Aerosol-Cloud Coupling in ICON#

Overview#

The interaction between aerosols and clouds influences both radiation (via indirect radiative effects) and microphysical processes. The ICON 1-moment microphysics scheme requires Cloud Droplet Number Concentration (CDNC) for calculating both effective radius and auto-conversion rates. The coupling method is controlled by the namelist parameter icpl_aero_gscp. The implementation is based on the Segal and Khain 2006 (SK2006) droplets activation parametrization, which in its complete form uses a 4D lookup table with four parameters: CCN concentration (range: 50-6400 cm⁻³), CCN mode radius (range: 0.02-0.04 µm), geometric standard deviation (range: 0.1-0.5), and updraft velocity at cloud base (range: 0.5-5.0 m/s). Different configuration options provide varying levels of complexity, from constant CDNC to full 4D parametrization. When using CAMS aerosols, the system processes 3D aerosol fields containing 11 tracers: three dust bins (DU), three sea salt bins (SS), sulphate (SU), hydrophobic and hydrophilic black carbon (BC), and hydrophobic and hydrophilic organic matter (OM).

Configuration Options#

icpl_aero_gscp=0: Constant CDNC - no aerosol-cloud interactions (Default)

Method: Initializes CDNC with a fixed value of 200 cm⁻³ that remains constant throughout the model run

icpl_aero_gscp=1: Simplified Tegen-SK2006 Coupling

Requirements: irad_aero=6, 7, or 9

Method: Implements the SK2006 parametrization with simplifications:

  • Derives CCN concentration from Tegen’s 2D AOD using prescribed vertical profiles

  • Combines 5 aerosol species into a single concentration using solubility-based weighting (0.1 for dust, 0.9 for organics)

  • Uses fixed parameters: mode radius = 0.03 µm, geometric standard deviation = 0.3, vertical wind speed = 0.25 m/s

  • Reduces the full 4D lookup table to 1D, making CDNC dependent solely on CCN concentration

  • Calculates CDNC at cloud base and extends to 3D based on pressure levels

icpl_aero_gscp=2: Full CAMS-SK2006 Coupling

Requirements: irad_aero=7 or 8

Method: Implements full 4D-LUT SK2006 parametrization

  • Species-specific number concentrations, mode radii, and geometric standard deviations

  • Hygroscopicity factors control each aerosol type’s contribution

  • Currently uses grid-scale updraft velocity

  • Direct 3D CDNC calculation without vertical profile assumptions

icpl_aero_gscp=3: External CDNC Climatology

Data source: External parameter file containing cloud-droplet number climatology (can be generated using Zonda)

Method: Reads 2D climatology and extends to 3D using the same approach as Option 1. For further information see here.

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