Ice fallout in convective storms

Layer-type 10 cm radar echoes from apparently 'clear air', extending to large distances ahead of convective storm clouds are examined for their origin and spatial dimensions. The echoes are attributed to Rayleigh scattering from ice crystals originating in the tops of Cb and anvil clouds. The quantity of ice in the echo volume is assessed from the measured reflectivity factor using mass-reflectivity relationships deduced from size and concentration of crystals sampled by others in clear air and non-raining clouds. The calculated amount of one to two million tonnes of ice contained in the average echo volume associated with moderate to severe storms, compares favourably with an estimate from considerations of water substance continuity, applying a convective precipitation model due to Kessler (1969). The crystal concentrations in the layer have not yet been established by in situ sampling in the vicinity of storms, but there is indirect evidence for particles in the diameter range 100 to 500µ to be sufficiently concentrated to play a part in natural seeding of middle level clouds.