Radar Requirements
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Transcript of Radar Requirements
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Radar Requirements
David J. StensrudNOAA/National Severe Storms Laboratory
2013 Warn-on-Forecast Workshop and Technical Guidance Meetings
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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.
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• 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.
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Tornado debris signature from dual-pol NWS radar in Huntsville, Alabama2 March 2012
Dual-Polarization Benefits
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Hail signature (large Z, low ZDR)
ZDR arc (high ZDR
indicating large raindrops)
Conceptual model of dual-pol signatures in thunderstorms being developed.
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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
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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
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Value of Phased Array Radar
Observing SystemSimulation Experiments
(Yussouf and Stensrud 2010 MWR)
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After 1 hour of Assimilation
YussoufandStensrud(2010 MWR)
Trut
h
Mod
el
Anal
ysis
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Bene
fits
of P
AR15
min
utes
of a
ssim
ilatio
n
Yussouf and Stensrud (2010)
Trut
h
PAR
Anal
ysis
WSR
88D
Anal
ysis
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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
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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
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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