AIITS: Preliminary light scattering data from Tropical Tropopause Layer cirrus

Abstract

The new optical particle spectrometer AIITS (Aerosol Ice Interface Transition Spectrometer) is the next instrument in the Small Ice Detector (SID) family. Like SID3, it acquires two-dimensional forward scattering patterns from particles in the size range from about one to a few hundred micrometers (depending on variable settings). The patterns allow quantifying the phase, habit and fine surface features of large aerosol and ice crystals, which are frequently too small to be adequately characterised using traditional imaging techniques.Two 2D-forward scattering patterns are recorded per particle using two high-resolution cameras. The cameras fire simultaneously, recording the scattering pattern via a beamsplitter. AIITS can be configured such that the cameras measure either perpendicular polarisations (i.e. P-polarisation with one camera, S-polarisation with the other) or to have a different gain setting on each camera to encompass a larger dynamic range. The incident beam can be either circularly or linearly polarised. Backscatter depolarisation is also measured. The camera and beam configuration must be selected pre-flight.The probe was deployed on board the NASA Global Hawk aircraft during a recent ATTREX/CAST campaign over the tropical eastern Pacific. We present preliminary results from a case study from the 5th of March 2015 which showed the existence of a variety of particles, including rough surfaced ice crystals, some regular, hexagonal ones, as well as particles with smooth, curved surfaces (but not spherical). We compare AIITS data with co-located particle imaging from the SPEC Hawkeye probe.The Hawkeye probe combines a 2D-Stereo optical array probe, a Cloud Particle Imager (CPI), and a Fast Cloud Droplet Probe (FCDP) to provide high resolution images (2.3 micron pixel resolution) and particle size distributions of concentration, area, and mass for particles with diameter between one micron and a few centimeters.The TTL is known to be of importance due to the presence of subvisual cirrus, which contributes to net climate radiative feedback. Knowledge of the processes involved in the creation and persistence of such clouds is limited due to sparse observational data.