Sensitivity of Contrail Radiative Forcing to Radiative Transfer Parameterization

Contrail cirrus, ice clouds produced when an airplane passes through cold, supersaturated air, make up over half of the effect of aviation on the global energy balance. In the shortwave spectrum, clouds (including contrails) reflect sunlight, exerting a negative forcing. Meanwhile, in the longwave spectrum, they exert a positive forcing, warming the planet due to their cold emission temperature. Thus, depending on atmospheric conditions, cloud properties, the time of day, and the season, the balance between the negative shortwave and positive longwave effects determines whether contrails cool or warm the planet overall. Furthermore, in the context of global climate modeling, the sign and magnitude of the net contrail radiative forcing can be sensitive to assumptions used in the radiation parameterization. For example, simplifications made to the cloud optical properties and assumptions about how clouds overlap within a model column increase the uncertainty in the calculated forcing. In this work, we examine the radiative forcing of contrails in idealized single-column simulations in order to isolate the effect of these assumptions on the calculated forcing. We compare calculations performed with the radiation parameterization used in the LMDZ climate model to a higher-complexity line-by-line code that serves as a reference calculation. Together these results allow us to identify the radiative transfer parameters and configurations that most strongly affect the magnitude and sign of the net contrail radiative forcing, which we will use to identify modeling priorities for the LMDZ contrail parameterization.