1Department of Physics, University of Oxford, Oxford, UK
2Met Office Hadley Centre, Exeter, UK
3National Centre for Atmospheric Science, University of Leeds, Leeds, UK
*now at: Department for Environment, Food & Rural Affairs, London, UK
**now at: Department of Meteorology, University of Reading, Reading, UK
Abstract. The activation of aerosols to form cloud droplets is dependent upon vertical velocities whose local variability is not typically resolved at the GCM grid scale. Consequently, it is necessary to represent the sub-grid-scale variability of vertical velocity in the calculation of cloud droplet number concentration.
This study uses the UK Chemistry and Aerosols community model (UKCA) within the Hadley Centre Global Environmental Model (HadGEM3), coupled for the first time to an explicit aerosol activation parameterisation, and hence known as UKCA-Activate. We explore the range of uncertainty in estimates of the indirect aerosol effects attributable to the choice of parameterisation of the sub-grid-scale variability of vertical velocity in HadGEM-UKCA. Results of simulations demonstrate that the use of a characteristic vertical velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs.
This study focuses on the effect of the variance (σw2) of a Gaussian pdf of vertical velocity. Fixed values of σw2 (spanning the range measured in situ by nine flight campaigns found in the literature) and a configuration in which σw2 depends on turbulent kinetic energy are tested. Results from the mid-range fixed σw2 and TKE-based configurations both compare well with observed vertical velocity distributions and cloud droplet number concentrations.
The radiative flux perturbation due to the total effects of anthropogenic aerosol is estimated at −1.4 W m−2 with σw2 = 0.1 m s−1, −1.7 W m−2 with σw2 derived from TKE, −1.9 W m−2 with σw = 0.4 m s−1 and −2.0 W m−2 with σw = 0.7 m s−1. The breadth of this range (0.6 W m−2) corresponds to almost a third of the total estimate of −1.9 W m−2, obtained with the mid-range value of σw = 0.4 m s−1, and is comparable to the total diversity of current aerosol forcing estimates. Reducing the uncertainty in the parameterisation of σw would therefore be an important step towards reducing the uncertainty in estimates of the indirect aerosol effects.
Detailed examination of regional radiative flux perturbations reveals that aerosol microphysics can be responsible for some climate-relevant radiative effects, highlighting the importance of including microphysical aerosol processes in GCMs.