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A study of the effects of cloud processing of aerosol on the microphysics of cloud

A field experiment was conducted in which measurements were made of the microphysics of stratocumulus cloud upwind and downwind of a hill cap cloud using the UK Meteorological Office C‐130 aircraft. At the same time, measurements of the cloud microphysics and cloud chemistry were made on the ground at the hill summit. It is shown that the stratocumulus downwind of the hill has a reduced liquid‐water content due to entrainment initiated by Kelvin—Helmholtz instability above the hill top, whilst the droplet effective radius is smaller than upwind for a given value of the liquid‐water content. These latter effects are attributed to aqueous phase sulphate production caused by the oxidation of SO2 by hydrogen peroxide and ozone in the hill cap cloud, increasing the number of cloud condensation nuclei (CCN) available to the stratocumulus cloud downwind of the hill. The production of CCN by processing of aerosol in clouds with a vigorous updraught near cloud base (such as convective clouds) may be an important source of CCN in the atmosphere, further very small particles activated in the high supersaturations present in such clouds will grow rapidly into the accumulation mode.