What Is Precipitation?
pre·cip·i·ta·tion
(n) water that falls from the clouds toward the ground, esp. as
rain or snow
(Definition from the Cambridge Dictionary of American English)
The description above sounds simple enough. Precipitation, however,
can take other forms in addition to rain and snow. Sleet, hail,
drizzle, and graupel (an amalgamation of hail and snow that resembles
round snow particles) are all forms of precipitation. Water in its
various forms covers most of the surface of our planet, and this
basic element has a profound impact on life on Earth. Water that
falls to the ground from clouds is essential to all living organisms,
but precipitation can also trigger catastrophes that can harm or
even obliterate life.
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For these very reasons, scientists strive to understand precipitation—when
it falls, where it falls, and why it falls. Greater knowledge
of precipitation mechanisms will allow researchers to increase
their understanding of the global water cycle, which is intimately
linked to changes in Earth’s climate system. Precipitation
data is utilized heavily in the models that scientists use
to predict our weather; more accurate precipitation data will
lead to enhanced weather prediction. Precipitation data with
a high temporal and spatial sampling rate will enable scientists
to improve life on Earth, and perhaps even help save lives.
Scientists will be able to refine flood and storm forecasts,
more accurately predict the availability of freshwater resources,
formulate agricultural plans that take predicted climate changes
into account, and more.
But why do we need the Global Precipitation Measurement (GPM)
initiative? Why can’t scientists just measure precipitation
on the ground, as it falls? Measurement of precipitation is
deceivingly complex. Unlike other meteorological phenomena
(e.g., pressure, temperature) precipitation is stochastic
(random) in nature, non-homogeneous, and rapidly changes.
You have probably witnessed evidence of this complexity yourself,
perhaps as a thunderstorm passed by, with its varying rates
of rainfall and even hail during a brief period of time.
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In addition, oceans cover most of the Earth’s surface. On
land, rain gauges and radar are often used to monitor precipitation
rates, but it is not feasible to utilize these methods over ocean
surfaces. Lastly, scientists desire global, unbiased, precipitation
estimates to attain the objectives described above. To effectively
reduce the bias in rainfall estimates, precipitation data must be
sampled relatively frequently (every three hours or better) at every
point on Earth.
The only viable means to collect precipitation data on a global
scale, with such a high temporal resolution, is to utilize space-borne
assets. Using current technology, GPM, with its constellation of
spacecraft, will be capable of acquiring the type of precipitation
data that scientists require. Furthermore, GPM provides for calibration
sites on the ground, which will provide scientists with an additional
means for refining precipitation data.
Thus, despite its simple-sounding definition, precipitation presents
a multifaceted, intricate challenge to the scientific community.
Implementation of GPM will arm researchers with the data they need
to more completely understand the complex workings of our planet’s
water system, and to ultimately improve the quality of life on Earth’s
surface.
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Who Is Involved?
Meet Dr. Eric A. Smith
NASA Project Scientist for GPM
In December 2000, the National Aeronautics and Space Administration
(NASA) Goddard Space Flight Center appointed Dr. Eric A. Smith
to serve as the Project Scientist for Global Precipitation
Measurement (GPM). In this capacity, Dr. Smith is responsible
for managing NASA’s scientific affairs for GPM, coordinating
NASA’s GPM scientific activities with the Japanese and
European space agencies as well as other space agency and
institutional partners, and conducting independent research
within the framework of NASA’s Global Water & Energy
Cycle (GWEC) research program.
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Dr. Smith has accumulated a wealth of scientific knowledge, computing
experience, and international familiarity during his career as a
research scientist and educator. Before he earned his Ph.D. in the
field of atmospheric science, he conducted research in satellite-based
meteorology and radiative transfer at the University of Wisconsin’s
Space Science and Engineering Center and at Colorado State University’s
(CSU) Atmospheric Sciences Department. While at CSU, he obtained
his doctoral degree in 1984.
In 1985, Dr. Smith became a professor in the Department of Meteorology
at Florida State University (FSU), where he also served as a Faculty
Associate of the Supercomputer Computations Research Institute.
