IN-SITU OBSERVATIONS OF THE MICROPHYSICAL PROPERTIES OF YOUNG CIRRUS CLOUDS

In situ measurements made in cold (-35 degrees to -60 degrees C) cirru s clouds over southern Germany in March 1994 are presented. The clouds appeared to be in an early stage of their life cycle and their proper ties in many ways resemble those reported for ice fogs. Crystal concen trations were high (median 2.5 cm(-1), STP) and sizes small with a dia meter of mean mass of typically 16 mu m. The cloud on 18 March present s an interesting case for modeling studies of cirrus formation. On;hat particular day, the bulk properties of the cloud appeared to be conne cted to wave structures in the vertical wind field consistent with the Brunt-Vaisala frequency, which gave a corresponding wavelength of 40- 50 km. Furthermore. analyses of potential temperature and vertical win d suggested that the vertical displacement producing these clouds was less than 100 m. Size distribution measurements of interstitial partic les and crystal residues (particles remaining after evaporation of the crystals) show that small aerosol particles (diameters <0.5 mu m) par ticipate in the nucleation of cirrus crystals at low temperatures. Bec ause the aerosol in this small size range is readily perturbed by anth ropogenic activity, it is important to study the link between upper tr opospheric aerosol properties and cirrus cloud microphysics. While the observations presented here are not adequate to quantify this link, t hey pave the way for modeling studies and would be interesting to comp are to cirrus observations from cleaner regions.