The Origin and Development of the Ice Phase in Convective Clouds Observed during COPE

In this presentation we present the study of the microphysics of convective clouds that formed in the summer along convergence lines over the south west peninsula of England. These microphysics measurements were embedded in a project that includes the larger-scale dynamics of convective clouds, as part of the COnvective Precipitation Experiment (COPE) together with comprehensive measurements of the aerosol on board aircraft and at the surface. The overall aim of this project is to improve predictions of severe convective rainfall that leads to flash flooding by addressing the combined problems of the microphysics and dynamics of precipitation development in convective clouds.

In order to address the origin of the ice phase in these deep convective clouds we made measurements of the properties of the aerosol particles, particularly soils, dust and biological material, on the ground and also in the boundary-layer with the FAAM BAe146 aircraft using a range of instrumentation.

Comprehensive measurements of the cloud microphysics were made by the BAe146 and the Wyoming King Air aircraft. They made penetrations of multiple clouds at different levels flying using a statistical flight pattern approach. New instruments that can detect and characterise small ice particles were deployed on the BAe146 aircraft. In this presentation we focus on measurements from the BAe146 aircraft.

We present results of the formation of droplets on cloud condensation nuclei and their growth by vapour diffusion and coalescence. We then consider the formation of the first ice in each cloud considered and its subsequent growth and development. The clouds studied had warm bases of around +10C and tops generally warmer than -15C so there was formation and growth of raindrops by coalescence before the first ice formed. It is shown that the presence of these supercooled raindrops played an important role in the development of the ice. The ice phase was observed in each of the clouds despite the relatively warm cloud tops. For each case study we infer the likely origin of the heterogeneous ice nuclei responsible for the first ice using the wide range of aerosol measurements made. After the first ice formed, rapid ice multiplication occurred by the Hallett-Mossop process and this ice controlled the subsequent glaciations of the cloud and precipitation formation.

It was further observed that each convective cell played a substantial role in transporting material into the free troposphere where it was detrained mostly at the level of the final cloud top. It was found that most of the sulphate, nitrate and ammonium aerosol was removed by passage through the cloud by a rainout process. However, the Black Carbon (BC) concentrations in the detrainment zone were higher than elsewhere in the free troposphere, meaning these particles must be cloud processed material lifted from the boundary layer to higher levels by the clouds, but interstitially to the activated cloud particles. The lifted BC particles were also larger and more thickly coated with organic material than below cloud, possibly as a result of their acting as condensation centres for organic vapours also lifted by clouds and/or alternatively expelled from activated cloud particles.