Satelite observation plan by international cooperation for grasp of rainfall ..high accuracy and high frequency...

Production of satellite-based global rainfall map

Advanced combination of microwave radiometer data obtained from multi-satellites will be a challenge for the GPM mission.

It had been difficult to produce homogeneous global rainfall distribution maps over land and the oceans, until satellite observation started. In the beginning, rainfall was estimated from cloud temperature (cloud-top height) observed by infrared imagers on board geo-stationary satellites, hypothesizing statistically constant relationships between cloud-top temperature (i.e., altitude of clouds) and surface rain rate. Distributions of clouds and precipitation, however, do not necessarily correspond to each other, and relationships between cloud-top temperature and rain rate are not always constant worldwide. On the other hand, microwave radiometers, which became available a little later, can measure microwave emission by rainfall more directly, and the accuracy of rainfall estimates has been improved compared to that by infrared imagers. Consequently, production of global rainfall map products using satellite observation has been ongoing since the late 1980s. In particular, there has been progress in production of various global rainfall maps based on several microwave radiometers (microwave imagers and/or sounders) since the launch of the TRMM satellite. Those achievements are precursors to the GPM mission, and those efforts are expected to be incorporated and improved in the GPM era.

Improvements of global rainfall maps

Since global rainfall map products developed before the TRMM’s launch intended mainly to obtain global “climatology” of precipitation, their temporal resolutions were several days or monthly, and were not available in near real time due to calibrating satellite data (infrared and microwave instruments) by ground-based rain gauge data.

Because of the launch of the TRMM satellite in 1997, there have been great improvements in rainfall retrieval algorithms for microwave radiometers. Because of the satellite’s low altitude, spatial resolution of the TMI on board the TRMM is one third that of SSM/I, which was standard for microwave imagers at the time, so the TMI can measure smaller structures of rainfall. Comparison of simultaneous observation among the TMI, PR, and VIRS instruments on the same platform also reveals some problems of assumptions in each rainfall retrieval algorithm, and improvements of the algorithms were made. The current state-of-the-art microwave imager is the AMSR-E, which was launched later in 2002 and is still in operation as of March 2009. Its spatial resolution is almost the same or finer than that of the TMI, but its observation swath is twice as wide as that of the TMI and it covers the globe because of altitude of approximately 700 km in a polar orbit. Due to this synergy, global rainfall map products in higher frequency and finer resolution have been developed and produced primarily in the United Sates. Products that are conscious of near real time availability have also increased substantially.

GSMaP and “JAXA Global Rainfall Watch”

Global Satellite Mapping of Precipitation (GSMaP) is a Japanese research project to produce global rainfall maps that are highly accurate and in high temporal and spatial resolution through the development of rain rate retrieval algorithms based on reliable precipitation physical models by using a number of microwave radiometer observation data, and comprehensive use of the PR and geostationary infrared imager data. It should also be noted that this rainfall map will be used in the GPM era.

One of major issues in production of frequent rainfall maps is regions spatially not observed by microwave radiometers during shorter periods, i.e., sampling errors. The GSMaP project solved this issue by developing methods to interpolate observation gaps by microwave radiometers. Hourly global rainfall maps in 0.1 degree latitude-longitude grid boxes can be produced using information of cloud moving vectors estimated by infrared imagers on board geostationary satellites and the Kalman filter.

Currently, data and browse images of global rainfall maps, which can be called pre-GPM products, based on GSMaP algorithms are distributed via the Internet. JAXA/EORC’s web site called “JAXA Global Rainfall Watch (GSMaP_NRT)” provides hourly global rainfall maps in 0.1-degree latitude-longitude grid boxes, which is estimated from TRMM and other satellite observation data, 4 hours after observation.

Distribution of GPM data and operational use

The data acquired by the GPM core satellite and the constellation satellites are transmitted to the ground stations of JAXA, NASA, and other partner agencies. The received data are immediately transferred to and stored in the GPM data processing systems in order to produce data for weather forecasts and a global precipitation map every 3 hours and to deliver these products in near real time. GPM standard products for geophysical research and data sets for more general use will also be processed at data centers and delivered to research organizations and the public via the Internet. Each agency will utilize those data and images to suit their needs, such as weather forecasting and flood prediction.

Real-time monitoring of "water" using satellites seeks to provide immediate and practical information for weather prediction, land management, agriculture, fisheries and, possibly, disaster prevention through improved weather forecasting and warning.

GPM data are expected to be used throughout the world not only for scientific research but also for social and economic activities, and are thus closely intertwined with our daily lives.

Utilization of GPM data in various fields

“Precipitation” is one of basic geophysical parameters in various fields. Less precipitation may cause decreases in dam storage and food production. If it continues longer and results in drought, it may also cause changes in ecosystems and exacerbation of desertification. On the other hand, much precipitation from heavy rainfall and tropical cyclones may cause floods by increasing river flow, and sometimes results in sediment disaster at areas of loosened ground. Anomalous precipitation could cause extensive damage in our societal life.

Even if timing of seasonal rainfall differs largely compared to normal years, it may affect plant growth and agricultural works and cause decreases in food production. Furthermore, recent predictions by climate models indicate that there could be influences consistent with global warming, such as acceleration of the water cycle and distribution change or centralization of rainfall. In this manner, even smaller fluctuations of rainfall compared to normal years, could affect the basis of our daily life, such as availability of drinking water and food production. It may also cause floods and droughts, and threaten our societal life.

The accurate and frequent precipitation data provided by the GPM will help researchers analyze the mechanism of the water cycle and improve the accuracy of both long-term and short-term weather forecasts. The GPM mission will also contribute to our life by improving water resource management in river control and irrigation systems for agriculture.