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GES DISC DAAC Data Guide: Chang SSM/I Derived Monthly Rain Indices Data Set

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Summary:

The Chang Special Sensor Microwave/Imager (SSM/I) Monthly Rainfall Indices were generated using the SSM/I brightness temperatures (TB) from the Defense Meteorological Satellite Program (DMSP) F8 and F11 satellites. The data set contains gridded ocean monthly total rainfall indices. The period of record for the data set is July, 1987 through December, 1995.

Table of Contents:

1. Data Set Overview:

Data Set Identification:

Chang SSM/I Derived Monthly Rain Indices

Data Set Introduction:

The Chang SSM/I Monthly Rainfall Indices were generated using the SSM/I brightness temperatures from the DMSP satellites F8 and F11. The data set contains gridded ocean monthly total rainfall indices. The data are on a 5 x 5 degree grid covering the area from 50N to 50S and 180E to 180W. Each grid cell value is the average of the satellite's ascending and descending estimates, corrected for beam fillings errors. Grid values are computed only over the oceans.
The data is binned into months using the Global Precipitation Climate Project (GPCP) convention. Using this convention, each month consists of six pentads (30 days total) except for August which contains seven pentads (35 days total).

Objective/Purpose:

This information is not available at this time.

Summary of Parameters:

Global ocean monthly rainfall indices

Discussion:

This information is not available at this time.

Related Data Sets:

SM/I Level 1b Sensor Counts from DMSP F8, F10, F11
SSM/I Pathfinder Data Sets
Wentz Antenna Temperatures
Wentz Geophysical Products
SSM/I Prabhkara Brightness Temperatures
MSU Daily Ocean Precipitation

2. Investigator(s):

Investigator(s) Name and Title:

Dr. Alfred Chang
Hydrological Sciences Branch, Code 974
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
U.S.A.
301-286-8997 (Voice)
301-286-1758 (Fax)
email address: ACHANG@RAINFALL.GSFC.NASA.GOV

Title of Investigation:

Retrieval of Monthly Rainfall Indices from Microwave Observations

Contact Information:

Please direct all queries to Goddard Space Flight Center (GSFC) DAAC Help Desk.
DAAC Help Desk:
The DAAC help desk also provides additional information on the Goddard DAAC system capabilities, and other supported datasets. The Help Desk can be reached at:
EOS Distributed Active Archive Center (DAAC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
Internet:daacuso@daac.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)

3. Theory of Measurements:

The information in section 3: Theory of Measurements is quoted from:

Wentz, F.J., User's Manual SSM/I Antenna Temperature Tapes. Santa Rosa, CA: Remote Sensing Systems, 1988, pp 9-13. The absolute brightness temperature of the scene (TB) incident upon the antenna is received and spatially filtered by the antenna to produce an effective input signal or antenna temperature (TA) at the input of the feedhorn antenna. The procedure for deriving Earth antenna temperatures is discussed below.

In computing the SSM/I antenna temperatures, the basic assumption is that the radiometer output voltage is linearly related to the input power at the mixer/preamplifier. Nonlinear effects such as imperfections in the square-law detector and Intermediate Frequency (IF) amplifier compression are assumed negligible. Expressing the input power in terms of radiation temperature gives

V(i) = ckBg(TA(i) + TN)                               (1)

where V(i) is the radiometer output voltage, TA(i) (Kelvin) is the radiation temperature entering the feedhorn, and TN (Kelvin) is the radiometer noise temperature. The constants are c = detector constant, k = Boltzmann constant, B = receiver predetection bandwidth, and g = receiver gain. The subscript i denotes either a cold-space observation (i = C), a hot-load observation (i = H), or an Earth observation (i = E). The gain and noise temperature are assumed stable over the period of one scan, which is 1.9 seconds. That is to say, g and TN are assumed to be the same for the cold-space, hot-load, and Earth observations. The output voltage (V) is converted to counts (C) by applying a scaling factor (w) and a constant offset (P):

C(i) = wV(i) - P                                      (2)

Combining (1) and (2) for i = C, H, and E gives

TA(E) = A * C(E) + B                 (3)

A = (TA(H) - TA(C)) / (C(H) - C(C)) (4)

B = ((TA(C) * C(H)) - (TA(H) * C(C))) / (C(H) - C(C)) (5)

For each SSM/I scan, five cold counts and five hot counts are recorded for each channel. (Only 85 GHz observations are taken on the B-scans). The quantities C(C) and C(H) are found by averaging the five individual counts. The cold space antenna temperature TA(C) is set to 2.7 K for all channels. The hot reference antenna temperature is found from

TA(H) = T0(H) + 0.01(TO(P) - TO(H))                   (6)

where TO(H) and TO(P) are the physical temperatures of the hot load and the radiator plate facing the hot load. The small correction for the radiator plate is to account for radiative coupling between the hot load and the top plate of the rotating drum assembly which faces the hot load. There are three thermistors on the hot load and one on the radiator plate. The temperature T0(H) is found by averaging the three thermistor readings. Thus the A and B coefficients can be computed, and equation (3) can be inverted to recover the original values for the Earth-viewing counts C(E).

