Dual-Polarization and Dual-Wavelength Radar Measurements
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Dual-Polarization and Dual-Wavelength Radar Measurements
VivekNational Center for Atmospheric Research
Boulder, Colorado
I. Polarization and dual-wavelength radar
II. Cloud microphysics
III. Examples of products and ka-band observations.
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Dual-wavelength radar: S-Polka
KA Band Antenna
Transmitter
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Microphysics
• Precipitation type: Ice or liquid?• Precipitation particle shape, size
and concentration.
• Sensitivity: -10 dBZ @ 50 km 0.1 mm droplet size100 per
liter
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S LW
Ir re gu lar Ic e Cr yst als
Ic e Cr ys tal s
Wet S no w
Dry Sn o w
Gra up el/R
ai n
Gra up el/S m
all Hail
Rain /H
ai l
Hail
Hea vy R
ain
Mod er at e R
ai n
Liht
Rain
Driz zl e
Clou d D
rop s
RHI scans of Z, ZDR and the corresponding particle classification results. The radar measurements were collected by NCAR S-Pol radar during the CASES 97 field program.
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.
(
ReflectivityZDR
Number concentration # m^-3
Shape of DSD
Rain rate mm/hr Median volumeDiameter, mm
Spatial distribution of polarization radar measurements, retrieved DSD
parameters, and rain rate
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Examples of Ka-band radar Measurements
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Sample Plane Parallel Indicator (PPI) Scan From WISP04, 2004.
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S-Band Reflectivity (dBZ) Ka-Band Reflectivity (dBZ)
Particle ID output Z difference (S-band – Ka-band)
Dry snowPristine Crystals
Irregular crystals
Ground clutter
Range (km)
Hei
ght (
km)
Side lobe artifact
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Advantages of dual-wavelength
system• Detection of cloud droplets.
• Estimate of size and LWC.
• Raindrop size distribution .
• Effect of Bragg scatter is less at Ka-band.
• Improved cloud microphysical retrieval using both dual-wavelength and dual-polarization observations.
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Knight et al. JAS, J. Atmos. Sci., 59, 1454-1472.
Data from the RHIs are fit to a
Gamma distribution and characteristics of the drops are displayed here as labeled at the left at the three times for the three rows.
The last row, , is an exponent in the gamma distribution.
When = 0 the distribution is exponential, and as increases it narrows.
The rainfall rate is calculated as if at the ground with zero vertical air velocity.
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Gro und C
lutter
Bir ds
Ins ects
SL
W
Irre gular Ice Cr ystals
Ice Cryst als
We t Sn
ow
Dry S
now
Gra up
el/Ra in
Gra up
el/Sm
all Hail
Rai n/H
ai l
Hai l
Hea vy R
a in
Mo derate R
ain
Lig ht R
ai n
Dri zzle
Clo ud D
r ops
S - P o l d a t a a n d p a r t i c l e I D r e s u l t s f r o m M A P ( I t a l y )
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Issues: Measurement
• Number of independent samples and standard error of radar observables.
• Calibration of radar measurements.
• How to combine forward scattering observations with backscatter measurements?
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Issues: Microphysics
• Drop shape and size distribution (DSD).
• Ice particle shape and density.
• How to separate DSD and particle shape effects in polarization measurements?
• How to detect super cooled droplets?