Ice nucleation by volcanic ash greatly alters cirrus cloud properties

Area of Science:

• Atmospheric Science
• Cloud Physics
• Climate Science

Background:

• Ice crystal formation in clouds critically influences atmospheric properties and global climate.
• Volcanic ash (VA) is known to efficiently nucleate ice in laboratory experiments, but its atmospheric role is unclear.

Purpose of the Study:

• To investigate the impact of volcanic ash on atmospheric ice crystal formation and cirrus cloud properties.
• To provide evidence for volcanic ash's role in heterogeneous ice nucleation in the atmosphere.

Main Methods:

• Analysis of satellite measurements of cirrus cloud properties following volcanic eruptions.
• Comparison of changes after ash-rich versus ash-poor eruptions.
• Cloud microphysical simulations to support proposed ice formation mechanisms.

Main Results:

• Observed abrupt changes in cirrus clouds after ash-rich volcanic eruptions: decreased ice crystal number, increased ice crystal size, and increased cirrus occurrence.
• No significant changes detected after ash-poor eruptions, highlighting the role of volcanic ash.
• Simulations confirmed that volcanic ash suppresses homogeneous freezing, favoring heterogeneous nucleation.

Conclusions:

• Volcanic ash acts as a heterogeneous ice nucleator in the atmosphere, altering cirrus cloud formation.
• This mechanism leads to fewer, larger ice crystals by suppressing homogeneous freezing.
• Findings enhance understanding of aerosol-ice cloud interactions and suggest potential for cirrus cloud geoengineering.