Variability of the properties of the distribution of the relative humidity with respect to ice: Implications for contrail formation

Abstract. Relative humidity with respect to ice (RHi) is a key variable in the formation of cirrus clouds and contrails. We document its probability density function (PDF) using long-term Measurement of Ozone and water vapour on Airbus aircraft In-service programme (MOZAIC) and the In-service Aircraft for a Global Observing System (IAGOS) observations over the period 1995–2022 in the upper troposphere (UT) and lower stratosphere (LS) between 325 hPa and 175 hPa. The characteristics of the RHi PDF differ in the UT and in LS of the high-latitudes (HL) and mid-latitudes (ML) regions of the Northern Hemisphere. In the LS, the probability (P) of observing a certain RHi decreases exponentially with increasing RHi. The rate of this decrease in P with increasing RHi is greater in supersaturated than in subsaturated conditions. In the UT, P first increases exponentially under subsaturated conditions then decreases exponentially in supersaturated conditions. Because of these different behaviours, the PDF for the combined UT and LS is bimodal. In contrast to the HL and the ML regions, P in the tropical troposphere decreases exponentially with increasing RHi. The different forms of PDF, in the tropics and in the higher latitude regions (ML and HL), lead to a global PDF of RHi in subsaturated tropospheric conditions that is almost uniform. This PDF shows a weak mode in the vicinity of 100 %, which can be associated essentially with the presence of cirrus clouds. These different characteristics of the RHi PDF exhibit some differences depending on the pressure level. These findings invite caution when using MOZAIC and IAGOS measurements to calibrate large-scale simulations of RHi. The variability of RHi properties associated with that of the temperature has implications for the formation of contrails. We examined the impact of switching from the current fuel of aircraft, kerosene, to bio-ethanol, or to liquid-hydrogen on the frequency of contrails using the Schmidt-Appleman criterion. We show that bio-ethanol and more so hydrogen would produce more contrails. The impact of a potential change from kerosene to one of these two alternative fuels decreases with the decreasing pressure level but increases when moving from the high-latitudes of the Northern Hemisphere to the tropics. We recommend that the comparison between models and observations be performed regionally and for the UT and LS separately. Finally, we emphasize that investigations on the impact on the contrail occurrence of switching from fossil kerosene to more sustainable fuels must be carried out in various climatic conditions.