Ice particle habits in Arctic clouds

Ice crystals in atmospheric clouds have shapes, which affect their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate change predictions, weather forecasting of precipitation, and remote sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are assumed. Using new technology imaging instrumentation with a resolution of 2.3 mu m, recent observations in Arctic clouds have shown that such pristine habits only describe approximately 3% of the particles. The measurements were made from an aircraft during April 1998 and cover a temperature range of 0 degrees C to -45 degrees C. Boundary layer, multi-layer and cirrus clouds were examined. The commonly observed irregularly shaped particles either consisted of faceted polycrystalline particles or sublimating (solid to vapor) ice particles with smooth curving sides and edges. Since climate warming is now predicted to be largest in the Arctic, and cloud properties significantly affect the radiation balance, it will be necessary to consider the effects of non-pristine ice particle habits in such calculations and predictions.