TMI

Introduction

The Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) is a multi-channel, dual polarized, conical scanning passive microwave radiometer designed to measure rain rates over a wide swath under the TRMM satellite.  The design of the instrument is similar to that of SSM/I but the resolution of data measurement is better due to the lower altitude of the satellite orbit.  TRMM was launched on November 27, 1997 into a semi-equatorial (+/- 35 deg) orbit carrying 5 instruments.  This type of orbit precesses allowing for TMI to sample the surface at all times of day as opposed to the twice-per-day sampling of other microwave radiometers in near-polar orbits.   TRMM is a joint program between NASA and the National Space Development Agency of Japan (NASDA, now JAXA).   In addition to rain rates, TMI can also measure sea surface temperature (SST), ocean surface wind speed, columnar water vapor, and cloud liquid water.  The SSTs from TMI were the first satellite microwave SSTs available and proved to be of great value to tropical cyclone intensity forecasting and research.

Instrument Description

TMI operates on the TRMM satellite in a semi-equatorial orbit.  TMI has 5 channels as outlined in the table below.  Two external calibration targets (one cold, one hot) are used to maintain consistency in measurements.  After a post-launch checkout period, data started on December, 7th 1997 and continues to the present.  TRMM underwent an orbital boost in August 2001 in order to increase the operational lifetime of the satellite/instruments.  This boost in altitude allowed for many more years of operation.  TRMM also routinely undergoes a 180 deg yaw maneuver every 2 to 4 weeks.

Band [GHz] Polarization Spatial Resolution (3-dB footprint size)
Pre-boost  /  Post-boost  [km x km]
10.65 V,H 63 x 37  /   72 x 43
19.35 V,H 30 x 18  /   35 x 21
21.3 V 23 x 18  /   26 x 21
37.0 V,H 16 x   9  /   18 x 10
85.5 V,H   7 x   5  /     8 x   6

 

We use brightness temperature data files which we reverse to radiometer counts, intercalibrate with other microwave radiometers and then forward process to provide a suite of products for distribution.  The RSS TMI data are provided as daily maps (separated into ascending and descending orbit segments), 3-day mean maps, weekly mean maps and monthly mean maps. All images cover a global region extending from 40S to 40N at a pixel resolution of 0.25 deg (~25 km). 

The initial production of this data set was 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 was a joint effort among NASA's Global Hydrology and Climate Center, the University of Alabama in Huntsville, and Remote Sensing Systems.  When the PM-ESIP project finished, we continued TMI data processing and product delivery  using funds from the NASA MEaSUREs program.

Data Processing

The measurement of sea-surface temperature (SST) through clouds by satellite microwave radiometers was an elusive goal for many years. The early radiometers in the 1980s (i.e., SMMR) were poorly calibrated, and the later radiometers (i.e., SSM/I) lacked the low frequency channels needed by the retrieval algorithm. Finally, in November 1997, the TMI radiometer with a 10.7 GHz channel was launched aboard the TRMM satellite. 

The important feature of microwave retrievals is that SST can be measured through clouds, which are nearly transparent at 10.7 GHz. This is a distinct advantage over the traditional infrared (IR) SST observations that require a cloud-free field of view. Ocean areas with persistent cloud coverage are viewed on a daily basis with TMI. Furthermore, the microwave retrievals are not affected by aerosols and are insensitive to atmospheric water vapor.  However, microwave retrievals are sensitive to sea-surface roughness, while the IR retrievals are not.

The microwave and infrared SST retrievals are very complementary and can be combined to obtain a reliable global data set. The algorithm for retrieving ocean measurements from TMI radiometer data is described in the "AMSR Ocean Algorithm" document.

 

RSS TMI Data Products

The RSS TMI product data files contain: sea surface temperatures, surface wind speeds, atmospheric water vapor, liquid cloud water, and rain rates. We use a robust radiative transfer model to aid in deriving the ocean measurements from the instrument brightness temperatures.  The approach is similar to that used for all other radiometers processed by RSS.  These algorithms are described in "A Well Calibrated Ocean Algorithm for SSM/I".  Appropriate adjustments have been made to account for small differences in GHz between the TMI and SSM/I channels and for the fact that TMI water vapor is measured at 21 GHz rather than 22.235 GHz as in all SSM/I. In addition, small offsets are added to the TMI antenna temperatures in order to intercalibrate them with the SSM/I instrument series.

The current version of TMI data are V4.  These data were first distributed in 2006.  We anticipate release of a V7 version to match other RSS products in the coming months.

We provide two surface wind speeds for TMI. The first is a surface wind speed derived primarily from the 11 GHz channel (not available on SSM/I). The second surface wind speed retrieval relies on measurements made by the 37 GHz channels and uses the SSM/I algorithm. We find these two wind products are very similar with some small differences when rain is nearby.

