Rain is the way the atmosphere sweats: water vapor condenses into liquid drops and thereby releases heat into the ambient air. Through this "latent" heating, global rainfall produces about three times as much energy as the other major source of atmospheric heating, direct shortwave solar radiation.

The energy produced by this condensation of water vapor is the dominant driver of global-scale motions in the troposphere (try the Q&A page for details about the role of rain in weather dynamics). Thus, to understand climate change in general, and the heat exchange between ocean, atmosphere, and land in particular, one must have accurate global measurements of rainfall. About two thirds of all rainfall takes place over the tropics, three quarters of which consist of oceans. Because of the inaccessibility and vastness of the area in question, one must resort to remote sensing techniques.

The Tropical Rainfall Measuring Mission (TRMM) satellite, a joint project between the United States (under the leadership of NASA's Goddard Space Flight Center, which hosts the main TRMM web site) and Japan (under the leadership of the National Space Development Agency), and the first spacecraft designed to monitor rain over the tropics, was successfully launched from Tanegashima, Japan, on November 27, 1997, at 13:27pm Los Angeles (California) time. It placed in low earth orbit the first precipitation radar (PR) to be flown in space, along with a 9-channel SSM/I-like passive microwave imager (TMI), an AVHRR-like visible-infrared radiometer (VIRS), a lightning sensor and a cloud sensor.

The PR measures the echo backscattered from rain: because the strength of the echo is roughly proportional to the square of the volume of falling water, the PR produces very accurate estimates of rain profiles. The TMI measures the microwave radiation emitted by Earth's surface and by cloud and rain drops. Because large ice particles (often present in upper cloud regions) tend to scatter this emitted radiation, the TMI uses its various channels along with cloud models to discriminate between these processes and quantify the rain and ice responsible for the observed microwave signatures.

One of the main challenges that the TRMM science team has had to face is to develop algorithms that can translate the electromagnetic measurements obtained from the PR, TMI and VIRS instruments together into estimates of the instantaneous rain rate profiles through the underlying precipitation systems.

The effort underway at JPL in support of TRMM is eight-fold:

1. Analysis of the interannual variability of global rainfall as estimated by TRMM (last update: November 11, 2004)
2. Development, validation and updating of the combined PR-TMI retrieval algorithm (last update: April 6, 1998)
3. Derivation of an optimal approach to combine the measurements of all instruments on the core spacecraft of the Global Precipitation Measurement mission (the TRMM follow-on), to develop a GPM core reference algorithm (last update: July 15, 2003)
4. Analysis of the effects of the raindrop size distribution on the TRMM and GPM retrieval algorithms (last update: July 2, 2003)
5. Design, manufacture, and deployment of airborne and spaceborne precipitation-profiling radars. (last update: September 23, 2003)
6. Development of synthetic-aperture Doppler processing to profile the vertical component of the wind. (last update: March 2, 2004)
7. Study of the SIR-C/X-SAR Rain Experiment results, including the first radar images of rain from space. (last update: July 10, 2003)
8. Comparison of different methods to derive non-paramteric radar-rain relations, and how they affect rain-retrieval algorithms. (last update: December 10, 1996)

Click here for a list of TRMM related links.