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[CIDC FTP Data]
[SSMI Precipitable WaterIDC Data on FTP]
Data Access
Total Atmospheric Precipitable Water Over Ocean from SSMI
[rule]
Readme Contents
Data Set Overview
Sponsor
Original Archive
Future Updates
The Data
Characteristics
Source
The Files
Format
Name and Directory Information
Companion Software
The Science
Theoretical Basis of Data
Processing Sequence and Algorithms
Scientific Potential of Data
Validation of Data
Contacts
Points of Contact
References
[rule]
Data Set Overview
This data set is a collection of monthly means of total
precipitable water over ocean during the period August 1987-
November 1991. It was generated from values obtained from the
Special Sensor Microwave/Imager (SSM/I). Precipitable water from
SSM/I is expected to be a primary source of long term measurements
of atmospheric moisture content throughout the 1990s.
Sponsor
The production and distribution of this data set are being funded
by NASA's Earth Science enterprise. The data are not copyrighted;
however, we request that when you publish data or results using
these data please acknowledge as follows:
The authors wish to thank the Distributed Active Archive
Center (Code 902.2) at Goddard Space Flight Center,
Greenbelt, MD, 20771, for producing the data in its
present format and distributing them. The original data
products were produced by Remote Sensing Systems, Santa
Rosa, CA, using an algorithm by Frank Wentz. Goddard's
share in these activities was sponsored by NASA's Earth
Science enterprise.
Original Archive
The geophysical data from which the Atmospheric Moisture data set
is derived were produced by Remote Sensing Systems, Santa Rosa,
California, using an algorithm by Frank Wentz. This data is
currently available from the Physical Oceanography DAAC at NASA
JPL.
Future Updates
This data set will be updated as new data are acquired and
processed.
The Data
Characteristics
* Parameters: Total Precipitable Water defined as the
vertically integrated water vapor in a column extending from
the surface to the top of the atmosphere
* Units: Centimeters (cm)
* Typical Range (yearly average):
60-90 deg latitude 0 - 1 cm
30-60 deg latitude 1 - 3 cm
0-30 deg latitude 2 - 6 cm
* Temporal Coverage: August 1987 - November 1991
* Temporal Resolution: All gridded values are monthly means
* Spatial Coverage: Global
* Spatial Resolution: 1 degree x 1 degree
There are no data for the period December 1987 or January 1988.
The instrument was off during December and the first part of the
January.
The Wentz algorithm produces valid precipitable water values only
over ocean that is clear of ice. The continental mask is generated
by the algorithm as it applies the fill value to each grid block
that contains a preponderance of land or sea ice.
Source
SSM/I is carried aboard Defense Meteorological Satellite Program
(DMSP) satellites DMSP F-8, DMSP F-10, and DMSP F-11.
Nominal orbit parameters for the satelllite DMSP F-10 are:
Launch date: June 19, 1987
Orbit: Circular, Sun synchronous
Nominal altitude: 883 km
Inclination: 98.7 degrees
Nodal period: 101 minutes
Equatorial crossing time: 6:12 AM (local time)
Microwave radiances emitted by the atmosphere, ocean, and terrain
are measured in 7 channels at 4 frequencies for both vertical and
horizontal polarizations. These radiometer measurements are used
to derive sea ice, total precipitable water and precipitation,
soil moisture, and various ocean parameters. The characteristics
of each channel are listed below.
Frequency (GHz) Wavelength (cm) Polarization
19.35 1.55 V/H
22.235 1.35 V
37.0 0.81 V/H
85.5 0.35 V/H
The official archive began on July 9, 1987, at 0000 GMT. From
December 3, 1987, through January 12, 1988, the sensor was turned
off because of overheating instrument components caused by solar
radiation. In late January 1989 the SSM/I 85 GHz vertical channel
began to demonstrate signs of failing to accurately record
radiances. The noise in this channel increased steadily until late
February 1989, when the data collected through this channel became
completely unusable.
