Measure rain and snow for the benefit of all.

  • Nowadays people watch the weather forecast as a way of life. Information from the weather forecast is used in our everyday lives and influences our lifestyle and behavior. For example, we may decide whether to bring an umbrella or not when we go outside based on a weather forecast. The weather decides what we will wear, what we want to eat, and even what products sell well. Just like an old saying expressing appreciation for rain, we should not forget that the almost all the water, which we use as drinking water, water for daily life, and agricultural and industrial water, comes from rainfall and snowfall. On the other hand, too much rainfall or snowfall may cause disasters. In recent years, we have greater opportunities to watch news of heavy rainfall, floods, and droughts that cause serious damage. Even if there was a small fluctuation in the amount and timing of rainfall or snowfall in areas, it may threaten human life and infrastructure. Furthermore, characteristics and the behavior of rainfall are becoming so different from our previous experiences.

  • Nowadays, our life and economic activities are becoming increasingly globalized. The severe flooding that occurred in Thailand in 2011 did not only threaten the lives of the local people, but also caused heavy damage and economic losses to factories of foreign companies. To reduce disaster damage and adapt to the changing Earth's environment, it is essential to comprehend environmental information for all of Earth. Satellite observations that can watch wide areas in a homogeneous way are the only efficient means to get global-scale rainfall information, and universal infrastructure of society.

The Global Precipitation Measurement (GPM) mission, which is led by Japan and United States and promoted trough a international collaboration framework, measures rain and snow for the benefit of all by conducting highly frequent and highly accurate global rainfall measurements using data from multi-satellites.

Why do we measure precipitation from space?


Water circulation in the atmosphere, land, and oceans is a key feature of the Earth's climate. “Precipitation” is one of most essential parameters which compose the water cycle. One of the reasons for this is that precipitation is closely related to our everyday lives ‒ precipitation is a source of fresh water which we use on a daily basis.

The significance of “precipitation”

The estimated amount of water for the entire globe is 1.4 billion cubic kilometers. However, 97.5 percent of this is salt water, and only 2.5 percent is fresh water. About 70 percent of the fresh water is in glacial ice and permanent snow cover;fresh water lakes and river storage account for only 0.3 percent of total fresh water. Precipitation on land is the source of renewable fresh water, but global precipitation is distributed unevenly and varies regionally and temporally. In addition, precipitation observation by rain gauges and ground radars only covers about 25 percent of the Earth's surface, since most regions are covered with water or inland areas that are difficult to access. In addition, precipitation observation by rain gauges and ground radars only covers about 25 percent of the Earth's surface, since most regions are covered with water or inland areas that are difficult to access.

Amounts of the Earth's Water

The figures show the total amounts of the Earth's water and its breakdown as a three-dimensional pie chart. The amounts of fresh water lakes and river storage, which we use in our daily lives, are only 0.3 percent of the total amounts of fresh water.

Global Hydrological Fluxes and Storages

The figure shows the global water cycle in the nature. The GPM mission targets to observe global precipitation in a highly frequent and highly accurate manner. Precipitation can be observed as flux quantities, and is a reference index for whether water cycle is accelerating or not.

Dense and highly frequent observation, about every 10 minute, usually cover limited regions in advanced countries, as do ground-based rainfall observation networks using rain gauges and/or weather radars. Ground observation data is extremely lacking in Asia and African regions even though these regions suffer water related disasters very often. Satellite-based precipitation data would be the only rainfall information in severe environments, where humans cannot enter, and in troubled regions. In addition, international rivers running through a number of countries are one of a source of conflicts regarding water acquisition, since countries located in the lower reaches of international rivers have serious problems in obtaining precipitation data from countries in the upper reaches.

The role of the satellites

Therefore, satellite observations that can consistently observe broad areas are a unique and effective means to achieve global scale rainfall measurement, and act as a universal “society's infrastructure.” Rainfall data observed by satellites provides basic information in various fields, such as weather, climate, disaster, ecosystem, and agriculture.

Rain Gauge Distribution and Coverage

The left figure shows the number of rain gauge available within 1-degree latitude/longitude (about 100 km) grid during December 2010. Rain gauge data was collected from all over the world and compiled by the Global Precipitation Climate Centre (GPCC). The right figure shows the zonal ratio of grids that have observations in percentages. While the distribution of rain gauges is comparatively concentrated in the European region, United States, Japan, and South Korea, large blank (non-observed) regions are found in the most part of Asia, Africa and South Africa. Although the ocean is a large part of the globe, only a few rain gauge observations of islands are available.

Weather forecast and the flood warnings


Utilization in weather forecasts

Satellite data is used commonly in weather forecasts. The Japan Meteorological Agency (JMA)and JAXA had been studied for the effective utilization of GPM observation data, which leads to more accurate weather forecasts. As the results, JMA started to use GPM observation data in their numerical prediction model operationally. Improvements in the accuracy of weather forecasts also directly contribute to weather information services and society. Weather information is used in routine work for the service and retail industries, traffics, agriculture, forestry and fisheries industries, and the infrastructure-related fields. Furthermore, improvements in the storm track forecast accuracy of tropical cyclones will largely contribute to protecting the human lives and property. It was estimated that the data from the Tropical Rainfall.

