Research Trends and Results
Upgrading Radar Precipitation Data by Converting C-Band Radar intoMP Radar
KAWASAKI Masaki, Head; TSUCHIYA Shuichi (D.Eng.), ResearcherWater Cycle Division, River Department
Keywords: radar precipitation data, C-band MP radar
1. Introduction
The Water and Disaster Management Bureau of theMLIT monitors rainfall across the country to use
precipitation data for river management, disasterprevention information to the public, etc. For this, it uses
26 radar rain gauges ("existing radar") that operate on theC-band (wavelength of approx. 5 cm), and 35
dual-polarization radar rain gauges ("XMP radar") that
operate on the X-band (wavelength of approx. 3 cm). The
existing radar has a wider range of observation but is lessaccurate than XMP radar. In order to upgrade radar
precipitation data, the existing radar, which is a single
polarization radar that transmits and receives one type ofradio wave, will be replaced by dual-polarization radar("CMP radar") that can transmit and receive two types ofpolarized waves, horizontal and vertical. (At present, twounits have been converted to MP [multi-parameter].)This paper examines the effort to convert radarobservation data into precipitation in order to improve theaccuracy of observation of existing radar infrastructure,by converting the existing radar into MP radar.
2. Rainfall observation with C-band MP radar
The Figure shows radar rainfall images from the 2012
Northern Kyushu Heavy Rain, taken by the existing radar(left), XMP radar (middle), and CMP radar (right). The
precipitation measured by CMP radar was calculated withthe precipitation calculation method studied by theNILIM. Note that the existing radar data was correctedusing the precipitation data measured on the ground but
no corrections were made to the data of XMP and CMPradars. With XMP radar, the whole rainy area was not
captured due to the unobservable area formed by radio
waves attenuated by intensive rain, while CMP and XMP
radars provided mostly consistent shapes of rainy areasand variations in the distribution of rainfall. In addition,
Existing
C-band radar
既存Cバンドレーダ

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as a result of comparison between the ground
precipitation and the time series precipitation by XMPand CMP radars, it was found that CMP radar is lessaccurate than XMP radar, but its data on precipitation at
the beginning, peak, and end of the rain was almost
consistent with the ground precipitation data, which
verifies that observation was done with good accuracy.
3. Future perspective
Since CMP radar allows for accurate observation
without the need for correction based on ground
precipitation data, distribution cycle, the time required
from observation to distribution can be reduced, which isexpected to lead to improved precipitation information.
In addition, joint use of CMP radar and XMP radar tosupplement unobservable areas under XMP radar or due
to rain attenuation is expected to improve wide-area
observation accuracy across the country, as well as stable
and highly accurate observation of urban areas whereXMP radar is provided. The existing radar units are to
be converted into MP radar as they come up forreplacement. In fiscal 2013, four units were converted
and, in fiscal 2014, one unit will be converted. The
rainfall calculation parameter of converted units will betuned by the NILIM and will be examined to ensure
observation accuracy.

X-bandMP radar
XバンドMPレーダ

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黒木C-bandMP radarCバンドMPレーダ

Radio
wave dissipation area
電波消散域（強雨(Rain attenuation by intensiveによる降雨減衰）rain)

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Figure: Radar Precipitation Image from 2012 Northern Kyushu Heavy Rain