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The aerosol indirect effect in mixed phase and ice clouds

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Detailed ground based studies of the initiation of the ice phase in clouds enveloping the Jungfraujoch in the Swiss Alps will be described. It is shown that a higher concentration of ice crystals is found in clouds forming in polluted air. The evidence is that these ice crystals are associated with carbonaceous aerosol. The origin and development of this aerosol is described. Another feature observed in these clouds, and also in layer clouds more generally, is that in mixed phase clouds fully glaciated regions are found in close proximity to regions consisting largely of supercooled water.

Finally the influence of pollution aerosol on the extent and ice microphysics of anvil cirrus forming above deep tropical convection observed during the ACTIVE programme will be discussed. It is found that the number of available cloud condensation nuclei can modulate the loading of supercooled water entering the homogeneous freezing zone. The sudden freezing of this water releases latent heat of fusion, which can increase the height of cloud top and the amount of water available to the anvil cloud. Further haze freezing appears to be an important source of ice crystals in anvil regions away from the main cloud updraft.

This talk is part of the British Antarctic Survey series.

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