Abstract Midlatitude cirrus cloud macrophysical and microphysical properties have been shown in previous studies to vary seasonally and in various large‐scale dynamical regimes, but relative humidity with respect to ice (RHI) within cirrus clouds has not been studied extensively in this context. Using a combination of radiosonde and millimeter‐wavelength cloud radar data, we identify 1076 cirrus clouds spanning a 7 year period from 2004 to 2011. These data are separated into five classes using a previously published algorithm that is based largely on synoptic conditions. Using these data and classification scheme, we find that RHI in cirrus clouds varies seasonally. Variations in cirrus cloud RHI exist within the prescribed classifications; however, most of the variations are within the measurement uncertainty. Additionally, with the exception of nonsummer class cirrus, these variations are not statistically significant. We also find that cirrus cloud occurrence is not necessarily correlated with higher observed values of RHI. The structure of RHI in cirrus clouds varies more in thicker clouds, which follows previous studies showing that macrophysical and microphysical variability increases in thicker cirrus clouds.

Contrail-cirrus is considered the most important component of aviation-induced climate impact. However, a reliable assessment requires better understanding of their radiative effects. Analysis of seven years of humidity observations by instrumented passenger aircraft shows that conditions promoting long-lived contrails are fulfilled most often in regions already covered by subvisible or visible cirrus: ~90% over the Northern midlatitudes and almost 100% in the Southeast Asian subtropics, approximately equally distributed among visible and subvisible cirrus clouds. A conceptual analysis shows that subvisible cirrus and clear-sky cover ~10% of the cruise altitude over Northern midlatitudes ( < 2% in the subtropics) and contrails within these regions are expected to cause additional warming. However, most contrails in the thicker, visible cirrus, only slightly enhance the cirrus warming effect or possibly reverse it to cooling. Our results suggest that potential flight rerouting concepts for contrail avoidance need to consider cirrus cloud coverage in addition to ice-supersaturation.

This work presents a modification of the Smith (1990) cloud scheme used in the ARPEGE (Action de Recherche Petite Echelle Grande Echelle) NWP global model in order to improve the forecast of relative humidity with respect to ice, with particular attention paid to supersaturation, a necessary condition for the persistence of aviation contrails. The modeling extends the Smith cloud scheme used in the operational ARPEGE by reworking the statistical concepts of Sommeria and Deardorff (1977) while including a temperature-based parametrization for the representation of homogenous nucleation. A notable point is that this modification can be implemented without major changes and does not require additional computational effort. Furthermore, it allows for extensions to other atmospheric models using a similar framework. The new forecasts are verified using in situ humidity observations made by IAGOS program aircraft and compared to ARPEGE operational forecasts, resulting in a better description of supersaturated regions. Further impacts on other general parameters (wind, temperature) are also presented in this study.

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.

upersaturation) is a frequent phenomenon in cold regions of the upper troposphere. Its existence is essential for the formation of ice clouds and a necessary condition for the persistence of contrails. Its spatial and temporal evolution is important for weather and climate. The ice saturation and supersaturation values are found in the upper tail of the probability density function (pdf) of upper tropospheric humidity with respect to ice (UTHi). Here, we analyse the changes in the frequency of occurrence of ice saturation and supersaturation from 1979 to 2020 and compare them to changes in the mean UTHi. Our results show that while the mean UTHi increases near-globally with a rate of about 0.15% per decade, high UTHi values exceeding the 70%, 80%, 90% and 100% thresholds increase faster than the mean, at rates of about 0.7%, 0.6%, 0.4% and 0.3% per decade, respectively. The increasing rates of values found in the upper tail of the UTHi pdf suggest that the ambient conditions for cirrus and contrail formation and persistence will be more favourable in the future and this is expected to further enhance the impact of aviation on climate. Supplementary Information The online version contains supplementary material available at 10.1038/s41598-024-75756-9. Subject terms: Atmospheric science, Climate change Introduction The distribution of water vapour in the upper troposphere and lower stratosphere (UT/LS) is of central importance in several ways: it plays a major role in the balance of planetary radiation; it influences and responds to atmospheric motions and plays a key role in the UT/LS chemistry 1 . In the upper troposphere where temperatures are persistently below − 40 °C, it is convenient to express the water vapour concentration as relative humidity with respect to ice (RHi). Based on tropical in situ observations, it has been found that upper tropospheric RHi is close to 100% over convective regions and RHi is often less than 10% in regions with subsidence 2 . The