INTRODUCTION:
As stated in the PACS scientific prospectus, a lack of
observational data is a major stumbling block in the validation of
models used to simulate tropical atmosphere-ocean interaction over
the east Pacific. This is also a factor limiting our understanding
of processes such as the North and South American monsoon systems.
In particular, the atmospheric hydrologic cycle is poorly observed
over many regions, yet determining the cycle of moisture in the
atmosphere is vitally important to understanding and predicting
climate variability over a variety of scales. This includes
determining the moisture sources contributing to the North and
South American monsoon systems and understanding teleconnections
between the east Pacific and the Americas associated with El
Niņo. Only satellite observations provide large-scale long-term
coverage which can furnish the necessary link between in-situ
observations and GCM scales. Such observations are necessary both
for climate studies and model validation efforts, especially over
regions such as the east Pacific where in-situ observations are
extremely limited.
PROJECT GOALS:
The objectives of this research activity are divided into two
areas. The first area focuses on satellite data set development
and subsequent use for model validation efforts and process
studies, emphasizing data sparse regions like the eastern tropical
Pacific. This involves the dissemination of real-time satellite
data sets and products through the web, the development of new
data products such as high-resolution multilevel water vapor
products, and working with other PACS investigators to facilitate
the use of these data sets for model validation and process
studies. The second area focuses on empirical studies using a
combination of the satellite observations and products with in-situ
and other data sources. This effort includes exploring differences
between satellite rainfall estimates over the east and west
Pacific, looking at interannual variability associated with El
Niņo and La Niņa, and investigating high-frequency moisture
variability associated with the North American monsoon system.
METHODOLOGY:
We have developed an archive of a large number of satellite
data sets and data products extending as far back as 1979 for HIRS
data and 1987 for SSM/I data and derived products. This gives us
the capability to investigate large-scale variations of a number
of atmospheric moisture products including rainfall, clouds,
surface winds, and water vapor at several levels to name a few. In
addition to regularly updating many of these data sets on our web
site, we are working to develop new products and to utilize them
in our studies of precipitation systems over the east Pacific,
moisture surges associated with the North American monsoon, and
direct comparisons with high resolution models.
RESULTS AND ACCOMPLISHMENTS:
Recent research efforts have focused on analyzing
precipitation systems over the east Pacific. As shown in Figure
1, substantial differences exist between current satellite
infrared and passive microwave rainfall estimates over the east
Pacific intertropical convergence zone (ITCZ). To understand the
reasons for this difference we have performed a comparison of
satellite retrievals from SSM/I with 3D radar reflectivities from
the August 1997 PACS Tropical Eastern Pacific Process Study (TEPPS)
cruise. We have also performed multiyear matches of SSM/I rainfall
estimates with GOES cloud-top temperatures over the east Pacific,
and comparisons with matches over the west Pacific warm pool
region. Although the TEPPS cruise took place during a strong El
Niņo event, several shallow precipitation events were identified
from the vertical radar reflectivity profiles which were matched
with SSM/I and GOES overpasses. One such example is shown in Figure
2. This comparison confirmed the ability of the SSM/I to
detect shallow precipitation events, which are not detected by
simple infrared retrieval techniques. Subsequent seasonal matches
of SSM/I rainfall estimates with cloud-top temperature information
from GOES over the east Pacific are shown in Figure
3. This figure shows the presence of a substantial number
of warm shallow rainfall events over the east Pacific during the
northern hemisphere fall (SON) and winter (DJF) of La Niņa or
normal years, but a dominance of cold convective events during the
strong 1997/98 El Niņo event. This result supports the hypothesis
that infrared techniques severely underestimate precipitation
along the east Pacific ITCZ, although this effect appears to have
been reversed between the east and west Pacific during the strong
1997/98 El Niņo.
Another area of investigation is the use of satellite water
vapor and precipitation estimates to track moisture sources for
the North American monsoon. Las Vegas, Nevada, was struck by flash
flood on July 8, 1999 which resulted in widespread flooding and
led to loss of life. SSM/I and GOES satellite observations shown
in Figure 4 and Figure
5 reveal that this event was associated with a moisture
surge originating in the tropical eastern Pacific several days
prior to the storm. This same surge produced intense rainfall
events at a number of locations along the Gulf of California as
revealed by station data shown in Figure 4. An evaluation of SSM/I
and GOES imagery from June through September of 1999 indicates
that similar moisture surges occurred repeatedly during the North
American monsoon season, seldom reaching Las Vegas but sometimes
pushing tropical moisture as far north as Lake Tahoe.
We continue to add real-time and near real-time
satellite images and products to our web site at
(http://www1.etl.noaa.gov/climsat/). This site contains current
daily images of SSM/I derived rainfall, cloud liquid water, and
integrated water vapor, the latest 3-hourly GOES visible, infrared
and water vapor imagery, and 5-day composite sea surface
temperatures from GOES and other sources. A variety of other
satellite images and derived products for global or other regions
and over 5-day and monthly time scales are also available at this
web site. In addition, several of the PACS products are archived
and distributed by the UCAR PACS Data Management Center.
FUTURE WORK:
We plan to expand our investigation of moisture surges
associated with the North American monsoon to include multiple
years in order to investigate the effects of interannual
variability on the monsoon. We also plan to look at other
potential data sources such as passive microwave observations of
tropospheric water vapor from AMSU-B on NOAA 15 and SSM/T2 on the
DMSP satellites. The relatively recent addition of AMSU-B and DMSP
F15 gives us the capability to look at high frequency (daily or
better) variability in water vapor, which is not obscured by
clouds as is the case with infrared observations. We also plan to
utilize TRMM data, including vertical profiles from the TRMM
radar, to further investigate differences between east and west
Pacific rainfall systems. We anticipate using a combination of
data from the new generation of sensors along with our long-term
archives of HIRS, SSM/I, and other operational sensors to further
investigate hydrologic cycle variability over the east Pacific and
the Americas.
PUBLICATIONS RESULTING FROM THIS RESEARCH:
Berg, W., D. Anderson, and J. Bates, 2000: Satellite
observations of a Pacific moisture surge associated with flooding
in Las Vegas, submitted to Geophysical Research Letters.
Berg, W., J.J. Bates, and D.L. Jackson, 1998:
Analysis of upper tropospheric water vapor brightness temperatures
from SSM/T2, HIRS and GMS-5 VISSR, J. Appl. Meteor., 38,
580-595.
Berg, W., 1999: An analysis of precipitation
systems along the east Pacific ITCZ, in preparation.
CONTACTS:
Principal Investigators:
John J. Bates
Jbates@etl.noaa.gov
Phone: (303) 497-6646
Fax: (303) 497-3794
Wesley Berg
Wberg@etl.noaa.gov
Phone: (303) 497-6066
Fax: (303) 497-3794
Institution:
NOAA/ERL
Environmental Technology Laboratory
R/ET1A
Boulder, CO 80303
LINKS:
http://www1.etl.noaa.gov/climsat/
(Satellite Climate Research Home Page)
http://www1.etl.noaa.gov/climsat/pacs/monsoon/monsoon.html
(Satellite observations of the North American monsoon)
http://www.joss.ucar.edu/pacs/
(PACS Data Management Center)
FIGURE CAPTIONS:
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