Alessandro Battaglia , Simone Tanelli , Gerry Heymsfield and Lin Tian
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Alessandro Battaglia,
Simone Tanelli, Gerry Heymsfield and Lin Tian
The Dual Wavelength Ratio knee: a signature of multiple scattering
in airborne Ku-Ka observations
Paper now out in J. Appl. Meteorol. Climatol., early on line release
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Focus and motivation• Convective towers heavy precipitation events
• CloudSat and TRMM providing single frequency observations of convective towers. GPM-DPR: first dual wavelength observations from space.
• CloudSat observations certainly affected by multiple scattering. For TRMM hints that rain rates were underestimated in deep convection (maximum not occurring under the convective core, Liu Chuntao)
CloudSat W-band
TRMM Ku-band
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GPM proxy: NASA-HIWRAP Ku-Ka MC3E observations
Region of strong attenuationVery likely
presence of hail (ground reports+
Ground-based S-pol radars)
Heymsfield et al., 2013: Airborne Radar Observations of Severe Hail Storms: Implications for Future Spaceborne Radar, J. Appl. Meteorol. Climatol.
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3dB/km
0.5dB/km1.6dB/km one way
High-density ice layer
S-band Z above 60 dBZ
only in the last 4 km
Vertical reflectivity profiles in sector II
iAnomalous sloping
DWR knee
Absence of surface return peak
Ka Ku
Inflexion point
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This feature was observed during MC3E in
severalconvective cells
The Dual Wavelength Ratio knee
In a singlescattering world …..
>10 dB
>12 dB
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Single scattering explanation?DWR sources from hail/graupel/rain
Only hail particles in this range of size But almost monodisperse can produce a
decrease in DWR. Rain and wet hail cannot!!
However quite hard to produce a 12 dB effect! Vertical microphysical processes not supported by S-band observations.
ways how to decrease DWR
Even if DWR can be explained there is no explanation of the disappearance of the surface with realistic s0 values
Differential attenuation
Mie effects
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The multiple scattering explanation
Anomalous sloping and surface peak disappearing have been already documented for CloudSat, but quite astonishing to see this features in Ku-Ka airborne observations. Ka is expected to be more heavily affected by multiple scattering than Ku.
Dense ice layer high SS albedo
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Simulation framework: single ice layer
Two features must be carefully treated:1) Antenna pattern2) Size of the convective cell
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Pulse stretching effect generated by an hail layer
The MS strength is a strong function of the horizontal extent of the system lateral diffusion allows radiation to re-enter the small footprint (whose 3 dB radius is of the order of 500 and 200 m for HIWRAP at 10 km distance for Ku and Ka, respectively) after being scattered away from it.
Ku profile
DWR profile
Pulse stretching
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Ku profile
Ka profile
Profile with hail explaining sector II HIWRAP+S-band observations
DWR profile Radiationheight
The radarIs loosing its
ranging capabilities
Grey-shaded envelopes=observed HIWRAP profiles within 1, 3, 5 km from the cluster center
Heuristic top-bottom
approach
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GPM configuration
3km10km
SS attenuatedSS effective
MDT MDT5km
• Maximum in DWR strongly suppressed• Appearance of a knee• Even negative DWR possible!
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ConclusionsA Ka-Ku DualWavelengthRatio knee was frequently observed during MC3E.
This signature is straightforwardly explained with the help of multiple scattering theory in presence of hail-bearing highly-diffusive convective cores with large horizontal extents Ka pulse stretching typically exceeding that occurring in the Ku channel anomalous sloping knee, visible thanks to the profile large attenuation and to the good HIWRAP sensitivity (10 and 0 dBZ at 20 km for Ku and Ka, respectively).
Multiple scattering effects are likely to be more pronounced in the upcoming space-borne GPM-DPR observations. Our forecast: DWR knees will be observed by the GPM radar when overflying high-density ice shafts embedded in large convective systems their explanation must not be sought in differential attenuation or differential Mie but via multiple scattering but interplay witn non uniform beam filling still under study. With real GPM-DPR data, we will be able to confirm or refute our conjecture.
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