The near-polar orbital characteristics of the satellite allow
global coverage every 3 days. Gaps in the data occur poleward of
87.6 degrees. Repeat coverage is possible in polar regions more
frequently because of overlaps in the orbital coverage.
The SSM/I sensor is directed 45 degrees to the rear of spacecraft
travel, yielding an angle of incidence to Earth's surface of 53.1
degrees. This results in a conical scanning pattern in which
radiance observations are taken on a 102.4 degree arc centered on
the spacecraft subtrack in the aft direction. This corresponds to
a 1400 km swath at ground level. During each scan, the 85 GHz
channels are sampled 128 times and the lower frequency channels 64
times over the 102.4 degree arc. The following table lists the
effective field of view for each frequency and polarization. The
first number is the along-track dimension and the second is the
cross-track dimension.
Frequency (GHz) Polarization FOV (km)
19.35 V 68.9 x 44.3
H 69.7 x 43.7
22.235 V 59.7 x 39.6
37.0 V 35.4 x 29.2
H 37.2 x 28.7
85.5 V 15.7 x 13.9
H 15.7 x 13.9
A detailed description of the SSM/I instrument and the DMSP series
of satellites is available on the Marshall Space Flight Center
Worldwide Web site.
The Files
This data set consists of 51 monthly mean data files from August
1987 through December 1991 and a collection of 51 gif images
derived from them.
Format
Data Files
* File Size: 259200 bytes, 64800 data values
* Data Format: IEEE floating point notation
* Headers, trailers, and delimiters: none
* Land or water mask: land mask, value -999.9
* Fill value: -999.9
* Image orientation: North to South
Start position: (179.5W, 89.5N)
End position: (179.5E, 89.5S)
Image Files
* Graphics Interchange Format (GIF)
Name And Directory Information Naming Convention
The file naming conventions for the Atmospheric Moisture data set
are
ssmi.prch2o.1nmego.[yymm].ddd
where:
ssmi = data product designator
prch2o = parameter name
1 = number of levels
n = vertical coordinate, n= not applicable
m = temporal period, m = monhtly
e = horizontal grid resolution, e = 1 x 1 degree
go = spatial coverage, go = global (ocean)
yy = year
mm = month
ddd = file type designation, (bin=binary, ctl=GrADS control
file)
Directory Path to Data Files
/data/inter_disc/hydrology/ssmi_wvap/yyyy/
where yyyy is year.
Directory Path to Image Files
/data/inter_disc/hydrology/ssmi_wvap/gif/
Companion Software
Several software packages have been made available on the CIDC
CD-ROM set. The Grid Analysis and Display System (GrADS) is an
interactive desktop tool that is currently in use worldwide for
the analysis and display of earth science data. GrADS meta-data
files (.ctl) have been supplied for each of the data sets. A GrADS
gui interface has been created for use with the CIDC data. See the
GrADS document for information on how to use the gui interface.
Decompression software for PC and Macintosh platforms have been
supplied for datasets which are compressed on the CIDC CD-ROM set.
For additional information on the decompression software see the
aareadme file in the directory:
software/decompression/
Sample programs in FORTRAN, C and IDL languages have also been
made available to read these data. You may also acquire this
software by accessing the software/read_cidc_sftwr directory on
each of the CIDC CD-ROMs
The Science
Theoretical Basis of Data
In the microwave region of the spectrum sensed by the SSM/I
instrument (19 GHz to 85 Ghz), water vapor, liquid water droplets,
and oxygen are the major atmospheric constituents responsible for
absorption of radiation emitted by the combined Earth-atmosphere
system. In particular, a weak water vapor absorption line exists
at 22 GHz which can be used in combination with other channels to
produce estimates of the total amount of precipitable water vapor
contained in an overhead column through the atmosphere. The
additional channels, primarily the 37 GHz channels, are needed to
account for variations of the ocean surface emissivity caused by
wind-induced roughness, which in turn can affect the accuracy of
the water vapor retrievals. In addition, use of the vertically
polarized component of the radiance at 22 GHz to deduce water
vapor abundance presents some advantages in that it is less
sensitive to surface wind and temperature effects as compared to
the horizontally polarized component.
