Aircraft observations of highly supercooled water in convective clouds (1.8 gm-3 at -37.5 ¿C) were reproduced by a numerical cloud model with explicit microphysical processes and turbulent effects. The model showed that large concentrations of small cloud droplets induced by high concentrations of CCN and a strong updraft were essential in reproducing the observations. The same model reproduced microphysically maritime clouds with fast depletion of cloud water by just changing the CCN spectrum. The constrains imposed by these extreme conditions revealed the weakness of existing cloud parameterizations, and the necessity of inclusion of very detailed explicit microphysical and turbulent cloud processes in cloud models. ¿ 2001 American Geophysical Union