T-dependent rate measurements of homogeneous ice nucleation in cloud droplets using a large atmospheric simulation chamber

The rates of photochemical reactions in the atmosphere depend on the optical properties and lifetimes of clouds. These are critically affected by the process of droplet freezing, because ice crystals can grow to large sizes at the expense of the metastable supercooled droplets, thereby initiating graupel formation and precipitation. The large evacuable and coolable aerosol chamber AIDA at Forschungszentrum Karlsruhe has been used to generate supercooled clouds under controlled conditions. Homogeneous freezing was detected below −35.5 °C, and nucleation rates J(T) were measured to about −37 °C. They vary between ∼106 cm−3 s−1 at the highest and ∼108 cm−3 s−1 at the lowest temperature, although the temperature dependence of the nucleation rate is not very well constrained by the measurements. The results agree within the combined error limits with recent literature data. Homogeneous ice nucleation, which sets a lower limit to cloud freezing temperatures when other nucleation mechanisms are inefficient in the atmosphere, is important in deep convective systems and in cirrus.