 2404-2421. DOI: 10.1002/jgrd.50184. ISSN 2169897X. Available at: http://doi.wiley.com/10.1002/jgrd.50184 [11] OCELÍK, Vojtěch. Optimalizace systému monitorování kondenzačních stop. Prague, 2017. Bachelor’s thesis. CTU in Prague, Faculty of Transportation Sciences. Supervisor Jakub Hospodka. [12] ZACH, Martin. Návrh nízkonákladového MLAT systému. Prague, 2015. Master’s thesis. CTU in Prague, Faculty of Transportation Sciences. Supervisor Stanislav Pleninger. [13] ICAO. Doc. 9684: Manual of the Secondary Surveillance Radar (SSR) Systems, AN/951. 3rd ed. 2004. ISBN 92-9194-333-9. [14] WORLD METEROLOGICAL ORGANIZATION. Guide to meteorological instruments and methods of observation [online]. 6th ed. Geneva, Switzerland: Secretariat of the World Meteorological Organization, 1996, 716 p. WMO (Series), no. 8. ISBN 92-631-6008-2. Available from: https://library.wmo.int/pmb_ged/wmo_8_en-2012.pdf

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