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Introduction to TRMM

Why Study Tropical Rainfall?

The Tropical Rainfall Measuring Mission (TRMM) is a joint U.S.-Japan satellite mission to monitor tropical and subtropical precipitation and to estimate its associated latent heating.Motivations include:

  • The tropics play an important role in the global hydrological cycle, and tropical rainfall is the critical component of this role. Three-fourth of the atmosphere's heat energy derives from the release of latent heat of condensation in the process of precipitation.Two-thirds of the global precipitation occurs in the tropics.
  • The variability of tropical rainfall affects the lives and economics of more than half of the world's population.
  • The large spatial and temporal variability of rainfall systems poses a major challenge to estimating global rainfall.

TRMM provides systematic, multi-year, visible, infrared, and microwave measurements of rainfall in the tropics as key inputs to weather and climate research.The satellite observations are complemented by ground radar and rain gauge measurements to validate the satellite rain estimation techniques.

What Are The TRMM Science Objectives?

  • To obtain and study multi-year science data sets of tropical and subtropical rainfall measurements.
  • To understand how interactions between the ocean, air, and land masses produce changes in global rainfall and climate.
  • To improve modelling of tropical rainfall processes and their influence on global circulation in order to predict rainfall and its variability at various space and time scales.
  • To test, evaluate, and improve satellite rainfall measurement techniques.

Tropical Rainfall Data at the Goddard DAAC

The Tropical Rainfall Measuring Mission (TRMM) satellite was launched on November 27, 1997 (EST). Data from TRMM are archived and distributed by the Goddard DAAC and can be accessed from the DAAC's TRMM Data Search and Order Web Interface.



What Instruments Are Carried By The TRMM Satellite?

The TRMM satellite carries three rain measuring instruments.NASA GSFC provided the TRMM Microwave Imager (TMI), the Visible Infrared Scanner (VIRS), and the observatory, and operates the TRMM satellite via the Tracking and Data Relay Satellite System (TDRSS). The The Japan Aerospace Exploration Agency (JAXA) of Japan provided the Precipitation Radar (PR), the first space-borne precipitation radar, and launched the TRMM observatory.

The TRMM orbit is circular, non-sun-synchronous, at an altitude of 350 km and an inclination of 35 degrees to the Equator.This orbit provides extensive coverage in the tropics and allows each location to be covered at a different local time each day.This kind of sampling enables the analysis of the diurnal cycle of precipitation.

The VIRS (of NOAA AVHRR heritage) is a five-channel, cross-track scanning radiometer operating at 0.63, 1.6, 3.75, 10.8, and 12 um, which provides high resolution observations on cloud coverage, cloud type, and cloud top temperatures.

The TMI (of DMSP SSM/I heritage) is a multichannel passive microwave radiometer operating at five frequencies:10.65, 19.35, 37.0, and 85.5 GHz at dual polarization and 22.235 GHz at single polarization.The TMI provides information on the integrated column precipitation content, cloud liquid water, cloud ice, rain intensity, and rainfall types (e.g., stratiform or convective).

The PR, the first of its kind in space, is an electronically scanning radar, operating at 13.8 GHz that measures the 3-D rainfall distribution over both land and ocean, and define the layer depth of the precipitation.

There are two other TRMM instruments whose data are not archived at the Goddard DAAC.The Lightning Imaging Sensor (LIS), already tested in space, is a calibrated optical sensor operating at 0.7774 um, and it observes the distribution and variability of lightning over Earth.The Clouds and Earth's Radiant Energy System (CERES), of NASA's Earth Radiation Budget Experiment (ERBE) heritage, is a broadband scanning radiometer with a total spectral range of 0.3 to 50 um; it measures emitted and reflected radiative energy from Earth's surface and the atmosphere and its constituents (e.g., clouds, aerosols).

The instrument characteristics for the VIRS, TMI, and PR are shown in the following table:

TRMM Instrument Characteristics


CharacteristicVisible Infrared Scanner TRMM Microwave ImagerPrecipitation Radar
Frequency/Wavelength0.63, 1.6, 3.75, 10.8, 12 um10.65, 19.35, 37.0, 85.5 GHz dual polarization, 22.235 GHz vertical polarization 13.8 GHz horizontal polarization
Scanning ModeCross trackConicalCross track
Ground Resolution2.1 kmRanges from 5 km at 85.5 GHz to 45 km at 10.65 GHz4.3 km at nadir
Swath Width720 km 760 km220 km

How Are Satellite Data Validated?

Ground Validation (GV) radar sites provide surface radar measurements to support the validation of the satellite-derived products.Validation sites include Darwin, Australia; Melbourne, Florida; Houston, Texas; Kwajalein Atoll, Republic of Marshall Islands; Tel Aviv, Israel; Sao Paolo, Brazil; Guam, Marianas Islands; Kaohsiung, Taiwan; Om Koi, Thailand; and two multi-radar sites at Florida and Texas. Additional information can be obtained from the TRMM Office.

How Are Science Data Processed?

TRMM satellite data for each orbit are stored on board and transmitted to the ground via the TDRSS.The TRMM science data are processed by the TRMM Science Data and Information System (TSDIS) into standard products.These products are transferred to the Goddard DAAC for archival and distribution.

What TRMM Standard Products Are Available at the Goddard DAAC?

The table below summarizes the TRMM standard products available at the Goddard DAAC. Level 1 products are the VIRS calibrated radiances, the TMI brightness temperatures, and the PR return power and reflectivity measurements.Level 2 products are derived geophysical parameters at the same resolution and location as those of the Level 1 source data.Level 3 products are the time-averaged parameters mapped onto a uniform space-time grid.

TRMM Products at the Goddard DAAC


LevelVisible Infrared Scanner TRMM Microwave ImagerPrecipitation RadarCombined ProductsGround Validation (GV)
Level 1Visible & IR radiancesMicrowave brightness temperatures Radar return power & reflectivity NACal. radar reflectivity at each GV site
Level 2NA TMI profile for CLW, prec. water, cloud ice, prec. ice, latent heat, & surface rainPR surface cross-section & path attenuation, rain type, storm, & freezing height; PR profile for rain rate, reflec., attenuation, & rain top/bottom height Rain rate, drop size dist. parameters, path integrated attenuationRain existence, rain map, rain type, 3-D reflectivity, rain gauge, disdrometer
Level 3NA TMI monthly rainfall, rain rate, rain frequency, & freezing heightPR monthly surface rain total, rain profile at 2, 4, 6, 10 & 15 km, fractional rain, storm height histogram, snow ice layer, surface rain rate, & path attenuationMonthly surface rainfall, CLW, rain water, cloud ice, & grauples; combined instruments calibration, global gridded rainfallRain map, 3-D map

How Can One Access Data and Services From the Goddard DAAC?

TRMM standard products can be ordered from the Goddard DAAC, via the TRMM Data Search and Order Web Interface. In addition to the standard products, the Goddard DAAC also provides, as part of its value-added TRMM support to facilitate analysis and processing by users, certain special products, including:

  1. Orbital gridded products, e.g., for the TMI Hydrometeor Profile standard product, a 0.5 degree by 0.5 degree resolution special product.
  2. Geographical subsets of the orbital gridded products.
  3. Parameter subsets of selected standard products.
  4. Satellite and ground radar coincidence subset products for the Ground Validation sites.
  5. Geographical Information System (GIS)-compatible TRMM products.

Data sets are distributed via FTP.Potential TRMM data users, especially those with specific needs, are urged to contact the Hydrology Data Support Team: .

Please send us your name and email so we can keep you informed.

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Last updated: May 18, 2010 10:20 AM ET