Measuring Rainfall – What is Radar/Raingauge-Analyzed Precipitation?

(Japan Meteorological Agency (JMA))
To help prevent disasters, JMA monitors rainfall using the observational equipment detailed below.

Equipment for observing precipitation
RadarsRaingauges

0The features of raingauges and radars are shown below.
Raingauges

RadarsAdvantages

Can measure actual amounts ofprecipitation.

Can observe large areas with higher
spatial resolution than the raingauge
network.

Disadvantages

Can observe precipitation at single
points only.

May produce readings different fromprecipitation observed on the ground,as it measures the amount of rainoverhead.

Radar/Raingauge-Analyzed Precipitation combines theadvantages of raingauges and radars
JMA has created an optimal mix of the advantages found in raingauge data and radar data.
Precipitation

Calibrated radar data

Radar data
Raingauge data

Precipitation amounts observed by radar
generally does not agree with thoseobserved by raingauges, and radar data are
therefore calibrated with raingauge data.
The calibrated radar data are then made intoa single composite data set.

Radar/Raingauge-Analyzed Precipitation
Radar/Raingauge-Analyzed Precipitation datadepicts hourly precipitation with high dimensional
accuracy, and is issued every thirty minutes with aspatial resolution of 1 km.

Information derived from Radar/Raingauge-Analyzed Precipitation
JMA issues Very-short-range Forecasts in the form of Precipitation and Precipitation Nowcasts,
and also produces/utilizes the Soil Water Index as a warning/advisory criterion. This information
is derived from Radar/Raingauge-analyzed Precipitation.

Very-short-range Forecasts of Precipitation
Data on precipitation is issued every thirty minutes to provide forecastsof hourly precipitation amounts for the next six hours with a spatial
resolution of 1 km. (http://www.jma.go.jp/en/radame/)

Precipitation Nowcasts
Precipitation Nowcasts provide ten-minute precipitation forecasts ofswiftly growing convections with a spatial resolution of 1 km up to anhour ahead to assist with disaster prevention activities. Nowcasts areissued every ten minutes within three minutes of each radar observation.
(http://www.jma.go.jp/en/radnowc/)

Soil Water Index
The Soil Water Index shows the risk of landslides* caused by heavy rain, and has aspatial resolution of 5 km up to six hours ahead.

What is the Soil Water Index?
The Soil Water Index shows the risk of landslides*, and is calculated from a tank model,
Radar/Raingauge-Analyzed Precipitation and Very-short-range Forecasts of Precipitation.
Higher index values indicate an increased risk of landslides*.
The Soil Water Index is equivalent to the total
storage volume of three serial tanks.

降った雨が土壌中を通って流れ出る様子
Image of precipitation advection in soil（イメージ）

Precipitation
First tankModeling

Surface runoff
Storage
Permeation
Second tank

Storage
Permeation

表面流出runoff
Surface

Shallow runoff
Storage
Permeation

表層浸透流出
Shallow runoff

Base rock

Groundwater
地下水流出runoff

Third tank
Groundwater
Storagerunoff
Permeation

Soil Water Index values arecalculated for individual 5-km
grid squares.
The whole nation is covered byapproximately 16,000 such squares.

To protect life, JMA alerts the public to natural hazards through warnings andadvisories including those for heavy rains. JMA monitors heavy rains using
Radar/Raingauge-Analyzed Precipitation, Very-short-range Forecasts ofPrecipitation and the Soil Water Index.
* The term “landslides” here refers to debris flows and concentrated slope failures.