Molecular dynamics simulations of homogeneous ice nucleation in extended
aqueous slabs show that freezing preferentially starts in the subsurface. The top
surface layer remains disordered during the freezing process. The subsurface
accommodates better than the bulk the increase of volume connected with freezing. It
also experiences strong electric fields caused by oriented surface water molecules,
which can enhance ice nucleation. Our computational results shed new light on the
experimental controversy concerning the bulk vs surface origin of homogeneous ice
nucleation in water droplets. This has important atmospheric implications for the
microphysics of formation of high altitude clouds.