Precipitable Water
A major objective of NASA's
Earth Science Enterprise
program is the description of the global hydrologic cycle. Since
water vapor is an important link connecting the various components
of the hydrological cycle, an understanding of the role of water
vapor in the hydrological climate system and its variability
on all scales will be necessary to meet this objective. A full
understanding of its role in regulating climate will require
a knowledge of its distribution, horizontal and vertical transport,
and interactive processes. An understanding of each of these
aspects will require measurements of water vapor on scales ranging
from local observational campaigns for improving climate model
parameterizations to global scales for the development of a global
climatology of water vapor. To adequately analyze global and
regional climate and hydrologic systems, observations will need
to employ a vertical resolution compatible with the inherent
vertical scales of atmospheric moisture structure and will require
global measurements on a comparable scale. Present satellite
observations are inadequate in this respect. Even radiosonde
observations produced by the operational international network
are often inadequate in terms of vertical resolution. In addition,
radiosonde observations provide poor coverage of large portions
of the planet. Regional (national) differences in humidity sensor
characteristics and performance is another serious problem. Further
refinements in present retrieval techniques, especially those
using combinations of data from various and planned observing
systems are expected to yield significant improvements.
An important water vapor
parameter currently being obtained from satellite and radiosonde
measurements is precipitable water. Precipitable water is the
total atmospheric water vapor contained in a vertical column
of unit cross-sectional area extending from the surface to the
top of the atmosphere. Precipitable water is commonly expressed
in terms of the height to which that water substance would stand
if completely condensed and collected in a vessel of the same
unit cross section. Climatologies of PW are currently being compiled
using measurements from the Defense Meteorological Satellite
Program (DMSP), the National Oceanic and Atmospheric Administration
(NOAA) polar orbiting satellites and the operational network
of radiosondes. These data sets are the first step in providing
information on the horizontal and temporal variability of water
vapor. Though these data sets do not provide the vertical distribution
of water vapor that will be needed for a complete understanding
of water vapor processes in the climate system, they do provide
important information. For example, climate models have shown
significant increases in water vapor in response to global warming.
PW climatologies may, therefore, be indicators of such climate
warming. PW climatologies would also provide verification of
climate model performance over multiyear periods.
One area of research that the Infrared Measurements
Group at GHCC is involved in is the determination of PW from
GOES. Current research activities include retrieval algorithm
development and evaluation, and PW data set development. Even
though GOES measurements are not global in extent, its high spatial
and temporal resolution can provide important applications. For
example, in order to improve the ability to predict climate variability,
better parameterizations of water vapor processes such as surface
evaporation, convection, and condensation and their interactions
with the circulation of the atmosphere will be needed in climate
models. Improvements in parameterizations of these processes
in climate models will depend on observations that quantify these
processes at various scales. PW data sets from GOES can provide
the temporal and spatial resolution to study these processes.
An example of a very important atmospheric forcing involving
the above processes is the diurnal cycle of PW. The ability of
climate models to capture this forcing is important. Analysis
has shown that when there is diurnal forcing present in the models,
land surfaces are cooler with more evaporation and less sensible
heat flux than when no forcing is present. Also precipitation
rate decreases over land particularly in the monsoon regions.
Determining the diurnal cycle of PW would be useful in model
analysis studies and parameterizations of this type. GOES is
the only satellite capable of high temporal resolution which
can provide the needed observations to characterize the diurnal
variability of PW.
An important aspect of the Earth Science Enterprise
program is its regional field measurement campaigns. One objective
of these campaigns is to provide qualitative evaluations of water
vapor measurements deduced from various satellite and radiosonde
observations. The results should help provide the basis or verification
of particular measuring system techniques or algorithms that
best meet observational requirements. GOES PW retrievals with
its temporal spatial resolution can provide an additional verification
for these global satellite measurement systems. Finally, GOES
PW data sets can provide additional data to PW climatologies
especially over land areas where measurements from satellite
microwave instruments are not presently successful. Though the
GOES PW data would not be global it would provide useful climatologies
of high interest regions such as those containing the Amazon
deforestation and the El Nino/ Southern Oscillation (ENSO).
Technical Information
- Retrieval Algorithm Development and Evaluation
- Data Set Development and Analysis
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Technical Contact: Dr. Gary J. Jedlovec (gary.jedlovec@nasa.gov)
Responsible Official: Dr. James L. Smoot (James.L.Smoot@nasa.gov)
Page Curator: Diane Samuelson (diane.samuelson@msfc.nasa.gov)
Last updated on: November 2, 1999 |