Dual-polarization C-band weather radar algorithms for rain rate estimation and hydrometeor classification in an alpine region

Adv. Geosci., 20, 3–8, 2009 www.adv-geosci.net/20/3/2009/ © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License. Advances in Geosciences Dual-polarization C-band weather radar algorithms for rain rate estimation and hydrometeor classification in an alpine region H. Paulitsch, F. Teschl, and W. L. Randeu Department of Broadband Communications, Graz University of Technology, Graz, Austria Received: 15 September 2008 – Revised: 2 February 2009 – Accepted: 25 February 2009 – Published: 9 March 2009 Abstract. Dual polarization is becoming the standard for new weather radar systems. In contrast to conventional weather radars, where the reflectivity is measured in one polarization plane only, a dual polarization radar provides transmission in either horizontal, vertical, or both polarizations while receiving both the horizontal and vertical channels simultaneously. Since hydrometeors are often far from being spherical, the backscatter and propagation are different for horizontal and vertical polarization. Comparing the reflected horizontal and vertical power returns and their ratio and correlation, information on size, shape, and material density of cloud and precipitation particles can be obtained. The use of polarimetric radar variables can therefore increase the accuracy of the rain rate estimation compared to standard Z−R relationships of non-polarimetric radars. It is also possible to derive the type of precipitation from dual polarization parameters, although this is not an easy task, since there is no clear discrimination between the different values. Fuzzy logic approaches have been shown to work well with overlapping conditions and imprecisely defined class output. In this paper the implementation of different polarization algorithms for the new Austrian weather radar on Mt. Valluga is described, and first results from operational use are presented. This study also presents first observations of rain events in August 2007 during the test run of the radar. Further, the designated rain rate estimation and hydrometeor classification algorithms are explained. 1 In summer 2006, the fifth Austrian C-band weather radar was installed to improve the measurement of rain in the western part of Austria. It is located on Mt. Valluga at 2809 m ASL at the border between the provinces Vorarlberg and Tyrol. The radar is operated by Austro Control, the Austrian air navigation services provider. As opposed to the existing radars, this radar is equipped with dual polarization capabilities. The Valluga Weather radar is an EEC SidPol version. It is equipped with an ortho-mode feed and electronically controlled waveguide signal routing to provide transmission in either horizontal, vertical, or both polarizations, while receiving both the horizontal and vertical channels simultaneously by two separate receive chains. A mode change from simultaneous transmission to either vertical or horizontal transmission is required for Linear Depolarization Rate (LDR) measurement. The characteristics of the radar are summarized in Table 1. Dual polarization radars transmit horizontally and vertically polarized electromagnetic pulses and measure the respective reflected powers. By comparing the reflected horizontal and vertical power returns and their ratio and correlation, information on the type, size and shape of cloud and precipitation particles can be obtained. In comparison with a conventional radar system dual polarization systems provide additional fundamental variables: 1.1 Correspondence to: H. Paulitsch () Introduction Differential reflectivity ZDR Differential reflectivity is the ratio of the horizontal and vertical power returns. ZDR provides information about particle properties, such as the shape of rain drops. If the majority of the particles in the measured radar volume have a nonspherical shape, and polarization is aligned with the particle axes, the power return will be greater for one polarization than for the other. For large rain drops with an oblate shape, Published by Copernicus Publications on behalf of the European Geosciences Union. 4 H. Paulitsch et al.: Dual-polarization C-band weather radar algorithms 0.8 Table 1. Characteristics of the Valluga radar during test run. Ah = 0.05 Kdp 0.7 2809 m (m.s.l.) 5.625 GHz 0.95◦ H/0.9◦ V Coaxial magnetron 250 kW 0.8 µs 1000 s−1 50 4 RPM (24◦ /sec) 125 m 960 120 km 14 from −1.8◦ to +90◦ (−1.8, −0.8, −0.1, 1.0, 2.0, 2.8, 4.2, 7.7, 10.2, 13.7, 19.7, 30.2, 60.0, 90.0) Adp = 0.01 Kdp 0.6 Ah, Adp (dB/km) Altitude Frequency Beamwidth Transmitter type Peak power Pulse length Pulse repetition rate Samples per integration Rotation rate Range resolution Number of range-gates Maximum range Elevation angles Ah = 0.073 Kdp^0.99 Adp = 0.013 Kdp^1.23 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 Kdp (deg/km) Fig. 1. Attenuation correction estimators for C-band. Fig. 1: Attenuation correction estimators for C-band. The correlation coefficient, ρH V , is a measure of the correlation between the reflected horizontal and vertical power returns. Generally in rain the correlation is high and ρH V is close to one. In regions where there is a mixture of precipitation types, such as rain and snow, or where the particle properties are highly irregular, the correlation is much lower and can result in ρH V values down to 0.8 for example for wet snow. 1.3 Differential phase shift φDP The differential phase shift, φDP , is the phase shift that occurs between the horizontally- and vertically- polarized pulses along the propagation path. The phase shift is caused by variations in the wave propagation speed, when the electromagnetic pulses encounter precipitation particles of different sizes and shapes. 1.4 Specific differential phase KDP The specific differential phase, KDP , is the range derivative of the differential phase φDP . Since the phase shift is influenced by propagation effects like attenuation or beam shielding, which reduce the power return, it can be used for attenuation correction. KDP is also a good estimator of rain rate. 1.5 Linear depolarization ratio LDR LDR is the ratio of the cross-polar to the co-polar power return from a horizontally or a vertically polarized pulse. It is Adv. Geosci., 20, 3–8, 2009 45 km 35 km 2 Attenuation correction At C-band wavelength, the attenuation along the propagation path due to precipitation particles can degrade radar measurements to a considerable degree. In order to make accurate rainfall estimates, an attenuation correction scheme for ZH (Horizontal Reflectivity) and ZDR should be used. The attenuation factors AH and ADP can be calculated using different methods depending on the type of measurements involved. Studies from Bringi et al. (1990) and Smyth and Illingworth (1998) show different correction methods. Typically methods using the specific differential phase (KDP ) are applied. The following relations 50 km 60 km 70 km 80 kmfrom Bring (...truncated)