Reply on RC2

Abstract

Contrail cirrus constitute the largest radiative forcing (RF) component of the aviation effect on climate. However, the difference of microphysical properties and radiative effects between contrails, contrail cirrus and natural cirrus clouds are still not completely resolved. Motivated by these uncertainties, we investigate the cirrus perturbed by aviation in the North Atlantic Region on 26 March 2014 during the Mid Latitude Cirrus (ML-CIRRUS) experiment. In the synoptic context of a ridge cirrus cloud, an extended thin ice cloud with many persistent contrails can be observed for many hours with the geostationary Meteosat Second Generation (MSG)/Spinning Enhanced Visible and InfraRed Imager (SEVIRI) from the morning hours until dissipation close to 14 UTC. Airborne lidar observations aboard the German High Altitude and LOng Range Research Aircraft (HALO) suggest that this cloud is mainly of anthropogenic origin. We develop a new method to distinguish between contrails, contrail cirrus and natural cirrus based on in situ measurements of ice number and NO gas concentrations. It turns out that effective radii (Reff) of contrail cirrus and contrails are in the range of 3 to 53 µm and about 18 % smaller than that of natural cirrus, hence a difference in Reff is still present. Ice particle sizes in contrail cirrus are on average 114 % larger than in contrails. The optical thickness of natural cirrus, contrail cirrus and contrails derived from satellite data has similar distributions with average values of 0.21, 0.24 and 0.15 for these three cloud types, respectively. As for radiative effects, a new method to estimate top-of-atmosphere instantaneous RF in the solar and thermal range is developed based on radiative transfer model simulations exploiting in situ and lidar measurements, satellite observations and ERA5 reanalysis data for both cirrus and cirrus-free regions. Broadband irradiances estimated from our simulations compare well with satellite observations from MSG and the Geostationary Earth Radiation Budget (GERB), indicating that our method provides a good representation of the real atmosphere and can thus be used to determine RF of ice clouds probed during this flight. Contrails net RF is smaller by a factor of 4 compared to contrail cirrus. On average, the net RF of contrails and contrail cirrus is more strongly warming than that of natural cirrus. For a larger spatial area around the flight path, the RF is well related to that along the flight track. We find warming contrail cirrus and cirrus in the early morning and cooling contrail cirrus and cirrus during the day. The results will be valuable for research to constrain uncertainties in the assessment of climate impacts of natural cirrus and contrail cirrus and for the formulation and evaluation of contrail mitigation options.