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R. Keränen (1) , Ylläsjärvi J. (2) , Passarelli R. (1) and Selzler J. (1)

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

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R. Keränen (1) , Ylläsjärvi J. (2) , Passarelli R. (1) and Selzler J. (1)

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  1. 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

  2. Introduction • Attenuation due to the liquid water major challenge at weather radar frequencies (X and C bands) • Traditionally, this has justified investments in S band technology in climates of heavy precipitation • This situation has changed: Modern Dual Polarization C band weather radars are an attractive choice in all climates • The cost issue: A turnkey-installed C band system is 1/2 to 1/3 the price of an S band system

  3. What is attenuation? • 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

  4. Typical values of attenuation at C band Cause of attenuation Hail Rain Mixed rain Snow Clouds Atmospheric gases Typical value (C-band)‏ ‏ ~ 10 - 30 dBZ ~ 2 - 25 dBZ ~ 1 - 5 dBZ ? ~ 0 - 3 dBZ ~ 0 - 3 dBZ ~ 0 - 2 dBZ Increasing attenuation Battan, L. J., 1973

  5. Example of attenuation at C band in Finland

  6. Example of attenuation at C band in Finland

  7. 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 attenuation • Not very widely used operationally

  8. Attenuation correction in dual polarization radars • Correction method is based on the measurement of differential phase (Φdp): 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

  9. How Differential Phase Works

  10. Case study of significant precipitation in Huntsville, Alabama, USA • The observations by the ARMOR polarimetric C Band radar (Petersen et al 2007) • The independent observations made at the NEXRAD S Band radar are used for comparison to an unattenuated reference • Squall line approaching Huntsville from north-west, maximum reflectivities exceed 58 dBZ

  11. C Band radar Reflectivity, dBZ S Band radar

  12. C Band radar, dBZ C Band radar, Φdp S Band radar

  13. C Band radar, uncorrected C Band radar, corrected Reflectivity, dBZ S Band radar

  14. A single ray comparison of C Band reflectivities, azimuth 40 degrees

  15. Conclusions • Dual polarization techniques applied in C Band weather radar specifies C Band radar to be useful in "all climate weather conditions" • Dual polarization C Band weather radars with advanced signal processing techniques offers new methods to mitigate attenuation • Attenuation is not anymore major limitation for the broader use of short wave lengths technology in climates of intense rain • Applying attenuation correction techniques demands modern weather radar with latest dual polarization technology

  16. Correction factor for rain intensity with different bias Correction factor Bias /dB

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