TRMM Microwave Imager (TMI) Wentz Ocean Products

Table of Contents

• 1. Instrument Description
• 2. File Naming Convention
• 3. Data Description
• 4. Data Format
• 5. Sample Read Routine
• 6. References
• 7. Contact Information

1. Instrument Description

In November 1997, the first microwave radiometer capable of accurately measuring SST through clouds was launched on the Tropical Rainfall Measuring Mission (TRMM) spacecraft. The TRMM Microwave Imager (TMI) is providing an unprecedented view of the oceans. Its lowest frequency channel (10.7 GHz) penetrates non-raining clouds with little attenuation, giving a clear view of the sea surface under all weather conditions except rain. Furthermore at this frequency, atmospheric aerosols have no effect, making it possible to produce a very reliable SST time series for climate studies. The one disadvantage of the microwave SST is spatial resolution. The radiation wavelength at 10.7 GHz is about 3 cm, and at these long wavelengths the spatial resolution on the Earth surface for a single TMI observation is about 50 km. Also, the TRMM orbit was selected for continuous monitoring of the tropics. To achieve this, a low inclination angle was chosen, confining the TRMM observations between 40°S and 40°N. Previous microwave radiometers were either too poorly calibrated or operated at too high of a frequency to provide a reliable estimate of SST. The early results for the TMI SST retrievals are quite impressive and are already leading to improved analyses in a number of important scientific areas, including tropical instability waves (TIWs) and tropical storms [Wentz et al., 1999]

The TRMM Microwave Imager (TMI) is a 5-channel, dual-polarized, passive microwave radiometer. Microwave radiation is emitted by the Earth's surface and by water droplets within clouds. The TMI is used to measure several important meteorological parameters over sea surfaces. The TMI, a successor to the SSM/I, measures radiation at frequencies of 10.7, 19.4, 21.3, 37, 85.5 GHz. It orbits at an altitude of 218 miles, much lower than the SSM/I, thus providing better resolution. From the 5 channels of data provided by TMI, scientists calculate several parameters over ocean surfaces. These are:

• sea surface temperature
• 11 GHz 10m wind speed
• 37 GHz 10m wind speed
• columnar water vapor (atmospheric water vapor, integrated water vapor)
• columnar cloud water (cloud liquid water, liquid cloud water)
• 19-37 GHz rain rate (precipitation rate)

Further information may be obtained from Remote Sensing Systems, Inc. (http://www.ssmi.com).

2. File Naming Convention

The TMI Swath Data Product is packaged in HDF-EOS format, one file per orbit. The file naming convention is

tmi_L2c_YYYY.JJJ_OOOOO_v0X.eos

where

YYYY is the year (e.g., 1999)
JJJ is the day-of-year (e.g., 032)
OOOOO is the orbit number (e.g., 00415)
v0X indicates the version number, currently at version 04.

3. Data Description

These ocean products were derived from observations made by a radiometer onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The TRMM Microwave Imager (TMI) is well-calibrated, similar to SSM/I, and contains lower frequency channels required for sea surface temperature retrievals. These products include sea surface temperatures (SST), surface wind speeds derived using two different TMI channels, atmospheric water vapor, liquid cloud water and rain rates. TRMM is a joint program between NASA and the National Space Development Agency of Japan (NASDA). The production of this data set is a collaborative effort with the TRMM Project at GSFC and the Passive Microwave Earth Science Information Partnership (ESIP) for Climate Studies. The Passive Microwave ESIP (PM-ESIP) was established to provide climate products derived from satellite microwave radiometers and is a joint effort among NASA's Global Hydrology and Climate Center, the University of Alabama in Huntsville, and Remote Sensing Systems These ocean products are produced by Remote Sensing Systems, and converted to HDF-EOS format here at the Global Hydrology Resource Center. RSS is currently using version-3a algorithm to produce the ocean products: surface wind speeds, atmospheric water vapor, liquid cloud water, and rain rates. More information about this algorithm can be found at: http://www.ssmi.com/tmi/tmi_description.html#intro A detailed description of the algorithm development can be found at: ftp://microwave.nsstc.nasa.gov/pub/doc/tmiwop/TMI_version_2_algorithm.pdf. The TMI algorithm was changed in September 2006 to version 04. A writeup by RSS (Remote Sensing Systems) describing the V04 changes can be found at: ftp://microwave.nsstc.nasa.gov/pub/doc/tmiwop/v04_TMI_RSS_update.pdf.

