"Mixing, correlations and fluctuations in the atmosphere"
Charles F Clement
5 March 2008
11am, Stephenson Building, room F16 (first floor)
Atmospheric trace gases and aerosols, including clouds, exhibit large fluctuations in space and time in which their concentrations in number and mass vary greatly. Fluctuations affect properties such as rainout and nucleation of new aerosol important to weather and climate. Processes which lead to these fluctuations include localized emissions and mixing of different air masses. Here we will describe results from recent research on these processes, and point out the need for more work on clouds and the formation of fluctuations by the condensation process.
Many fluctuations result from mixing, and we will show that remarkable correlations between concentrations of different constituents can arise from the mixing of uniform air masses. Correlations are often enhanced by averaging concentrations over larger time intervals, and a theory of this averaging is outlined. For localised plumes entering and mixing with the atmosphere, aerosol contained in the emitted plume does not mix well on small length scales because of the small diffusivity of aerosol particles. This means that there are very large local fluctuations in aerosol concentration which cannot be described by models containing “turbulent diffusivities”. An example is shown of the nucleation of a new sulphuric acid aerosol as a result of the mixing of a volcanic plume containing a dense aerosol with air.
A long-standing problem has been to explain fluctuations observed in clouds, the wide size distribution of cloud droplets, and rapid onset of rain from convective cumulus clouds. Fluctuations of water vapour saturation imposed in clouds have been shown to lead to wide and sometimes bimodal droplet size distributions, but the origins of such saturation fluctuations needs to be explained. Condensation in clouds is controlled by detailed coupling between heat and mass transfer and we examine the basic equations of these processes for which realistic calculations are still lacking.
Charles Clement graduated in Mathematics from Cambridge University, and received a Doctorate from Cambridge University in Theoretical Nuclear Physics in 1960. Most of his career was spent in Theoretical Physics Division at Harwell Laboratory from where he retired in 1993, but he also spent four years in the USA in teaching posts at the Universities of Princeton, Florida State and Minnesota. He has had nearly 50 years of research in both quantum and classical areas of theoretical physics including nuclear physics, heat and mass transfer theory, nuclear fuel modelling and physics related to nuclear safety. During the last 25 years he has worked mainly on the physics of aerosols, and was President of the Aerosol Society from 2002-6.