Physics of Clouds and Atmosphere

Water droplets, ice crystals, and aerosols (dust, volcanic ash, pollen, soot, pollutants, etc.), which we collectively refer as particles, are ubiquitous in Earth's atmosphere. Aerosols are the seeds (condensation nuclei), which are essential for the formation of water droplets and ice crystals.  Ice crystals and water droplets make up clouds, while aerosols influence their formation as cloud condensation nuclei. These particles affect precipitation and radiative transfer due to their light-scattering and absorbing properties, which has impacts on both local weather and global climate.
 Yet, much remains to be understood about the dynamics of cloud particles, in large part due to the large range of scales involved, from the size of the particles themselves (nanometres to millimetres) to thousands of kilometres. One of the problems concerns the transport and settling of non-spherical and irregular particles such as aerosols and ice crystals, which determines how far particles can be transported and how long they stay in the atmosphere before settling to the ground (or in the case of ice crystals, possibly sublimating before they reach the ground).
 In clouds, how particles grow is yet another open question. It starts with the activation of cloud aerosols where water vapour condenses onto them to form droplets and ice crystals. The growth dynamics of water droplets across the so called “size gap” between the diffusive growth regime dominated regime (d < 20µm) and differential gravitationally settling induced collision dominated regime (d > 100µm) remains unclear. These problems make accurate aerosol transport, rain, and snow prediction difficult and a detailed understanding is necessary for reliable climate prediction and weather forecasting. To better understand these processes, the LFPB investigates many aspects of these problems by carefully controlled laboratory experiments, numerical simulations and field measurements in atmospheric clouds. These range from experiments with small settling ellipsoidal particles and activation in the cloud aerosols in hydrometeor wakes all the way to direct measurement of water droplets at small scales in atmospheric clouds in the field.


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