Microwave emissivities for land surfaces vary over a significant
range (.50 -.98) depending on moisture content of the soil,
vegetation type, and snow and ice cover, while for ocean surfaces
the range is more restricted (.40 -.50) and depends on salinity,
surface roughness, foam, and sea surface temperature. Thus, over
land, the signal originating from atmospheric water vapor can be
severely masked by the potentially large and highly variable
surface emission term. For this reason, reliable estimates of the
total precipitable water are normally restricted to oceanic
regions; therefore, no water vapor retrievals were attempted over
continental areas in the SSM/I data sets.
Processing Sequence and Algorithms
The original level 2 SSM/I geophysical product contained liquid
water (see Wentz and Wentz et al. references), water vapor, and
marine wind speed computed using the combined algorithm developed
by Frank Wentz. The products are computed where the surface type
indicates ocean (plus coastal, sea ice, and possible sea ice
areas). The 22 GHz Vertical, 37 GHz Vertical, and 37 GHz
Horizontal channels are used in the algorithm. The algorithm fits
a radiative transfer model, parameterized in terms of the above
three quantities, to the 22 and 37 GHz observations (Wentz et al.
1986). An iterative process is used that solves for wind speed
first, then cloud and rain liquid water and columnar water vapor.
Absorption, emission, and sea surface roughness are accounted for
in this method, but not Mie scattering by raindrops or ice
particles.
The data consist of logical records that correspond to a single
SSM/I scan that contains 64 25 km by 25 km resolution cells. For
each cell the following information is given: time, latitude,
longitude, a classification index, antenna temperatures, and the
three geophysical parameters. The classification index is a flag
for surface type, i.e., water, land or sea-ice.
The eight classifications were defined as follows:
Class 0: Water surface, rain rate less then 1.5 mm/hr, and
cell is farfrom ice or land
Class 1: Water surface, rain rate less then 1.5 mm/hr, and
cell is closet o sea ice
Class 2: Water surface, rain rate less then 1.5 mm/hr, and
cell is close to land
Class 3: Water surface, rain rate greater then 1.5 mm/hr, and
cell is close to land
Class 4: Water surface, anomalous geophysical parameters, and
cell is far from ice or land--this class may represent
moderate to heavy rain with significant radiative scattering
Class 5: Sea ice concentration greater than about 10%
Class 6: Water surface, anomalous geophysical parameters, and
cell is close to land
Class 7: Over land
No geophysical data were calculated over land or sea-ice. In
addition, for precipitation rate greater than 1.5 mm/hr over
ocean, only water vapor is computed.
The water vapor for Classes 0, 1, and 2 should have an accuracy of
0.3 g/cm*2 or better, even in the presence of light rain. A water
vapor estimate is also given for Class 3; i.e., rain rates
exceeding 1.5 mm/hr. For moderate rain rates (1.5 mm/hr), the
water vapor estimate will be degraded but will probably still be
useful.
The gridded Atmospheric Moisture Product was created by the
Laboratory for Atmospheres (Code 910) at Goddard Space Flight
Center by extracting and mapping the Wentz precipitable water
vapor data to a 1 degree x 1 degree global grid. Equal weighting
was given to all level 2 data points for which the coordinates of
the center of the field of view were contained within a particular
grid cell's boundaries. Only observations with a classification
index of of 0, 1, 2, or 3 were included in the gridding procedure.
In general the satellite will observe a particular equatorial
location twice a day (except in higher latitudes where this is
significant overlap of individual orbital swaths). Thus, since the
ungridded data have a spatial resolution of about 25 km, the
number of observations constituting the monthly averaged
precipitable water value over a month should typically range
between 500 and 700 for lower latitude oceanic cells. Variations
may occur over persistent convective regions where the radiance
observations have been contaminated by moderate to heavy
precipitation.
