Cirrus Cloud Thinning - the benefits and options

Aviation exhaust emissions release greenhouse gases, particles and water vapour into the atmosphere. At high altitudes this can create linear clouds called contrails and cirrus clouds. These clouds add to cloud cover and indirectly modify properties of existing cirrus.

There may be no perfect natural analogues for CCT, but aviation-relevant cirrus cloud and the involved processes can help to understand how CCT might work in the real atmosphere. Within ACtIon4Cooling, we will use the existing airborne observations to focus on specific clouds where aviation emissions are dense and further derive their optical thicknesses and ice crystal number concentrations. From a statistical perspective, the project will also use satellite data to determine the optical and microphysical properties of cirrus clouds as a function of latitude and longitude as input for Earth system model studies.

The CCT technique is not well studied to date. Gasparini and Lohmann (2016) performed climate model simulations and concluded that cirrus cloud seeding cannot result in a significant cooling due to the large uncertainties of the complex microphysical mechanisms of those clouds. Penner et al., 2015 found that the cirrus cloud seeding cannot be considered as a viable climate intervention technique but they associate their conclusion to the uncertainties in the modelling and observations of cirrus clouds and particularly, the balance between homogeneous and heterogeneous ice nucleation.

Cirrus Cloud Thinning (CCT) Analogues

There are several best-known natural analogues of CCT: Volcanic eruptions inject sulfur dioxide and water vapor into the atmosphere, forming sulfate aerosols and affecting cirrus formation indirectly. During a mineral dust episode (like Saharan dust), a significant amount of mineral dust is lifted into troposphere, which may act as INPs. The consequent transport of dust particles, depending on meteorological factors, from their source regions across large distance will spread the influence into larger scales. Aircraft-emitted particles may also act as INPs, causing heterogeneous nucleation in regions with a favorable atmospheric state. It leads to the formation of contrails and exerts indirect effects on the existing cirrus clouds. In the frame of the current project, we will focus on the changes of cirrus cloud properties responding to aviation impact as a natural analogue of CCT.

Previous studies indicated that the enhanced heterogeneous nucleation caused by aviation exhaust particles can be responsible for the high values of PLDR of cirrus clouds (Urbanek et al., 2018; Li and Groß, 2021, 2022). Furthermore, cirrus clouds with enhanced PLDR exhibit larger effective ice particles and lower number concentrations (Groß et al., 2023). The findings provide strong support that changes in microphysical properties of cirrus clouds depending on aviation emissions can serve as a natural analogue of CCT.

References

Gasparini, B. and Lohmann, U. Why cirrus cloud seeding cannot substantially cool the planet. Journal of Geophysical Research: Atmospheres. 121: 4877–4893, https://doi.org/10.1002/2015JD024666 2016.

Groß, S., Jurkat-Witschas, T., Li, Q., Wirth, M., Urbanek, B., Krämer, M., Weigel, R., and Voigt, C.: Investigating an indirect aviation effect on mid-latitude cirrus clouds – linking lidar-derived optical properties to in situ measurements, Atmos. Chem. Phys., 23, 8369–8381, https://doi.org/10.5194/acp-23-8369-2023, 2023.

Li, Q., and Groß, S.: Changes in cirrus cloud properties and occurrence over Europe during the COVID-19-caused air traffic reduction, Atmos. Chem. Phys., 21, 14573-14590, https://doi.org/10.5194/acp-21-14573-2021, 2021.

Li, Q. and Groß, S.: Satellite observations of seasonality and long-term trends in cirrus cloud properties over Europe: investigation of possible aviation impacts, Atmos. Chem. Phys., 22, 15963–15980, https://doi.org/10.5194/acp-22-15963-2022, 2022.

Penner, J. E., Zhou, C. and Liu, X. Can cirrus cloud seeding be used for geoengineering? Geophysical Research Letters. 42: 8775–8782, https://doi.org/10.1002/2015GL065992, 2015.

Urbanek, B., Groß, S., Wirth, M., Rolf, C., Krämer, M., and Voigt, C.: High depolarization ratios of naturally occurring cirrus clouds near air traffic regions over Europe, Geophys. Res. Lett., 45, 13,166–13,172, https://doi.org/10.1029/2018GL079345, 2018.