Estimation of Rain Rate from Phased Array Weather Radar Using
X-Band Polarimetric Radar Measurements
1112*223Hajime Fukuroi , Katsura Adachi , Yusuke Otsubo , Naoki Inaba , Nozomu Takada , Sho Yoshida and Tomoo Ushio
1The Kansai Electric Power Company, Inc., Japan,2Meteorological Engineering Center, Inc., Japan, 3Osaka University, Japan・Recently, in a dam basin for hydraulic power generation that Kansai Electric Power manages, sudden floods with local heavy rain occur frequently.

(1) Time change of Z-R relation・Therefore, difficulty of the dam operation increases, and the need for accurate short term rainfall prediction technique becomes higher.

The right figure shows examples of time series of estimated 𝐵𝛽 from XRAIN and XPRD.

・To improve the accuracy of short term rainfall prediction, it is necessary to observe precipitation distribution in and around of the dam basin with high resolution andhigh accuracy.

B

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0Results are as follows:・In general, rain rate (R) can be estimated from 𝑍ℎ using fixed Z-R relations (𝑍ℎ = 𝐵𝑅 ). However, Z-R relations vary widely due to variations in drop size distribution (DSD).
・It is difficult to retrieve the DSD from non-polarimetric radar (PAWR), While the DSD can be retrieved by using not only 𝑍ℎ but also polarimetric radar parameters .

・ By using XRAIN data, we can express the variations of Z-R relation due to variations of theDSD.

・ we have developed rain rate estimation method from PAWR using DSD information retrieved by X-band polarimetric radar information network (XRAIN*).

Rain rate(mm/h)𝛽* XRAIN is x-band polarimetric radar network operated by Ministry of Land, Infrastructure and Transport and Tourism (MLIT) in Japan.

1.6

β200・ B value take low value in the time of heavy rainfall and is high in the time of light rainfall.

・X-band phased array weather radar (PAWR) can observe with high temporal and spatial resolution. However, PAWR can observe only reflectivity (𝑍ℎ ).

B

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5:00 JST

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β5.Results

1.Introduction

Fig. Time series of B𝜷 𝐚𝐧𝐝 𝐫𝐚𝐢𝐧 𝐫𝐚𝐭𝐞(𝐗𝐏𝐑𝐃) (date:2015/7/9)

The contents of “2.Method of retrieving DSD”

XRAINPolarimetricradar observation

Reflectivity 𝑍ℎ (dBZ)

Table. Specificatios of PAWR and XRAIN

Variations
in DSD

Spatialresolution𝑍ℎ = 200𝑅1.6

Rain rate R (mm/h)
Fig. Z-R relations ( after Battan 1973)

PAWR

XRAIN

100m

250m

Estimating
𝐵𝛽

Reflectivity 𝑍ℎPolarimetric parameters（𝑍𝐷𝑅 and 𝐾𝐷𝑃 ）Temporalresolution

30seconds

Observation
range

60km

80km

Observed
parameters𝑍ℎ𝑍ℎ , 𝑍𝐷𝑅 , 𝐾𝐷𝑃
and others

5minutes

PAWR
Non-polarimetricradar observation

Reflectivity 𝑍ℎFixed 𝑩𝜷(200,1.6)

The application of𝐁𝛃 estimated from XRAINWith XRAIN

We compared estimation accuracy of 10 minutes rainfall between the method with XRAIN and without XRAIN.
Results are as follows:
・ The method without XRAIN showed significant underestimation at all points.

BβDSD(2)Comparison of rainfall estimation accuracy

・ By the method with XRAIN, RMSE decreased at all points (especially at miyama).
・ Correlation coefficient of the method with XRAIN at Miyama and Syuchi are slightly greater than the method without XRAIN.
・ Correlation coefficient of both methods at Honjyo was less than other points (especially the method with XRAIN).