Improve accuracy of operating weather forecast by DPR

The Japan Meteorological Agency (JMA) started the DPR assimilation in the meso-NWP system. Word‘s first “operational” assimilation of spaceborne radar data in the NWP system of meteorological agencies! (Provided by JMA) Left and center panels show the 33-hour prediction without the DPR, and that with DPR, respectively. Right panel shows the ground observation data. It is found that the prediction accuracy is higher with DPR than without DPR.

Utilization in flood predictions and river managements

During 10-year from 1988 to 1997, two thirds of natural catastrophes worldwide were caused by floods and storms (World Water Council, 2000). The GSMaP product, which was developed in Japan for the GPM mission, provides an hourly global rainfall map four hours after observations. The flood alert system and tool, which use the GSMaP as inputs, are developed by the International Centre for Water Hazard and Risk Management (ICHARM), a UNESCO Category II Centre and hosted by the Public Works Research Institute (PWRI), and the Infrastructure Development Institute (IDI-JAPAN), the secretariat of International Flood Network (IFNet). The GSMaP has been used in operation. Since the satellite data is especially effective in the regions that are lacking in ground observations, efforts toward utilizing satellite data in flood predictions and river managements are ongoing in many Asian countries through funding from UNESCO, the Asian Development Bank, etc.

Flood Situation in Thailand in 2011 Observed by GSMaP

The figures show a comp arison of 3- month accumulated rainfall (July-September of 2008-2011) around the Chao Phraya River basin in Bangkok, Thailand, by the Global Satellite Mapping of Precipitation (GSMaP)data. Values under the figures indicate basin-average rainfall (region withinred lines). In the summer of 2011, more rainfall than the previous three years was observed and serious flood disasters occurred in the entire basin. The flooding did not only threaten the lives of the local people, but also caused heavy damage and economic losses to factories of foreign companies. Studies to prepare for future disasters are currently underway in collaboration with related agencies. (Images provided by IDI-JAPAN)

Integrated Flood Analysis System (IFAS)

The International Centre for Water Hazard and Risk Management (ICHARM) has distributed a tool called the Integrated Flood Analysis System (IFAS). IFAS uses the satellite-based global rainfall map and ground-based observation data as inputs to the river runoff model developed by using digital elevation data, and calculates river discharge. By introducing this system, local agencies can obtain information needed for flood prediction and/or alerts, and can disseminate evacuation instructions and information to residents.

Climate and water cycle variations


Utilization in global change and climate variation studies

Future projections of precipitation were reported by the Intergovernmental Panel on Climate Change (IPCC) Working Group I in September 2013 as a contribution to the Fifth Assessment Report that “Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases.” The latest simulations by climate models indicate that changes in the global water cycle in response to global warming, such as an increase in the contrast of precipitation between wet and dry regions and between wet and dry seasons. Current global climate models, however, still have uncertainty in projections in precipitation changes in response to global warming. Information from the GPM mission, especially highly accurate three-dimensional information of precipitation particles and systems derived from the Dual-frequency Precipitation Radar (DPR) will contribute in the validation of climate models and improvements in the precipitation processes. Another important role of satellite observation is to monitor long term changes in precipitation distribution, by combining various satellite and ground observation. To detect global-scale changes, global observations by satellites are essentia. Studies of precipitation system climatology have made substantial progress since the launch of the TRMM satellite and the Precipitation Radar (PR). Diurnal cycles of rainfall over the tropical regions were revealed by PR observations, as were the typical rainfall system of each region and statistics on extreme rainfall events. Observations that lead to a significant increase in the frequency of recent extreme rainfall events may be obtained by long term records, which will be carried on for more than 20 years though the TRMM to the GPM.

Utilization in water cycle studies

To assess the global water cycle quantitatively, observations of precipitation that is observable flux quantity is essential. Precipitation observations in the mid-latitude whose rainy areas are brought by extratropical cyclones follow that in the tropics and subtropics, and are critical as a new challenge in the GPM mission. It is expected that improvements in temporal and spatial resolution in GPM precipitation observations will contribute to the refinement of hydrological models. GPM data will quantify the water cycle and its variations, and be a big step to identify natural and anthropogenic variations in the water cycle. Studies to simulate river runoff and use satellite global rainfall map as inputs to land models are underway and at the evaluation stage for operational use in flood monitoring and water resource management.

