The indirect effects of anthropogenic aerosols—through their interactions with clouds—are currently one of the most uncertain perturbations to the radiative energy balance at the top of the atmosphere. A crucial link between aerosol and cloud is that aerosols can act as cloud condensation nuclei (CCN). This microphysical process must be parameterised if the large-scale effects are to be represented in a general circulation model (GCM). Theoretical work presented in this thesis highlights the importance of incorporating the kinetic limitations on droplet formation in aerosol activation parameterisations. HadGEM-UKCA is a GCM, capable of representing the chemical and microphysical aerosol processes required to model CCN accurately. The author has incorporated a Köhler theory based parameterisation of aerosol activation into HadGEM-UKCA, to facilitate quantitative predictions of the indirect aerosol effects. This thesis presents an estimate of the range of uncertainty in such predictions attributable to the choice of parameterisation of the sub-grid-scale variability of vertical velocity. Results of simulations demonstrate that the use of a characteristic updraught velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs. Consequently, work focuses on the effect of the variance (<var>σ</var><sub>w</sub><sup>2</sup>) of a Gaussian pdf of vertical velocity. Fixed values of <var>σ</var><sub>w</sub> and a configuration in which <var>σ</var><sub>w</sub> depends on turbulent kinetic energy are tested. Results from the mid-range fixed <var>σ</var><sub>w</sub> and TKE-based configurations both compare well with vertical velocity distributions and cloud droplet number concentrations measured in situ. However, the sparse set of available measurements does not provide enough of a constraint to recommend one or the other as the best configuration globally. The radiative flux perturbation (RFP) due to the total effects of anthropogenic aerosol is estimated at −1.7Wm<sup>−2</sup> for the TKE-based configuration. To the extent that it is valid to decouple the individual aerosol effects, the direct effect accounts for approximately −0.6Wm<sup>−2</sup> of the total, the cloud albedo effect −0.8Wm<sup>−2</sup> and the cloud lifetime effect −0.3Wm<sup>−2</sup>, indicating that these effects are additive within HadGEM-UKCA. Total aerosol RFP ranges from −1.4Wm<sup>−2</sup> from simulations with <var>σ</var><sub>w</sub>=0.1ms<sup>−1</sup>, up to −2.0Wm<sup>−2</sup> for <var>σ</var><sub>w</sub>=0.7ms<sup>−1</sup>. This range of 0.6Wm<sup>−2</sup> corresponds to almost a third of the total estimate of −1.9Wm<sup>−2</sup>, obtained with the mid-range value of <var>σ</var><sub>w</sub>=0.4ms<sup>−1</sup>. Reducing the uncertainty in the parameterisation of <var>σ</var><sub>w</sub> is therefore an important step towards reducing the uncertainty in estimates of the indirect aerosol effects.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:581167 |
| Date | January 2012 |
| Creators | West, Rosalind Eleanor Lunzer |
| Contributors | Stier, Philip; Grainger, Roy G.; Jones, Andy |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:bc1cc003-cf96-4b27-aad9-75ef7045dfc3 |
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