Anthropogenic Aerosol Indirect Effects in Cirrus Clouds.

Area of Science:

• Atmospheric Science
• Cloud Physics
• Climate Modeling

Background:

• Cirrus clouds play a crucial role in Earth's radiative balance.
• Accurate parameterization of ice formation is essential for climate models.
• Previous models struggled to accurately represent ice number concentrations.

Purpose of the Study:

• To implement a new parameterization for ice formation in cirrus clouds.
• To account for updraft variations and wave spectra in ice nucleation.
• To determine realistic ice number concentrations and their changes.

Main Methods:

• Developed a model incorporating updraft changes due to wave spectra.
• Accounted for homogeneous and heterogeneous freezing frequencies within model time steps.
• Compared model outputs to observational data for ice number concentrations.

Main Results:

• The model accurately fits ice number observations at the coldest tropical tropopause temperatures.
• Model overestimates ice number concentrations in tropical latitudes (195-215 K).
• Climate forcings from heterogeneous ice nuclei (INPs) are generally negative, influenced by soot and dust content.

Conclusions:

• The refined parameterization provides more realistic ice number concentrations.
• Anthropogenic aircraft soot exhibits a significant negative climate forcing (-0.2 ± 0.06 W/m²).
• Fossil/biofuel soot and biomass burning also contribute negative forcing, highlighting the climate impact of aerosols.