Precipitable Water Variability
A dataset of precipitable water
over the US for the summers of 1987-88 is being developed at
MSFC. This dataset is being generated using the Physical Split
Window (PSW) technique for deriving Precipitable Water (PW).
This technique utilizes differential absorption in the 11 and
12 10-6m (or µm) regions to obtain an PW estimate
(Guillory et al. 1993; Jedlovec et al. 1994). This study is using
GOES-7 VAS infrared data for the summer of 1988 in the calculations.
The domain of the dataset includes the continental United States,
Mexico, extreme southern Canada, and portions of the Caribbean.
Retrievals were made once per day at 1500 UTC for the period
June 1 - August 31, 1988. Gridded daily values for this dataset
were averaged over a smaller domain covering much of the eastern
and central U.S. Temporal and spatial analysis of this dataset
is planned as well as producing an equivalent dataset for the
summer of 1987. GOES IR data have the potential to provide very
high temporal and moderate spatial coverage of PW over the Americas
for the period beginning in 1981 and extending beyond the year
2000. Products such as the PSW PW dataset from GOES can be used
to study climate change and for GCM verification.
The general research objective
for this topic is to quantify regional-scale PW, its temporal
and spatial variability, and its relationship to observed cloud
cover, precipitation, storm formation, and surface features.
This work has just begun and the specific tasks are listed below
and supports NASA's GVaP and GCIP initiatives.
1.Generate accurate regional distributions of PW derived from
geostationary satellite data for extend periods of time. The
PSW algorithm will be used to derive PW at high spatial resolutions
over the continental U.S. and surrounding areas. Satellite data
sets will include historical data from the GOES pathfinder period
and/or future data from the GOES I imager/sounder.
2.Available "ground truth" data (e.g., radiosondes,
surface observations, special in situ or remote water
vapor measurements, etc.) will be utilized to validate the accuracy
and reliability of PW retrievals and the derived variability.
3.Regional water vapor variability will be quantified with
structure function analysis and with other statistical procedures
to document the variability over weeks, months, and seasons.
4.The time series of regional water vapor data will be correlated
to other hydrologic and climate parameters to study the inter-relationship
of these variables.
5.Higher resolution data sets from aircraft platforms will
be used to study very fine scale variability and local sources
Methodology and Results
The research described below
focuses on the application of the PSW retrieval technique (Jedlovec
1987; Guillory et al. 1993) for deriving PW, or precipitable
water, from the Geosychronous Operational Environmental Satellites
(GOES). Results will be compared to the NASA Water Vapor Project
(NVAP) PW dataset as well as radiosonde measurements.
GOES Pathfinder data from
the Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric
Sounder (VAS) are used in this study. These data were obtained
from the University of Wisconsin and are available for the benchmark
period (May 1987 - November 1988). The data gathered for this
study consisted of multispectral imaging (MSI) data for August
1988 which also included visible channel data.
A first guess field was derived
from radiosonde observations for August 1988. These resulting
data were objectively analyzed to a 4 x 4 grid encompassing the
region from 16 to 52 N and from 68 to 124 W using a 2-D Barnes
objective analysis scheme. Finally, these objectively analyzed
data were put through the "forward" code of a radiative
transfer model. This code takes a vertical profile of temperature
and mixing ratio and simulates the channel transmittances and
the observed brightness temperature at that point. This was done
for the entire Barnes' grid. These analyses were performed using
0000 UTC radiosondes for the day after the retrieval day, i.e.,
a first guess of 0000 UTC 2 August 1988 was employed as the first
guess for retrievals made on 1 August 1988. The 0000 UTC time
was chosen over 1200 UTC to reduce the effect of surface-based
inversions on the guess information (Suggs and Jedlovec 1996).
The retrieval algorithm employed
in this study is essentially the same as that used by Guillory
et al. (1993) with modifications described in Jedlovec et al.
(1994). Retrievals were made at 1500 UTC for each day in August
1988 (except for August 10, 15, and 18). These retrievals were
made at an approximate spacing of 120 km. Cloud filtering in
the algorithm is presently limited to using a simple 11 m brightness
temperature threshold. The retrieval data were then analyzed
to a 1 x 1 grid using the Barnes scheme so that a comparison
could be made with the NVAP and radiosonde products.
The NVAP dataset is a global
PW dataset that utilizes as weighted merging scheme of Special
Sensor Microwave/Imager (SSM/I) microwave retrievals, TIROS Operational
Vertical Sounder (TOVS) infrared retrievals and quality controlled
radiosonde retrievals. The final PW dataset was created by combining
all three of these input datasets, using a hierarchical weighting
scheme. This algorithm uses radiosonde data, when available as
truth, and then applies a weighting scheme to the TOVS and SSM/I
retrievals. Finally, linear and temporal interpolation is used
to fill in missing data points (Vonder Haar et al. 1994). The
NVAP data are on a 1 x 1 grid.
Examples of Results
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Technical Contact: Dr. Gary J. Jedlovec (firstname.lastname@example.org)
Responsible Official: Dr. James L. Smoot (James.L.Smoot@nasa.gov)
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Last updated on: November 2, 1999