Robin Hogan, Chris Westbrook Robin Hogan, Chris Westbrook University of Reading University of Reading Lin Tian Lin Tian NASA Goddard Space Flight Center NASA Goddard Space Flight Center Phil Brown Phil Brown Met Office Met Office Why it is important that ice Why it is important that ice particles are Smarties not particles are Smarties not Gobstoppers to a radar Gobstoppers to a radar
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Robin Hogan, Chris Westbrook University of Reading Lin Tian NASA Goddard Space Flight Center Phil Brown Met Office Why it is important that ice particles.
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Robin Hogan, Chris WestbrookRobin Hogan, Chris WestbrookUniversity of ReadingUniversity of Reading
Lin TianLin TianNASA Goddard Space Flight CenterNASA Goddard Space Flight Center
Phil BrownPhil BrownMet OfficeMet Office
Why it is important that ice Why it is important that ice particles are Smarties not particles are Smarties not
Gobstoppers to a radarGobstoppers to a radar
Introduction and overviewIntroduction 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
Dual-wavelength ratio Dual-wavelength ratio comparisoncomparison
• NASA ER-2 aircraft in tropical cirrus
10 GHz, 3 cm
94 GHz, 3.2 mm
10 GHz, 3 cm
94 GHz, 3.2 mm
Difference
Error 1: constant 5-dB overestimate of Rayleigh-
scattering reflectivity
Error 2: large overestimate in the dual-wavelength
ratio, or the “Mie effect”
Characterizing particle sizeCharacterizing 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=(Dlong+Dshort)/2
Error 1: Rayleigh Z Error 1: Rayleigh Z overestimateoverestimate
• Brown and Francis (1995) proposed mass[kg]=0.0185 Dmean[m]1.9
– 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
Particle shapeParticle shape• We propose ice is modelled as
Smarties rather than Gobstoppers!– Korolev and Isaac (2003) found
typical aspect ratio =Dshort/Dlong of 0.6-0.65
– Aggregate modelling by Westbrook et al. (2004) found a value of 0.65