Concepts of Weather Radar Weather radars measure the reflectivity and Doppler velocity from received power of electromagnetic waves backscattered by hydrometeor particles (raindrops, ice particles, graupels, hails) in the atmosphere and their velocity components along the radar beam using the Doppler effect. Frequency Echo Antenna Frequency 2 wind Transmitter & Receiver Processor Precipitation Display Echo Intensity Doppler Velocity Spectrum Width & Various Products
Family Tree of Remote Sensing - - - - - -,,,, Radiometer Sonic transducer RADAR Windprofiler IR Radar LIDAR SODAR RF IR Light ElectroMagnetic( ) Sonic( ) Passive( ) Remote Sensing Active( )
Weather Radar
Radar bands, frequencies & wavelength
X-band radar(3cm)
C-band radar(5cm)
S-band radar(10cm)
종류 파장 주파수 이용범위 장점 단점 국지 규모의 기상현상 연구목적으로 사용 일반적인 국지기상 경보 시스템으로 사용 가장 보편적으로 알려진 기상레이더의 종류 가격이 저렴 이동 및 운용 편리 비교적 운용이 편리하며 국지 경 보기능에 적합 탐지범위매우 좁음 감쇠효과매우 큼 탐지범위좁음 감쇠효과큼 호우 다발지역에 적합 탐지범위가 넓기 때문에 레이더 네트워킹 구성 탐지범위넓음 감쇠효과적음 가격이 비쌈
Polarization Concept
Radar Observables(moments) Polarimetric Radar Z DR, LDR, DP, K DP, hv, C RH, C LH
Φ dp Polarimetric Observables Φ dp K dp ρ hv C RH C LH
Zdr vs Hydrometeors and shape
The Joint Polarization Experiment (JPOLE) successfully demonstrated superb capabilities of a polarimetric prototype of the S-band WSR- 88D radar in operational environment Based on the JPOLE outcome, the US NWS has decided to add polarimetric diversity to all operational WSR-88D radars in near future Concurrently, national weather services around the world consider polarimetric upgrades of network weather radars including those operating at C band (e.g., Europe, Canada) Other efforts are directed towards possible utilization of inexpensive X-band polarimetric radars to complement existing WSR-88D radars in the areas of poor coverage (as gap fillers )
Problems at Shorter Radar Wavelengths Stronger attenuation in precipitation Stronger resonance scattering effects Interference of the EM waves reflected from the near and rear sides of the raindrop Stronger impact of nonuniform beam filling Perturbations of the radial profile of differential phase and radar reflectivity Z. Stronger cross-coupling between orthogonal polarizations due to simultaneous transmission / reception
Nonuniform beam filling (NBF) affects all radar variables Largest impact is on measurements of differential phase and cross-correlation coefficient. Much larger impact on the quality of polarimetric measurements at shorter wavelengths, particularly for smaller radars with broader beams
In this study Comparision between Measured polarimetric variables of S band Polarimetric variables of C, X bands by simulating realistic fields of variables Using Polarimetric algorithms for radar echo classification and DSD retrieval
Scattering Characteristics at S, C, and X bands Resonance effect : C T = 20ºC High Zdr Negative Kdp Negative Adp
Comparison of radar variables at S, C, and X bands (25920 DSDs)
Comparison of radar variables at S, C, and X bands (25920 DSDs) Cross-correlation coefficient Resonance effect C band Backscatter differential phase in Pure rain 0.98 (C band) larger than 5 mm 0.98 Cut-off (S and X)
Z DR and ρ hv at S, C, and X bands S band (measured) C band (simulated) X band (simulated)
The scatterplots of specific attenuations at horizontal and vertical polarizations versus specific differential phase at C band (25920 DSDs) T = 20ºC
