Modelling the climate effects of aviation in the ICOLMDZ climate model: From parameterising cirrus clouds and ice supersaturated regions to quantifying the radiative impact of contrails

Condensation trails from aviation have been shown to have a significant contribution to the total radiative impact of civil aviation. Consequently, their observation and modelling is an active field of research, so is the design of strategies to reduce the climate impact of aviation by avoiding contrail formation. However, the radiative efficacy of contrails in warming the climate is not well understood and constrained. Only a handful of studies have provided estimates of this key quantity so far. To better understand the impact of contrails on the climate, we developped a new parameterisation of contrails, cirrus clouds and ice supersaturation in the ICOLMDZ atmospheric general circulation model. This parameterisation is evaluated by estimating the value of radiative forcing of linear contrails and contrails evolving in cirrus clouds.

ICOLMDZ is the global atmospheric component of the IPSL-CM Earth System Model, actively and historically involved in the CMIP exercises. The standard version of ICOLMDZ is found to poorly represent cirrus clouds, and does not simulate ice supersaturated regions, which is a prerequisite for the formation of contrails. These regions are thermodynamically unstable, thus their modelling required a substantial revamp of the current parameterisation of cirrus clouds, which assumes thermodynamic equilibrium at all times. A new subgrid parameterisation that allows for ice supersaturation in both clear and cloudy sky was developped and implemented in ICOLMDZ.

This new parameterisation has then been adapted to simulate the effect of contrails and is used to assess the radiative forcing of contrails for a typical year, and a comparison with other state-of-the-art parameterisations in global climate models has been made. It will be used to estimate the efficacy of contrails to warm the Earth, using an ensemble of weakly nudged climate simulations with and without contrails.