Dual‐polarization airborne lidar observations of polar stratospheric cloud evolution

Dual‐polarization λ=0.532 μm lidar data show systematic polar stratospheric cloud (PSC) evolution along a portion of the Airborne Arctic Stratospheric Expedition DC‐8 flight of January 31, 1989. This flight leg was roughly aligned with air parcel motion on isentropic surfaces from 400–500K, where the local adiabatic cooling rate was ≈20 K/day. Type 1 PSCs show low depolarization ratios and scattering ratios which approach intermediate limiting values as ambient temperature decreases. These data suggest that Type 1 particles formed by rapid cooling may be nearly spherical and are restricted in size by partitioning of a limited HNO 3 vapor supply among many competing growth sites. Type 2 PSCs appear at temperatures below estimated local frost points with increases in depolarization and scattering typical of larger ice crystals.