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

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

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

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 - 2 dBZ Increasing attenuation Battan, L. J., 1973

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

Example of attenuation at C band in Finland

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

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

How Differential Phase Works

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

C Band radar Reflectivity, dBZ S Band radar

C Band radar, dBZ C Band radar, Φdp S Band radar

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

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

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

Correction factor for rain intensity with different bias Bias /dB