Using SPartICus measurements to study the occurrence and source of ice particle size distribution features in mid-latitude synoptic cirrus

The bulk properties of cirrus clouds, including radiative impact, are determined by the microphysical processes (heterogeneous nucleation, homogeneous nucleation, growth, evaporation, sedimentation and aggregation) that occur during cloud formation and evolution. The relative importance of these processes to the development of cirrus is poorly constrained, due in large part to unreliable or incomplete in-situ measurements of particle size distribution (PSD), ice crystal properties and atmospheric conditions. The Small Particles in Cirrus (SPartICus) campaign provides measurements from the 2-D Stereo Probe (2DS) and Cloud Particle Imager (CPI), and thus, simultaneous vertical profiles of PSD and ice crystal habit, as well as environmental parameters. Observed PSDs are unimodal at cloud tops and bimodal or trimodal at cloud bottoms with the highest concentrations observed in the small particle size range (diameter smaller than 50 microns). The number and properties of the PSD modes are commonly associated with differing combinations of microphysical processes. However, because the uncertainties associated with PSD measurements are not quantified spectrally, a determination of basic size distribution features, including the number of modes, remains difficult. Here we first determine which PSD features can be established as representative based on the SpartICus dataset. We then simulate PSDs with a size-resolved (bin) microphysics model with the objective of understanding the fundamental processes that lead to observed PSD features. We use ice properties derived from CPI images to develop ice crystal habit schemes and we use parcel and column simulations to test the sensitivity of PSD shape to habit and nucleation mode. We use a water activity based model for heterogeneous ice nucleation. Preliminary parcel simulations indicate that PSD is sensitive to ice crystal habit.