Freezing of droplets under mixed-phase cloud conditions

Mixed-phase clouds contain both liquid and ice particles. They have important roles in weather and climate and such clouds are thought to be responsible for a large proportion of precipitation. Their lifetime and precipitation rates are sensitive to the concentration of ice. This project focuses upon the formation of ice within clouds containing liquid droplets colder than 273 K. A new bench-top instrument has been developed to study ice nucleation in liquid droplets. \nPure water droplets of sizes relevant to clouds in the lower atmosphere do not freeze homogeneously until temperatures below ~237 K are reached. However, literature measurements of nucleation rates are scattered over two kelvin and there is uncertainty over the actual mechanism of ice formation in small droplets. The freezing of droplets with diameters equivalent to ~4 – 17 μm has been observed. It was found that ice nucleation rates in the smallest droplets of this size range were consistent with nucleation due to the droplet surface, but that surface nucleation does not occur at fast enough rates to be significant in the majority of tropospheric clouds. \nWater droplets can be frozen at higher temperatures than relevant for homogeneous freezing due to the presence of a class of aerosol particles called ice nuclei. Field observations of ice crystal residues have shown that mineral dust particles are an important group of ice nuclei, and the ice nucleating ability of seven of the most common minerals found in atmospheric dust has been described. In comparison to the other minerals, it was found that the mineral K-feldspar is much more efficient at nucleating ice. To relate this result to the atmosphere, a global chemical and aerosol transport modelling study was performed. This study concluded that dust containing feldspar emitted from desert regions reaches all locations around the globe. At temperatures below ~255 K, the modelled concentration of feldspar is sufficient to explain field observations of ice nuclei concentrations.