Contrails - Research, comments and links

Contrails and Aviation-induced Cirrus Clouds

KNMI (Royal Dutch Meteorological Institute)  (02)

( KNMI also does research on the climatological effects of aviation.)

Measuring and modelling the effects of aviation 
on the atmosphere

Michiel van Weele, Dominik Brunner, Hennie Kelder, Ernst Meijer, Veerle Pultau, Peter van Velthoven, Wiel Wauben
KNMI 1999


The use of aircraft in transporting people and goods is an important part of today's economy. But what is the environmental impact of all those airplanes, flying ceaselessly through the atmosphere? This contribution to the ‘Recent highlights’ aims to give a short overview of our present knowledge of the effects of aviation on the atmosphere and also of the work that has recently been done in this field in the Atmospheric Composition Division at KNMI.

regions and may even affect climate. The CO2 emitted by aircraft contributes 2-3% to the total anthropogenic CO2 emissions. Studies with global chemistry transport models show that aircraft emissions of nitrogen oxides NOx (= NO + NO2) in the North Atlantic Flight Corridor (NAFC) change the concentrations of long-lived greenhouse gases such as ozone and methane throughout the Northern Hemisphere.

When the aircraft emissions are injected in the stratosphere, they may affect the ozone concentrations in the ozone layer. Paul Crutzen [1] explained in the early seventies that ‘an artificial increase of nitrogen oxides in the stratosphere ... may lead to observable changes in the atmospheric ozone level’. Some years later these results became significant in the discussion on the effects of a fleet of supersonic aircraft flying in the stratosphere.

Aircraft NOx emissions account for only about 2-3% of the total anthropogenic NOx emissions. However, aircraft emissions have a larger impact than surface emissions due to the longer residence time of the emitted species at high altitudes. Furthermore, it is known that climate is especially sensitive to changes in atmospheric composition near the tropopause, which unfortunately coincides with the main cruise altitudes at mid-latitudes (8-13 km).

A potentially large climate effect of air traffic is the formation of contrails. Contrails are ice clouds (cirrus) that form in the wake of an aircraft. It is suspected that persistent contrails may initiate the formation of longer-lived cirrus cloud fields. Even a small increase in the frequency of occurrence of cirrus clouds would exert a large climate forcing.

The CO2 emissions, the effects of NOx emissions on the abundance of other greenhouse gases by chemical transformations, and the effects of water vapour exhaust via cloud formation may result in climate changes by air traffic. Due to the fast growth of air traffic around the globe (currently about 3-4% increase of fuel use per year), it is anticipated that the magnitude of these climate effects will increase rapidly during the next century.

Research on aircraft effects in the Atmospheric Composition Division is focused on:

  1. Participation in (inter-)national measurement campaigns
  2. Study of aircraft effects on chemistry and climate by model simulations



For further reading the IPCC Special Report ‘Aviation and the Global Atmosphere’ [4] is recommended. The report gives a detailed overview of our present knowledge on the effects of aircraft on the atmosphere. Additional information can be found in the recent review paper [5] and the references [6]-[11].

It is expected that more observations will become available in the future, both in-situ and by remote sensing. These observations will improve our knowledge on the global distribution and variation of NOx and ozone and other species in the upper troposphere and lower stratosphere. Current discrepancies between model calculations and measurements of aircraft effects are partly ascribed to insufficient knowledge of the natural variability of NOX, HNO3 and ozone, and partly to the coarse resolution of the global models. The latter may be improved when nested models become available. A correct description of the transport in convective events is also very important. The EULINOX project aims to reduce the uncertainty in the NOx production by lightning. The estimation of climate forcing by aircraft is still very uncertain. Given the fast growth in air traffic it is anticipated that research on the effects of aircraft emissions on atmospheric composition, chemistry and climate will intensify during the next century.


