WP L1, L2 (AIDA, LACIS)

Small Ice Detector (SID-2) - cloud particle measurement measures "spatial" scattering patterns from single particles

SID2 - in situ measurement Can determine both shape and size of particles Scattering patterns recorded using custom detector array the scattering patterns on the left have been recorded in the lab using intensified camera. SID-2 "simplifies" them because it has only 27 detector elements (instead of 0.5 million pixels of the camera). the pattern shown is from an elongated silicate particle

SID2 - in situ measurement Example scattering patterns - actual SID-2H screen shots (SID-2 version built for NCAR HIAPER) 2 mm sphere ellipsoid recorded in the lab, February 2007 OK down to about 0.3 - 0.5 mm size? Aerosol deliquescence?

SID2 - in situ measurement Example particle classification - droplets/crystals in HALO01 28 recorded in the lab, February 2007

SID2 - tests with ice analogues rosette columns Polar plots: amount of light falling on detector element  plot radius

SID2 - crystal shape classification AIDA HALO-01 campaign, experiment 18 20 minutes 30 minutes

In situ measurement - SID-3 (ready Nov./Dec. 2007) Measures high-resolution 2D patterns Example scattering patterns: ellipsoid salt crystal flat grain droplet fibre SID-3 will capture 2D patterns with fine detail

SID3 probe - inhomogeneous particles ... with sub-m inclusion 20 m droplet ... with inclusion 3  m droplet

2D scattering patterns: ice-analogues SID3 probe 2D scattering patterns: ice-analogues Smooth and rough rosette

2D scattering can be computed New scattering model: “Ray Tracing with Diffraction on Facets” column plate rosette droxtal

WP M3 (climate modelling)

Light scattering – computation Geometric optics, despite its severe limitations, is widely used to model scattering on atmospheric ice. Q1. Can geometric optics be combined with diffraction? Q2. Can facets be treated as apertures - for transmission and reflection? Q3. Can diffraction be described entirely by ray optics?! (models like Geometric Theory of Diffraction convert from wave to ray fields, and back again, which is inefficient) 3 x YES: wave fields can be replaced by ray fields by approximating the direction of lines of energy flow  Ray Tracing with Diffraction on Facets (RTDF) model the motivation for developing RTDF was that existing models are very slow or cannot be applied to certain sizes of particles

Light scattering – computation Diffraction on a half plane: lines of average energy flow reflected j Dy Postulate Diffraction angle f for half-plane: principle of RTDF. It was originally developed as an asymptotic approximation for Sommerfeld's exact theory of diffraction on a reflecting half-plane. RTDF approximates the deflection angle phi of lines of energy flow passing the edge of the half-plane using a simple analytic formula. A similar formula is valid for a slit, and ic can be generalized to any 2D aperture. Extension to slit of width 2d:

Light scattering – computation Near-field diffraction on half-plane - without interference exact geom. optics RTDF illuminated side shadow side model asymptotically convergent to exact theory in its version without phase tracing RTDF cannot predict interference effects (ripple on the illuminated side) by converges to exact theory on the shadow side of the half-plane. The blue line shows illumination as predicted by pure geometric optics. distance from edge (wavelengths)

Light scattering – computation Fraunhofer diffraction on a slit - with interference Fraunhofer RTDF with phase When phase tracing is added, RTDF converges nearly exactly to scalar diffraction theory, in this case Fraunhofer diffraction on a slit (here 5 wavelengths wide)

Light scattering – computation How good is the RTDF model for ice crystals? When phase tracing is added, RTDF converges nearly exactly to scalar diffraction theory, in this case Fraunhofer diffraction on a slit (here 5 wavelengths wide)

Ice Analogues - simple, smooth - SEM images

Ice Analogues - complex: rosettes

Light scattering – computation Comparison of lab experiment with RTDF model – hex. column Experiment RTDF model we will also model scattering using a 2D version of the RTDF model. this comparison is for an ice analogue prism shaped crystal that is being tilted over 90 degrees. The crystal is on a fibre which contributes the arc indicated by the blue arrows. Clarke et. al. 2005, J.Quant. Spectrosc. Rad. Transf.

Measurement of phase functions - levitation these procedures were originally developed for ice analogue crystals.

Ice-analogue aggregate Levitation - orientation randomization View from above it is also possible to produce pseudo-random orientation, to simulate the situation for non-aligned particles Ice-analogue aggregate - side view

Light scattering – comparison Measured phase function compared to the RTDF model Low angle - asymmetry parameter can be measured

Associated work

Ice analogues - testing of Cloud Particle Imager ?

Ice analogues - testing of CPI

Contributions In situ probes for particle characterization (size, shape): - SID-2 variants for AIDA, and LACIS(?) - SID-3 (complex, inhomogeneous, and/or rough particles Modelling of single scattering properties (phase functions, asymmetry parameter) of ice particles Light scattering computations for the interpretation of laboratory measurements, e.g. backscattering depolarization (SIMONE, LACIS) Characterization and calibration of existing and new particle probes using ice analogues

Thank you!

SID2 - shape classification using FFT HALO-01, exp. 18, 30 minutes Theory – best fits