It is now well established that atmospheric waves on a wide range of scales play key roles in regulating the occurrence and microphysical properties of cirrus clouds. Observations of cirrus and waves in the tropical tropopause layer (TTL) have shown that the locations where TTL cirrus form typically correspond to the cold phases of waves. Unlike liquid clouds in the lower troposphere, ice nucleation in cirrus occurs on only a small fraction of the available cirrus. As a result, the concentration of ice crystals is strongly sensitive to cooling rate, with rapid cooling driven by mesoscale gravity waves playing a particularly important role. In the absence of deep convection, homogeneous freezing driven by rapid, wave-driven cooling is the only way to explain the observations of numerous ice crystals in cirrus just after nucleation events. Recent studies have also shown that very high-frequency waves can quench ice nucleation under some circumstances. Current research is focused on understanding the overall impact of realistic spectra of waves and the potential sensitivity of cirrus (and their climate impacts) to changes in wave properties. Long-duration, super-pressure balloon measurements that provide close approximations to temperature variations experienced by air parcels are particularly useful for this research.

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*email: eric.j.jensen@nasa.gov
*Preference: Invited