Like the sea surface temperature maps, the 11GHz surface wind speed maps have small regions of missing data that occur. These regions are located where the sunlight reflects off the ocean surface (also called "sun glint") and affects the 11 GHz channel. Where there is sunglint, affected data are removed.

Gridded Binary Data Files

We produce daily, 3-day, weekly and monthly TMI data files.  The files contain no headers or metadata and are a simple set of single-byte arrays as described below.  Since the data are stored as single bytes, not byte-swapping is needed when transferring between computers. 

Gridded data are organized according to observation date. All dates and times are Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT), Zulu Time (Z), Universal Time (UT), and World Time.  The data products include daily and time averaged geophysical data as follows:

Daily   orbital data mapped to 0.25 degree grid
divided into 2 maps based on ascending and descending passes
early data may be overwritten by later data at high latitudes and daily "seam"
3-Day   average of 3 days ending on and including file date
Weekly   average of 7 days ending on and including the Saturday file date
Monthly   average of all data within the calendar month

There are gaps within these data. Missing data generally affect Daily and 3-Day products, but can also reduce the number of observations in Weekly and Monthly averages.

Data gaps are generally due to missing data upstream from our processing facility, such as the instrument being turned off. Occasionally, there are delays in obtaining and/or processing recently recorded data; beyond several weeks, it is unlikely that missing data will become available.

Each DAILY binary data file available from our ftp site consists of fourteen 0.25 x 0.25 degree grid (1440,320) byte maps. Seven ascending maps in the following order: Time (T), Sea Surface Temperature (S), 10-meter Surface Wind Speed using 11 GHz (Z), 10-meter Surface Wind Speed using 37 GHz (W), Atmospheric Water Vapor (V), Cloud Liquid Water (L), and Rain Rate (R), are followed by seven descending maps in the same order.

Acronym Product
Name
Product
Description
Scale Offset Valid Data Range Reason for No Data
TIME Time Minutes since midnight GMT
Fractional hour of day GMT
6.0
0.1
0.
0.
0 to 1440
0.0 to 24.0
no data
SST Sea surface temperature Temperature of top layer (skin) of
water ~1 mm thick
0.15 -3.0 -3 to 45 deg high winds (>20 m/s), sun glint, rain, RFI, near sea ice or land (~75 km)
WSPD_LF 10-m wind speed Wind speed using 10.7 GHz channel 0.2 0. 0. to 50.0 m/s sun glint, rain, RFI, near sea ice or land (~50 km)
WSPD_MF 10-m wind speed Wind speed using 18.7 GHz channel 0.2 0. 0. to 50.0 m/s sun glint, rain RFI, near sea ice or land (~50 km)
VAPOR Columnar atmospheric water vapor Total gaseous water contained in a
vertical column of atmosphere
0.3 0. 0. to 75.0 mm
1 gm/cm2= 10mm
heavy rain or near land (~25 km)
CLOUD Columnar cloud liquid water Total cloud liquid water contained in a
vertical column of atmosphere
0.01 0. 0 to 2.5 near land (~25 km)
RAIN Rain rate Rate of liquid water precipitation 0.1 0. 0. to 25.0 mm/hr near land (~25 km)

There are two cases for which neighboring data swaths overlap. At higher latitudes, orbit segments overlap within local regions and within a short measurement time. In these cases, parameter data are averaged. In the second case, there is a "seam" on each map where the first and last orbit segments of the day overlap. Data at this seam are not averaged. Here, later swath data overwrite data previously measured at the beginning of the day.

DAILY files are located in the directory /tmi/bmaps_v04/yYYYY/mMM. Each daily file name has the convention: yyyymmddv4.gz

yyyy   year 1997, 1998 etc.
mm   month 09, 10, etc.
dd  day  01, 02, etc.

 

As an example, TMI daily data for March 5, 1999 is located in: /tmi/bmaps_v04/y1999/m03/19990305v4.gz

The center of the first cell of the 1440 column and 320 row map is at 0.125 E longitude and -39.875 latitude. The center of the second cell is 0.375 E longitude, -39.875 latitude. The data values fall between 0 and 255. Specific values have been reserved:

0 to 250 =  valid geophysical data
251 = missing SST or wind speed due to rain, or missing water vapor due to heavy rain
252 =  *not used in this data set*
253 = TMI observations exist, but are bad
254 = no TMI observations
255 = land mass

The data values between 0 and 250 need to be scaled to obtain meaningful geophysical data. To scale the data, multiply by the scale factors listed in the table above:

The 3-DAY, WEEKLY and MONTHLY binary files available on our ftp site are similar to the the DAILY TMI binary files. They consist of six maps with a grid size of 0.25 by 0.25. Each file can be read as a 1440,320,6 array. There are only six maps because the Time layer is omitted. All other parameters are present and in the same order.

