Aerosol Cloud Coupling

Aerosol Cloud Coupling

The coupling between aerosols and clouds in HadGEM UKCA has been developed as collaborative effort between Dr Philip Stier's Climate Processes Group at Oxford and Dr Olivier Boucher, Dr Andy Jones and Dr Colin Johnson at the Met Office through joint CASE PhD studentships of Rosalind West (aerosol cloud coupling and indirect effects) and Zak Kipling (aerosol scavenging and cloud cycling). This work will be extended in the framework of the upcoming Natural Environment Research Council funded ACID-PROOF consortium.

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Aerosol Activation

Atmospheric aerosols have a significant effect on the Earth’s radiative balance both directly, by scattering and absorbing radiation, and indirectly, through their effects on cloud properties. A crucial link between aerosol and cloud is the ability of aerosols to act as cloud condensation nuclei (CCN) on which cloud droplets form, a process known as aerosol activation.

Aerosol activation is critically dependent on the size and composition of aerosols as well as the local supersaturation of water vapour. UKCA has been designed to explicitly represent all of these factors. In this sub-project, the dynamically-evolving two-moment-modal aerosol scheme GLOMAP-mode (Mann et al., 2010) has been coupled to a Köhler theory-based aerosol activation parameterisation (Abdul-Razzak and Ghan, 2000).

Variations in local vertical velocity have a significant effect on the maximum supersaturation achieved, and hence on the number of activated aerosols. In UKCA, the sub-grid-scale variability of updraught velocity is represented by probability density functions (pdf) derived from the turbulent kinetic energy in the boundary layer.

This diagnostic CDNC is then used to calculate the cloud droplet effective radius following Martin et al. (1994) which is passed to the Edwards-Slingo radiation code and hence used to determine the cloud albedo effect (first indirect aerosol effect).

UKCA is also coupled to the large-scale precipitation scheme due to the strong dependence on CDNC of the rate of autoconversion of cloud water to rain water (following the method of Tripoli and Cotton, 1980). This dependency permits estimation of the cloud lifetime effect and other secondary indirect aerosol effects in radiative flux perturbation calculations.

Aerosol Removal

Work is under way to couple large-scale scavenging more tightly with the cloud and precipitation microphysics.

Features: =

• Sub-grid scale probability density function for updraft velocities coupled to turbulent kinetic energy
• Coupling to cloud radiative properties (i.e. cloud albedo effects)
• Coupling to precipitation formation scheme (i.e. cloud lifetime effects)

Publications:

• West et al. (in preparation)