Also, since the SSM/I satellite observations at a particular
equatorial location for a day are representative of conditions at
6:12 AM and 6:12 PM sun time, the monthly gridded SSM/I
precipitable water values within a grid cell will be
representative of an average of the conditions at these two times
(as opposed to true diurnal averages). However, the deviation from
these two local times is considerable as the poles are approached,
and a grid cell may contain data averaged over a wide range of
local times in these cases.
Scientific Potential of Data
The SSM/I instrument provides continuous global measurements of
total precipitable water over oceanic regions. Because the water
vapor content of the atmosphere is highly variable, especially in
regions adjacent to continents, most atmospheric and oceanic
studies rely heavily on timely and consistent measurements. Some
examples of studies that benefit from global measurements of
moisture or precipitable water include
* Radiation budget studies (absorption and emission of solar
and longwave radiation by water vapor)
* Distribution of cloudiness and precipitation and their effect
on global climate and regional weather patterns
* Energetics of the atmosphere on both regional and global
scales. including ocean-to-atmosphere transport of heat and
moisture fluxes
* Thermodynamic and dynamic state of the atmospheric boundary
layer, where the bulk of the water vapor is situated
(Prabhakara et al. 1979)
* Effects of moisture fluxes on important periodic phenomena
such as El Nino, the Southern Oscillation and the winter and
summer monsoons (Liu 1988)
* Correlation of water vapor abundance with time-dependent
temperature changes (feedback mechanism in global warming
studies).
In addition, the total precipitable water derived from SSM/I can
be used to improve or assess the quality of other atmospheric or
surface data sets derived from satellite-borne instruments. As an
example, the interannual variability of SSM/I precipitable water
can be compared with that derived from the TOVS instruments aboard
NOAA Polar Orbiters in order to validate both the spatial patterns
and the amplitudes of the signal.
Reliable estimates of the total precipitable water can also be
used to atmospherically correct the infrared window radiances
measured by surface sensing instruments such as AVHRR to improve
the accuracy of sea surface temperature and vegetation
measurements (Justice et al. 1991).
Validation of Data
Not available at this revision.
Contacts
Points of Contacts
For information about or assistance in using any DAAC data,
contact
EOS Distributed Active Archive Center (DAAC)
Code 902.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
Internet: daacuso@daac.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)
References
Justice, C.O., T.F. Eck, D. Taure, and B.N. Holben. 1991. The
effect of water vapor on normalized difference vegetation index
derived for the Sahelian region from NOAA AVHRR data, Int. 5
Remote Sensing, 1165-1187.
Lau, K.M., and L. Peng. 1987. Origin of low-frequency
(intraseasonal) oscillations in the tropical atmosphere. Part I:
Basic theory. J. Atmos. Sci., 44:950-972.
Liu, W.T. 1988. Moisture and latent heat flux variabilities in the
tropical Pacific derived from satellite data. J. Geophys. Res.,
93:6749-6760.
Prabhakara,C., G. Dalu, R.C. Lo, and N.R. Nath. 1979. Remote
sensing of seasonal distribution of precipitable water vapor over
the oceans and the inference of boundary-layer structure. Mon.
Wea. Rev., 107:1388-1401.
Wentz, F.J. 1983. A model function for ocean microwave brightness
temperatures. J. Geophys. Res., 88(C3):892-1908.
Wentz, F.J., L.A. Mattox, and S. Peteherych. 1986. New algorithms
for microwave measurements of ocean winds: Applications to SEASAT
and the Special Sensor Microwave Imager. J. Geophys. Res.,
91(C2):2289-2307.
Wentz, Frank J. 1989. User's Manual SSM/I Geophysical Tapes, RSS
Technical Report 060989. Remote Sensing Systems, Santa Rosa, CA,
16 pp.
Wentz, Frank J. 1992. Revision-1 Update for SSM/I Geophysical
Tapes User's Manual, RSS Technical Report 040792. Remote Sensing
Systems, Santa Rosa, CA, 11 pp.
Wentz, Frank J. 1992. Measurement of oceanic wind vector using
satellite microwave radiometers, IEEE Transactions on Geoscience
and Remote Sensing, 30:960-972.
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