Water Vapor Transport
It is readily recognized that water vapor is
the most important greenhouse gas in the atmosphere. Water vapor
plays a critical role in climate processes at all scales by influencing
the distribution of OLR and as a
carrier of latent heat energy in the atmosphere. In a sense,
one can think of atmospheric water vapor as a thermostat on global
temperatures. Some scientists believe that increased amounts
of atmospheric water vapor (a product of potential global warming)
might lead to further warming due to water's absorption characteristics.
On the other hand, the presence of more water vapor (and hence
clouds) might limit the amount of incoming solar radiation thus
cooling the global temperature. Part of the uncertainty to the
role which water vapor plays in global climate changes is due
to the lack of high spatial and temporal measurements (especially
at upper levels). One of the ways to learn more about atmospheric
water vapor and its variability is to monitor it from satellites.
In this way, global and regional maps of atmospheric water vapor
can be derived and studied to understand its distribution, sources
and sinks, and its transport around the globe, all of which determine
its variability. This research focuses on atmospheric water vapor
in the upper troposphere (5-10 kilometers above the surface of
the Earth). Measurements from particular channels on satellites
provide information on the water vapor content in the atmosphere.
This research is currently using the infrared water vapor channels
on the GOES geostationary satellites
to measure water vapor in the upper troposphere and to monitor
its movement over the hemisphere.
GOES (VAS) Pathfinder data from May 1987 through
November 1988. A sequence of 3- hourly images centered about
1200 UTC are used for motions (winds) with the middle image used
for mositure and height assignment. This dataset corresponds
to the 1987-1988 ENSO period.
GOES (8/9/10) from May 1997 through April 1999.
A sequence of three images at 30-60 minute intervals are used.
This period corresponds to the strong 1997-1998 ENSO event. See
map for ENSO time series.
These datasets cover the Americas and the eastern
and central Pacific ocean (see map).
Winds are derived from a time sequence of GOES
imagery using the Marshall Automated
Wind (MAW) algorithm. Input parameters to the tracking algorithm
vary with satellite because of sensor spatial resolution and
time separation of available data.
The humidity fields are derived from the GOES
brightness temperatures using a modified
version of the Soden and Bretherton technique. The layer
relative humidity is converted to speficic humidity required
by the Water Vapor Transport Index formulation. The winds and
humidity are gridded and then combined to form the various layer
transport parameters used in the climate research. A height is
assigned to the wind and humidity based on comparing the channel
brightness temperature to a reference thermodynamic profile available
from a global forecast model. Currently, only winds and humidity
measurements from non-cloud environments are used in the analysis.
Details of this approach can be found in Jedlovec
et al. (1999).
A analysis of the daily and monthly transport
of upper-level water vapor for the 19-month GOES Pathfinder period
(May 1987 - November 1988) was examined preliminarily by Jedlovec et al. (1999) with
the summer months of 1987 and 1988, and in more detail for the
Ph.D. thesis of Dr. Jeff Lerner (Lerner
1999). The GOES-derived transport variables depicted the
annual migration of dominant westerlies across the tropical and
subtropical Americas and circulation features associated with
El Nino and La Nina climate anomalies. More intense subtropical
moisture transport occurred duriing the warm ENSO phase while
stronger tropical moisture transport occurred during the cold
phase. Spatial maps of seasonal meridional vapor transport revealed
that the north-south flux of water vapor does not adhere to the
classical Hadley cell circulation. On average, the meridional
moisture transport favors the winter and spring hemispheres such
that stronger southward (northward) transport is observed in
the Southern (Northern) hemisphere during their respective winter
and spring seasons.
Pressure velocity was estimated from the satellite
winds and pressure gradients using the adiabatic method to infer
vertical circulations over the cloud free tropical oceans. The
pressure velocites successfully portrayed ascending motions in
the vacinity of the Eastern Tropical Pacific ITCZ with persistent
subsidence over the south Pacific with stronger subsidence in
JJA of 1988 rather than the preceeding months.
Upper-level moisture transport compared favorably
with a similar product derived from the NCEP and Goddard DAO
reanalysis systems. The positioning of the vapor transport centers
were in good agreement, however, the gradients and magnitudes
of the transport values differed by a factor of two. The upper-troposphere
of the models tended to be too moist and at times under-estimated
the meridional component of the wind.
This work continues by focusing on the upper-level water vapor
transport associated with the 1997-1999
ENSO event. Data from the GOES 8/9/10 spacecraft are being
utilized to extend the data analysis
region westward to 170 W longitude. Daily products will be
produced for a 19-24 month period covering the ENSO event.
A CD ROM of the data products similar to that
produced for the 1987-1988 ENSO period will be available in late
This work is currently funded by NOAA's
Office of Global Programs under its Climate
Change Data and Detection Program.
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Technical Contact: Dr. Gary J. Jedlovec (email@example.com)
Responsible Official: Dr. James L. Smoot (James.L.Smoot@nasa.gov)
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Last updated on: January 17, 2002