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Effects of a spectrum of mesoscale gravity waves on homogeneous aerosol freezing in midlatitude cirrus are studied by means of parcel model simulations that are driven by random vertical wind speeds constrained by balloon measurements. Stochastic wave forcing with mean updraft speeds of 5-20 cm/s leads to substantial nucleated ice crystal number concentrations (ICNC) of 0.1-1 cm(-3) in situations with slow large-scale cooling, which by itself would generate fewer ice crystals. The stochastic nature of wave-driven air parcel temperatures enhances ICNC even further, but the times required to reach freezing conditions unsupported by large-scale cooling may vary widely. In the presence of wave forcing, ice crystals with low ICNC (<1-10 L-1) are also generated by homogeneous freezing, albeit only rarely. Comparisons with aircraft measurements suggest significant influences of heterogeneous ice-nucleating particles and ice crystal sedimentation on ICNC, but quantifying their individual contributions remains elusive.

Plain Language Summary Spontaneous freezing of airborne, water-containing particles below -38 degrees C is a fundamental pathway to form ice crystals in high-altitude cirrus clouds. This ice formation process has been well researched and was the first represented in weather forecast and climate models to advance cirrus predictions. One key characteristic is its strong dependence of the number of ice crystals formed on the cooling rate of air. Recent observations show that rapid cooling rates are generated by ubiquitous gravity waves. Here, we explore the rich suite of phenomena taking place during cirrus formation caused by a spectrum of gravity waves. We find that wave effects should be considered in future model simulations, when comparing model results with observations, and in parameterizations of cloud ice crystal formation.