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Centre for Atmospheric Science

Modelling the impact of aerosol properties on cloud formation and development

Atmospheric clouds can be composed of liquid droplets or ice particles depending on the prevailing temperature and composition of pre-existing aerosol particles. Consequently clouds are called warm, cold or mixed phase clouds. Properties, like reflectivity or the ability to form drizzle, of all three cloud types depends on the number concentrations of cloud droplets and ice particles present. Work is being undertaken within CAS focussing on the microphysics of cloud droplet and ice particle formation aiming to determine how aerosol properties, such as composition and size, are affecting cloud formation at different temperatures. This is being studied with an air parcel model describing the condensation (deposition) of water and semivolatile gases from the gas phase to liquid (solid) particles as the air parcel moves in the atmosphere.
In warm conditions the number of cloud droplets is mainly controlled by the updraft velocity of cloud forming air parcel, pre-existing aerosol size distribution and the amount of semivolatile gases (such as nitric acid) present. At the other extreme, in very cold conditions, ice particles are formed through homogeneous freezing affected by temperature and water saturation ratio on the surface of liquid aerosol particles. Between those two is the temperature regime, where ice particles are formed through heterogeneous freezing, which is mainly dependent on the properties of aerosol particles. Some particles may act as ice condensation nuclei as soon as the temperature drops few degrees below zero Celsius, as some other can freeze only when homogeneous freezing occurs. In order to study the effect of aerosol composition on the formation and persistence of mixed phase clouds, the latest results from ice chamber experiments are currently being parameterised. These parameterisations will be included into the air parcel model with a state-of-the art description of liquid phase thermodynamics, condensation and different heterogeneous freezing mechanisms, borrowing heavily from a model originally developed by Harri Kokkola at the University of Kuopio. This work is being carried out by Sami Romakkaniemi and will feed into model development within the NERC APPRAISE-funded core cloud modelling activity involving Dan Grosvenor, Dave Topping, Paul Connolly and Gordon McFiggans under the direction of Tom Choularton.