Comparison of Polarimetric C Band Doppler Radar Observations with Reflectivity Fields obtained at S Band: A Case Study of Water induced Attenuation R. Keränen (1) , Ylläsjärvi J. (2) , Passarelli R. (1) and Selzler J. (1) Heikki Pohjola, Vaisala (1) Vaisala, (2) Finnish Meteorolological Institute
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Comparison of Polarimetric C Band Doppler Radar Observations with Reflectivity Fields obtained at S Band: A Case Study of Water induced Attenuation R.
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Comparison of Polarimetric C Band Doppler Radar Observations with Reflectivity Fields obtained at S Band: A Case Study of Water induced AttenuationR. Keränen (1) , Ylläsjärvi J. (2) , Passarelli R. (1) and Selzler J. (1)
Heikki Pohjola, Vaisala
(1) Vaisala, (2) Finnish Meteorolological Institute
Attenuation is the reduction of intensity of electromagnetic wave along the path of propagation
It is caused by scattering and absorption by the propagation medium
Strongly dependent on wave lenght: C band (λ=5 cm) three times higher than S band (λ=10 cm) and X band (λ=3 cm) 30 times stronger than C band (λ=5 cm) (Doviak and Zrnic 1993)
In addition to drop size characteristics water induced attenuation depends on temperature: It is about 50% stronger at 0 C degrees compared to 20 C, in comparable rain
Attenuation correction in single polarization radars
Traditional correction method uses a power law relation between specific attenuation and reflectivity (single polarization radars) (Hitschfeld and Bordan, 1954)
This method is unstable High sensitivity to radar calibration errors Uncertainty in correction bigger or equal to errors due to the
Correction method is based on the measurement of differential phase (ΦΦdpdp): ): Phase difference between the received signals in horizontal and vertical channels (Bringi et al, 1990)
Nearly linear relationship between the specific attenuation (adp) and the evolution of Φdp in range (K dp ),
Rule of thumb: Change of 10 degrees in differential phase <-> path integrated attenuation of 1 dB
Sources of uncertainty: Sampling noise in Φdp , Non-Rayleigh scatters such as large drops, hail/graupel mixtures of rain, non-meteorological targets (clutter)
Can be diminished with advanced filtering techniques
Attenuation correction in dual polarization radars