Rain rate RMiyama

Without XRAIN

HonjyoSyuchiFig. Flow to estimate rain rate

2.Method of retrieving DSD

Without XRAIN

XRAIN
Polarimetricradar observation

In this study, DSD is retrieved byReflectivity 𝑍ℎPolarimetric parameters（𝑍𝐷𝑅 𝑎𝑛𝑑 𝐾𝐷𝑃 ）YM method for heavy rainfall data (𝐾𝐷𝑃 > 0.5) and CG method for light rainfall data (𝐾𝐷𝑃 ≤ 0.5).

(1)RMSE(Root Mean Squared Error)

𝑅𝑀𝑆𝐸 =(2) CORR(Correlation coefficient)(𝐹𝑖 − 𝐹 )(𝐴𝑖 − 𝐴)
𝐶𝑂𝑅𝑅 =( (𝐹𝑖 − 𝐹 )2 ( (𝐴𝑖 − 𝐴)2 ))1/2(1) CG method (Zhang et al., 2001)
・ The retrieval accuracy tends to decrease in heavy rainfall case due to the rainfall attenuation.

(Light rainfall)・ DSD is retrieved from 𝑍ℎ and 𝐾𝐷𝑃 using empirical formulas obtained from disdrometer observation.
・ The retrieval accuracy tends to decrease in light rainfall case due to the variations of 𝐾𝐷𝑃 .

Wachi dam are surrounded with high elevation area. Therefore, we used

4.Target events
Height(m)Wachi damHSM60kmoccurred in the Wachi dam basin.

formula.

Fig. Comparison of rainfall estimation accuracy

Table. List of target events.

In addition, we used XRAIN product rainfall data(XPRD)* at the same
*XPRD is directly estimated rainfall from 𝑍ℎ and 𝐾𝐷𝑃 using the empirical

𝐹𝑖 ：𝑅 𝑃𝐴𝑊𝑅 𝐴𝑖 ：𝑅 𝑋𝑃𝑅𝐷
𝐹 ：𝐴𝑣𝑒𝑟𝑎𝑔𝑒𝑑 𝑅 𝑃𝐴𝑊𝑅
𝐴：𝐴𝑣𝑒𝑟𝑎𝑔𝑒𝑑 𝑅(𝑋𝑃𝑅𝐷)
𝑁：𝑇ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑎𝑡𝑎

We selected following 4cases that local heavy rainfall had

PAWR data at altitude 4km.

altitude for true value.

YM method(Using 𝑍ℎ and 𝐾𝐷𝑃 )Fig. Flow to estimate DSD

3.Data(3)RATIORATIO > 1 ：Underestimation
RATIO < 1 ：Overestimation
𝐴𝑖
𝑅𝐴𝑇𝐼𝑂 =𝐹𝑖(Heavy rainfall)

Retrieving DSD

CG method(Using 𝑍ℎ and 𝑍𝐷𝑅 )(2) The method proposed by Yamaguchi et al., 2012 (YM method)

The ground observation points(Miyama(M), Honjyo(H), Syuchi(S)) near

𝐾𝐷𝑃 > 0.5

𝐾𝐷𝑃 ≤ 0.5

With XRAIN・ DSD is retrieved from 𝑍ℎ and 𝑍𝐷𝑅 using empirical formulas obtained from disdrometer observation.

(𝐹𝑖 − 𝐴𝑖 )𝑁No.

Period (JST)The cause of rain80km

1

2015/7/9 01:00～06:00

Stationary front■：XRAIN ●：PAWR▲： Ground observation

23

2015/7/28 16:00～21:00 Unstable atmosphere
Low pressure
2015/8/17 14:00～18:00

Fig. Map of Osaka4

2015/9/1 15:00～20:00

Stationary front

6.Conclusion and future works
・ We have developed a rain rate estimation method from PAWR using DSD information retrieved by XRAIN. As a result of application to Wachi dam basin inJapan, the new method showed improving underestimation against fixed Z-R relation (B = 200, 𝛽 = 1.6).
・ We will develop short time prediction method using 3D high resolution radar data observed by PAWR.