| |
 |
This simple diagram illustrates the global water cycle, depicting the path of water molecules as they travel from Earth's surface, to the atmosphere, and back to Earth. |
In addition, a very small portion of water vapor enters the atmosphere through sublimation, the process by which water changes from a solid (ice or snow) to a gas. (The gradual shrinking of snow banks, even though the temperature remains below the freezing point, results from sublimation.) The remaining 10 percent of the moisture found in the atmosphere is released by plants through transpiration. While evaporation from the oceans is the primary vehicle for driving the surface-to-atmosphere portion of the hydrologic cycle, transpiration is also significant. For example, a cornfield one acre in size can transpire as much as 4000 gallons of water every day.
After the water enters the lower atmosphere, rising air currents carry it upward, often high into the atmosphere, where the air cools and loses its capacity to support water vapor. As a result, the excess water vapor condenses (i.e., changes from a gas to a liquid) to form cloud droplets, which can eventually grow and produce precipitation (including rain, snow, sleet, freezing rain, and hail), the primary mechanism for transporting water from the atmosphere back to Earth's surface.
 |
In the global water cycle, water molecules enter the atmosphere via several means: evaporation, transpiration, and sublimation—the process by which water changes from a solid (ice or snow) to a gas. |
When precipitation falls over the land surface, it follows various routes. Some of it evaporates, returning to the atmosphere, and some seeps into the ground (as soil moisture or groundwater). Groundwater is found in two layers of the soil, the "zone of aeration," where gaps in the soil are filled with both air and water, and, further down, the "zone of saturation," where the gaps are completely filled with water. The boundary between the two zones is known as the water table, which rises or falls as the amount of groundwater increases or decreases. The rest of the water runs off into rivers and streams, and almost all of this water eventually flows into the oceans or other bodies of water, where the cycle begins anew (or, more accurately, continues). At different stages in the hydrologic cycle, humans or other life forms may intercept some of the water.
Even though the amount of water in the atmosphere is only 12,900 cubic kilometers (a minute fraction of Earth's total water supply that, if completely rained out, would cover Earth's surface to a depth of only 2.5 centimeters), some 495,000 cubic kilometers of water are cycled through the atmosphere every year—enough to uniformly cover Earth's surface to a depth of 97 centimeters. Because water continually evaporates, condenses, and precipitates—with evaporation on a global basis approximately equaling global precipitation—the total amount of water vapor in the atmosphere remains approximately the same over time.
However, over the continents, precipitation routinely exceeds evaporation, and conversely, over the oceans, evaporation exceeds precipitation. In the case of the oceans, the routine excess of evaporation over precipitation would eventually leave the oceans empty if they were not being replenished by additional means. Not only are they being replenished, largely through runoff from the land areas, but, over the past 100 years, they have been over-replenished, with sea level around the globe rising by a small amount. Sea level rises when the oceans are warmed, causing water expansion and thereby a volume increase, and when a greater mass of water enters the ocean than the amount leaving it through evaporation or other means. A primary cause for increased mass of water entering the ocean is the calving or melting of land ice (ice sheets and glaciers).
It is well recognized that natural and/or human-induced climate variability is revealed most significantly in the global water cycle. If Earth’s climate is changing (i.e., if global temperatures are increasing as suggested by some observations) resulting higher evaporation and precipitation rates might result in overall change to the global water cycle. The physical process that links these elements is precipitation, a critical indicator of the rate at which water is being cycled through the atmosphere.
Moreover, precipitation is the parameter that has direct and most significant influence on the quality of human lives in terms of availability of drinking water and agriculture. Therefore, high quality precipitation measurements with global, long-term coverage and frequent sampling are considered to be crucial to understanding and predicting Earth’s climate, weather, and water cycle processes, and their consequences to life on Earth. Obtaining such measurements is the motivation for Global Precipitation Measurement (GPM).
By Dr. J. Marshall Shepherd, Deputy Project Scientist for GPM
(with contributions from the NASA Earth Observatory staff)
GPM Report Series Now Online
The GPM Report Series, edited by Eric A. Smith and W. James Adams, is a collection of conference publications, technical memoranda, and technical reports documenting key issues pertaining to the formulation of the GPM. The publications are designed to serve as informational and reference material for interested scientists, engineers, industry, educators, and the public.
The subject material of the reports varies widely. For example, current report topics range from a summary of GPM Workshop Proceedings, to an explanation of the benefits of international partnership in GPM, to a discussion of potential tropical open ocean precipitation validation sites. Presently, there are six reports available, and several others are in the works.
The GPM Report Series is accessible via the online GPM Library at http://gpm.gsfc.nasa.gov/library.html. If you prefer to receive a hard copy of any of the reports, please contact Leslie Cusick (301-286-9094).
In each issue of The GPM Monitor, we will provide a brief summary of some of the available publications in the GPM Report Series.
GPM Report 1 – Summary of the First GPM Partners
Planning Workshop
This report provides a synopsis of the proceedings of the First
Global Precipitation Measurement (GPM) Partners Planning Workshop
held at the University of Maryland, College Park, on May 16-18,
2001. At this meeting, representatives from nations around the world
convened to stimulate existing partnerships, establish new partnerships,
provide input on science and technologies issues, and develop the
framework for an international GPM mission. The report summarizes
each of the Workshop’s technical sessions, including the Breakout
Group sessions where three separate working groups discussed Engineering
Issues, Retrieval and Calibration/Validation Requirements, and Interdisciplinary
Science Requirements, respectively. Technical sessions emphasized
the need to understand the Global Water and Energy Cycle (GWEC)
and how GPM’s capacity to frequently measure precipitation
on a global basis will significantly increase scientific knowledge
in this area. Speakers examined the overall concept and goals of
GPM, the GPM architecture (Core Spacecraft, Constellation Spacecraft,
Calibration/Validation Sites, and Global Precipitation Data System),
and potential applications of GPM data. The Workshop Agenda and
the List of Attendees are provided as Appendices to the report.
GPM Report 8 – White Paper
This report’s purpose is to inform the broad range of parties
interested in GPM about the concepts of the mission at a high level.
It is a comprehensive overall review of GPM, and although it describes
the international nature of the program, the report focuses on NASA’s
contributions to GPM. The paper lists the GPM science objectives,
linking them to the Earth Science Enterprise’s mission statement,
and associating them with science drivers that will help define
the scope of GPM. GPM elements reviewed include:
· System engineering of the space and ground segments
· Instruments to be placed on the various spacecraft
· Core Spacecraft and its launch
· NASA-provided Constellation Spacecraft
· NASA’s Mission Operations Center for its GPM satellites
· Ground validation of space-borne GPM measurements
· Precipitation Processing System which will produce and
distribute GPM data
This paper was authored during the Formulation phase of the mission, and while it presents an excellent overview of the mission concept, it is not meant to represent the final GPM baseline.