Robin Hogan, Chris Westbrook

1
The importance of ice particle shape and
orientation for spaceborne radar retrievals

• Robin Hogan, Chris Westbrook
• University of Reading
• Lin Tian
• NASA Goddard Space Flight Center
• Phil Brown
• Met Office

2
Introduction and overview

• To interpret 94-GHz radar reflectivity in ice
  clouds we need
• Particle mass Rayleigh scattering up to 0.5
  microns Z?? mass2
• Particle shape non-Rayleigh scattering above
  0.5 microns, Z also depends on the dimension of
  the particle in the direction of propagation of
  the radiation
• Traditional approach
• Ice particles scatter as spheres (use Mie theory)
• Diameter equal to the maximum dimension of the
  true particle
• Refractive index of a homogeneous mixture of ice
  and air
• New observations to test and improve this
  assumption
• Dual-wavelength radar and simultaneous in-situ
  measurements
• Differential reflectivity and simultaneous
  in-situ measurements
• Consequences
• Up to 5-dB error in interpretted reflectivity
• Up to a factor of 5 overestimate in the IWC of
  the thickest clouds

3
Dual-wavelength ratio comparison
10 GHz, 3 cm
10 GHz, 3 cm
94 GHz, 3.2 mm
94 GHz, 3.2 mm
Difference

• NASA ER-2 aircraft in tropical cirrus

4
Characterizing particle size

• An image measured by aircraft can be approximated
  by a...
• Sphere (but which diameter do we use?)
  Spheroid (oblate or prolate?)

Note Dmax ? Dlong Dmean(DlongDshort)/2
5
Error 1 Rayleigh Z overestimate

• Brown and Francis (1995) proposed masskg0.0185
  Dmeanm1.9
• Appropriate for aggregates which dominate most
  ice clouds
• Rayleigh reflectivity Z ? mass2
• Good agreement between simultaneous aircraft
  measurements of Z found by Hogan et al (2006)
• But most aircraft data world-wide characterized
  by maximum particle dimension Dmax
• This particle has Dmax 1.24 Dmean
• If Dmax used in Brown and Francis relationship,
  mass will be 50 too high
• Z will be too high by 126 or 3.6 dB
• Explains large part of ER-2 discrepancy

6
Particle shape
Randomly oriented in aircraft probe

• We propose ice is modelled as oblate spheroids
  rather than spheres
• Korolev and Isaac (2003) found typical aspect
  ratio a Dshort/Dlong of 0.6-0.65
• Aggregate modelling by Westbrook et al. (2004)
  found a value of 0.65

Horizontally oriented in free fall
7
Error 2 Non-Rayleigh overestimate
Spheroid
Sphere
8
Independent verification Z dr

• A scanning polarized radar measures differential
  reflectivity, defined as Zdr 10log10(Zh/Zv)

Dshort/Dlong
Solid-ice oblate spheroid
Dependent on both aspect ratio and density (or
ice fraction) If ice particles were spherical,
Zdr would be zero!
Solid-ice sphere
Sphere 30 ice, 70 air
9
Chilbolton 10-cm radar UK aircraft

• Reflectivity agrees well, provided Brown
  Francis mass used with Dmean

• Differential reflectivity agrees reasonably well
  for oblate spheroids

10
Z dr statistics

• One month of data from a 35-GHz (8-mm wavelength)
  radar at 45 elevation
• Around 75 of ice clouds sampled have Zdrlt 1 dB,
  and even more for clouds colder than -15C
• This supports the model of oblate spheroids
• For clouds warmer than -15C, much higher Zdr is
  possible
• Case studies suggest that this is due to
  high-density pristine plates and dendrites in
  mixed-phase conditions (Hogan et al. 2002, 2003
  Field et al. 2004)

11
Consequences for IWC retrievals

• Empirical formulas derived from aircraft will be
  affected, as well as any algorithm using radar

Raw aircraft data
Empirical IWC(Z,T) fit
Spheres with D Dmax Hogan et al. (2006) fit New
spheroids
Note the mass of the particles in these three
examples are the same