Production of the satellite global rainfall map

To produce a highly frequent global rainfall map, sampling errors caused by un-observed regions by microwave imagers is a problem. For the GPM mission, the Global Satellite Mapping of Precipitation (GSMaP) project in Japan has developed a method to interpolate observations between each microwave imager by utilizing information from the Infrared imagers on board the geostationary satellites, and achieved production of an hourly global rainfall map in 0.1-degree latitude/longitude grid. Currently, JAXA provides the near-real-time GSMaP product four hours after observation through the “JAXA Global Rainfall Watch” website. The GSMaP algorithm has been improved by refining rainfall retrievals over land, considered the orographic rainfall effects, added the rain gauge corrected rainfall product, and the information from the Dual-frequency Precipitation Radar (DPR) is compiled as a database.

The GPM mission and the Dual-frequency Precipitation Radar (DPR)


What is the GPM mission?

The Global Precipitation Measurement (GPM) mission has started in response to the success of the TRMM satellite, which is a U.S.-Japan joint project that was launched in November 1997. The GPM mission consists of the GPM Core Observatory jointly developed by U.S. and Japan and Constellation Satellites that carry microwave radiometers. The GPM mission is an international collaboration to achieve highly accurate and highly frequent global precipitation observations. Observation coverage is extended to the mid-to-high latitudes in the GPM era. The Dual-frequency Precipitation Radar (DPR) was developed by Japan, and installed on the GPM Core Observatory. It chooses a non-sun-synchronous orbit to carry on diurnal cycle observations of rainfall from the TRMM satellite, while the Constellation Satellites have been launched by each partner agency and contributed to expand observation coverage and increase observation frequency.

Role of the DPR

The DPR has two different frequencies to measure the three-dimensional structure of precipitation, and is capable of observing snowfall from space for the first time, in addition to observing both strong and weak rainfall. The DPR's observations provide global precipitation dataset including weak rainfall by extratropical cyclones in mid-to-high latitudes, in addition to carrying on the archive of long-term rainfall dataset in the tropics and the subtropics by PR. Furthermore, through its high-resolution and highly accurate observations, DPR plays a role of the reference standard for the microwave radiometers carrying on the Constellation Satellites who join the GPM mission, through the GPM Microwave Imager on board the same platform.

The Tropical Rainfall Measuring Mission (TRMM) satellite

The figure shows the three-dimensional rainfall structure of the Super Typhoon “KROSA” at 01Z on Oct. 5, 2007, observed by the PR.

The TRMM satellite, which was launched in November 1997 and a joint mission between U.S. and Japan, became a turning point of global rainfall observations by satellite remote sensing. The major objective of the TRMM satellite is to determine accurate rainfall amounts associated with tropical convective activities, which is a driving source of global atmospheric circulation. To this purpose, it focuses on rainfall observations, and carries the world's first satellite-borne Precipitation Radar (PR) developed by Japan, in addition to conventional instruments, such as an infrared imager and microwave imager. The combined use of the PR and microwave imager has greatly improved the estimation of rainfall amounts over the tropics and subtropics by one digit compare to previous satellite observations. It has also revealed the three-dimensional structure of typhoons over the ocean and climate variations such as El Niño and La Niña, which were rarely observed before the TRMM. The success of the TRMM satellite shows the potential of satellite remote sensing contributions for understanding the water cycle on Earth and improving weather forecasts.

Concept of the GPM Mission

The concept of the GPM mission is to apply highly accurate observations by the GPM Core Observatory to highly frequent observations by the Constellation Satellites. Targets of the Core Observatory, which carries the Dual-frequency Precipitation Radar and the multiple frequency microwave imager, are to understand the horizontal and vertical structure of precipitation systems, obtain information of precipitation particles, and improve accuracy of precipitation estimates by the Constellation Satellites. The Constellation Satellites have been launched by the GPM partner agencies around the same time as the launch of the Core Observatory, and enables global precipitation observations in highly frequent temporal resolution.

Observations by the Dual-frequency Precipitation Radar (DPR)

The DPR on board the GPM Core Observatory is composed of two radars: a Ku-band (13.6-GHz) Precipitation Radar (KuPR) and a Ka-band (35.5-GHz) Precipitation Radar (KaPR). The KaPR instrument aims at sensitive observations, and can detect weak rainfall and snowfall that cannot be measured by the KuPR. Since the KuPR instrument can detect heavier rainfall, simultaneous observations by the KaPR and KuPR enable accurate measurements of precipitation from heavy rainfall in the tropics to weak snowfall in high-latitudes.

Major Characteristics of the DPR

Name KuPR KaPR
Radar Type Active Phased Array Radar
Antenna Slotted Waveguide Antenna
Beam-matching Accuracy < 1,000 m
Frequency 13.597, 13.603 GHz 35.547, 35.553 GHz
Swath Width 245 km 125 km
Observation Altitude Up to 19 km
Horizontal Resolution 5.2 km (at nadir)
Range Resolution 250 m 50 m / 500 m
Minimum Detectable Rain Rate 0.5 mm/h ~ 0.2 mm/h ~
Power Consumption < 446 W orbit average < 344 W orbit average
Mass < 472 kg < 336 kg
Size 2.5m x 2.5m x 0.6m 1.4 m x 1.2 m x 0.8 m