  1. Crutzen, P., 1970. The influence of nitrogen oxides on the atmospheric ozone content. Quart. J. Roy. Meteor. Soc., 96, 320-325.
  2. Meijer, E.W., P.F.J. van Velthoven, A.M. Thompson, L. Pfister, H. Schlager, P. Schulte and H. Kelder, 1999. Model Calculations of the Impact of NOx from Air Traffic, Lightning and Surface Emissions, compared with measurements. J. Geoph. Res., Special Issue, submitted, 1999.
  3. Brunner, D., J. Staehelin and D. Jeker, 1998. Large-scale nitrogen oxide plumes in the tropopause region and implications for ozone. Science, 282, 1305-1309.
  4. IPCC Special Report ‘Aviation and the Global Atmosphere’, to appear early 1999.
  5. Fabian, P., and B. K�rcher, 1997. The impact of aviation upon the atmosphere; an assessment of present knowledge, uncertainties, and research needs. Phys. Chem. Earth, 22, 503-598.
  6. Fortuin, J.P.F., R. van Dorland, W.M.F. Wauben, and H. Kelder, 1995. Greenhouse effects of aircraft emissions as calculated by a radiative transfer model. Ann. Geophys., 13, 413-418.
  7. Meijer, E.W., P.F.J. van Velthoven, and H. Kelder, 1998a. Model calculations of the impact of air traffic, lightning, and surface emissions, compared with measurements. EU POLINAT-2 report.
  8. Meijer, E.W., J.P. Beck, G.J.M. Velders, P.F.J. van Velthoven, 1998b. The effect of the conversion of nitrogen oxides in aircraft exhaust plumes in global models. Geophys. Res. Lett., 24, 3013-3016.
  9. Pultau, V.E., 1998. Literature study of climate effects of contrails caused by aircraft emissions. KNMI Scientific Report WR 98-05.
  10. Velthoven, P.F.J. van, and several co-authors, 1997. The passive transport of NOx emissions from aircraft with a hierarchy of models. Atmos. Environ., 31, 1783-1799.
  11. Wauben, W.M.F., P.F.J. van Velthoven, and H. Kelder, 1997. A 3D chemistry transport model study of changes in atmospheric ozone due to aircraft NOx emissions. Atmos. Environ, 31, 1819-1836.

Last update: Monday, 15-Feb-99 16:19:12
Contact: Michiel van Weele

CLIWA-NET Project description

CLIWA-NET Homepage
Global observations are important for detecting climate change, understanding the present climate and predicting climate variability. Such observations, integrated into models provide immediate benefits to society in the form of improved forecasts of weather and climate. Clouds are a high priority problem for the planned Global Climate Observing System and for atmospheric models (GCM�s and weather forecast).

CLIWA-NET focuses on observations of cloud liquid water and vertical structures, and evaluation/improvement of parameterisations. A prototype of a European cloud observing system will be established. CLIWA-NET co-ordinates the use of existing, mostly operational, ground-based microwave radiometers and profiling instruments. The network data will be integrated with satellite estimates of cloud water. Based on these observations cloud parameterisations will be evaluated/improved.

The project is carried out in co-ordination with BALTEX.

Description of the work
The combination of vertical profiles of cloud water and temperature information will enable an accurate detection of super cooled water layers. These layers are responsible for in-flight icing, which is considered to be one of the major risks in today�s aviation..

  • investigation of the sensitivity of model cloud parameters to the employed horizontal grid spacing in the meso-scale range from (1-10 km)

Objectives and innovation

The prospect of global climate change resulting from increasing concentrations of greenhouse gases has become a major concern and has moved climate issues to the forefront of the international political agenda. Systematic and comprehensive global observations will lay the foundation for improving our capabilities for detecting climate change, understanding the present climate and predicting climate variability. Such observations, integrated into models of the climate system would provide immediate benefits to society in the form of improved understanding and forecast of climate.


  • Development of an adequate observing system for the detection of icing conditions for aircraft.

In-flight icing is considered to be one of the major risks in today�s aviation. Icing is the deposition of super cooled liquid water on the aircraft frame. Icing affects the aircraft�s flight characteristics. The identification of super cooled water layers is of utmost importance for aviation. The combination of vertical profiles of cloud water and temperature information will enable an accurate detection/prediction of these conditions. The results for the detection of super cooled water layers will be evaluated with representatives from the aviation authorities.


