A microphysical cloud model is employed to study the ice nucleation processes generating the extremely thin (∼10 m) cirrus cloud observed by the Airborne Tropical Tropopause Experiment in the Tropical Tropopause Layer. This cirrus is unique in its high concentration (104 L−1) of small (1.0-3.2 μm diameter) ice particles, suggesting that dynamical processes such as sedimentation and mixing have not yet occurred. This is advantageous for the investigation of cloud physical processes in numerical simulations. The maximum ice concentration (Nicemax) is calculated from a series of parameter sweep experiments by changing the cooling rate, aerosol particle size, initial water vapor mixing ratio, and accommodation coefficient, assuming homogeneous ice nucleation in the presence of monodisperse aerosol particles in an air parcel subject to isentropic uplift. The cooling rate necessary for the formation of this cirrus is investigated using the cross sections of Nicemax in parameter space. The required cooling rate, estimated to be a few Kelvin per hour under the observed atmospheric conditions, may exceed typical values associated with synoptic disturbances, but falls within typical values for mesoscale motion, although some uncertainty remains, especially in the value of the accommodation coefficient.
2016 by the Meteorological Society of Japan