GPM Science Objectives
Water is fundamental to life on Earth – so measuring and mapping precipitation gives insight into the behavior of our weather, climate and ecological systems. Tracking precipitation from space with satellites provides accurate information of when, where and how much it rains or snows anywhere in the world and helps to decipher how Earth systems works.
Because weather systems are complex and evolve rapidly, measuring rain, snow and other types of precipitation on the ground is difficult. 'Isolated showers' are exactly that – rain showers occur in one place and not another, which makes obtaining reliable ground-based precipitation measurements over entire regions a formidable challenge. Plus, most countries are not equipped with the instruments or systems to accurately and continuously measure rain or snow with rain gauges and/or radar. And due to large gaps in instrument coverage over oceans, which cover 70% of the Earth's surface, tracking rain and snowfall globally is exceptionally difficult.
Above the sky is a different story. Space-based remote sensing instruments present a way to provide more frequent and accurate measurements of rain and snow from the vantage point of a satellite. NASA's Global Precipitation Measurement (GPM) mission will join its predecessor the Tropical Rainfall measurement Mission (TRMM) in providing advanced information on rain and snow characteristics and detailed 3-D views of precipitation structure in the atmosphere. GPM and TRMM have a unique and innovative perspective on measuring precipitation from space because of how they collect satellite retrievals and convert the information into meaningful precipitation estimates, which are used for an array of scientific discoveries and societal applications.
The water cycle describes the movement of water on, above, and below the Earth's surface. Water is constantly changing states - from vapor to liquid to ice - and moves by precipitation, infiltration, transpiration, runoff, and subsurface flow. Its phase transitions among the gaseous, liquid and solid states dominate the behavior of the weather, climate, and ecological systems - the way water moves between all three phases is a powerful vehicle for rearranging Earth's energy budget.
A key to the connections between water and energy cycles is how the sun's heat affects the atmosphere. The sun's direct contribution explains only about 25% of the energy used by current global atmospheric dynamics. The other 75% is transferred to the atmosphere by water. This transfer happens when the heat of the ocean causes water to evaporate. This water vapor then condenses into clouds and/or rain and in doing so, releases its latent heat into the atmosphere. That latent heat drives atmospheric circulation, playing a major role not only in cloud formation and storm development, but in the motion of air around the world. TRMM created the first reliable global latent heating estimates ever made by measuring the profile of rain as it falls through the sky, as function of altitude.
GPM will expand these estimates beyond the tropics. It will measure the intensity and variability of 3-dimensional latent heating structures of precipitation systems as well as microphysics and surface water fluxes. The enhanced measurement and sampling capabilities of GPM helps us to understand how precipitation patterns change over time across local to regional and global scales. These patterns translate into changes in hydrologic fluxes and states - runoff, evapotranspiration, soil moisture, and groundwater recharge - both directly and in combination with land process models.
By providing more accurate estimates of the rate of transfer of water from the atmosphere to the surface, GPM reduces a significant source of uncertainty in the global water/energy budget. Scientists combine GPM observations with land surface data to provide better estimates for soil moisture, temperature, and snowpack, which leads to better predictions of vegetation cover, weather forecasts and integrated hydrologic models.
Hot tower structures in Hurricane Bonnie, as seen by the TRMM satellite.
Using satellites lets us observe changes in the precipitation structure over the life cycle of a storm, even if it moves over oceans and lands where conventional data are sparse. In particular, we now have insights into the dynamics of a storm, like how the eye of a hurricane stays stable as the storm moves across the Earth's surface, and how tropical cyclones intensify through the presence of "hot tower" structures.
A 'mesoscale convective system' is a cloud system that occurs in connection with a group of thunderstorms and produces an unbroken line of precipitation 100 kilometers or more across in at least one direction. The rain that falls from these systems can lead to dangerous flooding with tremendous socioeconomic impacts. Predicting and monitoring these high-impact natural hazard events depends on accurate and timely knowledge of precipitation at local and regional scales.
The area covered by a storm can vary greatly in size, depending on the type of the storm and the main force responsible for triggering it. A small rain event may occupy just a few kilometers while large-scale rain events may extend 1,000 kilometers or more and involve large frontal systems or cyclones and hurricanes.
