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Background
In 1988 the World Climate Research Programme (WCRP) initiated
GEWEX to observe, understand, and mode} the hydrological cycle and
energy fluxes in the atmosphere, at the land surface, and in the upper
oceans. GEWEX is an integrated program of research, observations,
and science activities with the ultimate objective of substantially
enhancing the ability to predict global and regional hydrologic
processes and water resources and their response to environmental
change.
In 1990 the GEWEX Science Steering Group (an international
advisory body) held a workshop in Easton, Maryland, titled "The Role
of Water Vapor in Climate Processes." The discussions and papers
presented at that workshop highlighted several key deficiencies in the
understanding of water vapor's spatiotemporal characteristics. It was
recognized that improvements in this understanding would be
necessary to realistically characterize fundamental aspects of the
atmospheric system, including radiative heating, precipitation, cloud
formation, and horizontal and vertical moisture transport and
convergence. Without improved understanding of these critical aspects
of the atmospheric system, the ability of models to accurately predict
weather and climate at all time scales will be significantly hampered.
To address this need, a GEWEX Global Water Vapor Project
(GVaP) was initiated, and in May 1991 a strategic research plan was
prepared and published by the National Aeronautics and Space
Administration (NASA), describing an approach for providing the data
to answer some of these critical questions. This initial plan was
designed to: (~) improve the accuracy and availability of global water
vapor data through the development of a global water vapor data set,
(2) establish reference observation stations, and (3) conduct
intercomparison studies among the existing water vapor data sets.
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In November 1991, a meeting was held in Columbia, Maryland, to
discuss the implementation of this initial plan. The results of these
discussions are described in a document titled "Implementation Plan for
the Pilot Phase of the GEWEX Water Vapor Project." This plan
included the addition of a fourth element, namely, the improvement and
standardization of radiosonde humidity sensors and data r~roc~e.~ina
procedures for worldwide use.
The cornerstone of this initial phase has been the production of the
NASA Water Vapor Project (NVAP) data set (Randel et al., 19961.
This is a global, 9-year (1988-1996), 1x1 degree resolution product that
quantifies both atmospheric water vapor and liquid water, with daily,
pentad (5-day), and monthly temporal resolutions for three layers
~ ~ 000-700, 700-500, and 500-300 mb), as well as the entire
atmospheric column. The NVAP data set, which is available on CD-
ROM, was constructed through a blending of radiosonde, Special
Sensor Microwave/Imager (SSM/I), and TIROS (Television and
Infrared Observation Satellite) Operational Vertical Sounder (TOYS)
water vapor soundings.
In August 1993, a GVaP workshop was held in Breckenridge,
Colorado, to discuss the current state of the art in satellite retrievals,
radiosonde climatologies, the GVaP pilot phase, and the NVAP data
set. In October ~ 994, an American Geophysical Union Chapman
Conference on Water Vapor was held on Jekyll Island, Georgia, to
review theoretical and observational aspects of water vapor and to
identify areas of future research.
The development of the NVAP data set and the advancement of
validation processes during the initial phase of GVaP led the GEWEX
Science Steering Group to conclude that the success of the pilot period,
coupled with recognition of the potential importance of upper
tropospheric and lower stratospheric water vapor for GEWEX, had set
the stage for GVaP to make major contributions to GEWEX. An
international planning workshop for GVaP was held in November 1996
in Geneva, Switzerland, at which representatives from WCRP, the
National Oceanic and Atmospheric Administration (NOAA), and
NASA endorsed the development of plans for the main phase of GVaP.
In 1997, the GEWEX Joint Steering Committee recommended a new,
7-year phase of GVaP with the participation of all WCRP programs,
including the Arctic Climate Systems Study (ACSYS), the Climate
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Investigate how the Serologic cycle will change during
periods of global warming
Characterize the role of clouds and other processes that
maintain the vertical distribution of water vapor.
Examine the direct and indirect water vapor feedbacks on the
climate system, including the relationship of water vapor to
other climate variables such as sea surface temperature,
cloudiness, andprecipitation.
· Document the three-dimensional distribution of water vapor at
time scales ranging from interannual down to short-term daily
variability.
Implementation of the main phase of GVaP will be organized around
four major thrusts, discussed in both the Science Plan (IGPO, 199Sa)
and the Implementation Plan (IGPO, 1998b):
1.
Develop and deploy the tools for serif cation, validation, and
calibration of observations for in situ and satellite retrievals of
water vapor.
Document the climatology of water vapor.
Address the GVaP research areas.
Develop and test new water vapor observing systems and
instruments.
GVaP will combine existing data, collected in NVAP, with a number of
new data collection systems, including retrievals from satellite systems
such as the Global Positioning System (GPS), the Advanced
Microwave Sounding Unit (AMSU-B), the Advanced infrared Sounder
(AIRS), and the Microwave Humidity Sensor (MHS). In addition,
GVaP will take advantage of data from the Water Vapor Sensing
System (WVSS), an aircraft-based component of the Meteorological
Data Collection and Reporting System (MDCRS).
The main phase of GVaP is being initiated with a series of
workshops and research meetings to advance knowledge of water vapor
data set calibration/validation issues, including algorithm development
and data intercomparison. The first of these was the GVaP upper
tropospheric humidity (UTH) Intercomparison Workshop held in June
1998 in Darmstadt, Germany, with the objective of quantifying existing
differences in top-of-the-atmosphere radiances in the 6.3,um water
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vapor absorption band as simulated by different radiative transfer
codes. Other workshops are planned to steer activities in the following
areas: lower tropospheric algorithm intercomparison, instrument
validation and intercomparison, GVaP data set development, veri-
f~cation of satellite water vapor, and water vapor science and app-
lications.
Validation-anc! calibration capabilities will be enhanced by the
development and testing of new instruments and systems such as
Differential Absorption Lidar (DIAL) and Raman Lidar, ground-based
microwave and infrared radiometers, GPS measurements, improved
radiosonde technologies, very long baseline interferometer (VERBS), and
aircraft reporting systems. The Department of Energy's (DOE)
Atmospheric Radiation Measurement (ARM) program will incorporate,
within the ARM structure, responsibility for coordination of the
calibration/validation activities and data collection and will undertake
fully instrumented calibration/validation measurements at 4-5 locations
around the world for verification of satellite retrievals and algorithm
development.
One of the key foci of GVaP will be documenting the climatology
of water vapor. Issues at stake include defining the appropriate spatial
and temporal resolutions for improving the understanding of various
atmospheric phenomena, such as greenhouse warming and large-scale
water vapor transport. in addition, the results of calibration/validation
work will be assessed and drawn upon to characterize the accuracy of
the data and to assist in developing data merging algorithms. This work
will lead to defining the inputs and processing of a second GVaP data
set (the follow-on to NVAP), which is proposed to include a minimum
of 4-5 vertical levels. It will also lead to the development of an
improved suite of water vapor products that include separate satellite-
only, ground-only, and blended data sets. Such a hierarchical approach
will allow independent comparison amongst the various data sources
and merged products. GVaP will also assess the use of new ob-
servational sources, such as those mentioned above, as they become
available.
The Implementation Plan (IGPO, 1998b) calls for these data sets to
be used as part of GVaP investigations into the GVaP research
objectives. The timeline given in the Implementation Plan indicates that
these investigations, as well as the final development of the GVaP data
sets, will be completed in 2005.
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Representative terms from entire chapter:
vapor data
