Publications

To find out more about the UKCA aerosol sub-model contact Graham Mann or visit the GLOMAP page at ResearchPages.net.

Aerosols: Why do we need UKCA?

Changes in the global aerosol can modify the earth's radiation budget through their ability to scatter and absorb solar and terrestrial radiation (direct effect) and by their ability to modify cloud properties via changed cloud condensation nucleii number. Additionally, aerosol particles provide surfaces for heterogeneous chemical reactions to take place, many of which are important in determining the tropospheric ozone burden. The online coupling of the model chemistry and aerosol schemes will enable oxidant changes to affect the evolution of the global aerosol and vice versa.

The current UM aerosol scheme represents ammonium sulfate, soot and biomass smoke aerosol as separate lognormal modes, whilst dust is carried in six size sections. Although these represent some important components of the global aerosol, there are additional chemical components of the aerosol that are now known to be important in the direct and indirect radiative forcing, but which are not included in the UM. Several aspects of the microphysical scheme will also be improved, such as the inclusion of mixed composition particles and the prognosis of particle number concentrations.

The IPCC third assessment report (2001) states that "the size distribution of aerosols is critical to all climate influences". One of the principal elements of the UKCA project is to improve on the current first generation aerosol scheme currently implemented in the UM.

Like most GCMs, CPU constraints have hitherto forced the UM aerosol scheme to only carry the mass in each aerosol mode, with the number of particles derived from an assumed fixed size distribution. Such "first generation" aerosol models have unwanted side-effects. For instance, an increase in particle mass caused by a microphysical process such as cloud processing results in a non-physical increase in the particle number concentration. Also, observations show large spatial and temporal variations in the mean size of Aitken and accumulation aerosol mode particles which cannot be captured by the current UM scheme.

A second area for improvement is the mixing state of particles. The existing UM aerosol scheme assumes each of the aerosol components to be externally mixed (particles consist of only one component). In reality, condensation and coagulation result in internal mixtures (e.g. soot and sulfate), whose direct and indirect radiative properties may differ substantially from a corresponding external mixture.

The online coupling of UKCA gas phase chemistry and the aerosol scheme will also improve the model considerably. In-cloud partitioning between sulfate, ammonium and nitrate aerosol will also be included in UKCA. Reduced chemistry schemes are being developed in Leeds to enable secondary organic aerosol to be included in UKCA. How is the UKCA aerosol scheme being developed?

ACMSU research scientist Dr Graham Mann and Prof Ken Carslaw at the University of Leeds are developing the main component of the UKCA aerosol sub-model.

The multi-component aerosol model combines a dynamically varying size distribution with a representation of composition and mixing state, at a cost compatible with GCM CPU constraints. The multi-component multi-modal UKCA aerosol scheme is initially being developed offline within the TOMCAT chemical transport model to facilitate comparison with observations and with the more detailed sectional multi-distribution multi-component GLOMAP model scheme.

At the Met Office, Dr Jamie Rae and Dr Colin Johnson are developing new cloud chemistry and multi-component aerosol chemistry schemes for incorporation into UKCA with input from DIAC scientist Dr Dave Topping and Dr. Gordon McFiggans at the University of Manchester. Met Office scientists Dr Jim Haywood and Dr. Nicholas Bellouin are also developing a new aerosol component of the UM radiation scheme which incorporates the varying size, composition and mixing state made possible by UKCA. A collaboration within the NERC QUEST programme with Dr Mat Evans and Prof Mike Pilling from the University of Leeds will also develop suitable emission, deposition and chemistry schemes to enable secondary organic aerosol to be incorporated into UKCA.

What Science can be done with UKCA?

The UKCA model will enable improved estimates of the aerosol direct and indirect effects on climate. The effect of previous assumptions of fixed-size and external mixtures on climate responses to changing anthropogenic emissions will also be able to be investigated. It is becoming increasingly clear that climate research must cover the entire earth system. UKCA will be implemented in the new HadGEM models in development, which will enable the study of global biogeochemical feedbacks on the climate system. For instance, will a warmer earth increase dust deposition into the ocean, resulting in increased oceanic dimethyl sulfide emissions and increased CCN number, and brighter, longer lived clouds reducing the warming signal? Also, how will predicted changes in land use feed back on climate due to secondary organic aerosol produced from condensation of low volatility oxidation products of monoterpene emissions from trees and vegetation.