- Total Atmospheric Precipitable Water Over Ocean from SSMI
- 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
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.
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.
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.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
Name And Directory Information Naming Convention
- Graphics Interchange Format (GIF)
- 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/
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
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).
Not available at this revision.
- 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)
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.
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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Last update:Fri Aug 22 18:01:05 EDT 1997