| Summary: | Thin and sub-visual cirrus cloud reside at or near the tropical tropopause layer (TTL) with high frequency and have a significant impact on climate due to their effect on the radiation balance relating to the microphysical properties. There is a significant lack of information about cirrus clouds including the conditions that form them as well as how these clouds evolve in time and difficulties in representing these clouds in models with high resolution. TTL observations show an atmospheric phenomenon of very low ice number concentration and substantial in-cloud super-saturations in cirrus cloud (0.005-0.2 cm-3 and RHj up to 130% respectively) which is at odds with homogeneous nucleation only. In this work, we have made use of recent experimental laboratory results (conducted at the AIDA chamber in Germany) which have indicated that aerosol solution particles can become glassy and therefore solid particles when the temperatures are low and similar to TTL temperatures. Glassy aerosols have the potential to form ice particles with much lower freezing thresholds than liquid aerosols need and there is likely an abundance of these particles in the TTL region, unlike typical IN particles. By incorporating the experimental results into the Advanced Particle Simulation Code (APSC), it has been shown that cirrus formed by only homogeneous freezing agrees with observations only for weak updrafts (up to 3 or 4 cm/s) and only for a deposition coefficient, (1, equal to 1.0. The simulations with glassy particles have shown good agreement of modelled cirrus cloud properties with TTL observation when forced by gravity waves whereas liquid aerosol nucleation is at odds with the observations. In this work, it has been shown that using the latest experimental results of glassy nucleation incorporated in a cirrus model shows that glassy aerosol nucleation is a likely explanation for TTL observations.
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