The scatterplots of specific attenuations at horizontal and vertical polarizations versus specific differential phase at X band (25920 DSDs) T = 20ºC
1. Perturbations of differential phase Nonuniform beam filling Φ 0.02Ω dz ( dθ dφ 2 HV DP DP + dθ dz HV dφ dφ dφ DP ) Z = 1/ 2 hv ρhv (Z hz v ) Z Ω is a 3 db one-way antenna pattern HV = 10 log(z hv ) dφ dθ DP dφ, dφ DP λ 1 2. Drop in cross-correlation coefficient ρ (b) hv ρ hv exp{ 0.045 Ω 2 dφ [ dθ DP 2 + dφ dφ DP 2 ]} 3. Increased standard errors of the Z DR and Φ DP estimates SD(Z DR SD(K 2 1 ρhv ) 3.51 σ vnm 3 SD(Φ ) 3 / 2 N r DP = 1/ 2 DP ) SD(Φ SD(K K DP DP DP ρ ) 24.7 σ ) 3 / 2 λ M - number of samples, N number of gates, r gate spacing 2 hv vn 1 M 1/ 2 σ vn = 2σ v T / λ
Measured and simulated Z and Z DR Measured fields of Z and Z DR at S band Simulated profiles of Z and Z DR at S, C, and X bands
Backscatter differential phase and nonuniform beam filling (NBF) Simulated radial profiles of Φ DP and ρ hv measured Φ DP simulated Φ DP (NBF is not accounted for) simulated Φ DP (NBF is accounted for)
Conclusions Resonance scattering effects in rain are more pronounced at C band than at X band In order to mitigate adverse effects of attenuation and resonance scattering it might be helpful to use radar reflectivity at vertical polarization instead of horizontal one at C band, especially in the areas of heavy attenuation. Effects of nonuniform beam filling are stronger at shorter wavelengths. They result in increased perturbations of the radial profile of differential phase and decrease in cross-correlation coefficient Adaptation of the S-band algorithms for rainfall estimation and radar echo classification to C and X bands is not straightforward (even after correction for attenuation is performed)
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Dual-polarization radar advantages For Better Rainfall Estimation For Radar Echo Classification
Benefits of Dual Polarization Much more information φ ρ φ ρ φ φ φ φ ρ φ ρ
NSSL
McGuil Univ.,Canada Polarimetric radar Radar rainfall estimation Target classification
Polarimetric radars in Japan (only cm wavelength radars for observing precipitation) MP-X of NIED : X-band - purpose: monitoring and forecasting of heavy rainfalls to prevent disasters - Location: easy movable (present location - Ebina city (35.4 N, 139.4 E), Kanagawa prefecture, Japan) COBRA of NICT : C-band - purpose: monitoring sub-tropical meteorological phenomena and validation of TRMM/PR - location: Nago City, Okinawa
MP-X : Multi-Parameter radar at X-band of NIED (Nation Research Institute for Earth Science and Disaster Prevention, Japan) Frequency 9.375 GHz Antenna Type Circular Parabola, 2.1 mφ Scan Range (Scan Rate): AZ Full Circle ( 36 deg/s) EL 2 to +92 deg ( 18 deg/s) Antenna Gain 41.6 db Beam Width 1.3 deg Transmitter Tube Magnetron Peak Power 50 kw Pulse Length 0.5 µs Pulse Repetition Frequency 1,800 Hz Pola riza tion H an d V Doppler Processing PPP, FFT Noise Figure 2.3 db Observation Range 80 km Outputs Z, V, W, ZDR, ρhv, ΦDP by 2002 since 2003 Manufactured by Mitsubishi, in 2000 one transmitter and two receivers fully remote control : radar operation and data acquisition real-time dissemination via Internet
COBRA: CRL Okinawa Bistatic Polarimetric Radar of Okinawa Subtropical Environment Remote-Sensing Center of NICT (National Institute of Information and Communications Technology, Japan) - CRL (Communication Research Lab.) is the old name of NICT - µ Manufactured by NEC, in 2002 two transmitters and two receivers