Continuous and reliable global precipitation information is crucial for myriad of weather, climate and hydrological applications. The importance of precipitation in the form of rain, hail, sleet, snow etc. is known to science and clear to a layman. However, it’s quite tricky to measure past precipitation trends or predicting accurate future forecasts. There are three main categories of precipitation data sets available: ground based, satellite-based and blended products of ground and space data (Climate Data Guide, 2014).
Sensors onboard satellites are currently the only instruments that can provide global, homogeneous, precipitation measurements, says Qiaohong Sun, a scientist at the Beijing Normal University in China (Sun et al. 2017, 79-107). For global precipitation measurements sensors are categorized in to three main categories: visible (VIS) and infrared (IR), passive or active microwave (MW) and finally data collected from a combination of VIS, IR and MW sensors. The latter are often merged to increase the accuracy, coverage and resolution of precipitation analysis.
The Global Precipitation Measurement (GPM) mission is an international network of satellites that provides the next-generation global observations of rain and snow reported on National Aeronautics and Space Administration (NASA) | Goddard Space Flight Center (GSFC) website (Precipitation Measurement Missions, 2011).
NASA and the Japan Aerospace Exploration Agency (JAXA) launched the GPM Core Observatory satellite in 2014. GPM constitutes of a core satellite, a constellation of pre-existing operational and experimental satellites, ground validation sites and a precipitation processing system, said Arthur Y. Hou, a scientist at the NASA|GSFC, Greenbelt, Maryland, USA (Hou et al. 2013, 701-722). GPM core satellite carries a combination of dual-frequency precipitation radar (DPR) and a Microwave Imager (GMI). This sensor package is a continuation of the Tropical Rainfall Measuring Mission (TRMM) instruments, with the primary objective to provide rainfall measurements over tropical and sub-tropical oceans, says Christian Kummerow, a scientist at the NASA|GSFC, Greenbelt, Maryland, USA (Kummerow et al. 1998, 809-817).
The GPM serves as a reference for unifying data from a constellation of partner satellites to provide next-generation, merged precipitation estimates and with high spatial and temporal resolutions globally, says Gail Skofronick-Jackson, a scientist at the NASA|GSFC, Greenbelt, Maryland, USA (Skofronick-Jackson et al. 2017, 1679-1695).
The Global Precipitation Measurements (GPM) mission comprises a consortium of U.S. and international space agencies, including Japan Space Agency (JAXA), French Centre National d’ Etudes Spatiales (CNES), European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), the Indian Space Research Organisation (ISRO), the U.S. Department of Defense, Defence Meteorological Satellite Program (DMSP) and the National Oceanic and Atmospheric Administration (NOAA).
The below figure shows data transfer from the GPM Core observatory via NASA’s Mission Operation Center (MOC) and the constellation satellites via partner’s ground systems to the Precipitation Processing System (PPS) at GSFC. The GPM mission ground system includes all the assets needed to command and operate GPM core observatory in orbit, as well as manage and distribute data received from the Core and other pre-existing operational and experimental satellites in the constellation (GPM mission Brochure, 2014). The GPM data from PPS are made available to GPM science team and users, as well as to the NASA and partner ground validation sites for integration of satellite precipitation measurements with ground observations (Precipitation Measurement Missions, 2011).
The GPM mission provides valuable and timely precipitation datasets. One data product based on the GPM mission is the Integrated Multi-satellite Retrievals for GPM (IMERG). An example of the IMERG, a near-real-time IMERG can be seen in the image below.
The near-real-time IMERG image (see main article image) shows liquid and frozen precipitation measurements for Jan 20, 2019 at 18:30:00 across the globe derived from the satellites in the GPM constellation and is calibrated with measurements from the GPM Core Observatory, including data from rain gauge networks around the world.
The GPM data play a key role for organizations monitoring food and water security such as the Famine Early Warning Systems Network (FEWSNET) (Famine Early Warning Systems Network, 2019).
Furthermore, GPM data combined with other datasets are used to determine the extent and availability of surface rainfall for areas as small as the size of a farm up to the area of entire countries. For example, a project by the Sustainability Satellites Water and Environment (SASWE) research group uses GPM data with model-based seasonal forecasting and other satellite-based information products to improve the management of water resources, and decision making of water management agencies in South Asian countries.
GPM is a science-based mission with specific objectives to quantify global precipitation with high accuracy, spatial and temporal coverage for scientific studies as well as for operational and non-operational user applications in numerical weather prediction, land surface and hydrological modelling in many regions around the world.
Gail Skofronick-Jackson et al. (2017, 1679-1695) conclude that the GPM mission provides fundamental knowledge of the water cycle and complements other NASA satellite missions, such as the Gravity Recovery and Climate Experiment (GRACE), which measures changes in groundwater levels in underground aquifers, or the Soil Moisture Active Passive (SMAP) satellite Aquarius, which observed ocean salinity while it was operating, and CloudSat, which measures the properties of clouds and light precipitation.