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Dual-Polarization Radar Radar reflectivity (Z) – backscattered power related to the drop-size distribution Radial velocity (V R ) – component of wind velocity oriented along the radar beam Differential reflectivity (Z DR ) - ratio of backscattered power at orthogonal polarizations – For targets with isotropic scattering properties such as spherical or chaotically tumbling particles, Z DR is 0 dB. High Z DR is mostly associated with large, wet hydrometeors. – Good indicator of median particle shape and can be useful to detect hail, updraft location and melting layer.
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Radar Requirements
David J. StensrudNOAA/National Severe Storms Laboratory
2013 Warn-on-Forecast Workshop and Technical Guidance Meetings
Dual-Polarization Radar• Radar reflectivity (Z) – backscattered power related to the drop-
size distribution• Radial velocity (VR) – component of wind velocity oriented along
the radar beam
• Differential reflectivity (ZDR) - ratio of backscattered power at orthogonal polarizations – For targets with isotropic scattering properties such as spherical or
chaotically tumbling particles, ZDR is 0 dB. High ZDR is mostly associated with large, wet hydrometeors.
– Good indicator of median particle shape and can be useful to detect hail, updraft location and melting layer.
• Specific differential phase (KDP) – range derivative of differential propagation phase.– KDP is a good indicator of the liquid water content within the
radar sampling volume, as it is nearly zero for heavily aggregated snow or dry graupel/hail.
– Strongly related to rain rate, so used for QPE.
• Correlation coefficient (ρHV) - correlation coefficient between the backscattered returns at horizontal and vertical polarizations at zero lag time.– approaches unity in pure rain or pure dry hail at S band, but is
decreased when a mixture of rain and hail is present.– Low values may indicate mixed precipitation types, non-
meteorological scatters or large hail.
Tornado debris signature from dual-pol NWS radar in Huntsville, Alabama2 March 2012
Dual-Polarization Benefits
Hail signature (large Z, low ZDR)
ZDR arc (high ZDR
indicating large raindrops)
Conceptual model of dual-pol signatures in thunderstorms being developed.
Dual-Polarization Benefits
• Tells us something about the characteristics of precipitation as seen by radar
• May be used to classify hydrometeor types• Should help with radar data quality control
• Benefits of direct assimilation of DP variables to Warn-on-Forecast are uncertain
Functional Radar Requirements 2030
• NOAA/NWS Functional Weather Radar Requirements Integrated Working Team was formed in late 2012
• FAA has December 2014 decision point– Near simultaneous volume scans every 1 or 2 minutes– Sample variances as new radar variables– Staggered pulse repetition times to improve radial
velocities– Improved quality control within signal processor or
radar system
Value of Phased Array Radar
Observing SystemSimulation Experiments
(Yussouf and Stensrud 2010 MWR)
After 1 hour of Assimilation
YussoufandStensrud(2010 MWR)
Trut
h
Mod
el
Anal
ysis
Bene
fits o
f PAR
15 m
inut
es o
f ass
imila
tion
Yussouf and Stensrud (2010)
Trut
h
PAR
Anal
ysis
WSR
88D
Anal
ysis
Real Data Case: 24 May 2011
• PAR data with 1-min volume scans, 10 tilts• Degrade to 88D using 2/3 tilts per minute, so
full volume scan in 5 minutes• Radar data thinned to 4 km grid
Courtesy of Lou Wicker
Assimilation Details• 60 members• LETKF + adaptive inflation• Perturbations to u,v profiles• Bubble initialization • 1.5 km horizontal/200 m vertical• NCOMMAS with LFO and turbulent mixing• Assimilate from 2020 to 2100 UTC• Assimilate 0 dBZ away from storms• Reflectivity data only assimilated every 5 minutes for all
experiments
Main Points
• Radar observations key to Warn-on-Forecast– Unique in-storm observations– Dual-pol should help with quality control, maybe
with direct assimilation or through use of HCA– PAR “snapshots” seem to yield better results
• Radar requirements for 2030 being discussed
