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)‏ ‏
~ dBZ
~ dBZ
~ dBZ ?
~ dBZ
~ dBZ
Increasing
attenuation
Battan, L. J., 1973

5. Example of attenuation at C band in Finland
Suuruusluokka, Gradun tavoite: saada statistiikkaa miten paljon vaimenemisilmiöitä tapahtuu jne.
Vaimennus merkittävää C-bandillä ei niinkään S-bandillä

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. Reflectivity, dBZ S Band radar C Band radar, corrected
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
Bias /dB