4. Equipment:

The information marked with an asterisk (*) in section 5: Equipment is quoted from:

Hollinger, J. P., J. L. Peirce, and G. A. Poe, 1990: SSM/I Instrument Evaluation. IEEE Transactions on Geoscience and Remote Sensing. 28(5),781-790.

Sensor/Instrument Description:

The SSM/I consists of an offset parabolic reflector illuminated by a corrugated broad-band, seven port horn antenna. The reflector and feed-horn antenna are mounted on a drum which contains the radiometers, digital data subsystem, mechanical scanning subsystem, and power subsystem. The entire reflector, feed horn, and drum assembly is rotated about the axis of the drum by a coaxially mounted bearing and power transfer assembly (BAPTA). All data, commands, timing and telemetry signals, and power pass through it on slip ring connectors to the rotating assembly. *

Collection Environment:

Source/Platform:

The Defense Meteorological Satellite Program (DMSP) Block 5D-2 spacecrafts fly in a near polar sun-synchronous orbit. The satellites complete 14.1 revolutions per day and the subsatellite ground track repeats approximately every 16 days.

Source/Platform Mission Objectives:

The mission of the DMSP is to provide global visual and infrared meteorological and oceanographic data required to support worldwide Department of Defense operations and high-priority programs. Timely data are supplied to Air Force Global Weather Central, the Navy Fleet Numerical Meteorology and Oceanography Center (FNMOC) and to deployed tactical receiving terminals worldwide.

Key Variables:

Operational parameters include the following orbital elements:
            DMSP F8 DMSP F11 Maximum Altitude 882 Km 878 Km Minimum Altitude 838 Km 841 Km Inclination 98.8 Degrees 98.8 Degrees Nodal Period 101.8 Min 101.9 Min Equator Crossing 19:42 17:04   
  (Ascending)

Principles of Operation:

The SSM/I is a seven-channel, four-frequency linearly polarized passive microwave radiometric system. The instrument measures brightness temperatures at 19.3, 22.2, 37.0 and 85.5 GHz.

Sensor/Instrument Measurement Geometry:

The SSM/I rotates at a uniform rate making one revolution in 1.9 seconds, during which time the satellite advances 12.5 km. The antenna beams are at an angle of 45 degrees to the BAPTA rotational axis, which is normal to the earth's surface. Thus, as the antenna rotates, the beams define the surface of a cone, and, from the orbital altitude of 833 km, make an angle of incidence of 53.1 degrees at the earth's surface.  The scene is viewed over a scan angle of 102.4 degrees centered on the ground track aft of the satellite, resulting in a scene swath width of approximately 1400 km.

For a discussion of channel sampling methods, please refer to Sensor Measurement Geometry.

Manufacturer of Sensor/Instrument:

Hughes Aircraft Company

Calibration:

Specifications:

A small mirror and hot reference absorber are mounted on the BAPTA and do not rotate with the drum assembly. They are positioned off-axis such that they pass between the feed horn and the parabolic reflector, occulting the feed once each scan. The mirror reflects cold sky radiation into the feed, thus serving, along with the hot reference absorber, as calibration references for the SSM/I.*

This scheme provides an overall absolute calibration which includes the feed horn. Corrections for spillover and antenna pattern effects from the parabolic reflector are incorporated in the data processing algorithms.*

For a complete discussion of the SSM/I calibration, please refer to SSM/I Instrument Evaluation (Hollinger et al. 1990).

Tolerance:

The tolerance is +/- 0.5 Kelvin for the lower frequencies and +/- 1 Kelvin for 85 Ghz.

Frequency of Calibration:

The antenna temperature of the SSM/I is calibrated each scan from the input to the feedhorn through the output of the A/D converter. For a complete discussion of the SSM/I calibration, please refer to SSM/I Instrument Evaluation (Hollinger et al. 1990).

Other Calibration Information:

Not applicable for this data set.

