1. WP L1, L2 (AIDA, LACIS)

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

3. 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

4. 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 mm size? Aerosol deliquescence?

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

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

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

8. 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

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

10. 2D scattering patterns: ice-analogues
SID3 probe
2D scattering patterns: ice-analogues
Smooth and rough rosette

11. 2D scattering can be computed
New scattering model: “Ray Tracing with Diffraction on Facets”
column
plate
rosette
droxtal

12. WP M3 (climate modelling)

13. 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

14. 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:

15. 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)

16. 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)

17. 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)

18. Ice Analogues - simple, smooth - SEM images

19. Ice Analogues - complex: rosettes

20. 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.

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

22. 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

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

24. Associated work

25. Ice analogues - testing of Cloud Particle Imager?

26. Ice analogues - testing of CPI

27. 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

28. Thank you!

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