Abstract/Summary

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 In-service airCraft 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, this PDF decreases exponentially with increasing RHi. In the UT, it first increases exponentially in 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, the RHi PDF in the tropical troposphere decreases exponentially with increasing RHi. The different forms of PDF, in the tropics and in the higher latitude regions, lead to a global PDF of RHi in subsaturated tropospheric conditions that is almost uniform. 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 temperature also has implications for the formation of contrails. We examined the impact of switching fuel (from kerosene to bio-ethanol or liquid-hydrogen) on the frequency of contrail formation using the Schmidt-Appleman criterion. We show that bio-ethanol and to a larger extent liquid-hydrogen would produce more contrails. The impact of a potential change from kerosene to these alternative fuels decreases with decreasing pressure but increases when moving from the high-latitudes of the Northern Hemisphere to the tropics. Finally, we emphasize that investigations of the impact on contrail occurrence frequency of switching from fossil kerosene to more sustainable fuels must be carried out in various meteorological conditions.