Contrail formation within cirrus: ICON-LEM simulations of the impact of cirrus cloud properties on contrail formation

Abstract

Abstract. Contrail formation within natural cirrus introduces large perturbations in cirrus ice crystal number concentrations, leading to modifications in cirrus microphysical and optical properties. The number of contrail ice crystals formed in an aircraft plume depends on the atmospheric state as well as aircraft and fuel properties. Our aim is to study contrail formation within cirrus and, in particular, the impact of pre-existing cirrus on the contrail formation processes. We implement a parameterization for contrail ice nucleation and the survival of contrail ice crystals within the vortex phase within the high-resolution ICON-LEM at a horizontal resolution of 625 m over Germany and modify the parameterizations in order to consider the impact of the pre-existing cirrus. We then simulate contrail formation and analyze the change in ice nucleation and survival due to the presence of cirrus ice crystals. We have selected two different synoptic situations to sample a large range of cirrus cloud properties from those representative of average thick in situ formed cirrus and liquid origin cirrus that are connected to a frontal system down to very thin cirrus within a high-pressure system. We find that contrail formation within cirrus mostly leads to increases in cirrus ice crystal numbers by a few orders of magnitude. Pre-existing cirrus has a discernible impact on the contrail formation threshold temperature and on contrail ice crystal number concentrations only if the cirrus ice water content is high and ice supersaturation is low. The cirrus ice crystals sucked into and subsequently sublimated within the aircraft's engine and the ice crystals mixed into the aircraft plume experiencing sublimation and later deposition lead to an increase in the contrail formation threshold temperature most of the time. Often this increase is negligible, but in the above atmospheric conditions the increase in the contrail formation threshold can be large, reaching maximum values of 2 K. Consequently, significant increases in ice nucleation are very seldom, but in areas of high cirrus ice water content and ice crystal number concentrations, contrail ice nucleation rates can be significantly increased, in particular at lower flight levels. Cirrus ice crystals can also lead to a reduction in ice nucleation, but negative changes are significantly smaller and less common. After nucleation cirrus and contrail ice crystals compete for water vapor, while the combined contrail and cirrus ice water mass grows. Once the aircraft plume gets trapped within the wake vortices and descends the plume relative humidity decreases so that eventually both cirrus and contrail ice crystals sublimate. We find that the impact of cirrus ice crystals on the contrail ice crystal loss within the descending vortices is negligible. Only for nucleation rates several orders of magnitude lower than the soot number emission index can the impact be noticeable, but the relevance of this impact is limited.

Differences in microphysical properties of cirrus at high and mid-latitudes

“…Following the same methodology, Marjani et al (2022) focused on the number concentration and found an increase within a few hundred meters behind the aircraft and below its flight track. In line with airborne observations, Verma and Burkhardt (2022) simulated contrail formation within pre-existing cirrus and found that contrails increased the ice crystal number concentration of cirrus by a few orders of magnitude. Not only the ice number concentration is modified, but also the effective diameter.…”

“…If we imagine the scenario in which an in situ origin cloud starts forming heterogeneously by the deposition of water vapor on INPs and an aircraft exhaust jet is introduced, the natural ice particles to be formed would be substituted by the freshly formed contrail affecting severely the resulting microphysical properties. Contrail formation within cirrus can increase the cirrus ice crystal number by a few orders of magnitude (Schröder et al, 2000;Voigt et al, 2017;, in particular if the pre-existing cirrus has high ice water content and the supersaturation is low (Verma and Burkhardt, 2022). In this sense, we can expect an influence of contrail formation and growth on the formation process of in situ origin cirrus at cruise altitudes by an increase in the N and reduction in ED.…”

Impact of Parametrizing Microphysical Processes in the Jet and Vortex Phase on Contrail Cirrus Properties and Radiative Forcing

“…Recently, many cloud related variables such as precipitation features, cloud vertical structure, cloud sizes, and daily cycles were shown to be improved in storm‐resolving simulations (Stevens et al., 2020) but uncertainties connected with the cloud microphysical schemes remain. Further improvements can be expected if future contrail cirrus studies can utilize the progress in storm‐resolving modeling not only in regional studies (Verma & Burkhardt, 2022) but also on the global scale.…”

Response of cirrus clouds to idealised perturbations from aviation

“…They explore the effects only on thin, mainly in situ origin cirrus, defined as those clouds with geometrical thickness smaller than 2 km. The primary effect of aircraft aerosols is to increase ICNC (Kärcher et al, 2021;Verma & Burkhardt, 2022). This is shown indirectly by studies examining the effect of reducing soot number in aviation fuel and consequently ICNC (e.g., Burkhardt et al, 2018;Voigt et al, 2021).…”

“…This is shown indirectly by studies examining the effect of reducing soot number in aviation fuel and consequently ICNC (e.g., Burkhardt et al, 2018;Voigt et al, 2021). Verma & Burkhardt (2022) use an LES model to show that contrail cirrus formation within existing cirrus can increase ICNC by a few orders of magnitude, especially in optically thick cirrus but also, to a lesser extent, in relatively thin cirrus.…”