Polarimetric Radar Observations with Multiparameter Phased-Array Radar (MP-PAWR) in the Tokyo Area

The radars using parabolic dish antenna generally require 5-10 minutes to mechanically rotate and to tilt upwards as they sample the atmosphere at several different elevation angles. The number of the observable elevation angles is about ten or so. The single polarization phased array weather radar (SP-PAWR), which has been developed in 2012 [1], was designed to conduct precipitation measurement of a radar reflectivity factor, in less than 10 or 30 seconds in a range of 20 or 60 km in real-time, respectively. The array antenna (horizontal polarization) consists of 128 elements that were arranged in tandem. There are analog-to-digital converters for each element. A digital beam forming is used in only elevation angle. As a transmitted waves of the radar, a fan-shaped transmission beam is used, with a narrow beamwidth (1.2 [deg]) in azimuth and a wider beamwidth (5.0 [deg]) in elevation. For elevation angles, Fourier beamforming method is used for electronic scanning, with 128 antenna elements. Consequently, the number of the observable elevation angles is about 100. The resolution in elevation angles is about 0.8 [deg]. The PAWR data are particularly useful to detect the phenomena are characterized by their short time scale (<several minutes in duration) [2]. In Japan, the five single polarization phased array weather radars (SP-PAWRs) with linear arrays are under operation. As a next radar development project, a dual polarimetric phased array weather radar is being developed. It can provide multi-parameter measurements that reveal detailed microphysics of storms in addition to accurate precipitation estimation, and can improve weather forecasts. The radar is termed “Multi-parameter phased array weather radar; MP-PAWR”.

The first made MP-PAWR, which simultaneously transmits pulses of horizontal (H) and vertical (V) polarized radiation, has been developed and installed in December 2017, at Saitama University, Japan. The center of frequency is 9.4GHz (X-band). The MP-PAWR　has a scanning scheme similar to the SP-PAWR, which uses the mechanical and electronic scanning in azimuth and elevation angles, respectively. The MP-PAWR provides the polarimetric precipitation measurements in three-dimensional volume scanning in less than 30 or 60 seconds in a range of 60 or 80 km in real-time, respectively.

In this presentation, we will show the initial observation results. To confirm reliability of the measurements and to evaluate the accuracy of the measurements by the MP-PAWR, the initial observation results of the MP-PAWR are compared with an existing X-band polarimetric multi-parameter radar data. In Japan, an X-band polarimetric multi-parameter radar information network (XRAIN) is used in urban areas such as Tokyo. The XRAIN is capable of estimating the rain rate with high accuracy using network and polarimetric observations. The temporal resolution of XRAIN is one minute for low elevation angles (0°–1°). On the other hand, for elevations ranging from 1° to 15°, the temporal resolution is 5 min. Park et al. [3] demonstrated the advantages of the specific differential phase approach for the X band by evaluating rain rate estimation with the XRAIN. They achieved rainfall estimates with a normalized bias of 1.1% and a normalized error of 14.8% in hourly rainfall accumulation. The XRAIN data are calibrated accurately and used broadly for weather forecast around the urban area in Japan.

Rainfall events were observed by the MP-PAWR from 05:39:53 to 07:00:17 on 3^rd May, 2018 (JST). We focused on the dataset and evaluated the observation accuracy of the MP-PAWR. In a radar reflectivity factor (Z), the values of the correlation coefficient (C.C.), bias, standard deviation (S.D.) between the MP-PAWR and XRAIN are 9.8, -0. 3 dB and 2.6 dB, respectively. The Z of MP-PAWR is in good agreement with that of the XRAIN from the result of the C.C.. However, the values of the bias and S.D. were slightly high. The difference seems to be caused by the difference of the methodology of the rainfall attenuation correction. In the XRAIN, the specific differential phase is used for the rainfall attenuation correction. On the other hand, the MP-PAWR does not considered any phase information for the attenuation correction at this time. An algorithm of the rainfall attenuation correction using the will be applied to the MP-PAWR in the near future. For the dual polarization parameters, in the differential reflectivity, the values of C.C., bias, and S.D. is 0.90, 0.25 dB, and 0.32 dB, respectively. Because of the same reason of the results of the Z, the bias and S.D were slightly high. The correlation coefficient indicated the similar characteristics of the both results. In this case, the values of differential phase shift and the specific differential phase couldn’t be evaluated because the Z of the rainfall events was not high to evaluate the accuracy of the phase information.

In the near future, the observation data by the MP-PAWR will be obtained and be evaluated the observation accuracy, quantitatively.

Acknowledgment

The authors wish to thank the party of Toshiba Corporation for valuable comment about the hardware of the phased array weather radar. The authors are grateful to the financial support from the Cross-ministerial Strategic Innovation Promotion Program operated by the cabinet office, the Government of Japan. XRAIN data sets used for this study are provided by Ministry Land, Infrastructure, transport and Tourism, Japan.

References

[1] F. Mizutani, T. Ushio, E. Yoshikawa, S. Shimamura, H. Kikuchi, M. Wada, S. Satoh and T. Iguchi, "Fast-Scanning Phased-Array Weather Radar With Angular Imaging Technique," in IEEE Transactions on Geoscience and Remote Sensing, vol. 56, no. 5, pp. 2664-2673, May 2018.

[2] T. Adachi, K. Kusunoki, S. Yoshida, K. I. Arai, S. Hayashi, and T. Ushio, " High-speed volumetric observation of downburst using X-band phased-array radar," in 37th Conference on Radar Meteorology, 2015, Sept. 2015

[3] S. Park, M. Maki, K. Iwanami, V. N. Bringi, and V. Chandrasekar, “Correction of Radar Reflectivity and Differential Reflectivity for Rain Attenuation at X Band. Part II: Evaluation and Application”, J. Atmos. Oceanic Technol., vol. 22, pp. 1633–1655, 2005.