David E. Steitz

Headquarters, Washington, DC

(Phone: 202/358-1730)

Allen Kenitzer

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-2806)

RELEASE: 00-75

A microwave imager onboard a NASA spacecraft can help improve
forecasts of hurricanes and severe storms, and monitor long-term
climate by seeing through clouds, new research shows.

The Tropical Rainfall Measuring Mission (TRMM) Microwave
Imager (TMI) represents the first microwave spacecraft sensor
capable of accurately measuring sea-surface temperatures through
clouds. These findings were reported today in the Journal
Science, by Frank Wentz and colleagues at Remote Sensing Systems,
Santa Rosa, CA, who also are TRMM Science Team members.

Science team members have found that data from the TRMM
Microwave Imaging (TMI) sensor onboard the spacecraft has great
potential to increase the accuracy of tropical storm and climate

Microwave radiation penetrates clouds with little loss of
signal, providing an uninterrupted view of the ocean surface,
whereas much of the infrared radiation, typically used for
measuring sea-surface temperatures from satellites, is blocked by
cloud cover.

“The microwave imager can give consistent readings of sea-
surface temperatures even through clouds,” said Wentz, director of
Remote Sensing Systems. “To date we’ve been limited by infrared
sensors. Having the complete picture of ocean surface
temperatures should greatly improve numerical models being run by
the National Weather Service.

“After a long wait the satellite technology for measuring the
ocean’s temperature through clouds is now operational,” said
Wentz. “We expect that this new satellite capability will have a
major impact on ocean sciences and storm forecasting.”

The first microwave radiometers operating at low frequencies
were flown on the SeaSat and Nimbus-7 missions launched in 1978.
Those instruments demonstrated the feasibility of measuring sea-
surface temperatures with microwaves. The usefulness of these
early radiometers was constrained by a limited calibration system.
Though subsequent microwave radiometers, such as the Special
Sensor Microwave Imager (SSM/I) have improved calibration systems,
they still lacked the low-frequency channels needed to accurately
retrieve sea-surface temperatures.

Sea-surface temperature plays a fundamental role in the
exchange of energy, momentum, and moisture between the ocean and
the atmosphere and is a central factor of air-sea interactions and
climate variability. A better understanding of air-sea dynamics
will translate into better weather forecasting.

“Better sea-surface temperature readings will help the models
determine if the storms will gain strength,” said Max Mayfield,
acting director, National Hurricane Center, Miami, FL. “Warm
tropical waters are a fuel for hurricanes and other storms —
helping them grow in intensity.”

This is particularly important when it comes to the
forecasting of El Nino and La Nina events, which have a profound
effect on the world’s climate, and are a dramatic manifestation of
the coupling of sea-surface temperature and atmospheric

TRMM has produced continuous data since December 1997.
Tropical rainfall, which typically falls between 35 degrees north
latitude and 35 degrees south latitude, comprises more than two-
thirds of the rainfall on Earth. TRMM is NASA’s first mission
dedicated to observing and understanding tropical rainfall and how
it affects the global climate.

In the future, this all-weather capability to observe sea-
surface temperature will continue from the NASA Earth Observing
satellite called “Aqua,” which is scheduled for launch late this

TRMM is a joint U.S.-Japanese mission and part of NASA’s
Earth Science Enterprise, a long-term research program designed to
study the Earth’s land, oceans, air, ice and life as a total
system. Information and images are available at URLs: