School of Earth and Environment

APPRAISE: A Next-Generation Model Framework for the Investigation of the Impact of Aerosol Properties on Mixed-Phase Cloud Processes

Principal Investigator: Prof Alan Blyth
Co-Investigator: Prof Ken Carslaw 
Sponsor: NERC

Start Date: 29 June 2007
End Date: 28 June 2010

Value to Leeds: £351,673

Institute: Institute for Climate and Atmospheric Science

Abstract

Clouds have a significant effect on the global albedo, play a key role in the hydrological cycle and offer an important loss route for aerosol particles through precipitation. To encapsulate such effects accurately in regional and global models we need to understand the key microphysical processes in the cloud and the interaction with aerosol particles and dynamics. Whilst there has been some progress on developing knowledge of such processes in warm clouds and cirrus, mixed-phase clouds are far less well understood. They are particularly important since they produce most of the precpitation and they cover a substantial fraction of the Earth’s surface.

Ice particles can influence the major features of the cloud. Of particular importance to APPRAISE is the interactions with radiation, latent heat release and cloud dynamics. Many mid-latitude clouds have extensive supercooled regions, lending themselves to copious ice production following heterogeneous nucleation. In this project we will build on our work in ICEPIC (Ice and Precipitation Initiation in Cumulus) and MMOCCAA (Modelling the Modification of Convective Clouds due to Anthropogenic Aerosol) to investigate the role of aerosol in the initiation and propagation of the ice phase in clouds through secondary ice processes.

Identifying where the first ice originates is difficult observationally, because initial concentrations are low and the small sizes and near spherical shapes of ice crystals makes them difficult to distinguish from water droplets. These poorly understood processes of ice initiation and secondary ice production in clouds produce large uncertainties in our ability to predict climate changes.

The goal of APPRAISE-Clouds is to determine how ice forms in clouds and in particular how ice formation is determined by the properties of aerosol entering into the cloud. The Leeds group is part of a UK consortium led by Professor Tom Choularton, University of Manchester. Field studies will be conducted over the UK in conjunction with laboratory work and modelling studies. Initial studies are also proposed to improve the treatment of ice formation in global models.

The objectives of the the APPRAISE-CLOUDS consortium are:

  • To determine the nucleating ability of specific ice nuclei and the initiation and development of ice in mixed phase clouds.
  • To determine how aerosol particles control the cloud microphysics, preciptiation and dynamics in mixed phase clouds.
  • To determine the type and phase partitioning of absorbing material above below and within clouds and the role of this material in ice nucleation.
  • To reduce the uncertainty in the contribution of indirect radiative forcing by better understanding of the role of aerosols in the microphysics of mixed phase cloud.

Some of these goals will be addressed using the model developed in the APPRAISE core position.