Operational Applications of Polarimetric Radar Steven A. Rutledge Department of Atmospheric Science Colorado State University Acknowledgements—All current and former staff and students of the Radar Meteorology Group, CSU-CHILL staff, Profs. Bringi and Chandra from CSU/ECE, funding agencies especially NSF, NASA and NOAA, and many colleagues in the community.
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Operational Applications of Polarimetric Radar
Steven A. RutledgeDepartment of Atmospheric ScienceColorado State University
Acknowledgements—All current and former staff and students of the Radar Meteorology Group, CSU-CHILL staff, Profs. Bringi and Chandra from CSU/ECE, funding agencies especially NSF, NASA and NOAA, andmany colleagues in the community.
Benefits of Polarization Diversity….(based on several decades of research)
• Identification of anomalous propagation and clutter (non-meteorological echo)---data quality control
• Improved rain rate estimation, especially in presence of ice
• Remotely sense cloud and precipitation processes, especially when combined with Doppler measurements
• Detection of severe weather including hail• Attenuation correction, especially critical at sub S-band
wavelengths
Radar[ ]
TMatrixn Propagatio
S
⎥⎦
⎤⎢⎣
⎡Matrix
Scattering
H
V
The observations are a combination of backscatter and propagation characteristics of precipitation. Use various polarization schemes to remotely sense the precipitation medium.
Basic measure of mean shape; median volume diameter (D0) can be retrieved. N0 fixed by Z.ZDR ~ 3 dB rainZDR ~ 0 dB hail/graupel
Differential Phase (ΨDP) ϕdp = ϕh - ϕv
Specific Differential Phase (KDP)Filtered, range derivative of ϕdpLWC, oblateness; isotropic vs. anisotropic scatterers--- KDP very different between rain/hail
Oblate Raindrop
Small RaindropHail/Graupel
Polarimetric Radar Variables
Linear Depolarization Ratio (LDR)Orientation, canting, melting—not possible withNEXRAD polarimetric configuration
Correlation Coefficient (ρHV); measure of correlation between estimates of ZH and ZV.Mixed phase, melting—strong function of howdiverse the particle shapes are in pulse volume. Clutter or AP have very low correlations—useful!
Z ∝ Σ [N(D) • D6 ]
R ∝ Σ [N(D) • D3.67 ]
Kdp ∝ Σ [N(D) • D4.24 ]
Advantages of using Kdp for rainfall estimationLess sensitive to variations in DSD than Z (4.24th moment of DSD is closer
to 3.67th than 6th !)Independent of power calibration-phase measurementLess sensitive to beam blockingImmune to attenuation—provided enough signal!
Issues regarding Kdp
Trade-off between accuracy and spatial resolution (rain estimation). Filtering required from a noisy field ϕdp
Backscatter differential phase (Mie targets), range effects, gradient regions, choice of drop shape model
Issues regarding ZDR
Reflectivity gradients—power received through sidelobes that are mismatched can produce spurious values of ZDR in low reflectivity regions next to strong cores
Antenna performance is critical, main beam H/V matching
Three body scattering
Differential attenuation
Presence of hail---GOOD and BAD
NCAR S-pol
CSU-CHILL
Intense storm, rain and hailas viewed by the CSU-CHILLradar
Polarimetric conversion of NEXRAD
• Single transmitter, simultaneous transmit, two receivers
Seliga and Bringi, 1976 (JAM)---first discussion of single transmitter, dual-receiver configuration---now known as STSR
1. Examine polarimetric parameters and temperature at each grid point
2. Score each hydrometeor category based on observations relative to known range of values for each hydrometeor class (determined from field obs, scatter modeling)
3. Highest score wins
Algorithm produces “dominant” hydrometeor type---thiscan be summed to provide storm volumes of hydrometeortype-crude information can be derived on mixing ratios.
June 29: STEPS Lim et al. (2005)C
B
Polarimetric dataused to diagnosehail and graupelcontents and relate toelectrical properties
K. Wiens and S. Tessendorf
Polarimetricradar as a cloud physicstool
Ryzhkov et al. 2005, J. Appl. Met.
PolarimetricTornado Detection
LoweredDiff ReflectivityandCorrelationCoefficient
Brandes and Ikeda, JAM, 2004
Mapping of meltinglevel
March 2003 blizzard, microphysical processesrevealed by polarimetric datacollected by the CSU-CHILLFacility—courtesy Pat Kennedy
CSU-CHILL, Z Rain-snow, Vr
ZDR LDR
Conclusions• Much research has been done to date using polarimetric radars
and much has been learned. More work is needed to investigate application of polarimetric rain estimators to broad spectrum ofrain regimes
• Use of polarimetric S-band radars for cool season precipitation is relatively unexplored compared to warm season precipitation—this work needs to be accelerated in advance of the NEXRAD polarimetric upgrade
• Also need to further explore role of surface based measurements such as disdrometers, profilers, mesonets, etc to enhance polarimetric NEXRAD network
• Need more study of short wavelength polarimetric radars combined with S-band data
S-Band and X-Band Polarimetric Radars Complement One Another
Example: Mesoscale Convective System
Trailing Stratiform
LeadingConvection
•X-band allows greater accuracy in light rain events (e.g., stratiform)•S-band provides excellent rainfall estimates in heavy rain (convective)