Model evaluation/improvement

  • Objective evaluation and improvement of present day state-of-the-art cloud parameterisations to demonstrate the usefulness of the observations.

The importance of cloud water observations and the information on the vertical structure of clouds will be demonstrated by the evaluation/ improvement of state of the art cloud parameterisations. Because of the known poor representation of clouds in the NWP models, the quality of cloud forecasts from models is still limited. This has large socio-economic impacts for business areas like: airports, solar energy, road construction, tourism, breweries, ice industry, etc..

Mainly the lack of adequate data hampers the development of better cloud parameterisations in NWP�s and climate models. At present, differences between the predictions from various atmospheric models can, to a large extent, be traced back to the differences in the treatment/representation of cloud processes.

As stated by the IPCC�95: "the most urgent scientific problems requiring attention to determine the rate and magnitude of climate change and sea level rise are the factors controlling the distribution of clouds and their radiative characteristics....". The same was concluded in the AMIP project (e.g. Gates et al., 1999) in which the outputs of 30 atmospheric models were compared for a ten-year run. The differences in the output and the effect of cloud parameters were enormous.

     3. Workplan

Clouds affect our daily life in many ways. Much more than air temperature clouds dominate our perception of weather and thus have an enormous influence on our daily activities and our health. This fact is completely at variance with our knowledge about clouds, their representation in climate and weather forecast models and our ability to predict clouds. It is their high variability in time and space, which makes clouds both hard to monitor and to model.

It is well-known, that clouds are directly linked to the dynamics of the atmosphere. The most important parameter linking dynamics to clouds, in both the real world and in forecast models is the water content of clouds. Passive microwave remote sensing is by far the most direct and accurate method to estimate cloud water content. Over the oceans microwave remote sensing from satellites has been proven to be the most accurate method to determine this parameter. Unfortunately, over land areas this technique fails. Here remote sensing methods must rely on very indirect information mainly taken from cloud reflection of solar radiation. Thus, important cloud information does not get into our models in the area where people live.

We propose to close the gap between models and observations by establishing a prototype of a European cloud water observing system. This will be achieved by co-ordinating the use of existing, mostly operational, ground-based passive microwave radiometers and profiling instruments. This network will feed high quality cloud information, with high temporal but poor spatial resolution, into the calibration of satellite-based estimates of cloud water content with high spatial resolution.

To demonstrate the potential and necessity of liquid water and cloud structure observations, cloud parameterisations from the major operational European weather forecast models are evaluated with this data. Possible improvements will be investigated.

A wide range of benefits will accrue from the network. Firstly, scientific and technical knowledge, so far unused, will be transferred into an operational environment. The expected improvement of cloud forecast will impact many areas. These range from the prediction of precipitation as the most important process influencing the hydrological cycle, water availability and quality, to solar radiation for solar energy use to UV-radiation influencing people's health, to mention but a few. Apart from this, the set-up of the network links together technical and scientific people in an area still rather poorly developed in Europe concentrating on a most important, but unique topic. Last but by no means least, the network will serve to monitor for the first time quantitatively the parameter most important for the human perception of global change, namely clouds.

Similar observing strategies can be used to identify super cooled water layers. The identification of super cooled water layers is of uttermost importance for aviation. Super cooled water layers are responsible for in-flight icing which is considered to be one of the major risks in today�s aviation. Icing is the deposition of super cooled liquid water on the aircraft frame and affects the aircraft�s flight characteristics seriously. The combination of vertical profiles of cloud water and temperature information will enable an accurate detection/prediction of these conditions. The results for the detection of super cooled water layers will be evaluated with representatives from the aviation authorities.


