Analysis of SPartiCus Data for Cirrus Modeling Case Studies

The U.S. Department of Energy (DOE) Small Particles in Cirrus (SPartICus) campaign sampled cirrus clouds above the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site from January to June 2010. The campaign was intended to improve measurements of particle size distributions (PSDs), especially focusing on reducing the error due to particle shattering, and to provide a comprehensive dataset, including measurements from the aircraft, ground-based radars and satellites, for the examination of microphysical processes and cloud-scale dynamical processes.

We present here observations of PSD evolution during the lifetime of cirrus from nucleation to dissipation. Preliminary analysis suggests that the shape of the PSD varies with the height in the cloud, from bimodal distributions with small and large particles at lower altitudes to more unimodal PSDs with solely small particles at higher altitudes. We analyzed six flights that completed spirals over the SGP site, over four separate days. PSD shape and atmospheric conditions are established from both aircraft and ground-based instruments. Aircraft instruments include the 2D Stereo Probe (2DS), Advanced Airborne Measurement Solutions probe (AIMMS-20), Deep-Cone Nevzorov, Diode Laser Hygrometer (DLH) and Cloud Particle Imager (CPI). Vertical velocity measurements retrieved from a Millimeter Cloud Radar (MMCR) are used to help establish the dynamical properties of the cirrus. To develop insight into the stage of development of each observed case, the cirrus cloud is broken up into five phases, or ages, based on the observed overlap of supersaturated regions and ice-containing regions. This information allows us to identify likely regions of recent ice nucleation occurring in a given cloud layer.

The means of nucleation is an outstanding problem in cirrus studies, and the concurrent roles of ice nucleation, growth, sedimentation, and aggregation within a cloud dynamical environment are challenging to evaluate from observations alone. The analyzed data will provide a basis for a future modeling analysis that will test whether the observed PSDs can be replicated with the given state parameters, and whether unimodality can be linked to homogeneous ice nucleation and bimodality to heterogeneous ice nucleation.