3-DAY files are located in the same directory as the daily data , i.e. /tmi/bmaps_v04/yYYYY/mMM.   The file names of the 3-DAY binary files consist of: yyyymmddv4_d3d.gz   3-DAY files are named according to the last day of data contributing to the average. (3 days ending on ...)

WEEKLY files are located in the directory /tmi/bmaps_v04/weeks.   The names of the weekly files are the year, month, and day of the last day (Saturday) of a 7-day period in the format: yyyymmddv4.gz. Striping is evident in the weekly wind speed and water vapor maps. This is due to high variability of these two parameters. 

MONTHLY files are located in the same directory as the daily data , i.e. /tmi/bmaps_v04/yYYYY/mMM.   The file names of the monthly binary files consist of: yyyymmv4.gz   Monthly averages contain data from all days within the calendar month.

All binary data files have zip compression to reduce size and decrease transfer time. If you only need one file, use the "Download Binary Data" button on the web page. If you are accessing a large number of files, it may be more convenient to use our anonymous ftp server: ftp.ssmi.com/tmi

Further help in using the TMI binary byte maps is located in the directory tmi_support on our ftp site. The directory contains sample Fortran, IDL, Matlab, and Python programs for reading the byte maps, plus a README file.

We process the incoming data in two stages.  The first results in temporary binary files that contain incomplete or intermediate data. This type of file can be distinguished from the more accurate reprocessed file by the letters 'rt' rather than 'v4' in the file name. An intermediate file for the above example would be named 19990305rt.gz These intermediate files exist for approximately 5 days until the final processing and quality checking is performed. The 'rt' files are in a state of flux, so exercise caution when using these data. For research, we always suggest just using the final 'v4' files.

Browse Images

Each Daily, 3-Day, Weekly or Monthly web page displays the derived ocean measurements. All data for a given map were collected on the GMT date. The graphic image maps were produced from TRMM TMI binary data files (described above) using IDL (an analysis and display tool from Research Systems, Inc.).  The maps are scaled to improve data visibility and do not show the entire range of data.  Be sure to check the scale located on the map for reference. Land regions on these maps are colored gray. The areas where TMI data are not available are black and include areas where the satellite did not make measurements or areas where there was rain.

Missing Data

There may be gaps within the provided data. If you select a date for which data are not available, a map with text stating 'Data currently not available' will be posted.  When browsing imagery, the navigation may skip dates with no data, or you may see a blank map stating that no data is available for that time. Binary data files for dates with completely missing data are not produced; they will be absent from our FTP server.

For official information on missing TMI data, visit the NASA Goddard DAAC TRMM Satellite Data Outages

Data Access

TMI data products are available for the time period from just after launch, December 7, 1997 to the present. As with all the other RSS satellite products, the data are provided in a gridded, binary format file. Graphic browse images of the ocean measurements can be viewed on our web page and the binary data files can be downloaded from our ftp server: ftp.ssmi.com/tmi.

Binary file read routines and verification files are available on our ftp server in the tmi_support directory.

Related Data Sets

The TMI sea surface temperature data are also available in GHRSST format L2P netCDF.

Level 1C TMI data files are available from NASA GHRC data center.  These files consist of individual orbits of ocean measurement data.

References

Gentemann, C. L., T. Meissner and F. J. Wentz, (2010) Accuracy of Satellite Sea Surface Temperatures at 7 and 11 GHz, IEEE Transactions on Geoscience and Remote Sensing, 48(3), 1009-1018.

Gentemann, C. L., F. J. Wentz, C. A. Mears and D. K. Smith, (2004) In Situ Validation of Tropical Rainfall Measuring Mission Microwave Sea Surface Temperatures, J. Geophys. Res., 109, C04021, doi:10.1029/2003JC002092.

Wentz, F. J., P. D. Ashcroft and C. L. Gentemann, (2001) Post-Launch Calibration of the TRMM Microwave Imager, IEEE Transactions on Geoscience and Remote Sensing, 39(2), 415-422.

Acknowledgements

TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project and the NASA Earth Science Physical Oceanography Program. RSS TMI data are available at www.remss.com.  We are grateful to NASA Goddard Space Flight Center for providing us access to TMI TDR data.

How to Cite These Data

Continued production of this data set requires support from NASA.  We need you to be sure to cite these data when used in your publications so that we can demonstrate the value of this data set to the scientific community.  Please include the following statement in the acknowledgement section of your paper:

"TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project. Data are available at www.remss.com. "

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