While at FSU, Dr. Smith taught classes in the physical program area
of meteorology, and conducted research on problems concerning atmospheric
remote sensing, particularly in the tropics, focusing on the Asian
Monsoon system, and on the development and application of physical
retrieval and process models involving radiative transfer, hydrometeorology,
and carbon assimilation.
During a two-year leave of absence from FSU in 1999, Dr. Smith
served as Director and Chief Scientist at the Global Hydrology and
Climate Center (GHCC) in Huntsville, AL—a joint institute
comprised of the Earth Science Division of the NASA Marshall Space
Flight Center (MSFC), the Departments of Atmospheric and Computer
Sciences at the University of Alabama-Huntsville (UAH), and the
MSFC/UAH Global Hydrology Resource Center (GHRC). Upon his decision
to serve as NASA Project Scientist for GPM in late 2000, Dr. Smith
resigned his position at GHCC and retired from his tenured position
at FSU in order to pursue new opportunities within NASA.
Dr. Smith’s previous and current research experience is complimentary
to GPM’s research program. His most recent research involves
the study of radiative energetics and the global water cycle, particularly
in the tropics, the Asian Monsoon system, and desert heat-low regimes.
In addition, he specializes in combining satellite observations
with linked hydrometeorological-carbon assimilation models to investigate
radiation, heat, water, and carbon budget processes. He has extensive
experience with active and passive radiative transfer modeling,
optical/infrared/microwave satellite measuring and associated data
processing techniques, field observing experiments throughout the
world, and advanced computing systems. Dr. Smith is the author of
over 100 refereed journal publications, numerous additional reports,
atlases, book chapters, and preprints. He has delivered invited
scientific addresses on seven continents.
Dr. Smith is fascinated by the development of GPM, noting that
since his initial experiences with passive microwave rain retrieval
in 1977, he has been intent on helping design a satellite measuring
program that could deliver science-quality global rainfall measurements
at a time scale concomitant with Earth’s daily solar cycle.
According to Dr. Smith, “This has been a frontier problem
of environmental remote sensing for some 30 years. In conjunction
with atmospheric temperature, humidity, and wind, precipitation
represents a foremost atmospheric variable requiring routine satellite
observations before the next significant upgrades in predictions
of global climate, weather, and hydrology can be achieved.”
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Meet Dr. Kenji Nakamura
Japanese GPM Science Lead
Dr. Kenji Nakamura received a Ph.D. in atmospheric dynamics
from the University of Tokyo in 1978. He joined Japan’s
Radio Research Laboratory [now called the Communications Research
Laboratory (CRL)] in 1977, and engaged in a microwave and
millimeter satellite-to-Earth radio wave propagation experiment.
Dr. Nakamura’s research work there also extended to
atmosphere probing by microwave techniques.
In addition, Dr. Nakamura has experience working at scientific
institutions in the United States. From 1985 to 1987 he worked
at the National Aeronautics and Space Administration (NASA)
Goddard Space Flight Center (GSFC), performing rain observations
using an airborne dual-frequency rain radar system. This activity
became a partial basis for the Tropical Rainfall Measuring
Mission (TRMM)—a joint project between Japan and the
United States.
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In 1994, Dr. Nakamura moved to the Institute for Hydrospheric-Atmospheric
Sciences, Nagoya University, part of which was later renamed the
Hydrospheric Atmospheric Research Center, Nagoya University, during
a reorganization. Dr. Nakamura is currently serving as the director
of the Center. While employed at the Center, he chaired the ground
validation team in Japan for TRMM, and also served as Japan’s
Project Scientist for TRMM in 1998-2000.
After the overwhelming success of TRMM, the United States and Japan
began to discuss a follow-on mission (ATMOS-A1). Dr. Nakamura chaired
the ATMOS-A1 team under the Earth Science and Technology Organization
Forum/Earth Observation Committee. The discussions of the ATMOS-A1
team are summarized in the Proposal for ATMOS-A1 Mission (January
2001). The ATMOS-A1 mission has now evolved into the GPM Core Spacecraft.
As the Japanese GPM Science Lead Dr. Nakamura continues to strive
for the realization of GPM.
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