Unit 3

The Cloud Microphysics module investigates the microscopic physical processes occurring inside a cloud. It seeks to address the following theoretical question: "How do embryonic liquid and solid hydrometeors form in the atmosphere and, remarkably, increase their size by many orders of magnitude to form precipitation, in time frames as short as ten minutes?" The conclusions drawn have some very useful applications to aviation and convective forecasting (in particular).

Following a brief review of moist thermodynamics, the nucleation of liquid water droplets and the role of atmospheric aerosol in this process is examined. Various means by which liquid water droplets can grow inside a cloud are considered and in particular, their significance in the formation of precipitation size drops. This leads to a set of general conclusions regarding the microscopic structure of liquid water clouds and their likelihood of producing precipitation.

Attention then turns to sub-freezing environments and under what circumstances the nucleation of ice particles can occur in clouds. This is followed by a look at the various growth processes of ice particles and the types of frozen precipitation that can result. With the presence of ice intrinsic to the formation of lightning, a model of charge separation within a cumulonimbus cloud is presented and the role of lightning in the global electric circuit is discussed.

Several useful forecasting applications emerge. These include rules-of-thumb for the likelihood of precipitation from cumulus clouds, cloud-top-temperature constraints on the occurrence of lightning and a detailed introduction to the aviation hazard of airframe icing and the extent to which its occurrence can be forecast.

This course supports the delivery of the oceanography cours on the Graduate Diploma of Meteorology Course. Class resources, additional activities, online assessments and links to additional resources will be added here prior to and during the delivery of the face-to-face courses.