Aircraft as a natural experiment on ice clouds

Aerosol impacts on ice clouds remain a highly uncertain component of the effective radiative forcing from aerosol-cloud interactions, with models simulating a wide range of responses. Developing observational constraints for these aerosol-cloud effects is challenging. The low aerosol concentrations involved hinder their direct observation and the meteorological conditions that affect cloud properties (such as temperature and updraught speed) also impact ice crystal number, limiting its use for inferring information about aerosol. Variations in meteorological conditions can also impact cloud and aerosol properties together, obscuring the causal impact of aerosol on cloud.

Similar to the use of ship emitted aerosol and the resulting 'shiptrack' cloud perturbation to understand aerosol-cloud interactions in liquid clouds, here we use aircraft to understand the response of ice clouds to aerosol perturbations.  Aircraft release water, aerosol and heat into the atmosphere as they fly, creating contrails in clear sky if conditions are suitable and perturbing existing clouds they fly through. The perturbation sizes vary with aircraft type, allowing a more detailed assessment of cloud responses.

Using a range of satellite data and ground-based radar observations, we composite contrails and aircraft impacts on existing clouds under a variety of conditions from a range of different aircraft types. We see that contrails formed from different aircraft types have varying lifetimes, consistent with an aerosol effect that increases cloud lifetime. Impacts on existing clouds vary significantly with time since the perturbation and meteorological conditions, highlighting the importance of the background cloud conditions. We also demonstrate how non-aerosol effects can be isolated and removed, to better constrain the impact of aerosols and aircraft on ice clouds and climate.