Description of TMI Data Products

Version Notes

Introduction

Sea Surface Temperatures

Other Geophysical Parameters

Binary Data Files

Graphic Image Maps

Acknowledgement

Version-4 TMI Ocean Products

The entire TMI ocean data set has been completely reprocessed. As of September 13, 2006, all TMI data files have been updated from Version-3 to Version-4. Version-3 TMI data have been archived off-line and are no longer available except by special request.

The byte map folder has changed from "bmaps_v03" to "bmaps_v04".
The final file designation has changed from "tm" to "v4".
No changes were made to the data file format.

More details on TMI Version-4 improvements:

The September 2006 Update to RSS Climate Data Records

Introduction

This web site offers geophysical parameters derived from observations made by a radiometer onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. This radiometer, the TRMM Microwave Imager (TMI) is well-calibrated, similar to SSM/I, and contains lower frequency channels required for sea surface temperature retrievals. The entire data set includes sea surface temperatures (SST), surface wind speeds derived using two different radiometer 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 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. The data are available from December 1997 to the present. All images cover a global region extending from 40S to 40N at a pixel resolution of 0.25 deg (~25 km). Unlike SSM/I on DMSP platforms, the TRMM satellite travels west to east in a semi-equatorial orbit. This produces data collected at changing local times for any given earth location between 40S and 40N. The DMSP platforms are in a near-polar, sun-synchronous orbit. Therefore, the SSM/I instruments collect data daily at the same local time. Due to these orbital differences, we offer morning/evening (local time) maps of SSM/I data and ascending/descending maps of TRMM data.

More information is provided on selected SST case study regions and on SST anomaly maps (TRMM SST minus Reynolds Climatology). Movies of equatorial SSTs highlight some of the changes that the world's oceans undergo.

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. The scientists working on the TRMM SST algorithm and analysis are Frank Wentz and Chelle Gentemann of Remote Sensing Systems.

Sea Surface Temperatures

The measurement of sea-surface temperature (SST) through clouds by satellite microwave radiometers has been 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 SST observations that require a cloud-free field of view. Ocean areas with persistent cloud coverage can now be viewed on a daily basis. Furthermore, microwave retrievals are not affected by aerosols and are insensitive to atmospheric water vapor. However, the microwave retrievals are sensitive to sea-surface roughness, while the infrared retrievals are not. A primary function of the TRMM SST retrieval algorithm is the removal of surface roughness effects. The microwave and infrared SST retrievals are very complementary and can be combined to obtain a reliable global data set. The algorithm for retrieving SSTs from radiometer data is described in "AMSR Ocean Algorithm" - PDF format (969 kb) or Word Doc format (1,374 kb).

We have significantly improved the SST data first made available on this web site in July 1998. The day to day variations in the SST images have been reduced and obvious striping along swath edges removed. The SST binary data are now available as part of the 7 parameter binary data file described below. We invite comments, suggestions, and critiques. Please address SST e-mail to .

Other Geophysical Parameters

In addition to SST retrievals, we are able to obtain from the TMI data: surface wind speeds, atmospheric water vapor, liquid cloud water, and rain rates. The algorithms used to retrieve the data are those used in SSM/I data processing. These algorithms are described in "A Well Calibrated Ocean Algorithm for SSM/I" - PS format, (Cannot perform fsize(): Win32 Error Code = 2
kb) or PDF format, (435 kb). 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.

Two surface wind speed retrievals are provided on the web page. 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. A cursory inspection suggests that these two products are very similar. We encourage users to compare the two products and provide us with feedback. Please address wind e-mail to .

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 this occurs, the data are removed.

Daily and Time Composite Data

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.

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

Missing Data

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

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.

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.

Binary Data Files

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.

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:

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 below:

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, and Matlab programs for reading the byte maps, plus a README file.

As of July 2000, we have started processing TMI data in near real-time mode. This process 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 suggest just using the final 'v4' files.

Graphic Image Maps

Each Daily, 3-Day, Weekly or Monthly web page displays the 6 derived parameters. All data for a given map were collected on the listed date according to Greenwich Mean Time (GMT). 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 as shown in the display 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.

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.

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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Last updated: November 13, 2006