On the transition of contrails into cirrus clouds

In situ observations of the microphysical properties of upper-tropospheric contrails and cirrus clouds have been performed during more than 15 airborn e missions over central Europe. Experimental and technical aspects concerni ng in situ characterization of ice clouds with the help of optical and nono ptical detection methods (preferably FSSP-300 and Hallet-type replicator) a re addressed. The development of contrails into cirrus clouds on the timesc ale of 1 h is discussed in terms of a representative set of number densitie s. and size distributions and surface area distributions of aerosols and cl oud elements. with special emphasis on small ice crystals (diameter <20 mu m). Contrails are dominated by high concentrations (>100 cm (3)) of nearly spherical ice crystals with mean diameters in the range 1-10 mu m Young cir rus clouds. which mostly contain small regularly shaped ice crystals in the range 10-20-mu m diameter and typical concentrations 2-5 cm(-3) have been observed. Measurement results are compared to simple parcel model calculati ons to identify parameters relevant for the contrail-cirrus transition. Obs ervations and model estimates suggest that contrail growth is only weakly, if at all, affected by preexisting cirrus clouds.