4. Data Format

• HDF-EOS objects

• Dimensions

• Geolocation fields

• Data fields

• Quality flag - Contains a quality flag stored as a 16-bit integer (INT16) in a vector dimensioned "Track". A value of zero indicates that the scan is good; a non-zero value indicates that the entire scan is invalid.
• Sun angle - Contains the sun angle stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Valid values range from 1 to 29 and are always odd. A value of 31 indicates that the sun angle is not valid.
• Adjacent rain flag - Contains a flag stored as an 8-bit integer (INT8) in an array dimensioned "Track" by "Xtrack". A non-zero value indicates adjacent rain.
• 37GHz wind QC flag - Contains a flag stored as an 8-bit integer (INT8) in array dimensioned "Track" by "Xtrack". A non- zero value indicates that the "37GHz 10 wind speed" datum is probably bad.
• Surface type - Contains the surface type stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". A value of zero indicates ocean; one indicates coast; and two indicates land.
• Sea surface temperature - Contains the sea surface temperature stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are degrees Celsius multiplied by 100. (E.g., 10 degrees C is stored as 1000.)
• 11 GHz 10m wind speed - Contains the wind speed stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are meters per second times 100. (E.g., 15 m/sec. is stored as 1500.)
• 37GHz 10m wind speed - Contains the wind speed derived from the 37GHz channel stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are meters per second times 100. (E.g., 15 m/sec. is stored as 1500.) See the "37GHzwind QC flag" for the validity of this field.
• Columnar water vapor - Contains the water vapor stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are millimeters * 100. (E.g., 3 mm is stored as 300.)
• Columnar cloud water - Contains the cloud water stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are millimeters * 100. (E.g., 3 mm is stored as 300.)
• 19-37GHz rain rate - Contains the rain rate stored as a 16-bit integer (INT16) in an array dimensioned "Track" by "Xtrack". Units are millimeters per hour times 100. (E.g., 5 mm/hr. is stored as 500.)
• (Empty field - No longer used.)

Array dimensions specified above refer to "C" order (last dimension varies fastest).

For all 16-bit fields, a value of -32768 indicates invalid data. For all 8-bit fields, a value of 255 (-128) indicates invalid data.

5. Sample read routine

A sample read routine may be found in tmireader.c. This routine must be compiled and linked with the HDF-EOS and HDF libraries. In addition, it requires the following include file:

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#ifndef _SDP_UTILITIES_
#define _SDP_UTILITIES_

#include <df.h>

/*  Return codes  */

#define SDP_S                   0x00000
#define SDP_M                   0x10000
#define SDP_W                   0x20000
#define SDP_E                   0x30000

#define SDP_S_SUCCESS                   (0 | SDP_S)
#define SDP_E_TOOLKIT                   (1 | SDP_E)

#define SDPTD                   0x100

#define SDPTD_M_LEAP_SEC_IGNORED        (1 | SDPTD | SDP_M)
#define SDPTD_M_ASCII_TIME_FMT_B        (2 | SDPTD | SDP_M)

#define SDPTD_W_PRED_LEAPS              (1 | SDPTD | SDP_W)
#define SDPTD_W_JD_OUT_OF_RANGE         (2 | SDPTD | SDP_W)
#define SDPTD_W_DATA_FILE_MISSING       (3 | SDPTD | SDP_W)

#define SDPTD_E_NO_LEAP_SECS            (1 | SDPTD | SDP_E)
#define SDPTD_E_TIME_VALUE_ERROR        (2 | SDPTD | SDP_E)
#define SDPTD_E_TIME_FMT_ERROR          (3 | SDPTD | SDP_E)

/*  Function declarations  */

int             SDP_TD_TAItoUTC (float64 tai, char *utc);
float64        *SDP_TD_TAItoTAIjd (float64 tai, float64 jd[2]);
int             SDP_TD_TAIjdtoUTCjd (float64 tai[2], float64 utc[2]);
int             SDP_TD_UTCjdtoUTC (float64 jd[], int onleap, char
utc[28]);
int             SDP_TD_LeapSec (float64 jd[2], float64 *leap,
                    float64 *lastchangeJD, float64 *nextChangeJD,
                    char *leapStatus);
float64        *SDP_TD_JulianDateSplit (float64 inJD[2], float64
outJD[2]);
int             SDP_TD_UTCjdtoTAIjd (float64 jdUTC[2], int onLeap,
                    float64 jdTAI[2]);
void            SDP_TD_calday (int32 julianDayNum, int32 *year, int32
*month,
                    int32 *day);
int             SDP_TD_UTCtoTAI (char asciiUTC[28], float64 *secTAI93);
int             SDP_TD_UTCtoUTCjd (char asciiUTC[28], float64 jdUTC[2]);
float64         SDP_TD_TAIjdtoTAI (float64 jdTAI[2]);
int             SDP_TD_timeCheck (char *asciiUTC);
int             SDP_TD_ASCIItime_BtoA (char asciiUTC_B[26],
                    char asciiUTC_A[28]);
int32           SDP_TD_julday (int32 year, int32 month, int32 day);

#endif /* _SDP_UTILITIES_ */

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6. References

HDF-EOS information may be found at http://hdfeos.gsfc.nasa.gov/hdfeos/index.cfm. This page contains software, instructions and documentation for HDF-EOS.

7. Contact Information

Data can be ordered and questions addressed at http://ghrc.nsstc.nasa.gov/.

To order this data or for further information, please contact: Global Hydrology Resource Center User Services 320 Sparkman Drive Huntsville, AL 35805 Phone: 256-961-7932 E-mail: ghrc@eos.nasa.gov

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NASA Information Contact: Michael Goodman, Global Hydrology and Climate Center GHRC Web Curator: GHRC Web Team Last update: Wednesday, 04-Oct-2006 10:32:45 CDT If you have trouble viewing or navigating this page, please contact GHRC User Services. U.S. Government Compliance report.