| Instrument Description | Data Processing | Binary Files | Data Access | Related Data | Data Use Policy |
Introduction
The Tropical Rainfall Measuring Mission (TRMM) was launched November 27, 1997 carrying 5 instruments, one of which is the TRMM Microwave Imager (TMI). 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 was similar to that of other satellite radiometers but the resolution of data measurement was better due to the lower altitude of the satellite orbit. TRMM traveled in a semi-equatorial orbit with an inclination of 35 deg. This type of orbit precessed approximately 7 degrees per day allowing for TMI to sample the surface at all times of day as opposed to the twice per day windows of the polar-orbiting radiometers. TRMM was a joint program between NASA and the Japan Aerospace Exploration Agency (JAXA). In addition to rain rates, TMI also measured sea surface temperatures (SST), ocean surface wind speeds, columnar water vapor, and cloud liquid water. The SSTs from TMI were the first satellite microwave SSTs available and have proven to be of great value to many areas of research.
The initial production of this data set was a collaborative effort with the TRMM Project at NASA Goddard Space Flight Center 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.
Instrument Description
For a detailed description of TRMM, see the eoPortal Directory. TMI operated on the TRMM satellite. It had 5 channels as outlined in the table below. Two external calibration targets (one cold, one hot) were used to maintain consistency in the measurements over time. After a post-launch checkout period, data started on December, 7th 1997 and ended on April 8th when the instrument was shut down. TRMM underwent an orbital boost in August 2001 in order to increase the lifetime of the satellite and instruments. This boost in altitude (from 350 km to 400 km) allowed for many more years of operation. However, starting in mid-2014 the TRMM orbit started to decline as there was insufficient fuel remaining to maintain the orbit. RSS continued routine data processing until April 6th 2015 just before the TRMM satellite was shut down on April 8th, 2015.
The table below provides the spatial resolution of each channel for both the pre and post boost altitudes. During decline in 2014-2015, the footprint sizes of the measurements changed constantly. The satellite routinely experienced a 180 deg yaw maneuver every 2 to 4 weeks. TRMM entered the atmosphere on June 16th, 2015.
| 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 |
Data Processing
We obtained TMI brightness temperature data files (TDRs) from NASA Goddard and reverse engineered the data back to raw radiometer counts. Using a consistent processing scheme and a robust radiative transfer model, we intercalibrated the TMI data with the other microwave radiometers, obtained brightness temperatures, and then produced our ocean measurement products for distribution. The current version of the RSS radiative transfer model and radiometer processing code is called Version-7 (V7). This carefully applied intercalibration yields consistent products from all microwave radiometer data processed at RSS.
A description of the updates to TMI Ocean Products summarizes the changes. Most importantly, the data file format has changed. A paper has been submitted to the Journal of Climate for publication that describes in detail the changes to the TMI intercalibration, geolocation, and data processing.
RSS TMI Data Products
The RSS TMI data files contain: sea surface temperatures, surface wind speeds, atmospheric water vapor, cloud liquid 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 consistent among all radiometers processed by RSS. Appropriate adjustments have been made to account for small differences in channel frequencies.
The current version of TMI data is V7.1 TMI v7.1 data were released in January 2015. The prior version (V4) should no longer be used. There are several improvements that make V7.1 the best data set that is most suitable for climate study. The significant changes made are summarized in the V7 document.
Similar to AMSR-E, AMSR2, and WindSat, two surface wind speeds are provided for TMI. The first is a surface wind speed derived primarily from the 11 GHz channel and above (we refer to this as the Low-Frequency wind product, WSPD-LF). The second surface wind speed retrieval relies on measurements made by the 37 GHz channels and above like those available for SSM/I (we refere to this as the Medium-Frequency wind product, WSPD-MF). We find these two wind products are very similar with some small differences near rain and land. When comparing winds from TMI and SSMI, the WSPD-MF values must be used. Like the sea surface temperatures, the 11GHz surface wind speeds have small regions of missing data that occur where the sunlight reflects off the ocean surface (also called "sun glint"). Where there is sunglint, affected data are removed.
Similar to other microwave radiometers, the growing problem of radio frequency interference (RFI) produces bad data if not removed. RFI occurs in large regions where geostationary tv broadcast or telecommunication signals in the microwave range reflect off the ocean and are observed by the radiometer. RFI can also occur in small regions due to emitting sources on the surface where microwave-range signals are used. We now remove any RFI-affected data using a semi-automated process consistently applied to all RSS data sets.