5. Data Acquisition Methods:

The Goddard DAAC obtains the monthly rainfall indices data set via an ftp transfer from Dr. Al Chang, NASA Goddard Space Flight Center.

6. Observations:

Data Notes:

No additional data notes are available.

Field Notes:

No field notes are available.

7. Data Description:

Spatial Characteristics:

Spatial Coverage:

The SSM/I produces a swath width of approximately 1400 km and approximately 14 orbits, comprised of ascending and descending passes, per day for global coverage.

Spatial Coverage Map:

spatial coverage

Spatial Resolution:

The swath width for the SSM/I is approximately 1400 kilometers. The Effective Field of View (EFOV) on the Earth Surface varies from 69 km along-track, 43 km cross-track for the 19 GHz channels to 15 km along-track, 13 km cross-track for the 85 GHz channels.

Projection:

For a desecription of the scan projection, see Sensor Measurement Geometry.

Grid Description:

The data are on a 5 x 5 degree grid for the area from 50N to 50S and 180W to 180E.

Temporal Characteristics:

Temporal Coverage:

The period of record for the data set is from July, 1987 through December, 1995. The DMSP F8 brightness temperatures were used to compute the rainfall indices from July, 1987 through December, 1991. The DMSP F11 brightness temperatures were used to compute the rainfall indices from January, 1992 through December, 1995.

Temporal Coverage Map:

A temporal coverage map is not available at this time.

Temporal Resolution:

The rainfall indices data files are in monthly files which contain 30 days of data, except for August which contains 35 days of data. During the leap years, the data for February will contain 31 days. The following is a list of which Julian days are included in each month.
Month Inclusive Dates Inclusive Julian Days January 01/01 - 01/30 001 - 030 February 01/31 - 03/01 031 - 060 **
March 03/02 - 03/31 061 - 090 April 04/01 - 04/30 091 - 120 May 05/01 - 05/30 121 - 150 June 05/31 - 06/29 151 - 180 July 06/30 - 07/29 181 - 210 August 07/30 - 09/02 211 - 245 September 09/03 - 10/02 246 - 275 October 10/03 - 11/01 276 - 305 November 11/02 - 12/01 306 - 335 December 12/02 - 12/31 336 - 365

** Note that leap year will have an extra day.

Data Characteristics:

Parameter/Variable:

The parameter for this data set is precipitation.

Variable Description/Definition:

Precipitation - Any or all forms of liquid or solid water particles that fall from the atmosphere and reach the surface of the Earth. It includes drizzle, rain, snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. Precipitation is also defined as a measurement of the quantity, expressed in centimeters or milliliters of liquid water depth, of the water substance that has fallen at a given location in a specified amount of time.

Unit of Measurement:

The rainfall indices are in units of millimeters per month.

Data Source:

The monthly rainfall indices were derived from Special Sensor Microwave/Imager instrument measurements.

Data Range:

The data ranges from -10.0 to 2378. The -10.0 value is a flag to denote land cells or cells that have island contamination or cells in which any rain retrieval that did not converge .

Sample Data Record:

The following is sample of a partial month of the rainfall indices data set.
Y87M07
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0 179.2 111.7 143.1 62.6 55.5 154.9 206.5 219.4 157.9 61.5 178.8 116.1 50.1 46.2 20.2 10.2 15.1 23.1   -10.0   -10.0   -10.0
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0
   -10.0 85.4 191.6 98.3 52.0 32.9 85.3 20.4 19.0 64.1 80.1   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0
   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0   -10.0 222.0
   -10.0 139.8 267.1 184.7 107.8 138.6 169.5 218.1 358.2 311.5

8. Data Organization:

Data Granularity:

A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.

The complete data set is contained in a single file that has a volume of approximately 1.18 Megabyte.

Data Format:

The Chang monthly rainfall indices data set is in ASCII format.
The file name is:
gpcp_ssmi_1295_5.0_v23

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

Each grid cell value is the average of the satellite ascending and descending estimates, corrected for beam filling errors. The equation used to calculate the estimates is
(ascending + descending) * 0.5 * 1.5.
The grid values are computed only over the oceans and the grid cells which correspond to land are set to a flag of -10.0.

Data Processing Sequence:

Processing Steps:

This information is not available at this time.

Processing Changes:

This information is not available at this time.

Calculations:

Special Corrections/Adjustments:

This information is not available at this time.

Calculated Variables:

Average Monthly Precipitation

Graphs and Plots:

No graphs or plots are available.

10. Errors:

Sources of Error:

No known sources of error exist at this time.