  • Microwave radiometers (MIUB, KNMI, CNRS, HUT, DWD, Chalmers, UNIBE and CCLRC)

Several types of passive microwave radiometers (see list of stations), which are otherwise used for different purposes (meteorology, geodesy or telecommunication) will be involved. Algorithms for the LWP retrieval from the measured brightness temperatures have to be adapted for each site due to the different frequencies, number of channels and the available additional instrumentation (see below). These measurements will not only be included in the LWP retrieval but also give important information on the vertical cloud structure

  • Cloud radars (GKSS, CCLRC and KNMI)

The vertical structure of clouds can be measured best by state-of-the-art cloud radars. These radars measure the radar reflectivity with a vertical resolution of typically 50 m. The radar systems operate at different frequencies: 3, 35, and 94 GHz. The observed reflectivity profiles can be converted to liquid water profiles. However, the occasional presence of single large drops in the cloud will cause a serious degradation of the accuracy of the retrieved liquid water profiles. For this reason it is important to have a collocated microwave radiometer next to a cloud radar, which give a constraint for the integrated liquid water content (LWC). Measurements of the doppler velocity and polarimetric quantities (ZDR, LDR) will help to distinguish between water and ice clouds.

  • Lidar ceilometers (MIUB, KNMI, GKSS, CCLRC, DWD and IFM)

Cloud base heights can be measured by lidar ceilometers. Laser pulses are transmitted and the instrument measures the power, which is reflected by clouds and/or aerosols. Internal algorithms are applied to derive the cloud base heights from these profiles. Up to three cloud layers can be identified. The algorithms are based on the detection of pre-defined features in the backscatter profile. Several types of lidar ceilometers will be used within the project. However all systems are comparable in wavelength (911nm), measurement range (7 or 12 km) and resolution (15 m). The instruments are fully operational and will be operated for 24 hours a day.

  • Infrared radiometers (KNMI, MIUB, UNIBE, IFM)

Infrared radiometers will be used to measure the cloud base temperatures. The wavelength range of the vertically pointing narrow beam infrared radiometer is 9.6-11.5 m m. The measurement range of the sky temperature is between +20 and -50oC. The instruments will be operated continuously. The observed cloud base temperatures have to be corrected for the atmospheric contribution. For this atmospheric correction the Modtran radiative transfer code will be used.

Analysis of observations (MIUB, GKSS and KNMI)

  While ice clouds are transparent for the microwave radiometers the multi-parameter cloud radar measurements show significant signatures especially in the case of melting layers. The data on the vertical structure of the clouds will help to identify multi layered clouds. This is very important information for the analysis of the satellite data. Furthermore, the observed vertical profiles will contribute strongly to studies on cloud overlap, an important ingredient of NWP model cloud parameterisations.

Super cooled water layers can be detected in several ways. In the CLARA and CLARE�98 cloud campaigns, layers of super cooled water were detected by analysing the ratio of the lidar and radar backscatter signals. Because of the large wavelength differences of these instruments, the ratio of the signals is very sensitive to the particle size (which is much smaller for water droplets then for ice). A second method is based on combining LWP information from the microwave radiometers, altitude information from the radar/lidar and temperature information from infrared radiometer data or Numerical Weather Prediction (NWP) model output. Both methods will be applied.

  • Model evaluation/improvement (WP4000, WP-Manager: KNMI)


With item 3) the following parameterisation issues will be considered, which are expected to be verifiable with the data-sets inferred from the observational periods within the CLIWA-NET project:

  1. Cloud amount, onset of cloud formation, and the role of "sub-grid" scale variance of total water, or similarly, of threshold relative humidity. In particular, it will be investigated how the total water variance can be predicted dynamically instead of prescribing its value externally.
  2. The temperature dependence of the ratio between cloud water content and cloud ice content. This is extremely important for radiation calculations since liquid water clouds have significantly higher albedo than ice clouds. It is also essential for the conversion into precipitation.
  3. Cloud overlap assumptions. This is important for radiation calculations but also in the account to the onset of precipitation by fall-out of rain or sedimentation of ice crystals from higher tropospheric layers.
  4. Cloud inhomogeneity and brokeness. Focus will be on the formulation of an effective cloud water path to account for the effect of cloud inhomogeneities on the radiative fluxes

Measuring and modelling the effects of aviation on the atmosphere (Review KNMI)

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