Gridded Binary Data Files
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 with valid data extending from 40S to 40N at a pixel resolution of 0.25 deg (~25 km). The files contain no headers or metadata and are a simple set of single-byte arrays. Since the data are stored as single bytes, no byte-swapping is needed when transferring between computers. These 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 at the 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 (and thereby the quality) 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.
Each DAILY binary data file available from our ftp site consists of fourteen 0.25 x 0.25 degree grid (1440,720) byte maps. Seven ascending maps in the following order: Time (T), Sea Surface Temperature (S), 10-meter Surface Wind Speed using 11 GHz (WSPD-LF), 10-meter Surface Wind Speed using 37 GHz (WSPD-MF), 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 34.5 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, sea ice or near land (~25 km) |
| CLOUD | Columnar cloud liquid water | Total cloud liquid water contained in a vertical column of atmosphere |
0.01 | -0.05 | -0.05 to 2.45 mm | near sea ice or land (~25 km) |
| RAIN | Rain rate | Rate of liquid water precipitation | 0.1 | 0. | 0. to 25.0 mm/hr | near sea ice or 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 provided they were measured by successive orbits (<50 minutes apart). 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, the last data from the day overwrite the first data measured at the beginning of the day.
DAILY, 3-day and monthly files are located in the directory /tmi/bmaps_v07.1/yYYYY/mMM. Each daily file name has the convention: yyyymmddv7.1.gz. The 3-day and monthly files have the conventions F12_yyyymmddv7.1_d3d.gz and F12_yyyymmv7.1.gz. Weekly files are located in the directory /tmi/bmaps_v07.1/weeks/, with the file name convention F12_yyyymmddv7.gz where
| fs | file specifier | F12 |
| 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_v07.1/y1999/m03/F12_19990305v7.1.gz
The center of the first cell of the 1440 column and 320 row map is at 0.125 E longitude and -89.875 latitude. The center of the second cell is 0.375 E longitude, -89.875 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 1440,720,6 array. There are only six maps because the Time layer is omitted. All other parameters are present and in the same order.
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.remss.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, C++, and Python programs for reading the byte maps, plus a README file.
Browse Images
Each Daily, 3-Day, Weekly or Monthly web page displays one ocean measurment: Sea Surface Temperature (SST), 10 meter Surface Wind Speed using low frequency channels (WSPD_LF), 10 meter Surface Wind Speed using medium frequency channels (WSPD_MF), Columnar Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), and Rain Rate (RAIN). All data for a given map were collected on that specific GMT date. The date at which the observation was made with respect to local time may vary depending on the grid longitude. The graphic image maps were produced from public V7.1 TMI binary data files (described above) using IDL. The maps are scaled to improve data visibility and do not show the entire valid 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 rain prohibited an accurate retrieval.
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 December 31, 2014. 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.remss.com/.
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 files from PO.DAAC. These are currently Version-4 TMI data.
Level 2C swath TMI geophysical data files are available from NASA GHRC data center. These files consist of individual orbits of ocean geophyscial products: SST, Wind, Vapor, Cloud and Rain Rates. The data set is currently only in V4. We expect to update to Version-7.1 soon.
References
Meissner, T., and F. J. Wentz, (2012), The emissivity of the ocean surface between 6 - 90 GHz over a large range of wind speeds and Earth incidence angles, IEEE TGRS, 50(8), 3004-3026.
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.
Hilburn, K. A., and F. J. Wentz, (2008), Intercalibrated passive microwave rain products from the unified microwave ocean retrieval algorithm, Journal of Applied Meteorology and Climatology, 47, 778-795.
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, Journal of Geophysical Research, 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 were produced by Remote Sensing Systems and sponsored by NASA. RSS TMI data are available at www.remss.com. We are grateful to NASA Goddard Space Flight Center for providing access to TMI TDR files.
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 were produced by Remote Sensing Systems and sponsored by the NASA Earth Sciences Program. Data are available at www.remss.com/missions/tmi. "
An official data citation for use in publications is given below. Insert the appropriate information in brackets.
Wentz, F.J.,C. Gentemann, K.A. Hilburn, 2015: Remote Sensing Systems TRMM TMI [indicate whether you used Daily, 3-Day, Weekly, or Monthly] Environmental Suite on 0.25 deg grid, Version 7.1, [indicate subset if used]. Remote Sensing Systems, Santa Rosa, CA. Available online at www.remss.com/missions/tmi. [Accessed dd mmm yyyy].