Quality Assessment:

Data Validation by Source:

This information is not available at this time.

Confidence Level/Accuracy Judgement:

This information is not available at this time.

Measurement Error for Parameters:

This information is not available at this time.

Additional Quality Assessments:

This information is not available at this time.

Data Verification by Data Center:

The GSFC DAAC assures that the data file can be read and contains all data within the specified period of record.

11. Notes:

Limitations of the Data:

The rainfall indices were computed only over the oceans for this data set. Therefore, this data set should not be used to examine land surface rainfall.

Known Problems with the Data:

There are 33 cells set to -10.0 due to island contamination. Any rain retrieval that did not converge for a given cell results in that cell being set to -10.0.

Usage Guidance:

This information is not available at this time.

Any Other Relevant Information about the Study:

This information is not available at this time.

12. Application of the Data Set:

This information is not available at this time.

13. Future Modifications and Plans:

The data producer provides periodic updates to the data set with increased temporal coverage. The GSFC DAAC will work with the data producer to include the additional data in the archive.

14. Software:

Software Description:

There is no software associated with this data set.

Software Access:

This information is not applicable to this data set.

15. Data Access:

Contact Information:

Goddard Space Flight Center DAAC User Services.

Data Center Identification:

GSFC Distributed Active Archive Center

Procedures for Obtaining Data:

This data set resides on DAAC Anonymous FTP.

GSFC DAAC User Services:

The DAAC User Services or Help Desk also provides additional information on the Goddard DAAC system capabilities, and other supported datasets. The Help Desk can be reached at:

     EOS Distributed Active Archive Center (DAAC)
     Code 610.2 NASA Goddard Space Flight Center Greenbelt, Maryland 20771 Internet:daacuso@daac.gsfc.nasa.gov
     301-614-5224 (voice)
     301-614-5268 (fax)

DAAC SEARCH AND ORDER SERVICES:

The Goddard DAAC maintains archives of these datasets. The data stored in our archives can be ordered using the following search and order utilities

ANONYMOUS FTP:

The Chang precipitation data set resides on DAAC anonymous FTP. The data files may be accessed from this document,

ftp gifChang SSM/I Derived Monthly Rain Data
or directly via FTP at
ftp disc1.gsfc.nasa.gov
login: anonymous
password: < your internet address >
cd data/hydrology/precip/chang

Data Center Status/Plans:

The GSFC DAAC will continue to archive and distribute updates received from the data producer. The Goddard DAAC plans to periodically check with the data producer regarding the data set revisions.

16. Output Products and Availability:

Data sets are available on 8mm tape, 4mm DAT, or anonymous ftp.

17. References:

Wilheit, T., A. Chang, L. Chiu, 1991: Retriecal of monthly rainfall indices from microwave radiometric measurements using probability distribution functions. Journal of Atmospheric and Oceanic Technology, 8, 118-136.
Chang, A., L. Chiu, T. Wilheit, 1993: Random errors of oceanic monthly rainfall derived from SSM/I using PDF. MWR, 121, 2351-2354.
Wentz, F. J., User's Manual SSM/I Antenna Temperature Tapes (Revision 1), RSS Technical Report 120191, 70 pp, Remote Sensing Systems, Santa Rosa, CA, 1 December 1991.
Hollinger, J. P., J. L. Peirce, and G. A. Poe, 1990: SSM/I Instrument Evaluation. IEEE Transactions on Geoscience and Remote Sensing. 28(5),781-790.

18. Glossary of Terms:

EOSDIS Glossary.

19. List of Acronyms:

DAAC

Distributed Active Archive Center

 

DMSP

Defense Meteorological Satellite Program

 

FNMOC

Fleet Numerical Meteorology and Oceanography Center

 

GPCP

Global Precipitation Climate Project

 

GSFC

Goddard Space Flight Center

 

MSU

Microwave Sounding Unit

 

SSM/I

Special Sensor Microwave/Imager

 

TB

Brightness Temperatures

 

URL

Uniform Resource Locator

EOSDIS acronyms

20. Document Information:

Document Revision Date:Fri May 10 11:51:31 EDT 2002

August 1, 1996

Document Review Date:

August 1, 1996

Document ID:

Not available at this time.

Citation:

Not available at this time.

Document Curator:

Suraiya Ahmad (ahmad@daac.gsfc.nasa.gov)

Document URL:

http://disc.sci.gsfc.nasa.gov/DATASET_DOCS/chang_gpcp_ssmi_precip_data.html
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