The Tropical Rainfall Measurement Mission (TRMM) was a revolution in terms of how it 'saw' tropical cyclones. It provides important information about the size and frequency of occurrence of rain storms in the tropics. TRMM fills a significant gap in our observations and increases our knowledge about the water cycle and atmospheric circulation over the globe. While small-scale rainstorms occur more frequently in the tropics, large events have a more significant impact on global circulation.
The Global Precipitation Measurement (GPM) mission will expand our view beyond the tropics, tracking tropical storms as they move into the mid-latitudes and, for the first time, providing a 3-dimensional view of the storms' structural changes as they move towards the poles. Storms can strengthen as they transition from tropical to mid-latitudes and GPM's improved tracking of the life-cycle of a storm will help us understand why some, but not all, storms change intensity as they move.
GPM's expansion of our precipitation database will help scientists understand regional variations of mesoscale convective systems, and GPM's enhanced instrument sensitivity adds to TRMM's data and improves the understanding of precipitation characteristics in mountainous regions.
For more information on tropical cyclones:
- TRMM Extreme Events
- Video: The Trouble with Irene
- National Hurricane Center (NHC)
- Joint Typhoon Warning Center (JTWC)
TRMM's Precipitation Radar (PR) was the first space-borne radar to see rain droplets in all 3-dimensions as they travel from the clouds to the ground. These measurements yield invaluable information on the intensity and distribution of the rain, on the type of rain, on the storm depth, and on the height at which snow melts into rain. Estimates of the heat released into the atmosphere at different heights based on these measurements factor in to models of global atmospheric circulation, which are used to model Earth's climate.
Watch "The Anatomy of a Raindrop" to learn more about how GPM will measure precipitation.
Download in HD from the Scientific Visualization Studio.
GPM's Dual-frequency Precipitation Radar (DPR) improves on the TRMM radar by having two radar frequencies. The second frequency is like taking a picture of rain from another angle, which ultimately gives scientists more accurate precipitation information and improves their ability to look at raindrop structure, intensity, and convective processes. Information on the distribution of the size of precipitation particles is added to measurements by GPM's Microwave Imager, which improves the accuracy of rain and snowfall estimates.
DPR measurements will offer insight into the microphysical processes of precipitation, including evaporation in the atmosphere and clouds and collision of raindrops to form bigger raindrops, among others and allow scientists to distinguish regions of rain, snow and sleet. It will also provide bulk precipitation properties such as intensity of rain, how much water falls, and humidity of the air. DPR's two channels significantly improve scientists' ability to detect thresholds for light rain and falling snow and helps researchers create links between rainfall and human impacts on the environment from pollution and urban areas.
The distribution of the world's water is shifting as our climate changes. Wet areas may become wetter, dry areas drier, storms more intense, and we may have more chaotic weather around the world. According to the Intergovernmental Panel on Climate Change (IPCC, 2011), an increase in the average global temperature is very likely to lead to changes in precipitation and atmospheric moisture, including shifts towards more extreme precipitation during storms.
TRMM climatology map showing the average global rainfall rates from 1998 to 2011.
As the lower atmosphere becomes warmer, evaporation rates increase, which leads to an increase in the amount of moisture circulating. This lower atmosphere is called the troposphere. When the troposphere has more moisture, more intense precipitation occurs, mainly over land areas, and thus more flooding occurs. Conversely in other areas, warmer temperatures may lead to increased drying, accelerating the onset of drought.
To predict future changes in climate, scientists use very sophisticated computer models that rely on available global data to describe climate as it is today and project how it may behave in the future. GPM data sets will help scientists estimate the rate of water transfer within the Earth's atmosphere and on the surface. It also reconciles the different parts of the overall water budget. By providing measurements of surface water fluxes, cloud/precipitation microphysics, and latent heat release in the atmosphere, GPM advances Earth system modeling and analysis. More accurate global precipitation estimates improve the accuracy and effectiveness of climate models and advance understanding of climate sensitivity.