Ice and Mixed-Phase Microphysics
How ice crystals form and grow, and how ice and supercooled liquid water interact to produce precipitation in cold clouds.
Definition
Ice and mixed-phase microphysics is the study of the nucleation, growth and interaction of ice particles, and their coexistence with supercooled liquid water, in clouds colder than the freezing point.
Scope
Covers ice nucleation modes, the growth of ice crystals by vapour deposition and the variety of crystal habits, the Wegener-Bergeron-Findeisen process by which ice grows at the expense of supercooled droplets, riming and graupel and hail formation, aggregation into snowflakes, and secondary ice production.
Core questions
- How does ice nucleate in clouds, and why does supercooled water persist?
- Why do ice crystals grow rapidly when supercooled droplets are present?
- How do riming and aggregation build graupel, hail and snowflakes?
Key theories
- Wegener-Bergeron-Findeisen process
- Because the saturation vapour pressure over ice is lower than over supercooled water, ice crystals grow by vapour deposition while nearby droplets evaporate, efficiently producing precipitation-sized ice.
Mechanisms
Cloud water can remain liquid well below freezing because ice nucleation requires either ice nuclei or, at very low temperatures, homogeneous freezing. Once ice forms, the lower saturation vapour pressure over ice than over supercooled water drives net vapour transfer from droplets to crystals, the Wegener-Bergeron-Findeisen process. Crystals adopt habits set by temperature and supersaturation, grow further by colliding with and freezing supercooled droplets (riming) to form graupel and hail, and aggregate into snowflakes that fall and may melt into rain.
Clinical relevance
Ice processes dominate midlatitude precipitation and strongly influence cloud lifetime and radiative properties, making mixed-phase microphysics a key uncertainty in weather and climate models.
History
Wegener noted the instability of ice and water coexistence, and Bergeron and Findeisen developed it in the 1930s into the ice-crystal theory of precipitation, long the prevailing explanation for rain in midlatitudes.
Key figures
- Tor Bergeron
- Walter Findeisen
- Alfred Wegener
Related topics
Seminal works
- pruppacher1997
- lamb2011
Frequently asked questions
- What is supercooled water?
- Supercooled water is liquid water that remains unfrozen below 0 degrees Celsius because it lacks effective ice nuclei; it is common in clouds and central to mixed-phase precipitation.